Radio-Activity




                       CAMBRIDGE PHYSICAL SERIES.

              GENERAL EDITORS:—F. H. NEVILLE, M.A., F.R.S.
                   AND W. C. D. WHETHAM, M.A., F.R.S.


                             RADIO-ACTIVITY




                  CAMBRIDGE UNIVERSITY PRESS WAREHOUSE
                          C. F. CLAY, MANAGER.
                       London: FETTER LANE, E.C.
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                                  ALSO

              London: H. K. LEWIS, 136, GOWER STREET, W.C.
                       Leipzig: F. A. BROCKHAUS.
                    New York: THE MACMILLAN COMPANY.
              Bombay and Calcutta: MACMILLAN AND CO., LTD.


                        [_All Rights reserved._]




                             RADIO-ACTIVITY


                                   BY

                 E. RUTHERFORD, D.Sc., F.R.S., F.R.S.C.
      MACDONALD PROFESSOR OF PHYSICS, McGILL UNIVERSITY, MONTREAL


                             SECOND EDITION


                               CAMBRIDGE
                        AT THE UNIVERSITY PRESS
                                  1905




                          _First Edition 1904_
                         _Second Edition 1905_




                             J. J. THOMSON

                 A TRIBUTE OF MY RESPECT AND ADMIRATION




                     PREFACE TO THE FIRST EDITION.


In this work, I have endeavoured to give a complete and connected
account, from a physical standpoint, of the properties possessed by the
naturally radio-active bodies. Although the subject is comparatively a
new one, our knowledge of the properties of the radio-active substances
has advanced with great rapidity, and there is now a very large amount
of information on the subject scattered throughout the various
scientific journals.

The phenomena exhibited by the radio-active bodies are extremely
complicated, and some form of theory is essential in order to connect in
an intelligible manner the mass of experimental facts that have now been
accumulated. I have found the theory that the atoms of the radio-active
bodies are undergoing spontaneous disintegration extremely serviceable,
not only in correlating the known phenomena, but also in suggesting new
lines of research.

The interpretation of the results has, to a large extent, been based on
the disintegration theory, and the logical deductions to be drawn from
the application of the theory to radio-active phenomena have also been
considered.

The rapid advance of our knowledge of radio-activity has been dependent
on the information already gained by research into the electric
properties of gases. The action possessed by the radiations from
radio-active bodies of producing charged carriers or ions in the gas,
has formed the basis of an accurate quantitative method of examination
of the properties of the radiations and of radio-active processes, and
also allows us to determine with considerable certainty the order of
magnitude of the different quantities involved.

For these reasons, it has been thought advisable to give a brief account
of the electric properties of gases, to the extent that is necessary for
the interpretation of the results of measurements in radio-activity by
the electric method. The chapter on the ionization theory of gases was
written before the publication of J. J. Thomson’s recent book on
“Conduction of Electricity through Gases,” in which the whole subject is
treated in a complete and connected manner.

A short chapter has been added, in which an account is given of the
methods of measurement which, in the experience of the writer and
others, are most suitable for accurate work in radio-activity. It is
hoped that such an account may be of some service to those who may wish
to obtain a practical acquaintance with the methods employed in
radio-active measurements.

My thanks are due to Mr W. C. Dampier Whetham, F.R.S., one of the
editors of the Cambridge Physical Series, for many valuable suggestions,
and for the great care and trouble he has taken in revising the proof
sheets. I am also much indebted to my wife and Miss H. Brooks for their
kind assistance in correcting the proofs, and to Mr R. K. McClung for
revising the index.

    E. R.

    MACDONALD PHYSICS BUILDING,
    MONTREAL,
    _February, 1904_.




                     PREFACE TO THE SECOND EDITION.


I feel that some apology is due to my readers for bringing out at such
an early date a new edition which includes so much new material, and in
which the rearrangement is so extensive as to constitute almost a new
work. Though only a year has passed since the book first made its
appearance, the researches that have been carried out in that time have
been too numerous and of too important a character to permit the
publishing of a mere reprint, unless the author were to relinquish his
purpose of presenting the subject as it stands at the present moment.

The three new chapters which have been added possibly constitute the
most important change in the work. These chapters include a detailed
account of the theory of successive changes and of its application to
the analysis of the series of transformations which occur in radium,
thorium, and actinium.

The disintegration theory, which was put forward in the first edition as
an explanation of radio-active phenomena, has in these later researches
proved to be a most powerful and valuable method of analysing the
connection between the series of substances which arise from the
transformation of the radio-elements. It has disclosed the origin of
radium, of polonium and radio-tellurium, and of radio-lead, and now
binds together in one coherent whole the large mass of apparently
heterogeneous experimental facts in radio-activity which have been
accumulating since 1896. The theory has received a remarkable measure of
verification in the past year, and, in many cases, has offered a
quantitative as well as a qualitative explanation of the connection
between the various properties exhibited by the radio-active bodies. In
the light of this evidence, radio-activity may claim to have assumed the
position of an independent subject, though one with close affinities to
physics on the one hand and to chemistry on the other.

The present edition includes a large amount of new material relating to
the nature and properties of the radiations and the emanations. In the
limits of this book, it would have been found impossible, even had it
been thought desirable, to include more than a brief sketch of the
physiological effects of the rays. The literature on this subject is
already large, and is increasing rapidly. For reasons of space, I have
not been able to refer more than briefly to the mass of papers that have
appeared dealing with the examination of various spring and well waters,
sediments, and soils, for the presence of radio-active matter.

In order to make the book more self-contained, a short account has been
given in Chapter II of the magnetic field produced by an ion in motion,
of the action of an external magnetic and electric field upon it, and of
the determination of the velocity and mass of the particles constituting
the cathode stream.

Two appendices have been added, one giving an account of some work upon
the α rays which was completed too late for inclusion in the subject
matter of the book, and the other containing a brief summary of what is
known in regard to the chemical constitution of the various radio-active
minerals, the localities in which they are found, and their probable
geologic age. For the preparation of the latter, I am indebted to my
friend Dr Boltwood of New Haven, who, in the course of his researches,
has had occasion to analyse most of these minerals in order to determine
their content of uranium and radium. I hope that this account of
radio-active minerals will prove of value to those who are endeavouring
to elucidate the connection between the various radio-active substances
and the inactive products which arise from their transformation.

For the convenience of those who have read the first edition, a list of
the sections and chapters which contain the most important additions and
alterations is added below the table of contents.

The writing of a complete account of a subject like radio-activity, in
which so much new work is constantly appearing, has been a matter of no
little difficulty. Among other things it has involved a continuous
revision of the work while the volume was passing through the press.

I wish to express my thanks to my colleague Professor Harkness for the
care and trouble he has taken in revising the proofs and for many useful
suggestions; also to Mr R. K. McClung for his assistance in correcting
some of the proofs and in preparing the index.

    E. R.


    MCGILL UNIVERSITY,
    MONTREAL,
    _9 May, 1905_.

                          Transcriber’s Note:

The plain text version of this ebook includes complex mathematical
formulas in TeX notation. These formulas are enclosed between double
dollar signs. The HTML version includes the same formulas rendered in
their original form.




                                ERRATA.


                          Transcriber’s Note:

These corrections have been applied to the text in the book.

    page 48, line 24 section 218 should read section 284
      „ 77, last line „ 263 „ „ „ 270
      „ 123, 5th line from bottom „ 254 „ „ „ 261
      „ 124, 10th „ „ „ „ 246 „ „ „ 253
      „ 151, line 3 „ 228 „ „ „ 229
      „ 156, 13th line from bottom „ 261 „ „ „ 268
      „ 200, line 9 „ 246 „ „ „ 253
      „ 216, line 3 „ 260 „ „ „ 267
      „ 184, at the top of 5th column of table the letter γ should be
         inserted.




                           TABLE OF CONTENTS.


I. Radio-active Substances 1

II. Ionization Theory of Gases 31

III. Methods of Measurement 82

IV. Nature of the Radiations 108

V. Properties of the Radiations 201

VI. Continuous Production of Radio-active Matter 218

VII. Radio-active Emanations 238

VIII. Excited Radio-activity 295

IX. Theory of Successive Changes 325

X. Transformation Products of Uranium, Thorium and Actinium 346

XI. Transformation Products of Radium 371

XII. Rate of Emission of Energy 418

XIII. Radio-active Processes 437

XIV. Radio-activity of the Atmosphere and of Ordinary Materials 501

      Appendix A. Properties of the α Rays 543

      Appendix B. Radio-active Minerals 554

      Index 559

Plate (Fig. 46A: Spectrum of Radium Bromide) _to face p._ 206

                  *       *       *       *       *

For the convenience of the reader, the sections and chapters which
contain mostly new matter, or have been either partly or wholly
rewritten, are appended below.

    Chap.  I.  Sections 18, 20–23.
     „    II.    „      48–52.
     „   III.    „      69.
     „    IV.    „      83–85, 92, 93, 103, 104, 106–108, 111, 112.
     „     V.    „      115, 117, 119, 122.
     „   VII.    „      171–173.
     „  VIII.    „      182–184, 190.
     „ IX-XIV. Mostly rewritten.




ABBREVIATIONS OF REFERENCES TO SOME OF THE JOURNALS.


    _Ber. d. deutsch. Chem. Ges._ Berichte der deutschen chemischen
    Gesellschaft. Berlin.

    _C. R._ Comptes Rendus des Séances de l’Académie des Sciences.
    Paris.

    _Chem. News._ Chemical News. London.

    _Drude’s Annal._ Annalen der Physik. Leipzig.

    _Phil. Mag._ Philosophical Magazine and Journal of Science. London.

    _Phil. Trans._ Philosophical Transactions of the Royal Society of
    London.

    _Phys. Rev._ Physical Review. New York.

    _Phys. Zeit._ Physikalische Zeitschrift.

    _Proc. Camb. Phil. Soc._ Proceedings of the Cambridge Philosophical
    Society. Cambridge.

    _Proc. Roy. Soc._ Proceedings of the Royal Society of London.

    _Thèses-Paris._ Thèses présentées à la Faculté des Sciences de
    l’Université de Paris.

    _Wied. Annal._ Annalen der Physik. Leipzig.




                               CHAPTER I.
                        RADIO-ACTIVE SUBSTANCES.


=1. Introduction.= The close of the old and the beginning of the new
century have been marked by a very rapid increase of our knowledge of
that most important but comparatively little known subject—the
connection between electricity and matter. No study has been more
fruitful in surprises to the investigator, both from the remarkable
nature of the phenomena exhibited and from the laws controlling them.
The more the subject is examined, the more complex must we suppose the
constitution of matter in order to explain the remarkable effects
observed. While the experimental results have led to the view that the
constitution of the atom itself is very complex, at the same time they
have confirmed the old theory of the discontinuous or atomic structure
of matter. The study of the radio-active substances and of the discharge
of electricity through gases has supplied very strong experimental
evidence in support of the fundamental ideas of the existing atomic
theory. It has also indicated that the atom itself is not the smallest
unit of matter, but is a complicated structure made up of a number of
smaller bodies.

A great impetus to the study of this subject was initially given by the
experiments of Lenard on the cathode rays, and by Röntgen’s discovery of
the X rays. An examination of the conductivity imparted to a gas by the
X rays led to a clear view of the mechanism of the transport of
electricity through gases by means of charged ions. This ionization
theory of gases has been shown to afford a satisfactory explanation not
only of the passage of electricity through flames and vapours, but also
of the complicated phenomena observed when a discharge of electricity
passes through a vacuum tube. At the same time, a further study of the
cathode rays showed that they consisted of a stream of material
particles, projected with great velocity, and possessing an apparent
mass small compared with that of the hydrogen atom. The connection
between the cathode and Röntgen rays and the nature of the latter were
also elucidated. Much of this admirable experimental work on the nature
of the electric discharge has been done by Professor J. J. Thomson and
his students in the Cavendish Laboratory, Cambridge.

An examination of natural substances, in order to see if they gave out
dark radiations similar to X rays, led to the discovery of the
radio-active bodies which possess the property of spontaneously emitting
radiations, invisible to the eye, but readily detected by their action
on photographic plates and their power of discharging electrified
bodies. A detailed study of the radio-active bodies has revealed many
new and surprising phenomena which have thrown much light, not only on
the nature of the radiations themselves, but also on the processes
occurring in those substances. Notwithstanding the complex nature of the
phenomena, the knowledge of the subject has advanced with great
rapidity, and a large amount of experimental data has now been
accumulated.

In order to explain the phenomena of radio-activity, Rutherford and
Soddy have advanced a theory which regards the atoms of the radio-active
elements as suffering spontaneous disintegration, and giving rise to a
series of radio-active substances which differ in chemical properties
from the parent elements. The radiations accompany the breaking-up of
the atoms, and afford a comparative measure of the rate at which the
disintegration takes place. This theory is found to account in a
satisfactory way for all the known facts of radio-activity, and welds a
mass of disconnected facts into one homogeneous whole. On this view, the
continuous emission of energy from the active bodies is derived from the
internal energy inherent in the atom, and does not in any way contradict
the law of the conservation of energy. At the same time, however, it
indicates that an enormous store of latent energy is resident in the
radio-atoms themselves. This store of energy has not been observed
previously, on account of the impossibility of breaking up into simpler
forms the atoms of the elements by the action of the chemical or
physical forces at our command.

On this theory we are witnessing in the radio-active bodies a veritable
transformation of matter. This process of disintegration was
investigated, not by direct chemical methods, but by means of the
property possessed by the radio-active bodies of giving out specific
types of radiation. Except in the case of a very active element like
radium, the process of disintegration takes place so slowly, that
hundreds if not thousands of years would be required before the amount
transformed would come within the range of detection of the balance or
the spectroscope. In radium, however, the process of disintegration
takes place at such a rate that it should be possible within a limited
space of time to obtain definite chemical evidence on this question. The
recent discovery that helium can be obtained from radium adds strong
confirmation to the theory; for helium was indicated as a probable
disintegration product of the radio-active elements before this
experimental evidence was forthcoming. Several products of the
transformation of the radio-active bodies have already been examined,
and the further study of these substances promises to open up new and
important fields of chemical enquiry.

In this book the experimental facts of radio-activity and the connection
between them are interpreted on the disintegration theory. Many of the
phenomena observed can be investigated in a quantitative manner, and
prominence has been given to work of this character, for the agreement
of any theory with the facts, which it attempts to explain, must
ultimately depend upon the results of accurate measurement.

The value of any working theory depends upon the number of experimental
facts it serves to correlate, and upon its power of suggesting new lines
of work. In these respects the disintegration theory, whether or not it
may ultimately be proved to be correct, has already been justified by
its results.


=2. Radio-active Substances.= The term “radio-active” is now generally
applied to a class of substances, such as uranium, thorium, radium, and
their compounds, which possess the property of _spontaneously_ emitting
radiations capable of passing through plates of metal and other
substances opaque to ordinary light. The characteristic property of
these radiations, besides their penetrating power, is their action on a
photographic plate and their power of discharging electrified bodies. In
addition, a strongly radio-active body like radium is able to cause
marked phosphorescence and fluorescence on some substances placed near
it. In the above respects the radiations possess properties analogous to
Röntgen rays, but it will be shown that, for the major part of the
radiations emitted, the resemblance is only superficial.

The most remarkable property of the radio-active bodies is their power
of radiating energy spontaneously and continuously at a constant rate,
without, as far as is known, the action upon them of any external
exciting cause. The phenomena at first sight appear to be in direct
contradiction to the law of conservation of energy, since no obvious
change with time occurs in the radiating material. The phenomena appear
still more remarkable when it is considered that the radio-active bodies
must have been steadily radiating energy since the time of their
formation in the earth’s crust.

Immediately after Röntgen’s discovery of the production of X rays,
several physicists were led to examine if any natural bodies possessed
the property of giving out radiations which could penetrate metals and
other substances opaque to light. As the production of X rays seemed to
be connected in some way with cathode rays, which cause strong
fluorescent and phosphorescent effects on various bodies, the substances
first examined were those that were phosphorescent when exposed to
light. The first observation in this direction was made by
Niewenglowski[1], who found that sulphide of calcium exposed to the
sun’s rays gave out some rays which were able to pass through black
paper. A little later a similar result was recorded by H. Becquerel[2]
for a special calcium sulphide preparation, and by Troost[3] for a
specimen of hexagonal blend. These results were confirmed and extended
in a later paper by Arnold[4]. No satisfactory explanations of these
somewhat doubtful results have yet been given, except on the view that
the black paper was transparent to some of the light waves. At the same
time Le Bon[5] showed that, by the action of sunlight on certain bodies,
a radiation was given out, invisible to the eye, but active with regard
to a photographic plate. These results have been the subject of much
discussion; but there seems to be little doubt that the effects are due
to short ultra-violet light waves, capable of passing through certain
substances opaque to ordinary light. These effects, while interesting in
themselves, are quite distinct in character from those shown by the
radio-active bodies which will now be considered.


=3. Uranium.= The first important discovery in the subject of
radio-activity was made in February, 1896, by M. Henri Becquerel[6], who
found that a uranium salt, the double sulphate of uranium and potassium,
emitted some rays which gave an impression on a photographic plate
enveloped in black paper. These rays were also able to pass through thin
plates of metals and other substances opaque to light. The impressions
on the plate could not have been due to vapours given off by the
substances, since the same effect was produced whether the salt was
placed directly on the black paper or on a thin plate of glass lying
upon it.

Becquerel found later that all the compounds of uranium as well as the
metal itself possessed the same property, and, although the amount of
action varied slightly for the different compounds, the effects in all
cases were comparable. It was at first natural to suppose that the
emission of these rays was in some way connected with the power of
phosphorescence, but later observations showed that there was no
connection whatever between them. The uranic salts are phosphorescent,
while the uranous salts are not. The uranic salts, when exposed to
ultra-violet light in the phosphoroscope, give a phosphorescent light
lasting about ·01 seconds. When the salts are dissolved in water, the
duration is still less. The amount of action on the photographic plate
does not depend on the particular compound of uranium employed, but only
on the amount of uranium present in the compound. The non-phosphorescent
are equally active with the phosphorescent compounds. The amount of
radiation given out is unaltered if the active body be kept continuously
in darkness. The rays are given out by solutions, and by crystals which
have been deposited from solutions in the dark and never exposed to
light. This shows that the radiation cannot be due in any way to the
gradual emission of energy stored up in the crystal in consequence of
exposure to a source of light.


=4.= The power of giving out penetrating rays thus seems to be a
specific property of the element uranium, since it is exhibited by the
metal as well as by all its compounds. These radiations from uranium are
persistent, and, as far as observations have yet gone, are unchanged,
either in intensity or character, with lapse of time. Observations to
test the constancy of the radiations for long periods of time have been
made by Becquerel. Samples of uranic and uranous salts have been kept in
a double box of thick lead, and the whole has been preserved from
exposure to light. By a simple arrangement, a photographic plate can be
introduced in a definite position above the uranium salts, which are
covered with a layer of black paper. The plate is exposed at intervals
for 48 hours, and the impression on the plate compared. No perceptible
weakening of the radiation has been observed over a period of four
years. Mme Curie[7] has made determinations of the activity of uranium
over a space of five years by an electric method described later, but
found no appreciable variation during that period.

Since the uranium is thus continuously radiating energy from itself,
without any known source of excitation, the question arises whether any
known agent is able to affect the rate of its emission. No alteration
was observed when the body was exposed to ultra-violet light or to
ultra-red light or to X rays. Becquerel states that the double sulphate
of uranium and potassium showed a slight increase of action when exposed
to the arc light and to sparks, but he considers that the feeble effect
observed was another action superimposed on the constant radiation from
uranium. The intensity of the uranium radiation is not affected by a
variation of temperature between 200° C. and the temperature of liquid
air. This question is discussed in more detail later.


=5.= In addition to these actions on a photographic plate, Becquerel
showed that uranium rays, like Röntgen rays, possess the important
property of discharging both positively and negatively electrified
bodies. These results were confirmed and extended by Lord Kelvin, Smolan
and Beattie[8]. The writer made a detailed comparison[9] of the nature
of the discharge produced by uranium with that produced by Röntgen rays,
and showed that the discharging property of uranium is due to the
production of charged ions by the radiation throughout the volume of the
gas. The property has been made the basis of a qualitative and
quantitative examination of the radiations from all radio-active bodies,
and is discussed in detail in chapter II.

The radiations from uranium are thus analogous, as regards their
photographic and electrical actions, to Röntgen rays, but, compared with
the rays from an ordinary X ray tube, these actions are extremely
feeble. While with Röntgen rays a strong impression is produced on a
photographic plate in a few minutes or even seconds, several days’
exposure to the uranium rays is required to produce a well-marked
action, even though the uranium compound, enveloped in black paper, is
placed close to the plate. The discharging action, while very easily
measurable by suitable methods, is also small compared with that
produced by X rays from an ordinary tube.


=6.= The rays from uranium show no evidence of direct reflection,
refraction, or polarization[10]. While there is no direct reflection of
the rays, there is apparently a diffuse reflection produced where the
rays strike a solid obstacle. This is due in reality to a secondary
radiation set up when the primary rays impinge upon matter. The presence
of this secondary radiation at first gave rise to the erroneous view
that the rays could be reflected and refracted like ordinary light. The
absence of reflection, refraction, or polarization in the penetrating
rays from uranium necessarily follows in the light of our present
knowledge of the rays. It is now known that the uranium rays, mainly
responsible for the photographic action, are deviable by a magnetic
field, and are similar in all respects to cathode rays, _i.e._ the rays
are composed of small particles projected at great velocities. The
absence of the ordinary properties of transverse light waves is thus to
be expected.


=7.= The rays from uranium are complex in character, and, in addition to
the penetrating deviable rays, there is also given off a radiation very
readily absorbed by passing through thin layers of metal foil, or by
traversing a few centimetres of air. The photographic action due to
these rays is very feeble in comparison with that of the penetrating
rays, although the discharge of electrified bodies is mainly caused by
them. Besides these two types of rays, some rays are emitted which are
of an extremely penetrating character and are non-deviable by a magnetic
field. These rays are difficult to detect photographically, but can
readily be examined by the electric method.


=8.= The question naturally arose whether the property of spontaneously
giving out penetrating radiations was confined to uranium and its
compounds, or whether it was exhibited to any appreciable extent by
other substances.

By the electrical method, with an electrometer of ordinary
sensitiveness, any body which possesses an activity of the order of
¹⁄₁₀₀ of that of uranium can be detected. With an electroscope of
special construction, such as has been designed by C. T. R. Wilson for
his experiments on the natural ionization of air, a substance of
activity ¹⁄₁₀₀₀₀ and probably ¹⁄₁₀₀₀₀₀ of that of uranium can be
detected.

If an active body like uranium be mixed with an inactive body, the
resulting activity in the mixture is generally considerably less than
that due to the active substance alone. This is due to the absorption of
the radiation by the inactive matter present. The amount of decrease
largely depends on the thickness of the layer from which the activity is
determined.

Mme Curie made a detailed examination by the electrical method of the
great majority of known substances, including the very rare elements, to
see if they possessed any activity. In cases where it was possible,
several compounds of the elements were examined. With the exception of
thorium and phosphorus, none of the other substances possessed an
activity even of the order of ¹⁄₁₀₀ of uranium.

The ionization of the gas by phosphorus does not, however, seem to be
due to a penetrating radiation like that found in the case of uranium,
but rather to a chemical action taking place at its surface. The
compounds of phosphorus do not show any activity, and in this respect
differ from uranium and the other active bodies.

Le Bon[11] has also observed that quinine sulphate, if heated and then
allowed to cool, possesses for a short time the property of discharging
both positively and negatively electrified bodies. It is necessary,
however, to draw a sharp line of distinction between phenomena of this
kind and those exhibited by the naturally radio-active bodies. While
both, under special conditions, possess the property of ionizing the
gas, the laws controlling the phenomena are quite distinct in the two
cases. For example, only one compound of quinine shows the property, and
that compound only when it has been subjected to a preliminary heating.
The action of phosphorus depends on the nature of the gas, and varies
with temperature. On the other hand, the activity of the naturally
radio-active bodies is spontaneous and permanent. It is exhibited by all
compounds, and is not, as far as is yet known, altered by change in the
chemical or physical conditions.


=9.= The discharging and photographic action alone cannot be taken as a
criterion as to whether a substance is radio-active or not. It is
necessary in addition to examine the radiations, and to test whether the
actions take place through appreciable thicknesses of all kinds of
matter opaque to ordinary light. For example, a body giving out short
waves of ultra-violet light can be made to behave in many respects like
a radio-active body. As Lenard[12] has shown, short waves of
ultra-violet light will ionize the gas in their path, and will be
absorbed rapidly in the gas. They will produce strong photographic
action, and may pass through _some_ substances opaque to ordinary light.
The similarity to a radio-active body is thus fairly complete as regards
these properties. On the other hand, the emission of these light waves,
unlike that of the radiations from an active body, will depend largely
on the molecular state of the compound, or on temperature and other
physical conditions. But the great point of distinction lies in the
nature of the radiations from the bodies in question. In one case the
radiations behave as transverse waves, obeying the usual laws of light
waves, while in the case of a naturally active body, they consist for
the most part of a continuous flight of material particles projected
from the substance with great velocity. Before any substance can be
called “radio-active” in the sense in which the term is used to describe
the properties of the natural radio-active elements, it is thus
necessary to make a close examination of its radiation; for it is
unadvisable to extend the use of the term “radio-active” to substances
which do not possess the characteristic radiating properties of the
radio-active elements which we have described, and the active products
which can be obtained from them. Some of the pseudo-active bodies will
however be considered later in chapter IX.


=10. Thorium.= In the course of an examination of a large number of
substances, Schmidt[13] found that thorium, its compounds, and the
minerals containing thorium, possessed properties similar to those of
uranium. The same discovery was made independently by Mme Curie[14]. The
rays from thorium compounds, like those from uranium, possess the
properties of discharging electrified bodies and acting on a
photographic plate. Under the same conditions the discharging action of
the rays is about equal in amount to that of uranium, but the
photographic effect is distinctly weaker.

The radiations from thorium are more complicated than those from
uranium. It was early observed by several experimenters that the
radiation from thorium compounds, especially the oxide, when tested by
the electrified method, was very variable and uncertain. A detailed
investigation of the radiations from thorium under various conditions
was made by Owens[15]. He showed that thorium oxide, especially in thick
layers, was able to produce conductivity in the gas when covered with a
large thickness of paper, and that the amount of this conductivity could
be greatly varied by blowing a current of air over the gas. In the
course of an examination[16] of this action of the air current, the
writer showed that thorium compounds gave out a material emanation made
up of very small particles _themselves radio-active_. The emanation
behaves like a radio-active gas; it diffuses rapidly through porous
substances like paper, and is carried away by a current of air. The
evidence of the existence of the emanation and its properties, is
considered in detail later in chapter VIII. In addition to giving out an
emanation, thorium behaves like uranium in emitting three types of
radiation, each of which is similar in properties to the corresponding
radiation from uranium.


=11. Radio-active minerals.= Mme Curie has examined the radio-activity
of a large number of minerals containing uranium and thorium. The
electrical method was used, and the current measured between two
parallel plates 8 cms. in diameter and 3 cms. apart, when one plate was
covered with a uniform layer of the active matter. The following numbers
give the order of the saturation current obtained in amperes.

        Pitchblende from Johanngeorgenstadt      8·3 × 10⁻¹¹
        „           Joachimsthal                 7·0   „
        „           Pzibran                      6·5   „
        „           Cornwall                     1·6   „
        Cleveite                                 1·4   „
        Chalcolite                               5·2   „
        Autunite                                 2·7   „
        Thorite                                  from 0·3 to 1·4 „
        Orangite                                 2·0   „
        Monazite                                 0·5   „
        Xenotine                                 0·03  „
        Aeschynite                               0·7   „
        Fergusonite                              0·4   „
        Samarskite                               1·1   „
        Niobite                                  0·3   „
        Carnotite                                6·2   „

Some activity is to be expected in these minerals, since they all
contain either thorium or uranium or a mixture of both. An examination
of the action of the uranium compounds with the same apparatus and under
the same conditions led to the following results:

     Uranium (containing a little carbon)     2·3 × 10⁻¹¹ amperes
     Black oxide of uranium                   2·6           „
     Green   „         „                      1·8           „
     Acid uranic hydrate                      0·6           „
     Uranate of sodium                        1·2           „
     Uranate of potassium                     1·2           „
     Uranate of ammonia                       1·3           „
     Uranous sulphate                         0·7           „
     Sulphate of uranium and potassium        0·7           „
     Acetate                                  0·7           „
     Phosphate of copper and uranium          0·9           „
     Oxysulphide of uranium                   1·2           „

The interesting point in connection with these results is that some
specimens of pitchblende have four times the activity of the metal
uranium; chalcolite, the crystallized phosphate of copper and uranium,
is twice as active as uranium; and autunite, a phosphate of calcium and
uranium, is as active as uranium. From the previous considerations, none
of the substances should have shown as much activity as uranium or
thorium. In order to be sure that the large activity was not due to the
particular chemical combination, Mme Curie prepared chalcolite
artificially, starting with pure products. This artificial chalcolite
had the activity to be expected from its composition, viz. about 0·4 of
the activity of the uranium. The natural mineral chalcolite is thus five
times as active as the artificial mineral.

It thus seemed probable that the large activity of some of these
minerals, compared with uranium and thorium, was due to the presence of
small quantities of some very active substance, which was different from
the known bodies thorium and uranium.

This supposition was completely verified by the work of M. and Mme
Curie, who were able to separate from pitchblende by purely chemical
methods two active bodies, one of which in the pure state is over a
million times more active than the metal uranium.

This important discovery was due entirely to the property of
radio-activity possessed by the new bodies. The only guide in their
separation was the activity of the products obtained. In this respect
the discovery of these bodies is quite analogous to the discovery of
rare elements by the methods of spectrum analysis. The method employed
in the separation consisted in examining the relative activity of the
products after chemical treatment. In this way it was seen whether the
radio-activity was confined to one or another of the products, or
divided between both, and in what ratio such division occurred.

The activity of the specimens thus served as a basis of rough
qualitative and quantitative analysis, analogous in some respects to the
indication of the spectroscope. To obtain comparative data it was
necessary to test all the products in the dry state. The chief
difficulty lay in the fact that pitchblende is a very complex mineral,
and contains in varying quantities nearly all the known metals.


=12. Radium.= The analysis of pitchblende by chemical methods, using the
procedure sketched above, led to the discovery of two very active
bodies, polonium and radium. The name polonium was given to the first
substance discovered by Mme Curie in honour of the country of her birth.
The name radium was a very happy inspiration of the discoverers, for
this substance in the pure state possesses the property of
radio-activity to an astonishing degree.

Radium is extracted from pitchblende by the process used to separate
barium, to which radium is very closely allied in chemical
properties[17]. After the removal of other substances, the radium
remains behind mixed with barium. It can, however, be partially
separated from the latter by the difference in solubility of the
chlorides in water, alcohol, or hydrochloric acid. The chloride of
radium is less soluble than that of barium, and can be separated from it
by the method of fractional crystallization. After a large number of
precipitations, the radium can be freed almost completely from the
barium.

Both polonium and radium exist in infinitesimal quantities in
pitchblende. In order to obtain a few decigrammes of very active radium,
it is necessary to use several tons of pitchblende, or the residues
obtained from the treatment of uranium minerals. It is thus obvious that
the expense and labour involved in preparation of a minute quantity of
radium are very great.

M. and Mme Curie were indebted for their first working material to the
Austrian government, who generously presented them with a ton of the
treated residue of uranium materials from the State manufactory of
Joachimsthal in Bohemia. With the assistance of the Academy of Science
and other societies in France, funds were given to carry out the
laborious work of separation. Later the Curies were presented with a ton
of residues from the treatment of pitchblende by the Société Centrale de
Produits Chimiques of Paris. The generous assistance afforded in this
important work is a welcome sign of the active interest taken in these
countries in the furthering of purely scientific research.

The rough concentration and separation of the residues was performed in
the chemical works, and there followed a large amount of labour in
purification and concentration. In this manner, the Curies were able to
obtain a small quantity of radium which was enormously active compared
with uranium. No definite results have yet been given on the activity of
pure radium, but the Curies estimate that it is about one million times
that of uranium, and may possibly be still higher. The difficulty of
making a numerical estimate for such an intensely active body is very
great. In the electric method, the activities are compared by noting the
relative strength of the maximum or saturation current between two
parallel plates, on one of which the active substance is spread. On
account of the intense ionization of the gas between the plates, it is
not possible to reach the saturation current unless very high voltages
are applied. Approximate comparisons can be made by the use of metal
screens to cut down the intensity of the radiations, if the proportion
of the radiation transmitted by such a screen has been determined by
direct experiment on impure material of easily measurable activity. The
value of the activity of radium compared with that of uranium will
however vary to some extent according to which of the three types of
rays is taken as a basis of comparison.

It is thus difficult to control the final stages of the purification of
radium by measurements of its activity alone. Moreover the activity of
radium immediately after its preparation is only about one-fourth of its
final value; it gradually rises to a maximum after the radium salt has
been kept in the dry state for about a month. For control experiments in
purification, it is advisable to measure the initial rather than the
final activity.

Mme Curie has utilized the coloration of the crystals of radiferous
barium as a means of controlling the final process of purification. The
crystals of salts of radium and barium deposited from acid solutions are
indistinguishable by the eye. The crystals of radiferous barium are at
first colourless, but, in the course of a few hours, become yellow,
passing to orange and sometimes to a beautiful rose colour. The rapidity
of this coloration depends on the amount of barium present. Pure radium
crystals do not colour, or at any rate not as rapidly as those
containing barium. The coloration is a maximum for a definite proportion
of radium, and this fact can be utilized as a means of testing the
amount of barium present. When the crystals are dissolved in water the
coloration disappears.

Giesel[18] has observed that pure radium bromide gives a beautiful
carmine colour to the Bunsen flame. If barium be present in any
quantity, only the green colour due to barium is observed, and a
spectroscopic examination shows only the barium lines. This carmine
coloration of the Bunsen flame is a good indication of the purity of the
radium.

Since the preliminary announcement of the discovery of radium,
Giesel[19] has devoted a great deal of attention to the separation of
radium, polonium and other active bodies from pitchblende. He was
indebted for his working material to the firm of P. de Haen, of Hanover,
who presented him with a ton of pitchblende residues. Using the method
of fractional crystallization of the bromide instead of the chloride, he
has been able to prepare considerable quantities of pure radium. By this
means the labour of final purification of radium has been much reduced.
He states that six or eight crystallizations with the bromide are
sufficient to free the radium almost completely from the barium.


=13. Spectrum of radium.= It was of great importance to settle as soon
as possible whether radium was in reality modified barium or a new
element with a definite spectrum. For this purpose the Curies prepared
some specimens of radium chloride, and submitted them for examination of
their spectrum to Demarçay, an authority on that subject. The first
specimen of radium chloride examined by Demarçay[20] was not very
active, but showed, besides the lines due to barium, a very strong new
line in the ultra-violet. In another sample of greater activity, the
line was still stronger and others also appeared, while the intensity of
the new lines was comparable with those present due to barium. With a
still more active specimen which was probably nearly pure, only three
strong lines of barium appeared, while the new spectrum was very bright.
The following table shows the wave-length of the new lines observed for
radium. The wave lengths are expressed in Ångström units and the
intensity of each ray is denoted by a number, the ray of maximum
intensity being 16.

               Wave length   Intensity  Wave length   Intensity
                 4826·3          10       4600·3           3
                 4726·9           5       4533·5           9
                 4699·6           3       4436·1           6
                 4692·1           7       4340·6          12
                 4683·0          14       3814·7          16
                 4641·9           4       3649·6          12

The lines are all sharply defined, and three or four of them have an
intensity comparable with any known lines of other substances. There are
also present in the spectrum two strong nebulous bands. In the visible
part of the spectrum, which has not been photographed, the only
noticeable ray has a wave length 5665, which is, however, very feeble
compared with that of wave length 4826·3. The general aspect of the
spectrum is similar to that of the alkaline earths; it is known that
these metals have strong lines accompanied by nebulous bands.

The principal line due to radium can be distinguished in impure radium
of activity 50 times that of uranium. By the electrical method it is
easy to distinguish the presence of radium in a body which has an
activity only ¹⁄₁₀₀ of uranium. With a more sensitive electrometer
¹⁄₁₀₀₀₀ of the activity of uranium could be observed. For the detection
of radium, the examination of the radio-activity is thus a process
nearly a million times more sensitive than spectrum analysis.

Later observations on the spectrum of radium have been made by
Runge[21], Exner and Haschek[22], with specimens of radium prepared by
Giesel. Crookes[23] has photographed the spectrum of radium in the
ultra-violet, while Runge and Precht[24], using a highly purified sample
of radium, observed a number of new lines in the spark spectrum. It has
been mentioned already that the bromide of radium gives a characteristic
pure carmine-red coloration to the Bunsen flame. The flame spectrum
shows two broad bright bands in the orange-red, not observed in
Demarçay’s spectrum. In addition there is a line in the blue-green and
two feeble lines in the violet.


=14. Atomic weight of radium.= Mme Curie has made successive
determinations of the atomic weight of the new element with specimens of
steadily increasing purity. In the first observation the radium was
largely mixed with barium, and the atomic weight obtained was the same
as that of barium, 137·5. In successive observations with specimens of
increasing purity the atomic weights of the mixture were 146 and 175.
The final value obtained recently was 225, which may be taken as the
atomic weight of radium on the assumption that it is divalent.

In these experiments about 0·1 gram of pure radium chloride was obtained
by successive fractionations. The difficulty involved in preparing a
quantity of pure radium chloride large enough to test the atomic weight
may be gauged from the fact that only a few centigrams of fairly pure
radium, or a few decigrams of less concentrated material, are obtained
from the treatment of about 2 tons of the mineral from which it is
derived.

Runge and Precht[25] have examined the spectrum of radium in a magnetic
field, and have shown the existence of series analogous to those
observed for calcium, barium, and strontium. These series are connected
with the atomic weights of the elements in question, and Runge and
Precht have calculated by these means that the atomic weight of radium
should be 258—a number considerably greater than the number 225 obtained
by Mme Curie by means of chemical analysis. Marshall Watts[26], on the
other hand, using another relation between the lines of the spectrum,
deduced the value obtained by Mme Curie. Runge[27] has criticised the
method of deduction employed by Marshall Watts on the ground that the
lines used for comparison in the different spectra were not homologous.
Considering that the number found by Mme Curie agrees with that required
by the periodic system, it is advisable in the present state of our
knowledge to accept the experimental number rather than the one deduced
by Runge and Precht from spectroscopic evidence.

There is no doubt that radium is a new element possessing remarkable
physical properties. The detection and separation of this substance,
existing in such minute proportions in pitchblende, has been due
entirely to the characteristic property we are considering, and is the
first notable triumph of the study of radio-activity. As we shall see
later, the property of radio-activity can be used, not only as a means
of chemical research, but also as an extraordinarily delicate method of
detecting chemical changes of a very special kind.


=15. Radiations from radium.= On account of its enormous activity, the
radiations from radium are very intense: a screen of zinc sulphide,
brought near a few centigrams of radium bromide, is lighted up quite
brightly in a dark room, while brilliant fluorescence is produced on a
screen of platino-barium cyanide. An electroscope brought near the
radium salt is discharged almost instantly, while a photographic plate
is immediately affected. At a distance of one metre, a day’s exposure to
the radium rays would produce a strong impression. The radiations from
radium are analogous to those of uranium, and consist of three types of
rays: easily absorbed, penetrating, and very penetrating. Radium also
gives rise to an emanation similar to that of thorium, but with a very
much slower rate of decay. The radium emanation retains its activity for
several weeks, while that of thorium lasts only a few minutes. The
emanation obtained from a few centigrams of radium illuminates a screen
of zinc sulphide with great brilliancy. The very penetrating rays of
radium are able to light up an X ray screen in a dark room, after
passage through several centimetres of lead and several inches of iron.

As in the case of uranium or thorium, the photographic action is mainly
due to the penetrating or cathodic rays. The radiographs obtained with
radium are very similar to those obtained with X rays, but lack the
sharpness and detail of the latter. The rays are unequally absorbed by
different kinds of matter, the absorption varying approximately as the
density. In photographs of the hand the bones do not stand out as in X
ray photographs.

Curie and Laborde have shown that the compounds of radium possess the
remarkable property of always keeping their temperature several degrees
above the temperature of the surrounding air. Each gram of radium
radiates an amount of energy corresponding to 100 gram-calories per
hour. This and other properties of radium are discussed in detail in
chapters V and XII.


=16. Compounds of radium.= When first prepared in the solid state, all
the salts of radium—the chloride, bromide, acetate, sulphate, and
carbonate—are very similar in appearance to the corresponding salts of
barium, but in time they gradually become coloured. In chemical
properties the salts of radium are practically the same as those of
barium, with the exception that the chloride and bromide of radium are
less soluble in water than the corresponding salts of barium. All the
salts of radium are naturally phosphorescent. The phosphorescence of
impure radium preparations is in some cases very marked.

All the radium salts possess the property of causing rapid colorations
of the glass vessel which contains them. For feebly active material the
colour is usually violet, for more active material a yellowish-brown,
and finally black.


=17. Actinium.= The discovery of radium in pitchblende gave a great
impetus to the chemical examination of uranium residues, and a
systematic search early led to the detection of several new radio-active
bodies. Although these show distinctive radio-active properties, so far
none of them have been purified sufficiently to give a definite spectrum
as in the case of radium. One of the most interesting and important of
these substances was discovered by Debierne[28] while working up the
uranium residues, obtained by M. and Mme Curie from the Austrian
government, and was called by him actinium. This active substance is
precipitated with the iron group, and appears to be very closely allied
in chemical properties to thorium, though it is many thousand times more
active. It is very difficult to separate from thorium and the rare
earths. Debierne has made use of the following methods for partial
separation:

(1) Precipitation in hot solutions, slightly acidulated with
hydrochloric acid, by excess of hyposulphite of soda. The active matter
is present almost entirely in the precipitate.

(2) Action of hydrofluoric acid upon the hydrates freshly precipitated,
and held in suspension in water. The portion dissolved is only slightly
active. By this method titanium may be separated.

(3) Precipitation of neutral nitrate solutions by oxygenated water. The
precipitate carries down the active body.

(4) Precipitation of insoluble sulphates. If barium sulphate, for
example, is precipitated in the solution containing the active body, the
barium carries down the active matter. The thorium and actinium are
freed from the barium by conversion of the sulphate into the chloride
and precipitation by ammonia.

In this way Debierne has obtained a substance comparable in activity
with radium. The separation, which is difficult and laborious, has not
yet been carried far enough to bring out any new lines in the spectrum.


=18.= After the initial announcement of the discovery of actinium,
several years elapsed before any definite results upon it were published
by Debierne. In the meantime, Giesel[29] had independently obtained a
radio-active substance from pitchblende which seemed similar in many
respects to the actinium of Debierne. The active substance belongs to
the group of cerium earths and is precipitated with them. By a
succession of chemical operations, the active substance is separated
mixed with lanthanum. While intensely active in comparison with thorium,
the new active substance closely resembles it in radio-active
properties, although, from the method of separation thorium cannot be
present except in minute quantity. Giesel early observed that the
substance gave off a radio-active emanation. On account of the intensity
of the emanation it emits, he termed it the “emanating substance.”
Recently this name has been changed to “emanium,” and under this title
preparations of the active substance prepared by Giesel have been placed
on the market.

Giesel found that the activity of this substance was permanent and
seemed to increase during the six months’ interval after separation. In
this respect it is similar to radium compounds, for the activity of
radium, measured by the electric method, increases in the course of a
month’s interval to four times its initial value at separation.

There can be no doubt that the “actinium” of Debierne and the “emanium”
of Giesel contain the same radio-active constituent, for recent work[30]
has shown that they exhibit identical radio-active properties. Each
gives out easily absorbed and penetrating rays, and emits a
characteristic emanation of which the rate of decay is the same for both
substances. The rate of decay of the emanation is the simplest method of
distinguishing actinium from thorium, which it resembles so closely in
radio-active as well as in chemical properties. The emanation of
actinium loses its radiating power far more rapidly than that of
thorium, the time taken for the activity to fall to half value being in
the two cases 3·7 seconds and 52 seconds respectively.

The rapid and continuous emission of this short-lived emanation is the
most striking radio-active property possessed by actinium. In still air,
the radio-active effects of this emanation are confined to a distance of
a few centimetres from the active material, as it is only able to
diffuse a short distance through the air before losing its radiating
power. With very active preparations of actinium, the material appears
to be surrounded by a luminous haze produced by the emanation. The
radiations produce strong luminosity in some substances, for example,
zinc sulphide, willemite and platinocyanide of barium. The luminosity
is especially marked on screens of zinc sulphide. Much of this effect is
due to the emanation, for, on gently blowing a current of air over the
substance, the luminosity is displaced at once in the direction of the
current. With a zinc sulphide screen, actinium shows the phenomena of
“scintillations” to an even more marked degree than radium itself.

The preparations of emanium are in some cases luminous, and a
spectroscopic examination of this light has shown a number of bright
lines[31].

The distinctive character of the emanation of actinium, as well as of
the other radio-active products to which it gives rise, coupled with the
permanence of its activity, renders it very probable that actinium will
prove to be a new radio-active element of very great activity. Although
very active preparations of actinium have been obtained, it has not yet
been found possible to free it from impurities. Consequently, no
definite observations have been made on its chemical properties, and no
new spectrum lines have been observed.

A more complete discussion of the radio-active and other properties of
actinium is given in later chapters.


=19. Polonium.= Polonium was the first of the active substances obtained
from pitchblende. It has been investigated in detail by its discoverer
Mme Curie[32]. The pitchblende was dissolved in acid and sulphuretted
hydrogen added. The precipitated sulphides contained an active
substance, which, after separation of impurities, was found associated
with bismuth. This active substance, which has been named polonium, is
so closely allied in chemical properties to bismuth that it has so far
been found impossible to effect a complete separation. Partial
separation of polonium can be made by successive fractionations based on
one of the following modes of procedure:

(1) Sublimation in a vacuum. The active sulphide is more volatile than
that of bismuth. It is deposited as a black substance at those parts of
the tube, where the temperature is between 250 and 300° C. In this way
polonium of activity 700 times that of uranium was obtained.

(2) Precipitation of nitric acid solutions by water. The precipitated
sub-nitrate is much more active than the part that remains in solution.

(3) Precipitation by sulphuretted hydrogen in a very acid hydrochloric
acid solution. The precipitated sulphides are much more active than the
salt which remains in solution.

For concentration of the active substance Mme Curie[33] has made use of
method (2). The process is, however, very slow and tedious, and is made
still more complicated by the tendency to form precipitates insoluble
either in strong or weak acids. After a large number of fractionations,
a small quantity of matter was obtained, enormously active compared with
uranium. On examination of the substance spectroscopically, only the
bismuth lines were observed. A spectroscopic examination of the active
bismuth by Demarçay and by Runge and Exner has led to the discovery of
no new lines. On the other hand Sir William Crookes[34] states that he
found one new line in the ultra-violet, while Berndt[35], working with
polonium of activity 300, observed a large number of new lines in the
ultra-violet. These results await further confirmation.

The polonium prepared by Mme Curie differs from the other radio-active
bodies in several particulars. In the first place the radiations include
only very easily absorbable rays. The two penetrating types of radiation
given out by uranium, thorium, and radium are absent. In the second
place the activity does not remain constant, but diminishes continuously
with the time. Mme Curie states that different preparations of polonium
had somewhat different rates of decay. In some cases, the activity fell
to half value in about six months, and in others, about half value in
eleven months.


=20.= The gradual diminution of the activity of polonium with time
seemed at first sight to differentiate it from such substances as
uranium and radium, the activity of which appeared fairly permanent.
This difference in behaviour is, however, one of degree rather than of
kind. We shall show later that there is present in pitchblende a number
of radio-active substances, the activity of which is not permanent. The
time taken for these bodies to lose half of their activity varies in
different cases from a few seconds to several hundreds of years. In
fact, this gradual loss of activity is an essential feature of our
theory of regarding the phenomena of radio-activity. No radio-active
substance, left to itself, can continue to radiate indefinitely; it must
ultimately lose its activity. In the case of bodies like uranium and
radium, the loss of activity is so slow that no sensible alteration has
been observed over a period of several years, but it can be deduced
theoretically that the activity of radium will eventually decrease to
half value in a period of about 1000 years, while in the case of a
feebly radio-active substance like uranium, more than a 100 million
years must elapse before the diminution of the activity becomes
appreciable.

It may be of interest here to consider briefly the suggestions advanced
at various times to account for the temporary character of the activity
of polonium. Its association with bismuth led to the view that polonium
was not a new active substance, but merely radio-active bismuth, that
is, bismuth which in some way had been made active by admixture with
radio-active bodies. It was known that a body placed in the vicinity of
thorium or radium became temporarily active. The same action was
supposed to take place when inactive matter was in solution with active
matter. The non-active matter was supposed to acquire activity by
“induction,” as it was called, in consequence of its intimate contact
with the active material.

There is no proof, however, that such is the case. The evidence points
rather to the conclusion that the activity is due, not to an alteration
of the inactive body itself, but to an admixture with it of a very small
quantity of intensely active matter. This active matter is present in
pitchblende and is separated with the bismuth but differs from it in
chemical properties.

The subject cannot be considered with advantage at this stage, but will
be discussed later in detail in chapter XI. It will there be shown that
polonium, that is, the radio-active constituent mixed with the bismuth,
is a distinct chemical substance, which is allied in chemical properties
to bismuth, but possesses some distinct analytical properties which
allow of a partial separation from it.

The polonium, if obtained in a pure state, should initially be several
hundred times as active as pure radium. This activity, however, is not
permanent; it decays with the time, falling to half value in about six
months.

The absence of any new lines in the spectrum of radio-active bismuth is
to be expected, for, even in the most active bismuth prepared, the
active matter exists in a very small proportion.


=21.= The discussion of the nature of polonium was renewed by the
discovery of Marckwald[36] that a substance similar to polonium can be
separated from pitchblende; the activity of this substance, he stated,
did not decay appreciably with the time. The method of separation from
the bismuth chloride solution, obtained from uranium residues, was very
simple. A rod of bismuth or antimony, dipped in the active solution,
rapidly became coated with a black deposit which was intensely active.
This process was continued until the whole of the activity was removed
from the solution. The active deposit gave out only easily absorbed
rays, and in that respect resembled the polonium of Mme Curie.

The active substance was found to consist mainly of tellurium, and for
this reason Marckwald gave it the name of radio-tellurium. In later
work, however, Marckwald[37] has shown that the active constituent has
no connection with tellurium, but can always be separated completely
from it by a simple chemical process.

In order to obtain a large amount of the active substance, 2000 kilos.
of pitchblende were worked up. This yielded 6 kilos. of bismuth
oxychloride, and from this was separated 1·5 grams of radio-tellurium.
The tellurium present was precipitated from a hydrochloric acid solution
by hydrazine hydrochloride. The precipitated tellurium still showed some
activity, but this was removed by repeating the process. The active
matter then remained in the filtrate, and, after evaporation, the
addition of a few drops of stannous chloride caused a small quantity of
a dark precipitate which was intensely active. This was collected on a
filter and weighed only 4 milligrams.

When plates of copper, tin or bismuth were dipped into an hydrochloric
acid solution of this active substance, the plates were found to be
covered with a very finely divided deposit. These plates were intensely
active, and produced marked photographic and phosphorescent action. As
an illustration of the enormous activity of this deposit, Marckwald
stated that a precipitate of ¹⁄₁₀₀ milligram on a copper plate, 4 square
centimetres in area, illuminated a zinc sulphide screen so brightly that
it could be seen by an audience of several hundred people.

The active substance of Marckwald is very closely allied in chemical and
radio-active properties to the polonium of Mme Curie. Both active
substances are separated with bismuth and both give out only easily
absorbed rays. The penetrating rays, such as are given out by uranium,
radium or thorium, are completely absent.

There has been a considerable amount of discussion as to whether the
active substance obtained by Marckwald is identical with that present in
the polonium of Mme Curie. Marckwald stated that his active substance
did not sensibly diminish in activity in the course of six months, but
it is doubtful whether the method of measurement used was sufficiently
precise.

The writer has found that radio-tellurium of moderate activity, prepared
after Marckwald’s method and sold by Dr Sthamer of Hamburg, undoubtedly
loses its activity with time. The radio-tellurium is obtained in the
form of a thin radio-active deposit on a polished bismuth rod or plate.
A bismuth rod was found to have lost half its activity in about 150
days, and a similar result has been recorded by other observers.

The two substances are thus similar in both radio-active and chemical
properties, and there can be no reasonable doubt that the active
constituent present in each case is the same. The evidence is discussed
in detail in chapter XI and it will there be shown that the active
substance present in the radio-tellurium of Marckwald is a slow
transformation product of radium.


=22. Radio-active lead.= Several observers early noticed that the lead
separated from pitchblende showed strong radio-active properties, but
considerable difference of opinion was expressed in regard to the
permanence of its activity. Elster and Geitel[38] found that lead
sulphate obtained from pitchblende was very active, but they considered
that the activity was probably due to an admixture of radium or polonium
with the lead, and, by suitable chemical treatment, the lead sulphate
was obtained in an inactive state. Giesel[39] also separated some
radio-active lead but found that its activity diminished with the time.
On the other hand, Hofmann and Strauss[40] obtained lead from
pitchblende whose activity seemed fairly permanent. They state that the
radio-active lead resembled ordinary lead in most of its reactions, but
showed differences in the behaviour of the sulphide and sulphate. The
sulphate was found to be strongly phosphorescent. These results of
Hofmann and Strauss were subjected at the time of their publication to
considerable criticism, and there is no doubt that the lead itself is
not radio-active but contains a small quantity of radio-active matter
which is separated with it. In later work[41], it has been shown that
radio-lead contains several radio-active constituents which can be
removed temporarily from it by suitable chemical methods.

There can be no doubt that the lead separated from pitchblende by
certain methods does show considerable activity and that this activity
is fairly permanent. The radio-active changes occurring in radio-lead
are complicated and cannot be discussed with advantage at this stage,
but will be considered in detail in chapter XI. It will there be shown
that the primary constituent present in lead is a slow transformation
product of radium. This substance then slowly changes into the active
constituent present in polonium, which gives out only easily absorbed
rays.

This polonium can be separated temporarily from the lead by suitable
chemical methods, but the radio-lead still continues to produce
polonium, so that a fresh supply may be obtained from it, provided an
interval of several months is allowed to elapse.

It will be calculated later that in all probability the radio-lead would
lose half of its activity in an interval of 40 years.

The constituent present in radio-lead has not yet been separated, but it
will be shown that, in the pure state, it should have an activity
considerably greater than that of radium itself. Sufficient attention
has not yet been paid to this substance, for, separated in a pure state,
it should be as useful scientifically as radium. In addition, since it
is the parent of polonium, it should be possible to obtain from it at
any time a supply of very active polonium, in the same way that a supply
of the radium emanation can be obtained at intervals from radium.

Hofmann and Strauss have observed a peculiar action of the cathode rays
on the active lead sulphate separated by them. They state that the
activity diminishes with time, but is recovered by exposure of the lead
for a short time to the action of cathode rays. No such action is shown
by the active lead sulphide. This effect is due most probably to the
action of the cathode rays in causing a strong phosphorescence of the
lead sulphate and has nothing to do with the radio-activity proper of
the substance.


=23. Is thorium a radio-active element?= The similarity of the chemical
properties of actinium and thorium has led to the suggestion at
different times that the activity of thorium is not due to thorium
itself, but to the presence of a slight trace of actinium. In view of
the difference in the rate of decay of the emanations of thorium and
actinium, this position is not tenable. If the activity of thorium were
due to actinium, the two emanations, as well as the other products
obtained from these substances, should have identical rates of decay.
Since there is not the slightest evidence that the rate of decay of
activity of the various products can be altered by chemical or physical
agencies, we may conclude with confidence that whatever radio-active
substance is responsible for the activity of thorium, it certainly is
not actinium. This difference in the rate of decay of the active
products is of far more weight in deciding the question whether two
bodies contain the same radio-active constituent than differences in
chemical behaviour, for it is quite probable that the active material in
each case may exist only in minute quantity in the matter under
examination, and, under such conditions, a direct chemical examination
in the first place is of little value.

Recent work of Hofmann and Zerban and of Baskerville, however, certainly
tends to show that the element thorium is itself non-radio-active, and
that the radio-activity observed in ordinary thorium compounds is due to
the admixture with it of an unknown radio-active element. Hofmann and
Zerban[42] made a systematic examination of the radio-activity of
thorium obtained from different mineral sources. They found generally
that thorium, obtained from minerals containing a large percentage of
uranium, were more active than those obtained from minerals nearly free
from uranium. This indicates that the radio-activity observed in thorium
may possibly be due to a transformation product of uranium which is
closely allied chemically to thorium and is always separated with it. A
small quantity of thorium obtained from the mineral gadolinite was found
by Hofmann to be almost inactive, whether tested by the electric or by
the photographic method. Later Baskerville and Zerban[43] found that
thorium obtained from a Brazilian mineral was practically devoid of
activity.

In this connection the recent work of Baskerville on the complexity of
ordinary thorium is of interest. By special chemical methods, he
succeeded in separating two new and distinct substances from thorium,
which he has named carolinium and berzelium. Both of these substances
are strongly radio-active, and it thus seems probable that the active
constituent observed in ordinary thorium may be due to one of these
elements.

If, as we have suggested, thorium itself is not active, it is certainly
a matter of surprise that ordinary commercial thorium and the purest
chemical preparations show about the same activity. Such a result
indicates that the methods of purification have not removed any of the
radio-active constituent originally present.

Whatever the radio-active constituent in thorium may ultimately prove to
be, it is undoubtedly not radium nor actinium nor any of the known
radio-active substances.

In later chapters, the radio-activity of thorium will, for simplicity,
be discussed on the assumption that thorium is itself a radio-active
element. The analysis of the changes which occur will thus not refer to
thorium itself but to the primary radio-active substance usually found
associated with it. The conclusions to be drawn from an examination of
the radio-active processes are for the most part independent of whether
thorium is itself radio-active or whether the radio-activity is due to
an unknown element. If thorium is not radio-active itself, it is not
possible to draw any conclusions upon the question of the duration of
the primary radio-activity associated with it. Such a deduction cannot
be made until the quantity of the radio-active element present in
thorium has been definitely determined.


=24.= If elements heavier than uranium exist, it is probable that they
will be radio-active. The extreme delicacy of radio-activity as a means
of chemical analysis would enable such elements to be recognized even if
present in infinitesimal quantities. It is probable that considerably
more than the three or four radio-elements at present recognized exist
in minute quantity, and that the number at present known will be
augmented in the future. In the first stage of the search, a purely
chemical examination is of little value, for it is not probable that the
new element should exist in sufficient quantity to be detected by
chemical or spectroscopic analysis. The main criteria of importance are
the existence or absence of distinctive radiations or emanations, and
the permanence of the radio-activity. The discovery of a radio-active
emanation with a rate of decay different from those already known would
afford strong evidence that a new radio-active body was present. The
presence of either thorium or radium in matter can very readily be
detected by observing the rate of decay of the emanations given out by
them. When once the existence of a new radio-element has been inferred
by an examination of its radio-active properties, chemical methods of
separation can be devised, the radiating or emanating property being
used as a guide in qualitative and quantitative analysis.

Footnote 1:

  Niewenglowski, _C. R._ 122, p. 385, 1896.

Footnote 2:

  Becquerel, _C. R._ 122, p. 559, 1896.

Footnote 3:

  Troost, _C. R._ 122, p. 564, 1896.

Footnote 4:

  Arnold, _Annal. d. Phys._ 61, p. 316, 1897.

Footnote 5:

  Le Bon, _C. R._ 122, pp. 188, 233, 386, 462, 1896.

Footnote 6:

  Becquerel, _C. R._ 122, pp. 420, 501, 559, 689, 762, 1086, 1896.

Footnote 7:

  Mme Curie, _Thèse présentée à la Faculté des Sciences de Paris_, 1903.

Footnote 8:

  _Nature_, 56, 1897; _Phil. Mag._ 43, p. 418, 1897; 45, p. 277, 1898.

Footnote 9:

  Rutherford, _Phil. Mag._ Jan. 1899.

Footnote 10:

  _Ibid._

Footnote 11:

  Le Bon, _C. R._ 130, p. 891, 1900.

Footnote 12:

  Lenard, _Annal. d. Phys._ 1, p. 498; 3, p. 298, 1900.

Footnote 13:

  Schmidt, _Annal. d. Phys._ 65, p. 141, 1898.

Footnote 14:

  Mme Curie, _C. R._ 126, p. 1101, 1898.

Footnote 15:

  Owens, _Phil. Mag._ Oct. 1899.

Footnote 16:

  Rutherford, _Phil. Mag._ Jan. 1900.

Footnote 17:

  M. and Mme Curie and G. Bemont, _C. R._ 127, p. 1215, 1898.

Footnote 18:

  Giesel, _Phys. Zeit._ 3, No. 24, p. 578, 1902.

Footnote 19:

  Giesel, _Annal. d. Phys._ 69, p. 91, 1890. _Ber. d. D. Chem. Ges._ p.
  3608, 1902.

Footnote 20:

  Demarçay, _C. R._ 127, p. 1218, 1898; 129, p. 716, 1899; 131, p. 258,
  1900.

Footnote 21:

  Runge, _Astrophys. Journal_, p. 1, 1900. _Annal. d. Phys._ No. 10, p.
  407, 1903.

Footnote 22:

  Exner and Haschek, _Wien. Ber._ July 4, 1901.

Footnote 23:

  Crookes, _Proc. Roy. Soc._ 72, p. 295, 1904.

Footnote 24:

  Runge and Precht, _Annal. d. Phys._ XIV. 2, p. 418, 1904.

Footnote 25:

  Runge and Precht, _Phil. Mag._ April, 1903.

Footnote 26:

  Watts, _Phil. Mag._ July, 1903; August, 1904.

Footnote 27:

  Runge, _Phil. Mag._ December, 1903.

Footnote 28:

  Debierne, _C. R._ 129, p. 593, 1899; 130, p. 206, 1900.

Footnote 29:

  Giesel, _Ber. d. D. Chem. Ges._ p. 3608, 1902; p. 342, 1903.

Footnote 30:

  Debierne, _C. R._ 139, p. 538, 1904. Miss Brooks, _Phil. Mag._ Sept.
  1904. Giesel, _Phys. Zeit._ 5, p. 822, 1904. _Jahrbuch. d.
  Radioaktivität_, no. 4, p. 345, 1904.

Footnote 31:

  Giesel, _Ber. d. D. Chem. Ges._ 37, p. 1696, 1904; Hartmann, _Phys.
  Zeit._ 5, No. 18, p. 570, 1904.

Footnote 32:

  Mme Curie, _C. R._ 127, p. 175, 1898.

Footnote 33:

  Mme Curie, _Thèse_, Paris, 1903.

Footnote 34:

  Crookes, _Proc. Roy. Soc._ May, 1900.

Footnote 35:

  Berndt, _Phys. Zeit._ 2, p. 180, 1900.

Footnote 36:

  Marckwald, _Phys. Zeit._ 4, No. 1 b, p. 51.

Footnote 37:

  Marckwald, _Ber. d. D. Chem. Ges._ p. 2662, No. 12, 1903.

Footnote 38:

  Elster and Geitel, _Annal. d. Phys._ 69, p. 83, 1899.

Footnote 39:

  Giesel, _Ber. d. D. Chem. Ges._ p. 3775, 1901.

Footnote 40:

  Hofmann and Strauss, _Ber. d. D. Chem. Ges._ p. 3035, 1901.

Footnote 41:

  Hofmann, Gonder and Wölfl, _Annal. d. Phys._ No. 13, p. 615, 1904.

Footnote 42:

  Hofmann and Zerban, _Ber. d. D. Chem. Ges._ No. 12, p. 3093, 1903.

Footnote 43:

  Baskerville and Zerban, _Amer. Chem. Soc._ 26, p. 1642, 1904.




                              CHAPTER II.
                      IONIZATION THEORY OF GASES.


=25. Ionization of gases by radiation.= The most important property
possessed by the radiations from radio-active bodies is their power of
discharging bodies whether positively or negatively electrified. As this
property has been made the basis of a method for an accurate
quantitative analysis and comparison of the radiations, the variation of
the rate of discharge under different conditions and the processes
underlying it will be considered in some detail.

In order to explain the similar discharging power of Röntgen rays, the
theory[44] has been put forward that the rays produce positively and
negatively charged carriers throughout the volume of the gas surrounding
the charged body, and that the rate of production is proportional to the
intensity of the radiation. These carriers, or ions[45] as they have
been termed, move with a uniform velocity through the gas under a
constant electric field, and their velocity varies directly as the
strength of the field.

[Illustration: Fig. 1.]

Suppose we have a gas between two metal plates _A_ and _B_ (Fig. 1)
exposed to the radiation, and that the plates are kept at a constant
difference of potential. A definite number of ions will be produced per
second by the radiation, and the number produced will depend in general
upon the nature and pressure of the gas. In the electric field the
positive ions travel towards the negative plate, and the negative ions
towards the positive, and consequently a current will pass through the
gas. Some of the ions will also recombine, the rate of recombination
being proportional to the square of the number present. For a given
intensity of radiation, the current passing through the gas will
increase at first with the potential difference between the plates, but
it will reach a limit when all the ions are removed by the electric
field before any recombination occurs.

This theory accounts also for all the characteristic properties of gases
made conducting by the rays from active substances, though there are
certain differences observed between the conductivity phenomena produced
by active substances and by _X_ rays. These differences are for the most
part the result of unequal absorption of the two types of rays. Unlike
Röntgen rays, a large proportion of the radiation from active bodies
consists of rays which are absorbed in their passage through a few
centimetres of air. The ionization of the gas is thus not uniform, but
falls off rapidly with increase of distance from the active substance.


=26. Variation of the current with voltage.= Suppose that a layer of
radio-active matter is spread uniformly on the lower of two horizontal
plates _A_ and _B_ (Fig. 1). The lower plate _A_ is connected with one
pole of a battery of cells the other pole of which is connected with
earth. The plate _B_ is connected with one pair of quadrants of an
electrometer, the other pair being connected with earth.

The current[46] between the plates, determined by the rate of movement
of the electrometer needle, is observed at first to increase rapidly
with the voltage, then more slowly, finally reaching a value which
increases very slightly with a large increase in the voltage. This, as
we have indicated, is simply explained on the ionization theory.

The radiation produces ions at a constant rate, and, before the electric
field is applied, the number per unit volume increases until the rate of
production of fresh ions is exactly balanced by the recombination of the
ions already produced. On application of a small electric field, the
positive ions travel to the negative electrode and the negative to the
positive.

Since the velocity of the ions between the plates is directly
proportional to the strength of the electric field, in a weak field the
ions take so long to travel between the electrodes that most of them
recombine on the way.

The current observed is consequently small. With increase of the voltage
there is an increase of speed of the ions and a smaller number
recombine. The current consequently increases, and will reach a maximum
value when the electric field is sufficiently strong to remove all the
ions before appreciable recombination has occurred. The value of the
current will then remain constant even though the voltage is largely
increased.

This maximum current will be called the “saturation” current, and the
value of the potential difference required to give this maximum current,
the “saturation P.D.”[47]

The general shape of the current-voltage curve is shown in Fig. 2, where
the ordinates represent current and the abscissae volts.

[Illustration: Fig. 2.]

Although the variation of the current with voltage depends only on the
velocity of the ions and their rate of recombination, the full
mathematical analysis is intricate, and the equations, expressing the
relation between current and voltage, are only integrable for the case
of uniform ionization. The question is complicated by the inequality in
the velocity of the ions and by the disturbance of the potential
gradient between the plates by the movement of the ions. J. J.
Thomson[48] has worked out the case for uniform production of ions
between two parallel plates, and has found that the relation between the
current _i_ and the potential difference _V_ applied is expressed by

                  _Ai² + Bi = V_

where _A_ and _B_ are constants for a definite intensity of radiation
and a definite distance between the plates.

[Illustration: Fig. 3.]

In certain cases of unsymmetrical ionization, which arise in the study
of the radiations from active bodies, the relation between current and
voltage is very different from that expressed by the above equation.
Some of these cases will be considered in section =47=.


=27.= The general shape of the current-voltage curves for gases exposed
to the radiations from active bodies is shown in Fig. 3.

This curve was obtained for ·45 grams of impure radium chloride, of
activity 1000 times that of uranium, spread over an area of 33 sq. cms.
on the lower of two large parallel plates, 4·5 cms. apart. The maximum
value of the current observed, which is taken as 100, was 1·2 × 10⁻⁸
amperes, the current for low voltages was nearly proportional to the
voltage, and about 600 volts between the plates was required to ensure
approximate saturation.

In dealing with slightly active bodies like uranium or thorium,
approximate saturation is obtained for much lower voltages. Tables I.
and II. show the results for the current between two parallel plates
distant 0·5 cms. and 2·5 cms. apart respectively, when one plate was
covered with a thin uniform layer of uranium oxide.

                                  TABLE I.

                               0·5 cms. apart

                             Volts Current
                              ·125      18
                               ·25      36
                                ·5      55
                                 1      67
                                 2      72
                                 4      79
                                 8      85
                                16      88
                               100      94
                               335     100

                                 TABLE II.

                               2·5 cms. apart

                             Volts Current
                                ·5       7·3
                                 1      14
                                 2      27
                                 4      47
                                 8      64
                                16      73
                                37·5    81
                               112      90
                               375      97
                               800     100

The results are shown graphically in Fig. 4.

[Illustration: Fig. 4.]

From the above tables it is seen that the current at first increases
nearly in proportion to the voltage. There is no evidence of complete
saturation, although the current increases very slowly for large
increases of voltage. For example, in Table I. a change of voltage from
·125 to ·25 volts increases the current from 18 to 36% of the maximum,
while a change of voltage from 100 to 335 volts increases the current
only 6%. The variation of the current per volt (assumed uniform between
the range of voltages considered) is thus about 5000 times greater for
the former change.

Taking into consideration the early part of the curves, the current does
not reach a practical maximum as soon as would be expected on the simple
ionization theory. It seems probable that the slow increase with the
large voltages is due either to an action of the electric field on the
rate of production of ions, or to the difficulty of removing the ions
produced near the surface of the uranium before recombination. It is
possible that the presence of a strong electric field may assist in the
separation of ions which otherwise would not initially escape from the
sphere of one another’s attraction. From the data obtained by Townsend
for the conditions of production of fresh ions at low pressures by the
movement of ions through the gas, it seems that the increase of current
cannot be ascribed to an action of the moving ions in the further
ionization of the gas.


=28.= The equation expressing the relation between the current and the
voltage is very complicated even in the case of a uniform rate of
production of ions between the plates. An approximate theory, which is
of utility in interpreting the experimental results, can however be
simply deduced if the disturbance of the potential gradient is
disregarded, and the ionization assumed uniform between the plates.

Suppose that the ions are produced at a constant rate _q_ per cubic
centimetre per second in the gas between parallel plates distant _l_
cms. from each other. When no electric field is applied, the number _N_
present per c.c., when there is equilibrium between the rates of
production and recombination, is given by _q_ = α_N_², where α is a
constant.

If a small potential difference _V_ is applied, which gives only a small
fraction of the maximum current, and consequently has not much effect on
the value of _N_, the current _i_ per sq. cm. of the plate, is given by

                                      _NeuV_
                                _i_ = -----
                                       _l_

where _u_ is the sum of the velocity of the ions for unit potential
gradient, and _e_ is the charge carried by an ion.

                                   _uV_
                                   ----
                                   _l_

is the velocity of the ions in the electric field of strength

                                    _V_
                                    ---
                                    _l_

The number of ions produced per second in a prism of length _l_ and unit
area of cross-section is _ql_. The maximum or saturation current _I_ per
sq. cm. of the plate is obtained when all of these ions are removed to
the electrodes before any recombination has occurred.

Thus

                             _I_ = _q . l . e_,

and

$$ \frac{i}{I} = \frac{NuV}{ql^2} = \frac{uV}{l^2\sqrt{q\alpha}} $$

This equation expresses the fact previously noted that, for small
voltages, the current _i_ is proportional to _V_.

Let

                                 _i/I_ = ρ,

then

$$ V = \frac {\rho l^2 \sqrt {q\alpha}} {u} $$

Now the greater the value of _V_ required to obtain a given value of ρ
(supposed small compared with unity), the greater the potential required
to produce saturation.

It thus follows from the equation that:

(1) For a given intensity of radiation, the saturation P.D. increases
with the distance between the plates. In the equation, for small values
of ρ, _V_ varies as _l²_. This is found to be the case for uniform
ionization, but it only holds approximately for non-uniform ionization.

(2) For a given distance between the plates, the saturation P.D. is
greater, the greater the intensity of ionization between the plates.
This is found to be the case for the ionization produced by radio-active
substances. With a very active substance like radium, the ionization
produced is so intense that very large voltages are required to produce
approximate saturation. On the other hand, only a fraction of a volt per
cm. is necessary to produce saturation in a gas where the ionization is
very slight, for example, in the case of the natural ionization observed
in a closed vessel, where no radio-active substances are present.

For a given intensity of radiation, the saturation P.D. decreases
rapidly with the lowering of the pressure of the gas. This is due to two
causes operating in the same direction, viz. a decrease in the intensity
of the ionization and an increase in the velocity of the ions. The
ionization varies directly as the pressure, while the velocity varies
inversely as the pressure. This will obviously have the effect of
causing more rapid saturation, since the rate of recombination is slower
and the time taken for the ions to travel between the electrodes is
less.

The saturation curves observed for the gases hydrogen and carbon
dioxide[49] are very similar in shape to those obtained for air. For a
given intensity of radiation, saturation is more readily obtained in
hydrogen than in air, since the ionization is less than in air while the
velocity of the ions is greater. Carbon dioxide on the other hand
requires a greater P.D. to produce saturation than does air, since the
ionization is more intense and the velocity of the ions less than in
air.


=29.= Townsend[50] has shown that, for low pressures, the variation of
the current with the voltage is very different from that observed at
atmospheric pressure. If the increase of current with the voltage is
determined for gases, exposed to Röntgen rays, at a pressure of about 1
mm. of mercury, it is found that for small voltages the ordinary
saturation curve is obtained; but when the voltage applied increases
beyond a certain value, depending on the pressure and nature of the gas
and the distance between the electrodes, the current commences to
increase slowly at first but very rapidly as the voltage is raised to
the sparking value. The general shape of the current curve is shown in
Fig. 5.

[Illustration: Fig. 5.]

The portion _OAB_ of the curve corresponds to the ordinary saturation
curve. At the point _B_ the current commences to increase. This increase
of current has been shown to be due to the action of the negative ions
at low pressures in producing fresh ions by collision with the molecules
in their path. The increase of current is not observed in air at a
pressure above 30 mms. until the P.D. is increased nearly to the value
required to produce a spark. This production of ions by collision is
considered in more detail in section 41.


=30. Rate of recombination of the ions.= A gas ionized by the radiation
preserves its conducting power for some time after it is removed from
the presence of the active body. A current of air blown over an active
body will thus discharge an electrified body some distance away. The
duration of this after conductivity can be examined very conveniently in
an apparatus similar to that shown in Fig. 6.

[Illustration: Fig. 6.]

A dry current of air or any other gas is passed at a constant rate
through a long metal tube _TL_. After passing through a quantity of
cotton-wool to remove dust particles, the current of air passes over a
vessel _T_ containing a radio-active body such as uranium, which does
not give off a radio-active emanation. By means of insulated electrodes
_A_ and _B_, charged to a suitable potential, the current between the
tube and one of these electrodes can be tested at various points along
the tube.

A gauze screen, placed over the cross-section of the tube at _D_, serves
to prevent any direct action of the electric field in abstracting ions
from the neighbourhood of _T_.

If the electric field is sufficiently strong, all the ions travel in to
the electrodes at _A_, and no current is observed at the electrode _B_.
If the current is observed successively at different distances along the
tube, all the electrodes except the one under consideration being
connected to earth, it is found that the current diminishes with the
distance from the active body. If the tube is of fairly wide bore, the
loss of the ions due to diffusion is small, and the decrease in
conductivity of the gas is due to recombination of the ions alone.

On the ionization theory, the number _dn_ of ions per unit volume which
recombine in the time _dt_ is proportional to the square of the number
present. Thus

                              _dn_
                              ----  =  α_n²_,
                              _dt_

where α is a constant.

Integrating this equation,

                            1      1
                           ---  − ---  = α_t_,
                           _n_    _N_

if _N_ is the initial number of ions, and _n_ the number after a time
_t_.

The experimental results obtained[51] have been shown to agree very well
with this equation.

In an experiment similar to that illustrated in Fig. 6, using uranium
oxide as a source of ionization, it was found that half the number of
ions present in the gas recombined in 2·4 seconds, and that at the end
of 8 seconds one-fourth of the ions were still uncombined.

Since the rate of recombination is proportional to the square of the
number present, the time taken for half of the ions present in the gas
to recombine decreases very rapidly with the intensity of the
ionization. If radium is used, the ionization is so intense that the
rate of recombination is extremely rapid. It is on account of this
rapidity of recombination that large voltages are necessary to produce
saturation in the gases exposed to very active preparations of radium.

The value of α, which may be termed the _coefficient of recombination_,
has been determined in absolute measure by Townsend[52], McClung[53] and
Langevin[54] by different experimental methods but with very concordant
results. Suppose, for example, with the apparatus of Fig. 6, the time
_T_, taken for half the ions to recombine after passing by the electrode
_A_, has been determined experimentally. Then

                                 1
                                --- =  α_T_,
                                _N_

where _N_ is the number of ions per c.c. present at _A_. If the
saturation current _i_ is determined at the electrode _A_, _i = NVe_,
where _e_ is the charge on an ion and _V_ is the volume of uniformly
ionized gas carried by the electrode _A_ per second. Then

                                   _Ve_
                               α = ----- .
                                   _iT_

The following table shows the value of α obtained for different gases.

                               _Value of_ α.

                 Gas      Townsend   McClung    Langevin


             Air          3420 × _e_ 3384 × _e_ 3200 × _e_

             Carbon       3500 × _e_ 3492 × _e_ 3400 × _e_
             Dioxide

             Hydrogen     3020 × _e_

The latest determination of the value of _e_ (see section 36) is 3·4 ×
10⁻¹⁰ E.S. units; thus α = 1·1 × 10⁻⁶.

Using this value, it can readily be shown from the equation of
recombination that, if 10⁶ ions are present per c.c., half of them
recombine in about 0·9 sec. and 99% in 90 secs.

McClung (_loc. cit._) showed that the value of α was approximately
independent of the pressure between ·125 and three atmospheres. In later
observations, Langevin has found that the value of α decreases rapidly
when the pressure is lowered below the limits used by McClung.


=31.= In experiments on recombination it is essential that the gas
should be free from dust or other suspended particles. In dusty air, the
rate of recombination is much more rapid than in dust-free air, as the
ions diffuse rapidly to the comparatively large dust particles
distributed throughout the gas. The effect of the suspension of small
particles in a conducting gas is very well illustrated by an experiment
of Owens[55]. If tobacco smoke is blown between two parallel plates as
in Fig. 1, the current at once diminishes to a small fraction of its
former value, although a P.D. is applied sufficient to produce
saturation under ordinary conditions. A much larger voltage is then
necessary to produce saturation. If the smoke particles are removed by a
stream of air, the current returns at once to its original value.


=32. Mobility of the ions.= Determinations of the mobility of the ions,
_i.e._ the velocity of the ions under a potential gradient of 1 volt per
cm., have been made by Rutherford[56], Zeleny[57], and Langevin[58] for
gases exposed to Röntgen rays. Although widely different methods have
been employed, the results have been very concordant, and fully support
the view that the ions move with a velocity proportional to the strength
of the field. On the application of an electric field, the ions almost
instantly attain the velocity corresponding to the field and then move
with a uniform speed.

Zeleny[59] first drew attention to the fact that the positive and
negative ions had different velocities. The velocity of the negative ion
is always greater than that of the positive, and varies with the amount
of water vapour present in the gas.

The results, previously discussed, of the variation of the current with
voltage and of the rate of recombination of the ions do not of
themselves imply that the ions produced in gases by the radiations from
active bodies are of the same size as those produced by Röntgen rays
under similar conditions. They merely show that the conductivity under
various conditions can be satisfactorily explained by the view that
charged ions are produced throughout the volume of the gas. The same
general relations would be observed if the ions differed considerably in
size and velocity from those produced by Röntgen rays. The most
satisfactory method of determining whether the ions are identical in the
two cases is to determine the velocity of the ions under similar
conditions.

In order to compare the velocity of the ions[60], the writer has used an
apparatus similar to that shown in Fig. 6 on p. 40.

The ions were carried with a rapid constant stream of air past the
charged electrode _A_, and the conductivity of the gas tested
immediately afterwards at an electrode _B_, which was placed close to
_A_. The insulated electrodes _A_ and _B_ were fixed centrally in the
metal tube _L_, which was connected with earth.

For convenience of calculation, it is assumed that the electric field
between the cylinders is the same as if the cylinders were infinitely
long.

Let _a_ and _b_ be the radii of the electrode _A_, and of the tube _L_
respectively, and let _V_ = potential of _A_.

The electromotive intensity _X_ (without regard to sign) at a distance
_r_ from the centre of the tube is given by

$$ X = \frac {V} {r \log_e \frac {b} {a}} $$

Let _u₁_ and _u₂_ be the velocities of the positive and negative ions
for a potential gradient of 1 volt per cm. If the velocity is
proportional to the electric force at any point, the distance _dr_
traversed by the negative ion in the time _dt_ is given by

_dr_ = _Xu₂_ _dt_,

or

$$ dt = \frac {\log_e \frac {b}{a} r dr} {Vu_{2}} $$

Let _r₂_ be the greatest distance measured from the axis of the tube
from which the negative ion can just reach the electrode _A_ in the time
_t_ taken for the air to pass along the electrode.

Then

$$ t = \frac {r_{2}^2 − a^2} {2 Vu_{2}} \log_e \frac {b}{a} $$

If ρ₂ be the ratio of the number of the negative ions that reach the
electrode _A_ to the total number passing by, then

$$ \rho_{2} = \frac {r_{2}^2 − a^2} {b^2 − a^2} $$

Therefore

$$ u_{2} = \frac {\rho_{2} (b^2 − a^2) \log_e \frac {b}{a}} {2 Vt} $$

Similarly the ratio ρ₁ of the number of positive ions that give up their
charge to the external cylinder to the total number of positive ions is
given by

$$ u_{1} = \frac {\rho_{1} (b^2 − a^2) \log_e \frac {b}{a}} {2 Vt} $$

In the above equations it is assumed that the current of air is uniform
over the cross-section of the tube, and that the ions are uniformly
distributed over the cross-section; also, that the movement of the ions
does not appreciably disturb the electric field. Since the value of _t_
can be calculated from the velocity of the current of air and the length
of the electrode, the values of the velocities of the ions under unit
potential gradient can at once be determined.

The equation (1) shows that ρ₂ is proportional to _V_,—_i.e._ that the
rate of discharge of the electrode _A_ varies directly as the potential
of _A_, provided that the value of _V_ is not large enough to remove all
the ions from the gas as it passes by the electrode. This was found
experimentally to be the case.

In the comparison of the velocities, the potential _V_ was adjusted to
such a value that ρ₂ was about one half, when uranium oxide was placed
in the tube at _L_. The active substance was then removed, and an
aluminium cylinder substituted for the brass tube. X rays were allowed
to fall on the centre of this aluminium cylinder, and the strength of
the rays adjusted to give about the same conductivity to the gas as the
uranium had done. Under these conditions the value of ρ₂ was found to be
the same as for the first experiment.

This experiment shows conclusively that the ions produced by Röntgen
rays and by uranium move with the same velocity and are probably
identical in all respects. The method described above is not very
suitable for an accurate determination of the velocities, but gave
values for the positive ions of about 1·4 cms. per second per volt per
centimetre, and slightly greater values for the negative ions.


=33.= The most accurate determinations of the mobility of the ions
produced by Röntgen rays have been made by Zeleny[61] and Langevin[62].
Zeleny used a method similar in principle to that explained above. His
results are shown in the following table, where _K₁_ is the mobility of
the positive ion and _K₂_ that of the negative ion.

  Gas                    _K₁_       _K₂_           _K₂/K₁_  Temperature

  Air, dry               1·36       1·87            1·375         13°·5 C.
  „   moist              1·37       1·51            1·10          14°
  Oxygen, dry            1·36       1·80            1·32          17°
  „     moist            1·29       1·52            1·18          16°
  Carbon dioxide,        0·76       0·81            1·07          17°·5
  dry
  „      „               0·81       0·75            0·915         17°
  moist
  Hydrogen, dry          6·70       7·95            1·15          20°
  „       moist          5·30       5·60            1·05          20°

Langevin determined the velocity of the ions by a direct method in which
the time taken for the ion to travel over a known distance was observed.

The following table shows the comparative values obtained for air and
carbon dioxide.

              Air _K₁_   Air _K₂_ Air      CO₂ _K₁_ CO₂ _K₂_ CO₂
                                  K₂/K₁                      K₂/K₁

   Direct     1·40       1·70     1·22     0·86     0·90     1·05
   method
   (Langevin)

   Current of 1·36       1·87     1·375    0·76     0·81     1·07
   gas
   (Zeleny)

These results show that for all gases except CO₂, there is a marked
increase in the velocity of the negative ion with the dryness of the
gas, and that, even in moist gases, the velocity of the negative ions is
always greater than that of the positive ions. The velocity of the
positive ion is not much affected by the presence of moisture in the
gas.

The velocity of the ions varies inversely as the pressure of the gas.
This has been shown by Rutherford[63] for the negative ions produced by
ultra-violet light falling on a negatively charged surface, and later by
Langevin[64] for both the positive and negative ions produced by Röntgen
rays. Langevin has shown that the velocity of the positive ion increases
more slowly with the diminution of pressure than that of the negative
ion. It appears as if the negative ion, especially at pressures of about
10 mm. of mercury, begins to diminish in size.


=34. Condensation experiments.= Some experiments will now be described
which have verified in a direct way the theory that the conductivity
produced in gases by the various types of radiation is due to the
production of charged ions throughout the volume of the gas. Under
certain conditions, the ions form nuclei for the condensation of water,
and this property allows us to show the presence of the individual ions
in the gas, and also to count the number present.

It has long been known that, if air saturated with water-vapour be
suddenly expanded, a cloud of small globules of water is formed. These
drops are formed round the dust particles present in the gas, which act
as nuclei for the condensation of water around them. The experiments of
R. von Helmholtz and Richarz[65] had shown that chemical reactions, for
example the combustion of flames, taking place in the neighbourhood,
affected the condensation of a steam-jet. Lenard showed that a similar
action was produced when ultra-violet light fell on a negatively charged
zinc surface placed near the steam-jet. These results suggested that the
presence of electric charges in the gas facilitated condensation.

A very complete study of the conditions of condensation of water on
nuclei has been made by C. T. R. Wilson[66]. An apparatus was
constructed which allowed a very sudden expansion of the air over a wide
range of pressure. The amount of condensation was observed in a small
glass vessel. A beam of light was passed into the apparatus which
allowed the drops formed to be readily observed by the eye.

Preliminary small expansions caused a condensation of the water round
the dust nuclei present in the air. These dust nuclei were removed by
allowing the drops to settle. After a number of successive small
expansions, the air was completely freed from dust, so that no
condensation was produced.

Let _v₁_ = initial volume of the gas in the vessel, _v₂_ = volume after
expansion.

If _v₂_/_v₁_ < 1·25 no condensation is produced in dust-free air. If
however _v₂_/_v₁_ > 1·25 and < 1·38, a few drops appear. This number is
roughly constant until _v₂_/_v₁_ = 1·38, when the number suddenly
increases and a very dense cloud of fine drops is produced.

If the radiation from an X ray tube or a radio-active substance is now
passed into the condensation vessel, a new series of phenomena is
observed. As before, if _v₂_/_v₁_ < 1·25 no drops are formed, but if
_v₂_/_v₁_ = 1·25 there is a sudden production of a cloud. The water
drops of which this cloud is formed are finer and more numerous the
greater the intensity of the rays. The point at which condensation
begins is very marked, and a slight variation of the amount of expansion
causes either a dense cloud or no cloud at all.

It now remains to be shown that the formation of a cloud by the action
of the rays is due to the productions of ions in the gas. If the
expansion vessel is provided with two parallel plates between which an
electric field can be applied, it is seen that the number of drops,
formed by the expansion with the rays acting, decreases with increase of
the electric field. The stronger the field the smaller the number of
drops formed. This result is to be expected if the ions are the centres
of condensation; for in a strong electric field the ions are carried at
once to the electrodes, and thus disappear from the gas. If no electric
field is acting, a cloud can be produced some time after the rays have
been cut off; but if a strong electric field is applied, under the same
conditions, no cloud is formed. This is in agreement with experiments
showing the time required for the ions to disappear by recombination. In
addition it can be shown that each one of the fine drops carries an
electric charge and can be made to move in a strong uniform electric
field.

The small number of drops produced without the action of the rays when
_v₂_/_v₁_ > 1·25 is due to a very slight natural ionization of the gas.
That this ionization exists has been clearly shown by electrical methods
(section 284).

The evidence is thus complete that the ions themselves serve as centres
for the condensation of water around them. These experiments show
conclusively that the passage of electricity through a gas is due to the
presence of charged ions distributed throughout the volume of the gas,
and verify in a remarkable way the hypothesis of the discontinuous
structure of the electric charges carried by matter.

This property of the ions of acting as nuclei of condensation gives a
very delicate method of detecting the presence of ions in the gas. If
only an ion or two is present per c.c., their presence after expansion
is at once observed by the drops formed. In this way the ionization due
to a small quantity of uranium held a yard away from the condensation
vessel is at once made manifest.


=35. Difference between the positive and negative ions.= In the course
of experiments to determine the charge carried by an ion, J. J.
Thomson[67] observed that the cloud formed under the influence of X rays
increased in density when the expansion was about 1·31, and suggested in
explanation that the positive and negative ions had different
condensation points.

[Illustration: Fig. 7.]

This difference in behaviour of the positive and negative ions was
investigated in detail by C. T. R. Wilson[68] in the following way. X
rays were made to pass in a narrow beam on either side of a plate _AB_
(Fig. 7) dividing the condensation vessel into two equal parts. The
opposite poles of a battery of cells were connected with two parallel
plates _C_ and _D_, placed symmetrically with regard to _A_. The middle
point of the battery and the plate _A_ were connected with earth. If the
plate _C_ is positively charged, the ions in the space _CA_ at a short
distance from _A_ are all negative in sign. Those to the right are all
positive. It was found that condensation occurred only for the negative
ions in _AC_ when _v₂_/_v₁_ = 1·25 but did not occur in _AD_ for the
positive ions until _v₂_/_v₁_ = 1·31.

Thus the negative acts more readily than the positive ion as a centre of
condensation. The greater effect of the negative ion in causing
condensation has been suggested as an explanation of the positive charge
always observed in the upper atmosphere. The negative ions under certain
conditions become centres for the formation of small drops of water and
are removed to the earth by the action of gravity, while the positive
ions remain suspended.

With the apparatus described above, it has been shown that the positive
and negative ions are equal in number. If the expansion is large enough
to ensure condensation on both ions, the drops formed on the right and
left of the vessel in Fig. 7 are equal in number and fall at the same
rate, _i.e._ are equal in size.

Since the ions are produced in equal numbers from a gas electrically
neutral, this experiment shows that the charges on positive and negative
ions are equal in value but opposite in sign.


=36. Charge carried by an ion.= For a known sudden expansion of a gas
saturated with water vapour, the amount of water precipitated on the
ions can be calculated readily. The size of the drops can be determined
by observing the rate at which the cloud settles under the action of
gravity. From Stokes’ equation, the terminal velocity _u_ of a small
sphere of radius _r_ and density _d_ falling through a gas of which the
coefficient of viscosity is μ is given by

                                     2 _dgr²_
                               _u_ = --------
                                     9 μ

where _g_ is the acceleration due to gravity. The radius of the drop and
consequently the weight of water in each drop can thus be determined.
Since the total weight of water precipitated is known, the number of
drops present is obtained at once.

This method has been used by J. J. Thomson[69] to determine the charge
carried by an ion. If the expansion exceeds the value 1·31, both
positive and negative ions become centres of condensation. From the rate
of fall it can be shown that approximately the drops are all of the same
size.

The condensation vessel was similar to that employed by C. T. R. Wilson.
Two parallel horizontal plates were fitted in the vessel and the
radiation from an X ray tube or radio-active substance ionized the gas
between them. A difference of potential _V_, small compared with that
required to saturate the gas, was applied between the parallel plates
distant _l_ cms. from each other. The small current _i_ through the gas
is given (section 28) by

                                      _NuVe_
                                _i_ = ------
                                      _l_

where

       _N_ = number of ions present in the gas,
       _e_ = charge on each ion,
       _u_ = sum of the velocities of the positive and negative ions.

Since the value of _N_ is the same as the number of drops, and the
velocity _u_ is known, the value of _e_ can be determined.

In his last determination J. J. Thomson found that

                   _e_ = 3·4 × 10⁻¹⁰ electrostatic units.

A very concordant value, namely, 3·1 × 10⁻¹⁰, has been obtained by H. A.
Wilson[70], by using a modified method of counting the drops. A check on
the size of the drops, determined by their rate of fall, was made by
observing the rate at which the drops moved in a strong electric field,
arranged so as to act with or against gravity.

J. J. Thomson found that the charge on the ions produced in hydrogen and
oxygen is the same. This shows that the nature of the ionization in
gases is distinct from that occurring in the electrolysis of solutions
where the oxygen ion always carries twice the charge of the hydrogen
ion.


=37. Diffusion of the ions.= Early experiments with ionized gases showed
that the conductivity was removed from the gas by passage through a
finely divided substance like cotton-wool, or by bubbling through water.
This loss of conductivity is due to the fact that the ions in passing
through narrow spaces diffuse to the sides of the boundary, to which
they either adhere or give up their charge.

A direct determination of the coefficient of diffusion of the ions
produced in gases by Röntgen rays or by the rays from active substances
has been made by Townsend[71]. The general method employed was to pass a
stream of ionized gas through a diffusion vessel made up of a number of
fine metal tubes arranged in parallel. Some of the ions in their passage
through the tubes diffuse to the sides, the proportion being greater the
slower the motion of the gas and the narrower the tube. Observations
were made of the conductivity of the gas before and after passage
through the tubes. In this way, correcting if necessary for the
recombination during the time taken to pass through the tubes, the
proportion _R_ of either positive or negative ions which are abstracted
can be deduced. The value of _R_ can be expressed mathematically by the
following equation in terms of _K_, the coefficient of diffusion of the
ions into the gas with which they are mixed[72],

$$ R = 4 (\cdot195e \frac{−3\cdot66KZ}{a^2V} + \cdot0243e
\frac{22\cdot3KZ}{a^2V} + &c.) $$

where

                _a_ = radius of the tube,
                _Z_ = length of the tube,
                _V_ = mean velocity of the gas in the tube.

Only the first two terms of the series need be taken into account when
narrow tubes are used.

In this equation _R_, _V_, and _a_ are determined experimentally, and
_K_ can thus be deduced.

The following table shows the results obtained by Townsend when X rays
were used. Almost identical results were obtained later, when the
radiations from active substances replaced the X rays.

              _Coefficients of diffusion of ions into gases._

              Gas             _K_    _K_    Mean   Ratio
                              for +  for −  value  of
                              ions   ions   of _K_ values
                                                   of _K_

              Air, dry        ·028   ·043   ·0347  1·54

              „   moist       ·032   ·035   ·0335  1·09

              Oxygen, dry     ·025   ·0396  ·0323  1·58

              „     moist     ·0288  ·0358  ·0323  1·24

              Carbonic acid,  ·023   ·026   ·0245  1·13
              dry

              „       „       ·0245  ·0255  ·025   1·04
              moist

              Hydrogen, dry   ·123   ·190   ·156   1·54

              „       moist   ·128   ·142   ·135   1·11

The moist gases were saturated with water vapour at a temperature of 15°
C.

It is seen that the negative ion in all cases diffuses faster than the
positive. Theory shows that the coefficients of diffusion should be
directly proportional to the velocities of the ions, so that this result
is in agreement with the observations on the greater velocity of the
negative ion.

This difference in the rate of diffusion of the ions at once explains an
interesting experimental result. If ionized gases are blown through a
metal tube, the tube gains a negative charge while the gas itself
retains a positive charge. The number of positive and negative ions
present in the gas is originally the same, but, in consequence of the
more rapid diffusion of the negative ions, more of the negative ions
than of the positive give up their charges to the tube. The tube
consequently gains a negative and the gas a positive charge.


=38.= A very important result can be deduced at once when the velocities
and coefficients of diffusion of the ions are known. Townsend (_loc.
cit._) has shown that the equation of their motion is expressed by the
formula

                         1           _dp_
                        --- _pu_ = − ---- + _nXe_,
                        _K_          _dx_

where _e_ is the charge on an ion,

                       _n_ = number of ions per c.c.,
                       _p_ = their partial pressure,

and _u_ is the velocity due to the electric force _X_ in the direction
of the axis of _x_. When a steady state is reached,

                         _dp_                 _nXeK_
                         ---- = 0  and  _u_ = ----,
                         _dx_                  _p_

Let _N_ be the number of molecules in a cubic centimetre of gas at the
pressure _P_ and at the temperature 15° C., for which the values of _u_
and _K_ have been determined. Then _N_/_P_ may be substituted for
_n_/_p_, and, since _P_ at atmospheric pressure is 10⁶,

                           3 × 10⁸_u₁_
                    _Ne_ = ----------     electrostatic units,
                             _K_

where _u₁_ is the velocity for 1 volt (_i.e._ ¹⁄₃₀₀ E. S. unit) per cm.

It is known that one absolute electromagnetic unit of electricity in
passing through water liberates 1·23 c.c. of hydrogen at a temperature
of 15° C. and standard pressure. The number of atoms in this volume is
2·46_N_, and, if _e´_ is the charge on the hydrogen atom in the
electrolysis of water,

                   2·46 _Ne´_ = 3 × 10¹⁰ E. S. units,
                        _Ne´_ = 1·22 × 10¹⁰ E. S. units.

                           _e_                _u₁_
                    Thus   --- = 2·46 × 10⁻²  ----
                           _e´_               _K_

For example, substituting the values of _u₁_ and _K_ determined in
moist air for the positive ion,

                     _e_       2·46     1·37
                     ---   =  ----- × ----- = 1·04.
                     _e´_      100     ·032

Values of this ratio, not very different from unity, are obtained for
the positive and negative ions of the gases hydrogen, oxygen, and carbon
dioxide. Taking into consideration the uncertainty in the experimental
values of _u₁_ and _K_, these results indicate that the _charge carried
by an ion in all gases is the same and is equal to that carried by the
hydrogen ion in the electrolysis of liquids_.


=39. Number of the ions.= We have seen that, from experimental data,
Townsend has found that _N_, the number of molecules present in 1 c.c.
of gas at 15° C. and standard pressure, is given by

                            _Ne_ = 1·22 × 10¹⁰.

Now _e_, the charge on an ion, is equal to 3·4 × 10⁻¹⁰ E. S. units;

                        thus      _N_ = 3·6 × 10⁻¹⁹.

If _I_ is the saturation current through a gas, and _q_ the total rate
of production of ions in the gas,

                                     _I_
                               _q_ = --- .
                                     _e_

The saturation current through air was found to be 1·2 × 10⁻⁸ amperes,
_i.e._ 36 E.S. units, for parallel plates 4·5 cms. apart, when ·45
gramme of radium of activity 1000 times that of uranium was spread over
an area of 33 sq. cms. of the lower plate. This corresponds to a
production of about 10¹¹ ions per second. Assuming, for the purpose of
illustration, that the ionization was uniform between the plates, the
volume of air acted on by the rays was about 148 c.c., and the number of
ions produced per c.c. per second about 7 × 10⁸. Since _N_ = 3·6 × 10¹⁹,
we see that, if one molecule produces two ions, the proportion of the
gas ionized per second is about 10⁻¹¹ of the whole. For uranium the
fraction is about 10⁻¹⁴, and for pure radium, of activity one million
times that of uranium, about 10⁻⁸. Thus even in the case of pure radium,
only about one molecule of gas is acted on per second in every 100
millions.

The electrical methods are so delicate that the production of one ion
per cubic centimetre per second can be detected readily. This
corresponds to the ionization of about one molecule in every 10¹⁹
present in the gas.


=40. Size and nature of the ions.= An approximate estimate of the mass
of an ion, compared with the mass of the molecule of the gas in which it
is produced, can be made from the known data of the coefficient _K_ of
inter-diffusion of the ions into gases. The value of _K_ for the
positive ions in moist carbon dioxide has been shown to be ·0245, while
the value of _K_ for the inter-diffusion of carbon dioxide with air is
·14. The value of _K_ for different gases is approximately inversely
proportional to the square root of the products of the masses of the
molecules of the two inter-diffusing gases; thus, the positive ion in
carbon dioxide behaves as if its mass were large compared with that of
the molecule. Similar results hold for the negative as well as for the
positive ion, and for other gases besides carbon dioxide.

This has led to the view that the ion consists of a charged centre
surrounded by a cluster of molecules travelling with it, which are kept
in position round the charged nucleus by electrical forces. A rough
estimate shows that this cluster consists of about 30 molecules of the
gas. This idea is supported by the variation in velocity, _i.e._ the
variation of the size of the negative ion, in the presence of water
vapour; for the negative ion undoubtedly has a greater mass in moist
than in dry gases. At the same time it is possible that the apparently
large size of the ion, as determined by diffusion methods, may be in
part a result of the charge carried by the ion. The presence of a charge
on a moving body would increase the frequency of collision with the
molecules of the gas, and consequently diminish the rate of diffusion.
The ion on this view may not actually be of greater size than the
molecule from which it is produced.

The negative and positive ions certainly differ in size, and this
difference becomes very pronounced for low pressures of the gas. At
atmospheric pressure, the negative ion, produced by the action of
ultra-violet light on a negatively charged body, is of the same size as
the ion produced by X rays, but at low pressures J. J. Thomson has shown
that it is identical with the corpuscle or electron, which has an
apparent mass of about ¹⁄₁₀₀₀ of the mass of the hydrogen atom. A
similar result has been shown by Townsend to hold for the negative ion
produced by X rays at a low pressure. It appears that the negative ion
at low pressure sheds its attendant cluster. The result of Langevin,
that the velocity of the negative ion increases more rapidly with the
diminution of pressure than that of the positive ion, shows that this
process of removal of the cluster is quite appreciable at a pressure of
10 mms. of mercury.

We must suppose that the process of ionization in gases consists in a
removal of a negative corpuscle or electron from the molecule of the
gas. At atmospheric pressure this corpuscle immediately becomes the
centre of an aggregation of molecules which moves with it and _is_ the
negative ion. After removal of the negative ion the molecule retains a
positive charge, and probably also becomes the centre of a cluster of
new molecules.

The terms electron and ion as used in this work may therefore be defined
as follows:—

The _electron_ or _corpuscle_ is the body of smallest mass yet known to
science. It carries a negative charge of value 3·4 × 10⁻¹⁰ electrostatic
units. Its presence has only been detected when in rapid motion, when,
for speeds up to about 10¹⁰ cms. a second, it has an apparent mass _m_
given by _e_/_m_ = 1·86 × 10⁷ electromagnetic units. This apparent mass
increases with the speed as the velocity of light is approached (see
section 82).

The ions which are produced in gases at ordinary pressure have an
apparent size, as determined from their rates of diffusion, large
compared with the molecule of the gas in which they are produced. The
negative ion consists of an electron with a cluster of molecules
attached to and moving with it; the positive ion consists of a molecule
from which an electron has been expelled, with a cluster of molecules
attached. At low pressures under the action of an electric field the
electron does not form a cluster. The positive ion is always atomic in
size, even at low pressures of the gas. Each of the ions carries a
charge of value 3·4 × 10⁻¹⁰ electrostatic units.


=41. Ions produced by collision.= The greater part of the radiation from
the radio-active bodies consists of a stream of charged particles
travelling with great velocity. In this radiation, the α particles,
which cause most of the ionization observed in the gas, consist of
positively charged bodies projected with a velocity about one-tenth the
velocity of light. The β rays consist of negatively charged particles,
which are identical with the cathode rays generated in a vacuum tube,
and travel with a speed about one-half the velocity of light (chapter
IV.). Each of these projected particles, in virtue of its great kinetic
energy, sets free a large number of ions by collision with the gas
molecules in its path. No definite experimental evidence has yet been
obtained of the number of ions produced by a single particle, or of the
way in which the ionization varies with the speed, but there is no doubt
that each projected body gives rise to many thousand ions in its path
before its energy of motion is destroyed.

It has already been mentioned (section 29) that at low pressures ions
moving under the action of an electric field are able to produce fresh
ions by collision with the molecules of the gas. At low pressures the
negative ion is identical with the electron set free in a vacuum tube,
or emitted by a radio-active substance.

The mean free path of the ion is inversely proportional to the pressure
of the gas. Consequently, if an ion moves in an electric field, the
velocity acquired between collisions increases with diminution of the
pressure. Townsend has shown that fresh ions are occasionally produced
by collision when the negative ion moves freely between two points
differing in potential by 10 volts. If the difference be about _V_ = 20
volts, fresh ions arise at each collision[73].

Now the energy _W_, acquired by an ion of charge _e_ moving freely
between two points at a difference of potential _V_, is given by

                               _W_ = _Ve_.

Taking _V_ = 20 volts = ²⁰⁄₃₀₀ E. S. units, and _e_ = 3·4 × 10⁻¹⁰, the
energy _W_ required in the case of a negative ion to produce an ion by
collision is given by

                          _W_ = 2·3 × 10⁻¹¹ ergs.

The velocity _u_ acquired by the ion of mass _m_ just before a collision
is given by

                              1
                             --- _mu²_ = _Ve_,
                              2

and

$$ u = \sqrt{\frac{2Ve}{m}} $$

Now _e_/_m_ = 1·86 × 10⁷ electromagnetic units for the electron at slow
speeds (section 82).

Taking _V_ = 20 volts, we find that

                       _u_ = 2·7 × 10⁸ cms. per sec.

This velocity is very great compared with the velocity of agitation of
the molecules of the gas.

In a weak electric field, the negative ions only produce ions by
collision. The positive ion, whose mass is at least 1000 times greater
than the electron, does not acquire a sufficient velocity to generate
ions by collision until an electric field is applied nearly sufficient
to cause a spark through the gas.

An estimate of the energy required for the production of an ion by X
rays has been made by Rutherford and McClung. The energy of the rays
was measured by their heating effect, and the total number of ions
produced determined. On the assumption that _all_ the energy of the rays
is used up in producing ions, it was found that _V_ = 175 volts—a value
considerably greater than that observed by Townsend from data of
ionization by collision. The ionization in the two cases, however, is
produced under very different conditions, and it is impossible to
estimate how much of the energy of the rays is dissipated in the form of
heat.


=42.= Variations are found in the saturation current through gases,
exposed to the radiations from active bodies, when the pressure and
nature of the gas and the distance between the electrodes are varied.
Some cases which are of special importance in measurements will now be
considered. With unscreened active material the ionization of the gas
is, to a large extent, due to the α rays, which are absorbed in their
passage through a few centimetres of air. In consequence of this rapid
absorption, the ionization decreases rapidly from the surface of the
active body, and this gives rise to conductivity phenomena different in
character from those observed with Röntgen rays, where the ionization is
in most cases uniform.


=43. Variation of the current with distance between the plates.= It has
been found experimentally[74] that the intensity of the ionization, due
to a large plane surface of active matter, falls off approximately in an
exponential law with the distance from the plate. On the assumption that
the rate of production of ions at any point is a measure of the
intensity _I_ of the radiation, the value of _I_ at that point is given
by

$$ \frac {i}{i₀} = 1 − e^{–λ x} $$

where λ is a constant, _x_ the distance from the plate, and _I₀_ the
intensity of the radiation at the surface of the plate.

While the exponential law, in some cases, approximately represents the
variation of the ionization with distance, in others the divergence from
it is wide. The ionization, due to a plane surface of polonium, for
example, falls off more rapidly than the exponential law indicates. The
α rays from an active substance like radium are highly complex; the law
of variation of the ionization due to them is by no means simple and
depends upon a variety of conditions. The distribution of ionization is
quite different according as a thick layer or a very thick film of
radio-active matter is employed. The question is fully considered at the
end of chapter IV., but for simplicity, the exponential law is assumed
in the following calculations.

Consider two parallel plates placed as in Fig. 1, one of which is
covered with a uniform layer of radio-active matter. If the distance _d_
between the plates is small compared with the dimensions of the plates,
the ionization near the centre of the plates will be sensibly uniform
over any plane parallel to the plates and lying between them. If _q_ be
the rate of production of ions at any distance _x_ and _q₀_ that at the
surface, then _q_ = _q₀__e_^{–λ_x_}. The saturation current _i_ per unit
area is given by

    $$ i = \int₀^d qe' dx $$, where _e´_ is the charge on an ion,

    $$ = q₀e' \int₀^d e^{–λ x} dx = \frac{q₀e'}{λ} (1 -
       e^{–λ d}) $$

hence, when λ_d_ is small, _i.e._ when the ionization between the plates
is nearly constant,

                             _i_ = _q₀e´d_.

The current is thus proportional to the distance between the plates.
When λ_d_ is large, the saturation current _i₀_ is equal to _q₀e´_/λ,
and is independent of further increase in the value of _d_. In such a
case the radiation is completely absorbed in producing ions between the
plates, and

$$ \frac {i}{i₀} = 1 − e^{–λ d} $$

For example, in the case of a thin layer of uranium oxide spread over a
large plate, the ionization is mostly produced by rays the intensity of
which is reduced to half value in passing through 4·3 mms. of air,
_i.e._ the value of λ is 1·6. The following table is an example of the
variation of _i_ with the distance between the plates.

                      Distance   Saturation Current

                        2·5 mms.             32
                        5    „               55
                        7·5  „               72
                       10    „               85
                       12·5  „               96
                       15    „              100

Thus the increase of current for equal increments of distance between
the plates decreases rapidly with the distance traversed by the
radiation.

The distance of 15 mms. was not sufficient to completely absorb all the
radiation, so that the current had not reached its limiting value.

When more than one type of radiation is present, the saturation current
between parallel plates is given by

$$ i = A (1 − e^{λ d}) + A_1 (1 − e^{–λ_1 d}) $$ &c.

where _A_, _A₁_ are constants, and λ, λ₁ the absorption constants of the
radiations in the gas.

Since the radiations are unequally absorbed in different gases, the
variation of current with distance depends on the nature of the gas
between the plates.


=44. Variation of the current with pressure.= The rate of production of
ions by the radiations from active substances is directly proportional
to the pressure of the gas. The absorption of the radiation in the gas
also varies directly as the pressure. The latter result necessarily
follows if the energy required to produce an ion is independent of the
pressure.

In cases where the ionization is uniform between two parallel plates,
the current will vary directly as the pressure; when however the
ionization is not uniform, on account of the absorption of the radiation
in the gas, the current does not decrease directly as the pressure until
the pressure is reduced so far that the ionization is sensibly uniform.
Consider the variation with pressure of the saturation current _i_
between two large parallel plates, one of which is covered with a
uniform layer of active matter.

Let λ₁ = absorption constant of the radiation in the gas for unit
pressure.

For a pressure _p_, the intensity _I_ at any point _x_ is given by

$$ \frac {I} {I₀} = e^{- p λ_1 x} $$

The saturation current _i_ is thus proportional to

    $$ \int₀^d pI dx = \int₀^d pI₀e^{-pλ_1 x} dx =
       \frac{I₀}{λ_1} (I − e^{pA_1d}) $$

If _r_ be the ratio of the saturation currents for the pressures _p₁_
and _p₂_,

$$ r = \frac {1 − e^{-p_1λ_1 d}} {1 − e^{-p_2λ_1 d}} $$

The ratio is thus dependent on the distance _d_ between the plates and
the absorption of the radiation by the gas.

The difference in the shape of the pressure-current curves[75] is well
illustrated in Fig. 8, where curves are given for hydrogen, air, and
carbonic acid for plates 3·5 cms. apart.

[Illustration: Fig. 8.]

For the purpose of comparison, the current at atmospheric pressure and
temperature in each case is taken as unity. The actual value of the
current was greatest in carbonic acid and least in hydrogen. In
hydrogen, where the absorption is small, the current over the whole
range is nearly proportional to the pressure. In carbonic acid, where
the absorption is large, the current diminishes at first slowly with the
pressure, but is nearly proportional to it below the pressure of 235
mms. of mercury. The curve for air occupies an intermediate position.

In cases where the distance between the plates is large, the saturation
current will remain constant with diminution of pressure until the
absorption is so reduced that the radiation reaches the other plate.

An interesting result follows from the rapid absorption of radiation by
the gas. If the current is observed between two fixed parallel plates,
distant _d₁_ and _d₂_ respectively from a large plane surface of active
matter, the current at first increases with diminution of pressure,
passes through a maximum value, and then diminishes. In such an
experimental case the lower plate through which the radiations pass is
made either of open gauze or of thin metal foil to allow the radiation
to pass through readily.

The saturation current _i_ is obviously proportional to

$$ \int_{d_1}^{d_2} pI₀e^{-pλ_1 d} $$,

i.e. to

$$ \frac{I₀}{λ_1} (e^{-pλ_1 d_1} − e^{-pλ_1 d_2}) $$

This is a function of the pressure, and is a maximum when

$$ \log_e \frac{d_1}{d_2} = − pλ_1 (d_2 − d_1) $$

For example, if the active matter is uranium, _p_λ₁ = 1·6 for the α rays
at atmospheric pressure. If _d₂_ = 3, and _d₁_ = 1, the saturation
current reaches a maximum when the pressure is reduced to about ⅓ of an
atmosphere. This result has been verified experimentally.


=45. Conductivity of different gases when acted on by the rays.= For a
given intensity of radiation, the rate of production of ions in a gas
varies for different gases and increases with the density of the gas.
Strutt[76] has made a very complete examination of the relative
conductivity of gases exposed to the different types of rays emitted by
active substances. To avoid correction for any difference of absorption
of the radiation in the various gases, the pressure of the gas was
always reduced until the ionization was directly proportional to the
pressure, when, as we have seen above, the ionization must everywhere be
uniform throughout the gas. For each type of rays, the ionization of air
is taken as unity. The currents through the gases were determined at
different pressures, and were reduced to a common pressure by assuming
that the ionization was proportional to the pressure.

With unscreened active material, the ionization is almost entirely due
to α rays. When the active substance is covered with a layer of
aluminium ·01 cm. in thickness, the ionization is mainly due to the β or
cathodic rays, and when covered with 1 cm. of lead, the ionization is
solely due to the γ or very penetrating rays. Experiments on the γ rays
of radium were made by observing the rate of discharge of a special
gold-leaf electroscope filled with the gas under examination and exposed
to the action of the rays. The following table gives the relative
conductivities of gases exposed to various kinds of ionizing radiations.

           Gas             Relative  α     β     γ    Röntgen
                           Density  rays  rays  rays   rays


           Hydrogen        0·0693   0·226 0·157 0·169  0·114

           Air             1·00     1·00  1·00  1·00   1·00

           Oxygen          1·11     1·16  1·21  1·17   1·39

           Carbon dioxide  1·53     1·54  1·57  1·53   1·60

           Cyanogen        1·86     1·94  1·86  1·71   1·05

           Sulphur dioxide 2·19     2·04  2·31  2·13   7·97

           Chloroform      4·32     4·44  4·89  4·88  31·9

           Methyl iodide   5·05     3·51  5·18  4·80  72·0

           Carbon          5·31     5·34  5·83  5·67  45·3
           tetrachloride

With the exception of hydrogen, it will be seen that the ionization of
gases is approximately proportional to their density for the α, β, γ
rays of radium. The results obtained by Strutt for Röntgen rays are
quite different; for example, the relative conductivity produced by them
in methyl iodide was more than 14 times as great as that due to the rays
of radium. The relative conductivities of gases exposed to X rays has
been recently re-examined by McClung[77] and Eve[78], who have found
that the conductivity depends upon the penetrating power of the X rays
employed. The results obtained by them will be discussed later (section
107).

This difference of conductivity in gases is due to unequal absorptions
of the radiations. The writer has shown[79] that the total number of
ions produced by the α rays for uranium, when completely absorbed by
different gases, is not very different. The following results were
obtained:

               Gas                                 Total
                                              Ionization

               Air                                   100

               Hydrogen                               95

               Oxygen                                106

               Carbonic acid                          96

               Hydrochloric acid gas                 102

               Ammonia                               101

The numbers, though only approximate in character, seem to show that the
energy required to produce an ion is probably not very different for the
various gases. Assuming that the energy required to produce an ion in
different gases is about the same, it follows that the relative
conductivities are proportional to the relative absorption of the
radiations.

A similar result has been found by McLennan for cathode rays. He proved
that the ionization was directly proportional to the absorption of the
rays in the gas, thus showing that the same energy is required to
produce an ion in all the gases examined.


=46. Potential Gradient.= The normal potential gradient between two
charged electrodes is always disturbed when the gas is ionized in the
space between them. If the gas is uniformly ionized between two parallel
plates, Child and Zeleny have shown that there is a sudden drop of
potential near the surface of both plates, and that the electric field
is sensibly uniform for the intermediate space between them. The
disturbance of the potential gradient depends upon the difference of
potential applied, and is different at the surface of the two plates.

In most measurements of radio-activity the material is spread over one
plate only. In such a case the ionization is to a large extent confined
to the volume of the air close to the active plate. The potential
gradient in such a case is shown in Fig. 9. The dotted line shows the
variation of potential at any point between the plates when no
ionization is produced between the plates; curve _A_ for weak
ionization, such as is produced by uranium, curve _B_ for the intense
ionization produced by a very active substance. In both cases the
potential gradient is least near the active plate, and greatest near the
opposite plate. For very intense ionization it is very small near the
active surface. The potential gradient varies slightly according as the
active plate is charged positively or negatively.

[Illustration: Fig. 9.]


=47. Variation of current with voltage for surface ionization.=

Some very interesting results, giving the variation of the current with
voltage, are observed when the ionization is intense, and confined to
the space near the surface of one of two parallel plates between which
the current is measured.

The theory of this subject has been worked out independently by
Child[80] and Rutherford[81]. Let _V_ be the potential difference
between two parallel plates at a distance _d_ apart. Suppose that the
ionization is confined to a thin layer near the surface of the plate _A_
(see Fig. 1) which is charged positively. When the electric field is
acting, there is a distribution of positive ions between the plates _A_
and _B_.

    Let
    _n₁_
    = number of positive ions per unit volume at a distance _x_ from the
       plate _A_,

    _K₁_
    = mobility of the positive ions,

    _e_ = charge on an ion.

The current _i₁_ per square centimetre through the gas is constant for
all values of _x_, and is given by

$$ i_1 = K_1n_1e \frac{dV}{dx} $$

By Poisson’s equation

$$ \frac {d^2 V} {da^2} = 4\pi n_1 e $$

Then

$$ i_1 = \frac {K_1} {4\pi} \frac {dV} {dx} \frac {d^2 V} {dx^2} $$

Integrating

$$ (\frac {dV} {dx})^2 = \frac {8\pi i_1x} {K_1} + A $$

where _A_ is a constant. Now _A_ is equal to the value of

                                   _dV_
                                   ----
                                   _dx_

when _x_ = 0. By making the ionization very intense, the value of

                                   _dV_
                                   ----
                                   _dx_

can be made extremely small.

Putting _A_ = 0, we see that

$$ \frac {dV} {dx} = \pm \sqrt {\frac {8\pi i_1 x} {K_1}} $$

This gives the potential gradient between the plates for different
values of _x_.

Integrating between the limits 0 and _d_,

$$ V = \pm \frac{2}{3} \sqrt {\frac {8\pi i_1} {K_1} d^{ rac{3}{2}}} $$

or

$$ i_1 = \frac {9V^2} {32\pi d^3} K_1 $$

If _i₂_ is the value of the current when the electric field is reversed,
and _K₂_ the velocity of the negative ion,

$$ i_2 = \frac {9V^2} {32 \pi d^3} K_2 $$

and

                               _i₁_   _K₁_
                               ---- = ---- .
                               _i₂_   _K₂_

The current in the two directions is thus directly proportional to the
velocities of the positive and negative ions. The current should vary
directly as the square of the potential difference applied, and
inversely as the cube of the distance between the plates.

The theoretical condition of surface ionization cannot be fulfilled by
the ionization due to active substances, as the ionization extends some
centimetres from the active plate. If, however, the distance between the
plates is large compared with the distance over which the ionization
extends, the results will be in rough agreement with the theory. Using
an active preparation of radium, the writer has made some experiments on
the variation of current with voltage between parallel plates distant
about 10 cms. from each other[82].

The results showed

(1) That the current through the gas for small voltages increased more
rapidly than the potential difference applied, but not as rapidly as the
square of that potential difference.

(2) The current through the gas depended on the direction of the
electric field; the current was always smaller when the active plate was
charged positively on account of the smaller mobility of the positive
ion. The difference between _i₁_ and _i₂_ was greatest when the gas was
dry, which is the condition for the greatest difference between the
velocities of the ions.

An interesting result follows from the above theory. For given values of
_V_ and _d_, the current cannot exceed a certain definite value, however
much the ionization may be increased. In a similar way, when an active
preparation of radium is used as a source of surface ionization, it is
found that, for a given voltage and distance between the plates, the
current does not increase beyond a certain value however much the
activity of the material is increased.


=48. Magnetic field produced by an ion in motion.= It will be shown
later that the two most important kinds of rays emitted by radio-active
substances consist of electrified particles, spontaneously projected
with great velocity. The easily absorbed rays, known as α rays, are
positively electrified atoms of matter; the penetrating rays, known as β
rays, carry a negative charge, and have been found to be identical with
the cathode rays produced by the electric discharge in a vacuum tube.

The methods adopted to determine the character of these rays are very
similar to those first used by J. J. Thomson to show that the cathode
rays consisted of a stream of negatively electrified particles projected
with great velocity.

The proof that the cathode rays were corpuscular in character, and
consisted of charged particles whose mass was very small compared with
that of the hydrogen atom, marked an important epoch in physical
science: for it not only opened up new and fertile fields of research,
but also profoundly modified our previous conceptions of the
constitution of matter.

A brief account will accordingly be given of the effects produced by a
moving charged body, and also of some of the experimental methods which
have been used to determine the mass and velocity of the particles of
the cathode stream[83].

Consider an ion of radius _a_, carrying a charge of electricity _e_, and
moving with a velocity _u_, small compared with the velocity of light.
In consequence of the motion, a magnetic field is set up around the
charged ion, which is carried with it. The charged ion in motion
constitutes a current element of magnitude _eu_, and the magnetic field
_H_ at any point distant _r_ from the sphere is given by

                                  _eu_ sin θ
                            _H_ = ---------
                                     _r²_

where θ is the angle the radius vector makes with the direction of
motion. The lines of magnetic force are circles around the axis of
motion. When the ion is moving with a velocity small compared with the
velocity of light, the lines of electric force are nearly radial, but as
the speed of light is approached, they tend to leave the axis of motion
and to bend towards the equator. When the speed of the body is very
close to that of light, the magnetic and electric field is concentrated
to a large extent in the equatorial plane.

The presence of a magnetic field around the moving body implies that
magnetic energy is stored up in the medium surrounding it. The amount of
this energy can be calculated very simply for slow speeds.

In a magnetic field of strength _H_, the magnetic energy stored up in
unit volume of the medium of unit permeability is given by

                                    _H²_
                                    ----
                                     8π

Integrating the value of this expression over the region exterior to a
sphere of radius _a_, the total magnetic energy due to the motion of the
charged body is given by

    $$ \int_a^{\infty} \frac{H^2}{8\pi} d(vol) = \frac{e^2 u^2}{8\pi}
       \int₀^{2\pi} \int₀^{\pi} \int_a^{\infty} \frac{\sin^2
       \theta}{r^4} r \sin \theta d\phi rd\theta dr $$
    $$ = \frac{e^2 u^2}{4} \int₀^{\pi} \int_a^{\infty} \frac{(1−\cos^2
       \theta)}{r^2} \sin \theta d\theta . dr $$
    $$ = \frac{e^2 u^2}{3} \int_a^{\infty} \frac{dr}{r^2} = \frac{e^2
       u^2}{3a} $$

The magnetic energy, due to the motion, is analogous to kinetic energy,
for it depends upon the square of the velocity of the body. In
consequence of the charge carried by the ion, additional kinetic energy
is associated with it. If the velocity of the ion is changed, electric
and magnetic forces are set up tending to stop the change of motion, and
more work is done during the change than if the ion were uncharged. The
ordinary kinetic energy of the body is

                                  1
                                 --- _mu²_
                                  2

In consequence of its charge, the kinetic energy associated with it is
increased by

                                   _e²u²_
                                   ------
                                    3_a_

It thus behaves as if it possessed a mass _m_ + _m₁_ where _m₁_ is _the
electrical mass_, with the value

                                   2_e²_
                                   ----
                                   3_a_

We have so far only considered the electrical mass of a charged ion
moving with a velocity small compared with that of light. As the speed
of light is approached, the magnetic energy can no longer be expressed
by the equation already given. The general values of the electrical mass
of a charged body for speed were first worked out by J. J. Thomson[84]
in 1887. A more complete examination was made in 1889 by Heaviside[85],
while Searle[86] worked out the case for a charged ellipsoid. Recently,
the question was again attacked by Abraham[87]. Slightly different
expressions for the variation of electrical mass with speed have been
obtained, depending upon the conditions assumed for the distribution of
the electricity on the sphere. The expression found by Abraham, which
has been utilized by Kaufmann to show that the mass of the electron is
electromagnetic in origin, is given later in section 82.

All the calculations agree in showing that the electrical mass is
practically constant for slow speeds, but increases as the speed of
light is approached, and is theoretically infinite when the speed of
light is reached. The nearer the velocity of light is approached, the
greater is the resisting force to a change of motion. An infinite force
would be required to make an electron actually attain the velocity of
light, so that, according to the present theory, it would be impossible
for an electron to move faster than light, _i.e._ faster than an
electromagnetic disturbance travels in the ether.

The importance of these deductions lies in the fact that an electric
charge in motion, quite independently of any material nucleus, possesses
an apparent mass in virtue of its motion, and that this mass is a
function of the speed. Indeed, we shall see later (see section 82) that
the apparent mass of the particles constituting the cathode stream can
be explained in virtue of their charge, without the necessity of
assuming a material body in which the charge is distributed. This has
led to the suggestion that all mass may be electrical in origin, and due
purely to electricity in motion.


=49. Action of a magnetic field on a moving ion.= Let us consider the
case of an ion of mass _m_ carrying a charge _e_ and moving freely with
a velocity _u_. If _u_ is small compared with the velocity of light, the
ion in motion corresponds to a current element of magnitude _eu_. If the
ion moves in an external magnetic field of strength _H_, it is acted on
by a force at right angles both to the direction of motion, and to that
of the magnetic force and equal in magnitude to _Heu_ sin θ, where θ is
the angle between the direction of the magnetic force and the direction
of motion. Since the force due to the magnetic field is always
perpendicular to the direction of motion, it has no effect upon the
velocity of the particle, but can only alter the direction of its path.

If ρ is the radius of curvature of the path of the ion, the force along
the normal is equal to

                                    _mu²_
                                    -----,
                                     ρ

and this is balanced by the force _Heu_ sin θ.

If

                                      π
                                 θ = ---,
                                      2

_i.e._ if the ion is moving at right angles to the direction of the
magnetic field

                                     _mu²_
                             _Heu_ = -----
                                       ρ

or

                                    _m_
                             _H_ρ = ----  _u_
                                    _e_

Since _u_ is constant, ρ is also constant, _i.e._ the particle describes
a circular orbit of radius ρ. The radius of the circular orbit is thus
directly proportional to _u_, and inversely proportional to _H_.

If the ion is moving at an angle θ with the direction of the magnetic
field, it describes a curve which is compounded of a motion of a
particle of velocity _u_ sin θ perpendicular to the field and _u_ cos θ
in the direction of the field. The former describes a circular orbit of
radius ρ, given by

                                 _m_
                          _H_ρ = ---  _u_ sin θ;
                                 _e_

the latter is unaffected by the magnetic field and moves uniformly in
the direction of the magnetic field with a velocity _u_ cos θ. The
motion of the particle is in consequence a helix, traced on a cylinder
of radius

                                  _mu_ sin θ
                              ρ = ----------,
                                     _eH_

whose axis is in the direction of the magnetic field. Thus an ion
projected obliquely to the direction of a uniform magnetic field always
moves in a helix whose axis is parallel to the lines of magnetic
force[88].


=50. Determination of e/m for the cathode stream.= The cathode rays,
first observed by Varley, were investigated in detail by Crookes. These
rays are projected from the cathode in a vacuum tube at low pressure.
They travel in straight lines, and are readily deflected by a magnet,
and produce strong luminosity in a variety of substances placed in their
path. The rays are deflected by a magnetic field in the same direction
as would be expected for a negatively charged particle projected from
the cathode. In order to explain the peculiar properties of these rays
Crookes supposed that they consisted of negatively electrified
particles, moving with great velocity and constituting, as he
appropriately termed it, “a new or fourth state of matter.” The nature
of these rays was for twenty years a subject of much controversy, for
while some upheld their material character, others considered that they
were a special form of wave motion in the ether.

Perrin and J. J. Thomson showed that the rays always carried with them a
negative charge, while Lenard made the important discovery that the rays
passed through thin metal foil and other substances opaque to ordinary
light. Using this property, he sent the rays through a thin window and
examined the properties of the rays outside the vacuum tube in which
they were produced.

The absorption of the rays by matter was shown to be nearly proportional
to the density over a very wide range, and to be independent of its
chemical constitution.

The nature of these rays was successfully demonstrated by J. J.
Thomson[89] in 1897. If the rays consisted of negatively electrified
particles, they should be deflected in their passage through an electric
as well as through a magnetic field. Such an experiment had been tried
by Hertz, but with negative results. J. J. Thomson, however, found that
the rays were deflected by an electric field in the direction to be
expected for a negatively charged particle, and showed that the failure
of Hertz to detect the same was due to the masking of the electric field
by the strong ionization produced in the gas by the cathode stream. This
effect was got rid of by reducing the pressure of the gas in the tube.

The experimental arrangement used for the electric deflection of the
rays is shown in Fig. 10.

The cathode rays are generated at the cathode _C_, and a narrow pencil
of rays is obtained by passing the rays through a perforated disc _AB_.
The rays then passed midway between two parallel insulated plates _D_
and _E_, _d_ centimetres apart, and maintained at a constant difference
of potential _V_. The point of incidence of the pencil of rays was
marked by a luminous patch produced on a phosphorescent screen placed at
_PP´_.

The particle carrying a negative charge _e_ in passing between the
charged plates, is acted on by a force _Xe_ directed towards the
positive plate, where _X_, the strength of the electric field, is given
by

                                  _V_
                                 ---- .
                                  _d_

[Illustration: Fig. 10.]

The application of the electric field thus causes the luminous patch to
move in the direction of the positive plate. If now a uniform magnetic
field is applied at the plates _D_ and _E_, perpendicular to the pencil
of rays, and parallel to the plane of the plates, and in such a
direction that the electric and magnetic forces are opposed to one
another, the patch of light can be brought back to its undisturbed
position by adjusting the strength of the magnetic field. If _H_ is the
strength of the magnetic field, the force on the particle due to the
magnetic field is _Heu_, and when a balance is obtained

                              _Heu_ = _Xe_,

                              or
                                      _X_
                              _u_ =   ---       (1).
                                      _H_

Now if the magnetic field _H_ is acting alone, the curvature ρ of the
path of the rays between the plates can be deduced from the deflection
of the luminous patch. But we have seen that

                                   _mu_
                             _H_ = -----       (2).
                                   _e_

From equations (1) and (2), the value of _u_ and _e_/_m_ for the
particle can be determined.

The velocity _u_ is not constant, but depends upon the potential
difference between the electrodes, and this in turn depends upon the
pressure and nature of the residual gas in the tube.

By altering these factors, the cathode particles may be made to acquire
velocities varying between about 10⁹ and 10¹⁰ cms. per second. This
velocity is enormous compared with that which can be impressed
ordinarily upon matter by mechanical means. On the other hand, the value
of _e_/_m_ for the particles is sensibly constant for different
velocities.

As a result of a series of experiments the mean value _e_/_m_ = 7·7 ×
10⁶ was obtained. The value of _e_/_m_ is independent of the nature or
pressure of the gas in the vacuum tube and independent of the metal used
as cathode. A similar value of _e_/_m_ was obtained by Lenard[90] and
others.

Kaufmann[91] and Simon[92] used a different method to determine the
value of _e_/_m_. The potential difference _V_ between the terminals of
the tube was measured. The work done on the charged particle in moving
from one end of the tube to the other is _Ve_, and this must be equal to
the kinetic energy

                                  1
                                  -- _mu²_
                                  2

acquired by the moving particle. Thus

                            _e_     _u²_
                            ---   = ----      (3).
                            _m_     2_V_

By combination of this equation with (2) obtained by measurement of the
magnetic deflexion, both _u_ and _e_/_m_ can be determined.

Simon found by this method that

                             _e_
                             ---  = 1·865 × 10⁷.
                             _m_

It will be seen later (section 82) that a similar value was deduced by
Kaufmann for the electrons projected from radium.

These results, which have been based on the effect of a magnetic and
electric field on a moving ion, were confirmed by Weichert, who
determined by a direct method the time required for the particle to
traverse a known distance.

The particles which make up the cathode stream were termed “corpuscles”
by J. J. Thomson. The name “electron,” first employed by Johnstone
Stoney, has also been applied to them and has come into general use[93].

The methods above described do not give the mass of the electron, but
only the ratio of the charge to the mass. A direct comparison can,
however, be made between the ratio _e_/_m_ for the electron and the
corresponding value for the hydrogen atoms set free in the electrolysis
of water. Each of the hydrogen atoms is supposed to carry a charge _e_,
and it is known that 96,000 coulombs of electricity, or, in round
numbers, 10⁴ electromagnetic units of quantity are required to liberate
one gram of hydrogen. If _N_ is the number of atoms in one gram of
hydrogen, then _Ne_ = 10⁴. But if _m_ is the mass of a hydrogen atom,
then _Nm_ = 1. Dividing one by the other _e_/_m_ = 10⁴. We have seen
already that a gaseous ion carries the same charge as a hydrogen atom,
while indirect evidence shows that the electron carries the same charge
as an ion, and consequently the same charge as the atom of hydrogen.
Hence we may conclude that the apparent mass of the electron is only
about ¹⁄₁₀₀₀ of the mass of the hydrogen atom. The electron thus behaves
as the smallest body known to science.

In later experiments J. J. Thomson showed that the negative ions set
free at low pressures by an incandescent carbon filament, and also the
negative ions liberated from a zinc plate exposed to the action of
ultra-violet light, had the same value for _e_/_m_ as the electrons
produced in a vacuum tube. It thus seemed probable that the electron was
a constituent of all matter. This view received strong support from
measurements of quite a different character. Zeeman in 1897 found that
the lines of the spectrum from a source of light exposed in a strong
magnetic field were displaced and doubled. Later work has shown that the
lines in some cases are trebled, in others sextupled, while, in a few
cases, the multiplication is still greater. These results received a
general explanation on the radiation theories previously advanced by
Lorenz and Larmor. The radiation, emitted from any source, was supposed
to result from the orbital or oscillatory motion of the charged parts
constituting the atom. Since a moving ion is acted on by an external
magnetic field, the motion of the charged ions is disturbed when the
source of light is exposed between the poles of a strong magnet. There
results a small change in the period of the emitted light, and a bright
line in the spectrum is, in consequence, displaced by the action of the
magnetic field. According to theory, the small change in the wave-length
of the emitted light depends upon the strength of the magnetic field and
on the ratio _e_/_m_ of the charge carried by the ion to its mass. By
comparison of the theory with the experimental results, it was deduced
that the moving ion carried a negative charge, and that the value of
_e_/_m_ was about 10⁷. The charged ion, responsible for the radiation
from a luminous body, is thus identical with the electron set free in a
vacuum tube.

It thus seems reasonable to suppose that the atoms of all bodies are
complex and are built up, in part at least, of electrons, whose apparent
mass is very small compared with that of the hydrogen atom. The
properties of such disembodied charges has been examined mathematically
among others by Larmor, who sees in this conception the ultimate basis
of a theory of matter. J. J. Thomson and Lord Kelvin have investigated
mathematically certain arrangements of a number of electrons which are
stable for small disturbances. This question will be discussed more in
detail in section 270.


=51. Canal rays.= If a discharge is passed through a vacuum tube
provided with a perforated cathode, within certain limits of pressure,
luminous streams are observed to pass through the holes and to emerge on
the side of the cathode remote from the anode. These rays were first
observed by Goldstein[94] and were called by him the “Canal-strahlen.”
These rays travel in straight lines and produce phosphorescence in
various substances.

Wien[95] showed that the canal rays were deflected by strong magnetic
and electric fields, but the amount of deflection was very small
compared with that of the cathode rays under similar conditions. The
deflection was found to be opposite in direction to the cathode rays,
and this indicates that the canal rays consist of positive ions. Wien
determined their velocity and the ratio _e_/_m_, by measuring the amount
of their magnetic and electric deflection. The value of _e_/_m_ was
found to be variable, depending upon the gas in the tube, but the
maximum value observed was 10⁴. This shows that the positive ion, in no
case, has a mass less than that of the hydrogen atom. It seems probable
that the canal rays consist of positive ions, derived either from the
gas or the electrodes, which travel towards the cathode, and have
sufficient velocity to pass through the holes of the cathode and to
appear in the gas beyond.

It is remarkable that, so far, no case has been observed where the
carrier of a positive charge has an apparent mass less than that of the
hydrogen atom. Positive electricity always appears to be associated with
bodies atomic in size. We have seen that the process of ionization in
gases is supposed to consist of the expulsion of an electron from the
atom. The corresponding positive charge remains behind on the atom and
travels with it. This difference between positive and negative
electricity appears to be fundamental, and no explanation of it has, as
yet, been forthcoming.


=52. Radiation of energy.= If an electron moves uniformly in a straight
line with constant velocity, the magnetic field, which travels with it,
remains constant, and there is no loss of energy from it by radiation.
If, however, its motion is hastened or retarded, the magnetic field is
altered, and there results a loss of energy from the electron in the
form of electromagnetic radiation. The rate of loss of energy from an
accelerated electron was first calculated by Larmor[96] and shown to be

                          2_e²_
                          ----   × (acceleration)²,
                          3_V_

where _e_ is the charge on the electron in electromagnetic units, and
_V_ the velocity of light.

Any alteration in the velocity of a moving charge is thus always
accompanied by a radiation of energy from it. Since the electron, set
free in a vacuum tube, increases in velocity in passing through the
electric field, energy must be radiated from it during its passage from
cathode to anode. It can, however, readily be calculated that, in
ordinary cases, this loss of energy is small compared with the kinetic
energy acquired by the electron in passing through the electric field.

An electron moving in a circular orbit is a powerful radiator of energy,
since it is constantly accelerated towards the centre. An electron
moving in an orbit of radius equal to the radius of an atom (about 10⁻⁸
cms.) would lose most of its kinetic energy of motion in a small
fraction of a second, even though its velocity was originally nearly
equal to the velocity of light. If, however, a number of electrons are
arranged at equal angular intervals on the circumference of a circle and
move with constant velocity round the ring, the radiation of energy is
much less than for a single electron, and rapidly diminishes with an
increase in the number of electrons round the ring. This result,
obtained by J. J. Thomson, will be discussed in more detail later when
the stability of systems composed of rotating electrons is under
consideration.

Since the radiation of energy is proportional to the square of the
acceleration, the proportion of the total energy radiated depends upon
the suddenness with which an electron is started or stopped. Now some of
the cathode ray particles are stopped abruptly when they impinge on the
metal cathode, and, in consequence, give up a fraction of their kinetic
energy in the form of electromagnetic radiation. Stokes and Weichert
suggested that this radiation constituted the X rays, which are known to
have their origin at the surface on which the cathode rays impinge. The
mathematical theory has been worked out by J. J. Thomson[97]. If the
motion of an electron is suddenly arrested, a thin spherical pulse in
which the magnetic and electric forces are very intense travels out from
the point of impact with the velocity of light. The more suddenly the
electron is stopped, the thinner and more intense is the pulse. On this
view the X rays are not corpuscular like the cathode rays, which produce
them, but consist of transverse disturbances in the ether, akin in some
respects to light waves of short wave-length. The rays are thus made up
of a number of pulses, which are non-periodic in character, and which
follow one another at irregular intervals.

On this theory of the nature of the X rays, the absence of direct
deflection, refraction, or polarization is to be expected, if the
thickness of the pulse is small compared with the diameter of an atom.
It also explains the non-deflection of the path of the rays by a
magnetic or electric field. The intensity of the electric and magnetic
force in the pulse is so great that it is able to cause a removal of an
electron from some of the atoms of the gas, over which the pulse passes,
and thus causes the ionization observed.

The cathode rays produce X rays, and these in turn give rise to a
secondary radiation whenever they impinge on a solid body. This
secondary radiation is emitted equally in all directions, and consists
partly of a radiation of the X ray type and also of electrons projected
with considerable velocity. This secondary radiation gives rise to a
tertiary radiation and so on.

Barkla[98] has shown that the secondary radiation emitted from a gas
through which the rays pass consists in part of scattered X rays of
about the same penetrating power as the primary rays as well as some
easily absorbed rays.

Part of the cathode rays is diffusely reflected on striking the cathode.
These scattered rays consist in part of electrons of the same speed as
in the primary beam, but also include some others of much less velocity.
The amount of diffuse reflection depends upon the nature of the cathode
and the angle of incidence of the rays.

We shall see later (chapter IV.) that similar effects are produced when
the rays from radio-active substances impinge upon solid bodies.

                  *       *       *       *       *

In this chapter an account of the ionization theory of gases has been
given to the extent that is necessary for the interpretation of the
measurements of radio-activity by the electric method. It would be out
of place here to discuss the development of that theory in detail, to
explain the passage of electricity through flames and vapours, the
discharge of electricity from hot bodies, and the very complicated
phenomena observed in the passage of electricity through a vacuum tube.

For further information on this important subject, the reader is
referred to J. J. Thomson’s _Conduction of Electricity through Gases_,
in which the whole subject is treated in a full and complete manner. A
simple account of the effect of moving charges and the electronic theory
of matter was given by the same author in the Silliman Lectures of Yale
University and published under the title _Electricity and Matter_
(Scribner, New York, 1904).

Footnote 44:

  J. J. Thomson and Rutherford, _Phil. Mag._ Nov. 1896.

Footnote 45:

  The word ion has now been generally adopted in the literature of the
  subject. In using this word, it is not assumed that the ions in gases
  are the same as the corresponding ions in the electrolysis of
  solutions.

Footnote 46:

  A minute current is observed between the plates even if no
  radio-active matter be present. This has been found to be due mainly
  to a slight natural radio-activity of the matter composing them. (See
  chapter XIV.)

Footnote 47:

  This nomenclature has arisen from the similarity of the shape of the
  current-voltage curves to the magnetization curves for iron. Since, on
  the ionization theory, the maximum current is a result of the
  _removal_ of all the ions from the gas, before recombination occurs,
  the terms are not very suitable. They have however now come into
  general use and will be retained throughout this work.

Footnote 48:

  J. J. Thomson, _Phil. Mag._ 47, p. 253, 1899; _Conduction of
  Electricity through Gases_, p. 73, 1903.

Footnote 49:

  Rutherford, _Phil. Mag._ Jan. 1899.

Footnote 50:

  Townsend, _Phil. Mag._ Feb. 1901.

Footnote 51:

  Rutherford, _Phil. Mag._ Nov. 1897, p. 144, Jan. 1899.

Footnote 52:

  Townsend, _Phil. Trans._ A, p. 157, 1899.

Footnote 53:

  McClung, _Phil. Mag._ March, 1902.

Footnote 54:

  Langevin, _Thèse présentée à la Faculté des Sciences_, p. 151, Paris,
  1902.

Footnote 55:

  Owens, _Phil. Mag._ Oct. 1899.

Footnote 56:

  Rutherford, _Phil. Mag._ p. 429, Nov. 1897.

Footnote 57:

  Zeleny, _Phil. Trans._ A, p. 193, 1901.

Footnote 58:

  Langevin, _C. R._ 134, p. 646, 1902.

Footnote 59:

  Zeleny, _Phil. Mag._ July, 1898.

Footnote 60:

  Rutherford, _Phil. Mag._ Feb. 1899.

Footnote 61:

  Zeleny, _Phil. Trans._ 195, p. 193, 1900.

Footnote 62:

  Langevin, _C. R._ 134, p. 646, 1902, and Thesis, p. 191, 1902.

Footnote 63:

  Rutherford, _Proc. Camb. Phil. Soc._ 9, p. 410, 1898.

Footnote 64:

  Langevin, Thesis, p. 190, 1902.

Footnote 65:

  Helmholtz and Richarz, _Annal. d. Phys._ 40, p. 161, 1890.

Footnote 66:

  Wilson, _Phil. Trans._ p. 265, 1897; p. 403, 1899; p. 289, 1900.

Footnote 67:

  Thomson, _Phil. Mag._ p. 528, Dec. 1898.

Footnote 68:

  Wilson, _Phil. Trans._ A, 193, p. 289, 1899.

Footnote 69:

  Thomson, _Phil. Mag._ p. 528, Dec. 1898, and March, 1903. _Conduction
  of Electricity through Gases_, Camb. Univ. Press, 1903, p. 121.

Footnote 70:

  Wilson, _Phil. Mag._ April, 1903.

Footnote 71:

  Townsend, _Phil. Trans._ A, p. 129, 1899.

Footnote 72:

  Townsend, _loc. cit._ p. 139.

Footnote 73:

  Some difference of opinion has been expressed as to the value of _V_
  required to produce ions at each collision. Townsend considers it to
  be about 20 volts; Langevin 60 volts and Stark about 50 volts.

Footnote 74:

  Rutherford, _Phil. Mag._ Jan. 1899.

Footnote 75:

  Rutherford, _Phil. Mag._ Jan. 1899.

Footnote 76:

  Strutt, _Phil. Trans._ A, p. 507, 1901 and _Proc. Roy. Soc._ p. 208,
  1903.

Footnote 77:

  McClung, _Phil. Mag._ Sept. 1904.

Footnote 78:

  Eve, _Phil. Mag._ Dec. 1904.

Footnote 79:

  Rutherford, _Phil. Mag._ p. 137, Jan. 1899.

Footnote 80:

  Child, _Phys. Rev._ Vol. 12, 1901.

Footnote 81:

  Rutherford, _Phil. Mag._ p. 210, August, 1901; _Phys. Rev._ Vol. 13,
  1901.

Footnote 82:

  Rutherford, _Phil. Mag._ Aug. 1901.

Footnote 83:

  A simple and excellent account of the effects produced by the motion
  of a charged ion and also of the electronic theory of matter was given
  by Sir Oliver Lodge in 1903 in a paper entitled “Electrons”
  (_Proceedings of the Institution of Electrical Engineers_, Part 159,
  Vol. 32, 1903). See also J. J. Thomson’s _Electricity and Matter_
  (Scribner, New York, 1904).

Footnote 84:

  J. J. Thomson, _Phil. Mag._ April, 1887.

Footnote 85:

  Heaviside, _Collected Papers_, Vol. II. p. 514.

Footnote 86:

  Searle, _Phil. Mag._ Oct. 1897.

Footnote 87:

  Abraham, _Phys. Zeit._ 4, No. 1 b, p. 57, 1902.

Footnote 88:

  A full account of the path described by a moving ion under various
  conditions is given by J. J. Thomson, _Conduction of Electricity in
  Gases_ (Camb. Univ. Press, 1903), pp. 79–90.

Footnote 89:

  J. J. Thomson, _Phil. Mag._ p. 293, 1897.

Footnote 90:

  Lenard, _Annal. d. Phys._ 64, p. 279, 1898.

Footnote 91:

  Kaufmann, _Annal. d. Phys._ 61, p. 544; 62, p. 596, 1897; 65, p. 431,
  1898.

Footnote 92:

  Simon, _Annal. d. Phys._ 69, p. 589, 1899.

Footnote 93:

  A complete discussion of the various methods employed to measure the
  velocity and mass of electrons and also of the theory on which they
  are based will be found in J. J. Thomson’s _Conduction of Electricity
  through Gases_.

Footnote 94:

  Goldstein, _Berlin Sitzber._ 39, p. 691, 1896; _Annal. d. Phys._ 64,
  p. 45, 1898.

Footnote 95:

  Wien, _Annal. d. Phys._ 65, p. 440, 1898.

Footnote 96:

  Larmor, _Phil. Mag._ 44, p. 593, 1897.

Footnote 97:

  J. J. Thomson, _Phil. Mag._ Feb. 1897.

Footnote 98:

  Barkla, _Phil. Mag._ June, 1903.




                              CHAPTER III.
                        METHODS OF MEASUREMENT.


=53. Methods of Measurement.= Three general methods have been employed
for examination of the radiations from radio-active bodies, depending on

(1) The action of the rays on a photographic plate.

(2) The ionizing action of the rays on the surrounding gas.

(3) The fluorescence produced by the rays on a screen of platinocyanide
of barium, zinc sulphide, or similar substance.

The third method is very restricted in its application, and can only be
employed for intensely active substances like radium or polonium.

The photographic method has been used very widely, especially in the
earlier development of the subject, but has gradually been displaced by
the electrical method, as a quantitative determination of the radiations
became more and more necessary. In certain directions, however, it
possesses distinct advantages over the electrical method. For example,
it has proved a very valuable means of investigating the curvature of
the path of the rays, when deflected by a magnetic or electric field,
and has allowed us to determine the constants of these rays with
considerable accuracy.

On the other hand, as a general method of study of the radiations, it is
open to many objections. A day’s exposure is generally required to
produce an appreciable darkening of the sensitive film when exposed to a
weak source of radiation like uranium and thorium. It cannot, in
consequence, be employed to investigate the radiations of those active
products which rapidly lose their activity. Moreover, W. J. Russell has
shown that the darkening of a photographic plate can be produced by many
agents which do not give out rays like those of the radio-active bodies.
This darkening of the plate is produced under the most varied
conditions, and very special precautions are necessary when long
exposures to a weak source of radiation are required.

The main objection to the photographic method, however, lies in the fact
that the radiations which produce the strongest electrical effect are
very weak photographically. For example, Soddy[99] has shown that the
photographic action of uranium is due almost entirely to the more
penetrating rays, and that the easily absorbed rays produce in
comparison very little effect. Speaking generally, the penetrating rays
are the most active photographically, and, under ordinary conditions,
the action on the plate is almost entirely due to them.

Most of the energy radiated from active bodies is in the form of easily
absorbed rays which are comparatively inactive photographically. These
rays are difficult to study by the photographic method, as the layer of
black paper which, in many cases, is required in order to absorb the
phosphorescent light from active substances, cuts off at the same time
most of the rays under examination. These easily absorbed rays will be
shown to play a far more important part in the processes occurring in
radio-active bodies than the penetrating rays which are more active
photographically.

The electrical method, on the other hand, offers a rapid and accurate
method of quantitatively examining the radiations. It can be used as a
means of measurement of all the types of radiation emitted, excluding
light waves, and is capable of accurate measurement over an extremely
wide range. With proper precautions it can be used to measure effects
produced by radiations of extremely small intensity.


=54. Electrical Methods.= The electrical methods employed in studying
radio-activity are all based on the property of the radiation in
question of ionizing the gas, _i.e._ of producing positively and
negatively charged carriers throughout the volume of the gas. The
discussion of the application of the ionization theory of gases to
measurements of radio-activity has been given in the last chapter. It
has been shown there that the essential condition to be fulfilled for
comparative measurements of the intensity of the radiations is that the
electrical field shall in all cases be strong enough to obtain the
maximum or saturation current through the gas.

The electric field required to produce practical saturation varies with
the intensity of the ionization and consequently with the activity of
the preparations to be examined. For preparations which have an activity
not more than 500 times that of uranium, under ordinary conditions, a
field of 100 volts per cm. is sufficient to produce a practical
saturation current. For very active samples of radium, it is often
impossible to obtain conveniently a high enough electromotive force to
give even approximate saturation. Under such conditions comparative
measurement can be made by measuring the current under diminished
pressure of the gas, when saturation is more readily obtained.

The method to be employed in the measurement of this ionization current
depends largely on the intensity of the current to be measured. If some
very active radium is spread on the lower of two insulated plates as in
Fig. 1, and a saturating electric field applied, the current may readily
be measured by a sensitive galvanometer of high resistance. For example,
a weight of ·45 gr. of radium chloride of activity 1000 times that of
uranium oxide, spread over a plate of area 33 sq. cms., gave a maximum
current of 1·1 × 10⁻⁸ amperes when the plates were 4·5 cms. apart. In
this case the difference of potential to be applied to produce practical
saturation was about 600 volts. Since most of the ionization is due to
rays which are absorbed in passing through a few centimetres of air, the
current is not much increased by widening the distance between the two
plates. In cases where the current is not quite large enough for direct
deflection, the current may be determined by connecting the upper
insulated plate with a well insulated condenser. After charging for a
definite time, say one or more minutes, the condenser is discharged
through the galvanometer, and the current can readily be deduced.


=55.= In most cases, however, when dealing with less active substances
like uranium or thorium, or with small amounts of active material, it is
necessary to employ methods for measuring much smaller currents than can
be detected conveniently by an ordinary galvanometer. The most
convenient apparatus to employ for this purpose is one of the numerous
types of quadrant electrometer or an electroscope of special design. For
many observations, especially where the activity of the two substances
is to be compared under constant conditions, an electroscope offers a
very certain and easy method of measurement. As an example of a simple
apparatus of this kind, a brief description will be given of the
electroscope used by M. and Mme Curie in many of their earlier
observations.

[Illustration: Fig. 11.]

The connections are clearly seen from Fig. 11. The active material is
placed on a plate laid on top of the fixed circular plate _P_, connected
with the case of the instrument and with earth. The upper insulated
plate _P´_ is connected with the insulated gold-leaf system _LL´_. _S_
is an insulating support and _L_ the gold-leaf.

The system is first charged to a suitable potential by means of the rod
_C_. The rate of movement of the gold-leaf is observed by means of a
microscope. In comparisons of the activity of two specimens, the time
taken by the gold leaf to pass over a certain number of divisions of the
micrometer scale in the eye-piece is observed. Since the capacity of the
charged system is constant, the average rate of movement of the
gold-leaf is directly proportional to the ionization current between _P_
and _P´_, _i.e._ to the intensity of the radiation emitted by the active
substance. Unless very active material is being examined, the difference
of potential between _P_ and _P´_ can easily be made sufficient to
produce saturation.

When necessary, a correction can be made for the rate of leak when no
active material is present. In order to avoid external disturbances, the
plates _PP´_ and the rod _C_ are surrounded by metal cylinders, _E_ and
_F_, connected with earth.


=56.= A modified form of the gold-leaf electroscope can be used to
determine extraordinarily minute currents with accuracy, and can be
employed in cases where a sensitive electrometer is unable to detect the
current. A special type of electroscope has been used by Elster and
Geitel, in their experiments on the natural ionization of the
atmosphere. A very convenient type of electroscope to measure the
current due to minute ionization of the gas is shown in Fig. 12.

[Illustration: Fig. 12.]

This type of instrument was first used by C. T. R. Wilson[100] in his
experiments of the natural ionization of air in closed vessels. A brass
cylindrical vessel is taken of about 1 litre capacity. The gold-leaf
system, consisting of a narrow strip of gold-leaf _L_ attached to a flat
rod _R_, is insulated inside the vessel by the small sulphur bead or
piece of amber _S_, supported from the rod _P_. In a dry atmosphere a
clean sulphur bead or piece of amber is almost a perfect insulator. The
system is charged by a light bent rod _CC´_ passing through an ebonite
cork[101]. The rod _C_ is connected to one terminal of a battery of
small accumulators of 200 to 300 volts. If these are absent, the system
can be charged by means of a rod of sealing-wax. The charging rod _CC´_
is then removed from contact with the gold-leaf system. The rods _P_ and
_C_ and the cylinder are then connected with earth.

The rate of movement of the gold-leaf is observed by a reading
microscope through two holes in the cylinder, covered with thin mica. In
cases where the natural ionization due to the enclosed air in the
cylinder is to be measured accurately, it is advisable to enclose the
supporting and charging rod and sulphur bead inside a small metal
cylinder _M_ connected to earth, so that only the charged gold-leaf
system is exposed in the main volume of the air.

In an apparatus of this kind the small leakage over the sulphur bead can
be eliminated almost completely by keeping the rod _P_ charged to the
average potential of the gold-leaf system during the observation. This
method has been used with great success by C. T. R. Wilson (_loc.
cit._). Such refinements, however, are generally unnecessary, except in
investigations of the natural ionization of gases at low pressures, when
the conduction leak over the sulphur bead is comparable with the
discharge due to the ionized gas.


=57.= The electric capacity _C_ of a gold-leaf system about 4 cms. long
is usually about 1 electrostatic unit. If _V_ is the decrease of
potential of the gold-leaf system in t seconds, the current i through
the gas is given by

                                    _CV_
                              _i_ = ----
                                    _t_.

With a well cleaned brass electroscope of volume 1 litre, the fall of
potential due to the natural ionization of the air was found to be about
6 volts per hour. Since the capacity of the gold-leaf system was about 1
electrostatic unit

                   6
    _i_ = 1 × ----------    = 5·6 × 10⁻⁶ E.S. units = 1·9 × 10⁻¹⁵ amperes.
              3600 × 300

With special precautions a rate of discharge of ⅒ or even ¹⁄₁₀₀ of
this amount can be measured accurately.

The number of ions produced in the gas can be calculated if the charge
on an ion is known. J. J. Thomson has shown that the charge _e_ on an
ion is equal to 3·4 × 10⁻¹⁰ electrostatic units or 1·13 × 10⁻¹⁹
coulombs.

     Let _q_ = number of ions produced per second per cubic centimetre
               throughout the volume of the electroscope,

         _S_ = volume of electroscope in cubic centimetres.

If the ionization be uniform, the saturation current _i_ is given by _i_
= _qSe_.

Now for an electroscope with a volume of 1000 c.c., _i_ was equal to
about 1·9 × 10⁻¹⁵ amperes. Substituting the values given above

               _q_ = 17 ions per cubic centimetre per second.

With suitable precautions an electroscope can thus readily measure an
ionization current corresponding to the production of 1 ion per cubic
centimetre per second.

The great advantage of an apparatus of this kind lies in the fact that
the current measured is due to the ionization inside the vessel and is
not influenced by the ionization of the external air or by electrostatic
disturbances[102]. Such an apparatus is very convenient for
investigating the very penetrating radiations from the radio-elements,
since these rays pass readily through the walls of the electroscope.
When the electroscope is placed on a lead plate 3 or 4 mms. thick, the
ionization in the electroscope, due to a radio-active body placed under
the lead, is due entirely to the very penetrating rays, since the other
two types of rays are completely absorbed in the lead plate. If a
circular opening is cut in the base of the electroscope and covered with
thin aluminium of sufficient thickness to absorb the α rays,
measurements of the intensity of the β rays from an active substance
placed under it, can be made with ease and certainty.


=58.= A modified form of electroscope, which promises to be of great
utility for measuring currents even more minute than those to be
observed with the type of instrument already described, has recently
been devised by C. T. R. Wilson[103]. The construction of the apparatus
is shown in Fig. 13.

The case consists of a rectangular brass box 4 cms. × 4 cms. × 3 cms. A
narrow gold-leaf _L_ is attached to a rod _R_ passing through a clean
sulphur cork. Opposite the gold-leaf is fixed an insulated brass plate
_P_, placed about 1 mm. from the wall of the box. The movement of the
gold-leaf is observed through two small windows by means of a microscope
provided with a micrometer scale. The plate _P_ is maintained at a
constant potential (generally about 200 volts). The electrometer case is
placed in an inclined position as shown in the figure, the angle of
inclination and the potential of the plate being adjusted to give the
desired sensitiveness. The gold-leaf is initially connected to the case,
and the microscope adjusted so that the gold-leaf is seen in the centre
of the scale. For a given potential of the plate, the sensitiveness
depends on the angle of tilt of the case. There is a certain critical
inclination below which the gold-leaf is unstable. The most sensitive
position lies just above the critical angle. In a particular experiment
Wilson found that with an angle of tilt of 30° and with the plate at a
constant potential of 207 volts, the gold-leaf, when raised to a
potential of one volt above the case, moved over 200 scale divisions of
the eye-piece, 54 divisions corresponding to one millimetre.

[Illustration: Fig. 13.]

In use, the rod _R_ is connected with the external insulated system
whose rise or fall of potential is to be measured. On account of the
small capacity of the system and the large movement of the gold-leaf for
a small difference of potential, the electroscope is able to measure
extraordinarily minute currents. The apparatus is portable. If the plate
_P_ be connected to one pole of a dry pile the gold-leaf is stretched
out towards the plate, and in this position can be carried without risk
of injury.


=59. Electrometers.= Although the electroscope can be used with
advantage in special cases, it is limited in its application. The most
generally convenient apparatus for measurement of ionization currents
through gases is one of the numerous types of quadrant electrometer.
With the help of auxiliary capacities, the electrometer can be used to
measure currents with accuracy over a wide range, and can be employed
for practically every kind of measurement required in radio-activity.

The elementary theory of the symmetrical quadrant electrometer as given
in the text-books is very imperfect. It is deduced that the sensibility
of the electrometer—measured by the deflection of the needle for 1 volt
P.D. between the quadrants—varies directly as the potential of the
charged needle, provided that this potential is high compared with the
P.D. between the quadrants. In most electrometers however, the
sensibility rises to a maximum, and then decreases with increase of
potential of the needle. For electrometers in which the needle lies
close to the quadrants, this maximum sensibility is obtained for a
comparatively low potential of the needle. A theory of the quadrant
electrometer, accounting for this action, has been recently given by G.
W. Walker[104]. The effect appears to be due to the presence of the air
space that necessarily exists between adjoining quadrants.

[Illustration: Fig. 14.]

Suppose that it is required to measure with an electrometer the
ionization current between two horizontal metal plates _A_ and _B_ (Fig.
14) on the lower of which some active material has been spread. If the
saturation current is required, the insulated plate _A_ is connected
with one pole of a battery of sufficient E.M.F. to produce saturation,
the other pole being connected to earth. The insulated plate _B_ is
connected with one pair of quadrants of the electrometer, the other pair
being earthed. By means of a suitable key _K_, the plate _B_ and the
pair of quadrants connected with it may be either insulated or connected
with earth. When a measurement is to be taken, the earth connection is
broken. If the positive pole of the battery is connected with _A_, the
plate _B_ and the electrometer connections immediately begin to be
charged positively, and the potential, if allowed, will steadily rise
until it is very nearly equal to the potential of _A_. As soon as the
potential of the electrometer system begins to rise, the electrometer
needle commences to move at a uniform rate. Observations of the angular
movement of the needle are made either by the telescope and scale or by
the movement of the spot of light on a scale in the usual way. If the
needle is damped so as to give a uniform motion over the scale, the rate
of movement of the needle, _i.e._ the number of divisions of the scale
passed over per second, may be taken as a measure of the current through
the gas. The rate of movement is most simply obtained by observing with
a stop-watch the time taken for the spot of light, after the motion has
become steady, to pass over 100 divisions of the scale. As soon as the
observation is made, the plate _B_ is again connected with earth, and
the electrometer needle returns to its original position.

In most experiments on radio-activity, only comparative measurements of
saturation currents are required. If these measurements are to extend
over weeks or months, as is sometimes the case, it is necessary to adopt
some method of standardizing the electrometer from day to day, so as to
correct for variation in its sensibility. This is done most simply by
comparing the current to be measured with that due to a standard sample
of uranium oxide, which is placed in a definite position in a small
testing vessel, always kept in connection with the electrometer. Uranium
oxide is a very constant source of radiation, and the saturation current
due to it is the same from day to day. By this method of comparison
accurate observations may be made on the variation of activity of a
substance over long intervals of time, although the sensibility of the
electrometer may vary widely between successive measurements.


=60. Construction of electrometers.= As the quadrant electrometer has
gained the reputation of being a difficult and uncertain instrument for
accurate measurements of current, it may be of value to give some
particular details in regard to the best method of construction and
insulation. In most of the older types of quadrant electrometers the
needle system was made unnecessarily heavy. In consequence of this, if a
sensibility of the order of 100 mms. deflection for 1 volt was required,
it was necessary to charge the Leyden jar connected to the needle to a
fairly high potential. This at once introduced difficulties, for at a
high potential it is not easy to insulate the Leyden jar satisfactorily,
or to charge it to the same potential from day to day. This drawback is
to a large extent avoided in the White pattern of the Kelvin
electrometer, which is provided with a replenisher and attracted disc
for keeping the potential of the needle at a definite value. If
sufficient trouble is taken in insulating and setting up this type of
electrometer, it proves a very useful instrument of moderate
sensibility, and will continue in good working order for a year or more
without much attention.

Simpler types of electrometer of greater sensibility can however be
readily constructed to give accurate results. The old type of quadrant
electrometer, to be found in every laboratory, can readily be modified
to prove a useful and trustworthy instrument. A light needle can be made
of thin aluminium, of silvered paper or of a thin plate of mica, covered
with gold-leaf to make it conducting. The aluminium wire and mirror
attached should be made as light as possible. The needle should be
supported either by a fine quartz fibre or a long bifilar suspension of
silk. A very fine phosphor bronze wire of some length is also very
satisfactory. A magnetic control is not very suitable, as it is
disturbed by coils or dynamos working in the neighbourhood. In addition,
the zero point of the needle is not as steady as with the quartz or
bifilar suspension.

When an electrometer is used to measure a current by noting the rate of
movement of the needle, it is essential that the needle should be damped
sufficiently to give a uniform motion of the spot of light over the
scale. The damping requires fairly accurate adjustment. If it is too
little, the needle has an oscillatory movement superimposed on the
steady motion; if it is too great, it moves too sluggishly from rest and
takes some time to attain a state of uniform motion. With a light
needle, very little, if any, extra damping is required. A light platinum
wire with a single loop dipping in sulphuric acid is generally
sufficient for the purpose.

With light needle systems and delicate suspensions, it is only necessary
to charge the needle to a potential of a few hundred volts to give a
sensibility of several thousand divisions for a volt. With such low
potentials, the difficulty of insulation of the condenser, with which
the needle is in electrical connection, is much reduced. It is
convenient to use a condenser such that the potential of the needle does
not fall more than a few per cent. per day. The ordinary short glass jar
partly filled with sulphuric acid is, in most cases, not easy to
insulate to this extent. It is better to replace it by an ebonite (or
sulphur) condenser[105] such as is shown in Fig. 15.

[Illustration: Fig. 15.]

A circular plate of ebonite about 1 cm. thick is turned down until it is
not more than ½ mm. thick in the centre. Into this circular recess a
brass plate _B_ fits loosely. The ebonite plate rests on another brass
plate _C_ connected with earth. The condenser thus formed has a
considerable capacity and retains a charge for a long time. In order to
make connection with the needle, a small glass vessel _D_, partly filled
with sulphuric acid, is placed on the plate _B_ and put in connection
with the needle by means of a fine platinum wire. The platinum wire from
the needle dips into the acid, and serves to damp the needle. In a dry
atmosphere, a condenser of this kind will not lose more than 20 per
cent. of its charge in a week. If the insulation deteriorates, it can
readily be made good by rubbing the edge of the ebonite _A_ with
sand-paper, or removing its surface in a lathe.

If a sufficient and steady E.M.F. is available, it is much better to
keep the battery constantly connected with the needle, and to avoid the
use of the condenser altogether. If a battery of small accumulators is
used, their potential can be kept at a constant value, and the
electrometer always has a constant sensibility.


=61.= A very useful electrometer of great sensibility has been devised
by Dolezalek[106]. It is of the ordinary quadrant type with a very light
needle of silvered paper, spindle shaped, which lies fairly close to the
quadrants. A very fine quartz suspension is employed. In consequence of
the lightness of the needle and its nearness to the quadrants, it acts
as its own damper. This is a great advantage, for difficulties always
arise when the wire dips into sulphuric acid, on account of the thin
film which collects after some time on the surface of the acid. This
film obstructs the motion of the platinum wire dipping into the acid,
and has to be removed at regular intervals. These instruments can
readily be made to give a sensibility of several thousand divisions for
a volt when the needle is charged to about one hundred volts. The
sensibility of the electrometer passes through a maximum as the
potential of the needle is increased. It is always advisable to charge
the needle to about the value of this critical potential. The capacity
of the electrometer is in general high (about 50 electrostatic units)
but the increased sensibility more than compensates for this. The needle
may either be charged by lightly touching it with one terminal of a
battery, or it may be kept charged to a constant potential through the
quartz suspension.

Dolezalek states that the fibre can be made sufficiently conducting for
the purpose by dipping it into a dilute solution of calcium chloride or
phosphoric acid. I have not found this method satisfactory in dry
climates as in many cases the fibre practically loses its conductivity
after a few days exposure to dry air.

In addition to its great sensibility, the advantage of this instrument
is in the steadiness of the zero and in the self-damping.

A sensibility of 10,000 millimetre divisions per volt can be readily
obtained with this electrometer, if a very fine fibre be used. The use
of such high sensibilities cannot, however, be recommended except for
very special experiments. The period of swing of the needle under these
conditions is several minutes and the natural leak of the testing
vessels employed, as well as electrostatic and other disturbances, make
themselves only too manifest. If measurements of minute currents are
required, an electroscope of the type described in Section 56 is much to
be preferred to a very sensitive electrometer. The electroscope readings
in such a case are more accurate than similar measurements made by an
electrometer.

For most measurements in radio-activity, an electrometer which has a
sensibility of 100 divisions per volt is very suitable, and no advantage
is gained by using an electrometer of greater sensibility. If still
smaller effects require to be measured, the sensibility may be increased
to several thousand divisions per volt.


=62. Adjustment and screening.= In adjusting an electrometer, it is
important to arrange that the needle shall lie symmetrically with regard
to the quadrants. This is best tested by observing whether the needle is
deflected on charging, the quadrants all being earthed. In most
electrometers there is an adjustable quadrant, the position of which may
be altered until the needle is not displaced on charging. When this
condition is fulfilled, the zero reading of the electrometer remains
unaltered as the needle loses its charge, and the deflection on both
sides of the zero should be the same for equal and opposite quantities
of electricity.

The supports of the quadrants require to be well insulated. Ebonite rods
are as a rule more satisfactory for this purpose than glass. In testing
for the insulation of the quadrants and the connections attached, the
system is charged to give a deflection of about 200 scale divisions. If
the needle does not move more than one or two divisions after standing
for one minute, the insulation may be considered quite satisfactory.
When a suitable desiccator is placed inside the tight-fitting
electrometer case, the insulation of the quadrants should remain good
for months. If the insulation of the ebonite deteriorates, it can easily
be made good by removing the surface of the ebonite in a lathe.

In working with a sensitive instrument like the Dolezalek electrometer,
it is essential that the electrometer and the testing apparatus should
be completely enclosed in a screen of wire-gauze connected with earth,
in order to avoid electrostatic disturbances. If an apparatus is to be
tested at some distance from the electrometer, the wires leading to it
should be insulated in metal cylinders connected with earth. The size of
the insulators used at various points should be made as small as
possible, in order to avoid disturbances due to their electrification.
In damp climates, paraffin, amber, or sulphur insulates better than
ebonite. The objection to paraffin as an insulator for sensitive
electrometers lies in the difficulty of getting entirely rid of any
electrification on its surface. When paraffin has been once charged, the
residual charge, after diselectrifying it with a flame, continues to
leak out for a long interval. All insulators should be diselectrified by
means of a spirit-lamp or still better by leaving some uranium near
them. Care should be taken not to touch the insulation when once
diselectrified.

In accurate work it is advisable to avoid the use of gas jets or Bunsen
flames in the neighbourhood of the electrometer, as the flame gases are
strongly ionized and take some time to lose their conductivity. If
radio-active substances are present in the room, it is necessary to
enclose the wires leading to the electrometer in fairly narrow tubes,
connected with earth. If this is not done, it will be found that the
needle does not move at a constant rate, but rapidly approaches a steady
deflection where the rate of loss of charge of the electrometer and
connections, due to the ionization of the air around them, is balanced
by the current to be measured. This precaution must always be taken when
observations are made on the very penetrating rays from active
substances. These rays readily pass through ordinary screens, and ionize
the air around the electrometer and connecting wires. For this reason it
is impossible to make accurate measurements of small currents in a room
which is used for the preparation of radio-active material. In course of
time the walls of the room become radio-active owing to the
dissemination of dust and the action of the radio-active
emanations[107].


=63. Electrometer key.= For work with electrometers of high sensibility,
a special key is necessary to make and break from a distance the
connection of the quadrants with earth in order to avoid electrostatic
disturbances at the moment the current is to be measured. The simple key
shown in Fig. 16 has been found very satisfactory for this purpose. A
small brass rod _BM_, to which a string is attached, can be moved
vertically up and down in a brass tube _A_, which is rigidly attached to
a bent metal support connected with earth. When the string is released,
this rod makes contact with the mercury _M_, which is placed in a small
metal vessel resting on a block of ebonite _P_. The electrometer and
testing vessel are connected with the mercury. When the string is
pulled, the rod _BM_ is removed from the mercury and the earth
connection of the electrometer system is broken. On release of the
string, the rod _BM_ falls and the electrometer is again earthed. By
means of this key, which may be operated at any distance from the
electrometer, the earth connection may be made and broken at definite
intervals without any appreciable disturbance of the needle.

[Illustration: Fig. 16.]


=64. Testing apparatus.= The arrangement shown in Fig. 17 is very
convenient for many measurements in radio-activity. Two parallel
insulated metal plates _A_ and _B_ are placed inside a metal vessel _V_,
provided with a side door. The plate _A_ is connected with one terminal
of a battery of small storage cells, the other pole of which is earthed;
the plate _B_ with the electrometer, and the vessel _V_ with earth. The
shaded areas in the figure indicate the position of ebonite insulators.
The active material to be tested is spread uniformly in a shallow groove
(about 5 cms. square and 2 mms. deep) in the brass plate _A_. In order
to avoid breaking the battery connection every time the plate _A_ is
removed, the wire from the battery is permanently connected with the
metal block _N_ resting on the ebonite support. In this arrangement
there is no possibility of a conduction leak from the plate _A_ to _B_,
since the earth-connected vessel _V_ intervenes.

[Illustration: Fig. 17.]

An apparatus of this kind is very convenient for testing the absorption
of the radiations by solid screens, as well as for making comparative
studies of the activity of different bodies. Unless very active
preparations of radium are employed, a battery of 300 volts is
sufficient to ensure saturation when the plates are not more than 5
centimetres apart. If substances which give off a radio-active emanation
are being tested, the effect of the emanation can be eliminated by
passing a steady current of air from a gas bag between the plates. This
removes the emanation as fast as it is produced.

If a clean plate is put in the place of _A_, a small movement of the
electrometer needle is always observed. If there is no radio-active
substance in the neighbourhood, this effect is due to the small natural
ionization of the air. We can correct for this natural leak when
necessary.


=65.= We have often to measure the activity due to the emanations of
thorium or radium, or the excited activity produced by those emanations
on rods or wires. A convenient apparatus for this purpose is shown in
Fig. 18. The cylinder _B_ is connected with the battery in the usual
way, and the central conductor _A_ with the electrometer. This central
rod is insulated from the external cylinder by an ebonite cork, which is
divided into two parts by a metal ring _CC´_ connected to earth. This
ring acts the part of a guard-ring, and prevents any conduction leak
between _B_ and _A_. The ebonite is thus only required to insulate
satisfactorily for the small rise of potential produced on _A_ during
the experiment. In all accurate measurements of current in
radio-activity the guard-ring principle should always be used to ensure
good insulation. This is easily secured when the ebonite is only
required to insulate for a fraction of a volt, instead of for several
hundred volts, as is the case when the guard-ring is absent.

[Illustration: Fig. 18.]


=66.= For measurements of radio-activity with an electrometer, a steady
source of E.M.F. of at least 300 volts is necessary. This is best
obtained by a battery of small cells simply made by immersing strips of
lead in dilute sulphuric acid, or by a battery of small accumulators of
the usual construction. Small accumulators of capacity about one-half
ampere-hour can now be obtained at a moderate price, and are more
constant and require less attention than simple lead cells.

In order to measure currents over a wide range, a graduated series of
capacities is required. The capacity of an electrometer and testing
apparatus is usually about 50 electrostatic units or ·000056
microfarads. Subdivided condensers of mica are constructed in which
capacities varying from ·001 to ·2 microfarads are provided. With such a
condenser, another extra capacity is required to bridge over the gap
between the capacity of the electrometer and the lowest capacity of the
condenser. This capacity of value about 200 electrostatic units can
readily be made by using parallel plates or still better concentric
cylinders. With this series of capacities, currents may be measured
between 3 × 10⁻¹⁴ and 3 × 10⁻⁸ amperes—a range of over one million.
Still larger currents can be measured if the sensibility of the
electrometer is reduced, or if larger capacities are available.

In a room devoted to electrometer measurements of radio-activity, it is
desirable to have no radio-active matter present except that to be
tested. The room should also be as free from dust as possible. The
presence of a large quantity of dust in the air (see section 31) is a
very disturbing factor in all radio-active measurements. A larger E.M.F.
is required to produce saturation on account of the diffusion of the
ions to the dust particles. The presence of dust in the air also leads
to uncertainty in the distribution of excited activity in an electric
field (see section 181).


=67. Measurement of Current.= In order to determine the current in the
electrometer circuit by measuring the rate of movement of the needle, it
is necessary to know both the capacity of the circuit and the
sensibility of the electrometer.

    Let _C_ = capacity of electrometer and its connections in E.S.
       units,
        _d_ = number of divisions of the scale passed over per second,
        _D_ = sensibility of the electrometer measured in scale
           divisions
                    for 1 volt P.D. between the quadrants.

The current _i_ is given by the product of the capacity of the system
and the rate of rise of potential.

                     Thus
                                _Cd_
                          _i_ = -----   E.S. units,
                                300_D_

                                   _Cd_
                              = -----------    amperes.
                                9 × 10¹¹ _D_

Suppose, for example,

                     _C_ = 50, _d_ = 5, _D_ = 1000;
                     then   _i_ = 2·8 × 10⁻¹³ amperes.

Since the electrometer can readily measure a current corresponding to a
movement of half a scale division per second, we see that an
electrometer can measure a current of 3 × 10⁻¹⁴ amperes, which is
considerably below the range of the most sensitive galvanometer.

The capacity of the electrometer itself must not be considered as equal
to that of the pair of quadrants and the needle when in a position of
rest. The actual capacity is very much larger than this, on account of
the motion of the charged needle. Suppose, for example, that the needle
is charged to a high negative potential, and kept at the zero position
by an external constraint. If a quantity _Q_ of positive electricity is
given to the electrometer and its connections, the whole system is
raised to a potential _V_, such that _Q_ = _CV_, where _C_ is the
capacity of the system. When however the needle is allowed to move, it
is attracted into the charged pair of quadrants. This corresponds to the
introduction of a negatively charged body between the quadrants, and in
consequence the potential of the system is lowered to _V´_. The actual
capacity _C´_ of the system when the needle moves is thus greater than
_C_, and is given by

                               _C´V´_ = _CV_.

Thus the capacity of the electrometer is not a constant, but depends on
the potential of the needle, _i.e._ on the sensibility of the
electrometer.

An interesting result of practical importance follows from the variation
of the capacity of the electrometer with the potential of the needle. If
the external capacity attached to the electrometer is small compared
with that of the electrometer itself, the rate of movement of the needle
for a constant current is, in some cases, independent of the
sensibility. An electrometer may be used for several days or even weeks
to give nearly equal deflections for a constant current, without
recharging the needle, although its potential has been steadily falling
during the interval. In such a case the decrease in sensibility is
nearly proportional to the decrease in capacity of the electrometer, so
that the deflection for a given current is only slightly altered. The
theory of this action has been given by J. J. Thomson[108].


=68.= The capacity of the electrometer and its connections cannot be
measured by any of the commutator methods used for the determination of
small capacities, for in such cases the needle does not move, and the
capacity measured is not that of the electrometer system when in actual
use. The value of the capacity may, however, be determined by the method
of mixtures.

            Let _C_ = capacity of electrometer and connections,
                _C₁_ = capacity of a standard condenser.

The electrometer and its connections are charged to a potential _V₁_ by
a battery, and the deflection _d₁_ of the needle is noted. By means of
an insulated key, the capacity of the standard condenser is added in
parallel with the electrometer system. Let _V₂_ be the potential of the
system, and _d₂_ the new deflection.

                   Then _CV₁_ = (_C_ + _C₁_) _V₂_,
                   _C_ + _C₁_      _V₁_       _d₁_
                   --------      = ----    = -----
                    _C_            _V₂_      _d₂_

                                     _d₂_
                   and _C_ = _C₁_ ----------
                                  _d₁_ − _d₂_

[Illustration: Fig. 19.]

A simple standard capacity for this purpose can be constructed of two
concentric brass tubes the diameters of which can be accurately
measured. The external cylinder _D_ (Fig. 19) is mounted on a wooden
base, which is covered with a sheet of metal or tinfoil connected to
earth. The tube _C_ is supported centrally on ebonite rods at each end.
The capacity is given approximately by the formula

               $$ C = \frac {l} {2 \log_e \frac {b} {a}} $$,

where _b_ is the internal diameter of _D_, _a_ the external diameter of
_C_, and _l_ the length of the tubes.

The following method can be used in some cases with advantage. While a
testing vessel is in connection with the electrometer, a sample of
uranium is placed on the lower plate _A_. Let _d₂_ and _d₁_ be the
number of divisions passed over per second by the needle with and
without the standard capacity in connection.

                           _C_ + _C₁_         _d₁_
                    Then   ----------     =  ----,
                              _C_             _d₂_

                                         _d₂_
                    and   _C_ = _C₁_ ---------
                                     _d₁_ − _d₂_

This method has the advantage that the relative capacities are expressed
in terms of the motion of the needle under the actual conditions of
measurement.


=69. Steady deflection method.= The methods of measurement previously
described depend upon the rate of angular movement of a suspended
gold-leaf or of an electrometer needle. The galvanometer can only be
employed for measurements with intensely active matter. A need, however,
has long been felt for a method in which ordinary ionization currents
can be measured by means of a steady deflection of an electrometer
needle. This is especially the case, where measurements have to be made
with active substances whose activity alters rapidly in the course of a
few minutes.

This can obviously be secured if the electrometer system (one pair of
quadrants being earthed) is connected to earth through a suitable high
resistance. A steady deflection of the electrometer needle will be
obtained when the rate of supply of electricity to the electrometer
system is balanced by the loss due to conduction through the resistance.
If the high resistance obeys Ohm’s law, the deflection should be
proportional to the ionization current to be measured.

A simple calculation shows that the resistance required is very great.
Suppose, for example, that a current is to be measured corresponding to
a rate of movement of the needle of 5 divisions per second, with a
sensibility of 1000 divisions per volt, and where the capacity of the
electrometer system is 50 electrostatic units. This current is equal to
2·8 × 10⁻¹³ amperes. If a steady deflection of 10 divisions is required,
which corresponds to a rise of potential of the system of ¹⁄₁₀₀ of a
volt, the resistance should be 36,000 megohms. For a deflection of 100
divisions, the resistance should be 10 times as large. Dr Bronson[109],
working in the laboratory of the writer, has recently made some
experiments in order to devise a practical method for measurements of
this character. It is difficult to obtain sufficiently high and constant
resistances to answer the purpose. Tubes of xylol had too great a
resistance, while special carbon resistances were not sufficiently
constant. The difficulty was finally got over by the use of what may be
called an “air resistance.” The arrangement of the experiment is shown
in Fig. 20.

[Illustration: Fig. 20.]

The electrometer system was connected with the upper of two insulated
parallel plates _AB_, on the lower of which was spread a layer of a very
active substance. An active bismuth plate, coated with radio-tellurium,
which had been obtained from Sthamer of Hamburg, proved very convenient
for this purpose.

The lower plate _B_ was connected to earth. The charge communicated to
the upper plate of the testing vessel _CD_ and the electrometer system
leaked away in consequence of the strong ionization between the plates
_AB_, and a steady deflection was obtained when the rate of supply was
equal to the rate of discharge.

This air resistance obeyed Ohm’s law over a considerable range, _i.e._
the steady deflection was proportional to the current. It is advisable,
in such an arrangement, to test whether the deflection is proportional
to the ionization current over the range required for measurement. This
can readily be done by the use of a number of metal vessels filled with
a constant radio-active substance like uranium oxide. The effect of
these, when placed in the testing vessel, can be tested separately and
in groups, and in this way the scale can be calibrated accurately.

The plates _AB_ were placed inside a closed vessel to avoid air
currents. The contact difference of potential between the plates _AB_,
which shows itself by a steady deflection when no radio-active matter is
present in _CD_, was for the most part eliminated by covering the
surface of the plates _A_ and _B_ with very thin aluminium foil.

This method proved very accurate and convenient for measurement of rapid
changes in activity, and possesses many advantages over the ordinary
rate-method of use of an electrometer. A thin layer of radium of
moderate activity would probably serve in place of the radio-tellurium,
but the emanation and the β and γ rays emitted from it would be a
possible source of disturbance to the measurements. The deflection of
the electrometer needle in this arrangement is independent of the
capacity of the electrometer system, and thus comparative measurements
of current can be made without the necessity of determining the capacity
in each case.


=70.= =Quartz piezo-electrique.= In measurements of the strength of
currents by electrometers, it is always necessary to determine the
sensibility of the instrument and the capacity of the electrometer and
the apparatus attached thereto. By means of the quartz piezo-electrique
devised by the brothers MM. J. and P. Curie[110], measurements of the
current can be made with rapidity and accuracy over a wide range. These
measurements are quite independent of the capacity of the electrometer
and external circuit.

The essential part of this instrument consists of a plate of quartz
which is cut in a special manner. When this plate is placed under
tension, there is a liberation of electricity equal in amount but
opposite in sign on the two sides of the plate. The plate of quartz _AB_
(Fig. 21) is hung vertically and weights are added to the lower end. The
plate is cut so that the optic axis of the crystal is horizontal and at
right angles to the plane of the paper.

[Illustration: Fig. 21.]

The two faces _A_ and _B_ are normal to one of the binary axes (or
electrical axes) of the crystal. The tension must be applied in a
direction normal to the optic and electric axes. The two faces _A_ and
_B_ are silvered, but the main portion of the plate is electrically
insulated by removing a narrow strip of the silvering near the upper and
lower ends of the plate. One side of the plate is connected with the
electrometer and with the conductor, the rate of leak of which is to be
measured. The quantity of electricity set free on one face of the plate
is accurately given by

                                       _L_
                            _Q_ = ·063 ----  _F_
                                       _b_


where _L_ is the length of the insulated portion of the plate, _b_ the
thickness _AB_, and _F_ the weight attached in kilogrammes. _Q_ is then
given in electrostatic units.

Suppose, for example, that it is required to measure the current between
the plates _CD_ (Fig. 21) due to some radio-active material on the plate
_C_, for a given difference of potential between _C_ and _D_. At a given
instant the connection of the quadrants of the electrometer with the
earth is broken. The weight is attached to the quartz plate, and is held
in the hand so as to apply the tension gradually. This causes a release
of electricity opposite in sign to that given to the plate _D_. The
electrometer needle is kept at the position of rest as nearly as
possible by adjusting the tension by hand. The tension being fully
applied, the moment the needle commences to move steadily from zero is
noted. The current between the plates _CD_ is then given by _Q_/_t_
where _t_ is the time of the observation. The value of _Q_ is known from
the weight attached.

In this method the electrometer is only used as a detector to show that
the system is kept at zero potential. No knowledge of the capacity of
the insulated system is required. With practice, measurements of the
current can be made in this way with rapidity and certainty.

Footnote 99:

  Soddy, _Trans. Chem. Soc._ Vol. 81, p. 860, 1902.

Footnote 100:

  Wilson, _Proc. Roy. Soc._ Vol. 68, p. 152, 1901.

Footnote 101:

  If the apparatus is required to be air-tight, the gold-leaf system can
  be charged by means of a piece of magnetized steel wire, which is made
  to touch the rod _R_ by the approach of a magnet.

Footnote 102:

  It is sometimes observed that the motion of the gold-leaf, immediately
  after charging, is irregular. In many cases, this can be traced to air
  currents set up in the electroscope in consequence of unsymmetrical
  heating by the source of light used for illumination.

Footnote 103:

  Wilson, _Proc. Camb. Phil. Soc._ Vol. 12, Part II. 1903.

Footnote 104:

  Walker, _Phil. Mag._ Aug. 1903.

Footnote 105:

  Strutt, _Phil. Trans._ A, p. 507, 1901.

Footnote 106:

  Dolezalek, _Instrumentenkunde_, p. 345, Dec. 1901.

Footnote 107:

  It is very desirable that care should be taken not to release large
  quantities of the radium emanation inside a laboratory. This emanation
  has a slow rate of decay and is carried by currents of air throughout
  the whole building and finally leaves behind an active deposit of very
  slow rate of change (see chapter XI.). Eve (_Nature_, March 16, 1905)
  has drawn attention to the difficulty of making refined radio-active
  measurements under such conditions.

Footnote 108:

  J. J. Thomson, _Phil. Mag._ 46, p. 537, 1898.

Footnote 109:

  Bronson, _Amer. Journ. Science_, Feb. 1905.

Footnote 110:

  J. and P. Curie, _C. R._ 91, pp. 38 and 294, 1880. See also Friedel
  and J. Curie, _C. R._ 96, pp. 1262 and 1389, 1883, and Lord Kelvin,
  _Phil. Mag._ 36, pp. 331, 342, 384, 414, 453, 1893.




                              CHAPTER IV.
                       NATURE OF THE RADIATIONS.


                                PART I.
                     Comparison of the Radiations.


=71. The Three Types of Radiation.= All the radio-active substances
possess in common the power of acting on a photographic plate and of
ionizing the gas in their immediate neighbourhood. The intensity of the
radiations may be compared by means of their photographic or electrical
action; and, in the case of the strongly radio-active substances, by the
power they possess of lighting up a phosphorescent screen. Such
comparisons, however, do not throw any light on the question whether the
radiations are of the same or of different kinds, for it is well known
that such different types of radiations as the short waves of
ultra-violet light, Röntgen and cathode rays, all possess the property
of producing ions throughout the volume of a gas, lighting up a
fluorescent screen, and acting on a photographic plate. Neither can the
ordinary optical methods be employed to examine the radiations under
consideration, as they show no trace of regular reflection, refraction,
or polarization.

Two general methods can be used to distinguish the types of the
radiations given out by the same body, and also to compare the
radiations from the different active substances. These methods are as
follows:

    (1) By observing whether the rays are appreciably deflected in a
    magnetic field.

    (2) By comparing the relative absorption of the rays by solids and
    gases.

Examined in these ways, it has been found that there are three different
types of radiation emitted from radio-active bodies, which for brevity
and convenience have been termed by the writer the α, β, and γ rays.

(i) The α rays are very readily absorbed by thin metal foil and by a few
centimetres of air. They have been shown to consist of positively
charged bodies projected with a velocity of about ⅒ the velocity of
light. They are deflected by intense magnetic and electric fields, but
the amount of deviation is minute in comparison with the deviation,
under the same conditions, of the cathode rays produced in a vacuum
tube.

(ii) The β rays are far more penetrating in character than the α rays,
and consist of negatively charged bodies projected with velocities of
the same order as the velocity of light. They are far more readily
deflected than the α rays, and are in fact identical with the cathode
rays produced in a vacuum tube.

(iii) The γ rays are extremely penetrating, and non-deviable by a
magnetic field. Their true nature is not definitely settled, but they
are analogous in most respects to very penetrating Röntgen rays.

The three best known radio-active substances, uranium, thorium, and
radium, all give out these three types of rays, each in an amount
approximately proportional to its relative activity measured by the α
rays. Polonium stands alone in giving only the α or easily absorbed
rays[111].


=72. Deflection of the rays.= The rays emitted from the active bodies
thus present a very close analogy with the rays which are produced in a
highly exhausted vacuum tube when an electric discharge passes through
it. The α rays correspond to the canal rays, discovered by Goldstein,
which have been shown by Wien to consist of positively charged bodies
projected with great velocity (see section 51). The β rays are the same
as the cathode rays, while the γ rays resemble the Röntgen rays. In a
vacuum tube, a large amount of electric energy is expended in producing
the rays, but, in the radio-active bodies, the rays are emitted
spontaneously, and at a rate uninfluenced by any chemical or physical
agency. The α and β rays from the active bodies are projected with much
greater velocity than the corresponding rays in a vacuum tube, while the
γ rays are of much greater penetrating power than Röntgen rays.

The effect of a magnetic field on a pencil of rays from a radio-active
substance giving out the three kinds of rays is very well illustrated in
Fig. 22[112].

[Illustration: Fig. 22.]

Some radium is placed in the bottom of a narrow cylindrical lead vessel
_R_. A narrow pencil of rays consisting of α, β, and γ rays escapes from
the opening. If a strong uniform magnetic field is applied at right
angles to the plane of the paper, and directed towards the paper, the
three types of rays are separated from one another. The γ rays continue
in a straight line without any deviation. The β rays are deflected to
the right, describing circular orbits the radii of which vary within
wide limits. If the photographic plate _AC_ is placed under the radium
vessel, the β rays produce a diffuse photographic impression on the
right of the vessel _R_. The α rays are bent in the direction opposite
to that of the β rays, and describe a portion of the arc of a circle of
large radius, but they are rapidly absorbed after traversing a distance
of a few centimetres from the vessel _R_. The amount of the deviation of
the α rays compared with that of the β rays is much exaggerated in the
figure.


=73. Ionizing and penetrating power of the rays.= Of the three kinds of
rays, the α rays produce most of the ionization in the gas and the γ
rays the least. With a thin layer of unscreened active material spread
on the lower of two parallel plates 5 cms. apart, the amount of
ionization due to the α, β, and γ rays is of the relative order 10,000,
100, and 1. These numbers are only rough approximations, and the
differences become less marked as the thickness of the radio-active
layer increases.

The average penetrating power of the rays is shown below. In the first
column is given the thickness of the aluminium, which cuts each
radiation down to half its value, and in the second the relative power
of penetration of the rays.

             Radiation    Thickness of            Relative
                          Aluminium in cms.       power of
                          which cuts off half  penetration
                          the radiation


             α rays       0·0005 cms.                    1

             β   „        0·05 cms.                    100

             γ  „         8 cms.                     10000

The relative power of penetration is thus approximately inversely
proportional to the relative ionization. These numbers, however, only
indicate the order of relative penetrating power. This power varies
considerably for the different active bodies.

The α rays from uranium and polonium are the least penetrating, and
those from thorium the most. The β radiations from thorium and radium
are very complex, and consist of rays widely different in penetrating
power. Some of the β rays from these substances are much less and others
much more penetrating than those from uranium, which gives out fairly
homogeneous rays.


=74. Difficulties of comparative measurements.= It is difficult to make
quantitative or even qualitative measurements of the relative intensity
of the three types of rays from active substances. The three general
methods employed depend upon the action of the rays in ionizing the gas,
in acting on a photographic plate, and in causing phosphorescent or
fluorescent effects in certain substances. In each of these methods the
fraction of the rays which is absorbed and transformed into another form
of energy is different for each type of ray. Even when one specific kind
of ray is under observation, comparative measurements are rendered
difficult by the complexity of that type of rays. For example, the β
rays from radium consist of negatively charged particles projected with
a wide range of velocity, and, in consequence, they are absorbed in
different amounts in passing through a definite thickness of matter. In
each case, only a fraction of the energy absorbed is transformed into
the particular type of energy, whether ionic, chemical, or luminous,
which serves as a means of measurement.

The rays which are the most active electrically are the least active
photographically. Under ordinary conditions, most of the photographic
action of uranium, thorium, and radium, is due to the β or cathodic
rays. The α rays from uranium and thorium, on account of their weak
action, have not yet been detected photographically. With active
substances like radium and polonium, the α rays readily produce a
photographic impression. So far the γ rays have been detected
photographically from radium only. That no photographic action of these
rays has yet been established for uranium and thorium is probably due
merely to the fact that the effect sought for is very small, and during
exposures for long intervals it is very difficult to avoid fogging of
the plates owing to other causes. Considering the similarity of the
radiations in other respects, there can be little doubt that the γ rays
do produce some photographic action, though it is too small to observe
with certainty.

These differences in the photographic and ionizing properties of the
radiations must always be taken into account in comparing results
obtained by the two methods. The apparent contradiction of results
obtained by different observers using these two methods is found to be
due to their differences in relative photographic and ionizing action.
For example, with the unscreened active material, the ionization
observed by the electrical method is due almost entirely to α rays,
while the photographic action under the same condition is due almost
entirely to the β rays.

It is often convenient to know what thickness of matter is sufficient to
absorb a specific type of radiation. A thickness of aluminium or mica of
·01 cms. or a sheet of ordinary writing-paper is sufficient to absorb
completely all the α rays. With such a screen over the active material,
the effects are due only to the β and γ rays, which pass through with a
very slight absorption. Most of the β rays are absorbed in 5 mms. of
aluminium or 2 mms. of lead. The radiation passing through such screens
consists very largely of the γ rays. As a rough working rule, it may be
taken that a thickness of matter required to absorb any type of rays is
inversely proportional to the density of the substance, _i.e._ the
absorption is proportional to the density. This rule holds approximately
for light substances, but, in heavy substances like mercury and lead,
the radiations are about twice as readily absorbed as the density rule
would lead us to expect.


                                PART II.



                        The β or Cathodic Rays.


=75. Discovery of the β rays.= A discovery which gave a great impetus to
the study of the radiations from active bodies was made in 1899, almost
simultaneously in Germany, France, and Austria. It was observed that
preparations of radium gave out some rays which were deviable by a
magnetic field, and very similar in character to the cathode rays
produced in a vacuum tube. The observation of Elster and Geitel that a
magnetic field altered the conductivity produced in air by radium rays,
led Giesel[113] to examine the effect of a magnetic field on the
radiations. In his experiments, the radio-active preparation was placed
in a small vessel between the poles of an electromagnet. The vessel was
arranged to give a pencil of rays which was approximately perpendicular
to the field. The rays caused a small fluorescent patch on the screen.
On exciting the electromagnet, the fluorescent zone was observed to
broaden out on one side. On reversing the field, the extension of the
zone was in the opposite direction. The deviation of the rays thus
indicated was in the same direction and of the same order of magnitude
as that for cathode rays.

S. Meyer and Schweidler[114] also obtained similar results. They showed,
in addition, the deviation of the rays by the alteration of the
conductivity of the air when a magnetic field was applied.
Becquerel[115], a little later, showed the magnetic deflection of the
radium rays by using the photographic method. P. Curie[116], by the
electrical method, showed furthermore that the rays from radium
consisted of two kinds, one apparently non-deviable and easily absorbed
(now known as the α rays), and the other penetrating and deviable by a
magnetic field (now known as the β rays). The ionization effect due to
the β rays was only a small fraction of that due to the α rays. At a
later date Becquerel, by the photographic method, showed that uranium
gave out some deflectable rays. It had been shown previously[117] that
the rays from uranium consisted of α and β rays. The deflected rays in
Becquerel’s experiment consisted entirely of β rays, as the α rays from
uranium produce no appreciable photographic action. Rutherford and
Grier[118], using the electric method, showed that compounds of thorium,
like those of uranium, gave out, besides α rays, some penetrating β
rays, deviable in a magnetic field. As in the case of radium, the
ionization due to the α rays of uranium and thorium is large compared
with that due to the β rays.


=76. Examination of the magnetic deviation by the photographic method.=
Becquerel has made a very complete study, by the photographic method, of
the β rays from radium, and has shown that they behave in all respects
like cathode rays, which are known to be negatively charged particles
moving with a high velocity. The motion of a charged ion acted on by a
magnetic field has been discussed in section 49. It has been shown that
if a particle of mass _m_ and charge _e_ is projected with a velocity
_u_, at an angle α with the direction of a uniform field of strength
_H_, it will describe a helix round the magnetic lines of force. This
helix is wound on a cylinder of radius _R_, with the axis parallel to
the field, where _R_ is given by

                                   _mu_
                             _R_ = ----  sin α.
                                   _He_

When α = π/2, _i.e._ when the rays are projected normally to the field,
the particles describe circles of radius

                                     _mu_
                               _R_ = ----
                                     _He_

The planes of these circles are normal to the field. Thus, for a
particular velocity _u_, the value of _R_ varies inversely as the
strength of the field. In a uniform field the rays projected normally to
the field describe circles, and their directions of projection are the
tangents at the origin.

This conclusion has been verified experimentally by Becquerel for the β
rays of radium, by an arrangement similar to that shown in Fig. 23.

[Illustration: Fig. 23.]

A photographic plate _P_, with the film downwards, is enveloped in black
paper and placed horizontally in the uniform horizontal magnetic field
of an electromagnet. The magnetic field is supposed to be uniform, and,
in the figure, is at right angles to the plane of the paper. The plate
was covered with a sheet of lead, and on the edge of the plate, in the
centre of the magnetic field, is placed a small lead vessel _R_
containing the radio-active matter.

On exciting the magnet, so that the rays are bent to the left of the
figure, it is observed that a photographic impression is produced
directly below the source of the rays, which have been bent round by the
magnetic field. The active matter sends out rays equally in all
directions. The rays perpendicular to the field describe circles, which
strike the plate immediately under the source. A few of these rays,
_A₁_, _A₂_, _A₃_, are shown in the figure. The rays, normal to the
plate, strike the plate almost normally, while the rays nearly parallel
to the plate strike the plate at grazing incidence. The rays, inclined
to the direction of the field, describe spirals and produce effects on
an axis parallel to the field passing through the source. In consequence
of this, any opaque screen placed in the path of the rays has its shadow
thrown near the edge of the photographic plate.


=77. Complexity of the rays.= The deviable rays from radium are complex,
_i.e._ they are composed of a flight of particles projected with a wide
range of velocity. In a magnetic field every ray describes a path, of
which the radius of curvature is directly proportional to the velocity
of projection. The complexity of the radiation has been shown very
clearly by Becquerel[119] in the following way.

An uncovered photographic plate, with the film upwards, was placed
horizontally in the horizontal uniform magnetic field of an
electromagnet. A small, open, lead box, containing the radio-active
matter, was placed in the centre of the field, on the photographic
plate. The light, due to the phosphorescence of the radio-active matter,
therefore, could not reach the plate. The whole apparatus was placed in
a dark room. The impression on the plate took the form of a large,
diffuse, but continuous band, elliptic in shape, produced on one side of
the plate.

Such an impression is to be expected if the rays are sent out in all
directions, even if their velocities of projection are the same, for it
can readily be shown theoretically, that the path of the rays is
confined within an ellipse whose minor axis, which is at right angles to
the field, is equal to 2_R_, and whose major axis is equal to π_R_. If,
however, the active matter is placed in the bottom of a deep lead
cylinder of small diameter, the rays have practically all the same
direction of projection, and in that case each part of the plate is
acted on by rays of a definite curvature.

In this case also, a diffuse impression is observed on the plate,
giving, so to speak, a continuous spectrum of the rays and showing that
the radiation is composed of rays of widely different curvatures. Fig.
24 shows a photograph of this kind obtained by Becquerel, with strips
of paper, aluminium, and platinum placed on the plate.

[Illustration: Fig. 24.]

If screens of various thickness are placed on the plate, it is observed
that the plate is not appreciably affected within a certain distance
from the active matter, and that this distance increases with the
thickness of the screen. This distance is obviously equal to twice the
radius of curvature of the path of the rays, which are just able to
produce an impression through the screen.

These experiments show very clearly that the most deviable rays are
those most readily absorbed by matter. By observations of this kind
Becquerel has determined approximately the inferior limit of the value
of _HR_ for rays which are transmitted through different thicknesses of
matter.

The results are given in the table below:

               Substance       Thickness  Inferior limit
                               in mms.       of _HR_ for
                                             transmitted
                                                    rays


               Black paper     0·065                 650

               Aluminium       0·010                 350

               „               0·100                1000

               „               0·200                1480

               Mica            0·025                 520

               Glass           0·155                1130

               Platinum        0·030                1310

               Copper          0·085                1740

               Lead            0·130                2610

If _e_/_m_ is a constant for all the rays, the value of _HR_ is
proportional to the velocity of the rays, and it follows from the table
that the velocity of the rays which just produce an effect on the plate
through ·13 mms. of lead is about 7 times that of the rays which just
produce an impression through ·01 mm. of aluminium. It will be shown,
however, in section 82, that _e_/_m_ is not a constant for all speeds,
but decreases with increase of velocity of the rays. The difference in
velocity between the rays is in consequence not as great as this
calculation would indicate. On examination of the rays from uranium,
Becquerel found that the radiation is not as complex as that from
radium, but consists wholly of rays for which the value of _HR_ is about
2000.


=78. Examination of the β rays by the electric method.= The presence of
easily deviable rays given off from an active substance can most readily
be shown by the photographic method, but it is necessary, in addition,
to show that the penetrating rays which produce the ionization in the
gas are the same as those which cause the photographic action. This can
be conveniently tested in an arrangement similar to that shown in Fig.
25.

[Illustration: Fig. 25.]

The radio-active matter _A_ is placed on a lead block _B´´_ between the
two parallel lead plates _BB´_. The rays pass between the parallel
plates and ionize the gas between the plates _PP´_ of the testing
vessel. The magnetic field is applied at right angles to the plane of
the paper. The dotted rectangle _EEEE_ represents the position of the
pole piece. If a compound of radium or thorium is under investigation, a
stream of air is required to prevent the diffusion of the radio-active
emanations into the testing vessel. When a layer of uranium, thorium or
radium compound is placed at _A_, the ionization in the testing vessel
is due mainly to the action of the α and β rays. The α rays are cut off
by adding a layer of aluminium ·01 cm. thick over the active material.
When the layer of active matter is not more than a few millimetres
thick, the ionization due to the γ rays is small compared with that
produced by the β rays, and may be neglected. On the application of a
magnetic field at right angles to the mean direction of the rays, the
ionization in the testing vessel due to the rays steadily decreases as
the strength of the field increases, and in a strong field it is reduced
to a very small fraction of its original value. In this case the rays
are bent so that none of them enter the testing vessel.

Examined in this way, it has been found that the β rays of uranium,
thorium, and radium consist entirely of rays readily deflected by a
magnetic field. The rays from polonium consist entirely of α rays, the
deviation of which can be detected only in very intense magnetic fields.

When the screen covering the active material is removed, in a strong
magnetic field, the ionization in the vessel is mainly due to the α
rays. On account of the slight deviation of the α rays under ordinary
experimental conditions, a still greater increase of the magnetic field
does not appreciably alter the current due to them in the testing
vessel.

The action of a magnetic field on a very active substance like radium is
easily shown by the electrical method, as the ionization current due to
the deviable rays is large. With substances of small activity like
uranium and thorium, the ionization current due to the deviable rays is
very small, and a sensitive electrometer or an electroscope is required
to determine the variation, in a magnetic field, of the very small
current involved. This is especially the case for thorium oxide, which
gives out only about ⅕ of the amount of deviable rays given out by the
same weight of uranium oxide.


=79. Experiments with a fluorescent screen.= The β rays from a few
milligrams of pure radium bromide produce intense fluorescence in barium
platinocyanide and other substances which can be made luminous under
the influence of the cathode rays. Using a centigram of radium bromide,
the luminosity on a screen, placed upon it, is bright enough to be
observed in daylight. With the aid of such a screen in a dark room many
of the properties of the β rays may be simply illustrated and their
complex nature clearly shown. A small quantity of radium is placed in
the bottom of a short, narrow, lead tube open at one end. This is placed
between the pole pieces of an electromagnet, and the screen placed below
it. With no magnetic field, a faint luminosity of the screen is observed
due to the very penetrating γ rays which readily pass through the lead.
When the magnetic field is put on, the screen is brightly lighted up on
one side over an area elliptical in shape (section 77). The direction of
deviation is reversed by reversal of the field. The broad extent of the
illumination shows the complex nature of the β rays. On placing a
metallic object at various points above the screen, the trajectory of
the rays can readily be traced by noticing the position of the shadow
cast upon the screen. By observing the density of the shadow, it can be
seen that the rays most easily deviated are the least penetrating.


              Comparison of the β rays with cathode rays.


=80. Means of comparison.= In order to prove the identity of the β rays
from active bodies with the cathode rays produced in a vacuum tube, it
is necessary to show

    (1) That the rays carry with them a negative charge;

    (2) That they are deviated by an electric as well as by a magnetic
    field;

    (3) That the ratio _e_/_m_ is the same as for the cathode rays.


=Electric charge carried by the β rays.= The experiments of Perrin and
J. J. Thomson have shown that the cathode rays carry with them a
negative charge. In addition, Lenard has shown that the rays still carry
a charge after traversing thin layers of matter. When the rays are
absorbed, they give up their charge to the body which absorbs them. The
total amount of charge carried by the β rays from even a very active
preparation of radium is, in general, small compared with that carried
by the whole of the cathode rays in a vacuum tube, and can be detected
only by delicate methods.

                  *       *       *       *       *

Suppose that a layer of very active radium is spread on a metal plate
connected to earth, and that the β rays are absorbed by a parallel plate
connected with an electrometer. If the rays are negatively charged, the
top plate should receive a negative charge increasing with the time. On
account, however, of the great ionization produced by the rays between
the plates, any charge given to one of them is almost instantly
dissipated. In many cases, the plate does become charged to a definite
positive or negative potential depending on the metal, but this is due
to the contact difference of potential between the plates, and would be
produced whether the rays were charged or not. The ionization of the gas
is greatly diminished by placing over the active material a metal screen
which absorbs the α rays, but allows the β rays to pass through with
little absorption.

The rapid loss of any charge communicated to the top plate can be very
much reduced, either by diminishing the pressure of the gas surrounding
it or by enclosing the plate with suitable insulators. In their
experiments to determine the amount of charge carried by the radium
rays, M. and Mme Curie[120] used the second method.

A metal disc _MM_ (Fig. 26) is connected with an electrometer by the
wire _T_. The disc and wire are completely surrounded by insulating
matter _ii_. The whole is surrounded by a metal envelope _EEEE_
connected with earth. On the lower side of the disc, the insulator and
the metallic covering are very thin. This side is exposed to the rays of
the radium _R_ placed in a depression in a lead plate _AA_.

[Illustration: Fig. 26.]

The rays of the radium pass through the metal cover and insulator with
little absorption, but they are completely absorbed by the disc _MM_. It
was observed that the disc received a negative charge which increased
uniformly with the time, showing that the rays carry with them a
negative charge. The current observed was very small. With an active
preparation of radium[121], forming a layer 2·5 sq. cms. in area and 2
mms. thick, a current of the order of 10⁻¹¹ amperes was observed after
the rays had traversed a layer of aluminium ·01 mm. thick and a layer of
ebonite ·3 mm. thick. The current was the same with discs of lead,
copper, and zinc, and also when the ebonite was replaced by paraffin.

Curie also observed in another experiment of a similar character that
the radium itself acquired a positive charge. This necessarily follows
if the rays carry with them a negative charge. If the β rays alone
carried with them a charge, a pellet of radium, if perfectly insulated,
and surrounded by a non-conducting medium, would in the course of time
be raised to a high positive potential. Since, however, the α rays carry
with them a charge opposite in sign to the β rays, the ratio of the
charge carried off by the two types of rays must be determined, before
it can be settled whether the radium would acquire a positive or a
negative charge. If, however, the radium is placed in an insulated metal
vessel of a thickness sufficient to absorb all the α rays, but not too
thick to allow most of the β rays to escape, the vessel will acquire a
positive charge in a vacuum.

An interesting experimental result bearing upon this point has been
described by Dorn[122]. A small quantity of radium was placed in a
sealed glass tube and left for several months. On opening the tube with
a file, a bright electric spark was observed at the moment of fracture,
showing that there was a large difference of potential between the
inside of the tube and the earth.

In this case the α rays were absorbed in the walls of the tube, but a
large proportion of the β rays escaped. The inside of the tube thus
became charged, in the course of time, to a high positive potential; a
steady state would be reached when the rate of escape of negative
electricity was balanced by the leakage of positive electricity through
the walls of the tube. The external surface of the glass would be always
practically at zero potential, on account of the ionization of the air
around it.

Strutt[123] has recently described a simple and striking experiment to
illustrate still more clearly that a radium preparation acquires a
positive charge, if it is enclosed in an envelope thick enough to absorb
all the α particles, but thin enough to allow most of the β particles to
escape. The experimental arrangement is clearly seen in Fig. 27. A
sealed tube _AA_ containing the radium, was attached at one end to a
pair of thin gold leaves in metallic connection with the radium, and was
insulated inside a larger tube by means of a quartz rod _B_. The inner
surface of the tube was coated with tinfoil _EE_ connected to earth. The
glass surface of _AA_ was made conducting by a thin coating of
phosphoric acid. The air in the outer tube was exhausted as completely
as possible by means of a mercury pump, in order to reduce the
ionization in the gas, and consequently the loss of any charge gained by
the gold leaves. After an interval of 20 hours, the gold leaves were
observed to diverge to their full extent, indicating that they had
acquired a large positive charge. In this experiment Strutt used ½ gram
of radiferous barium of activity only 100 times that of uranium.

[Illustration: Fig. 27.]

If the tube is filled with 30 mgrs. of pure radium bromide, the leaves
diverge to their full extent in the course of about a minute. If it is
arranged that the gold leaf, at a certain angle of divergence, comes in
contact with a piece of metal connected with earth, the apparatus can be
made to work automatically. The leaf diverges, touches the metal, and at
once collapses, and this periodic movement of the leaf will continue, if
not indefinitely, at any rate as long as the radium lasts. This “radium
clock” should work at a sensibly uniform rate for many years, but, from
evidence considered later (Section 261), there is reason to believe that
the number of β particles emitted would decrease exponentially with the
time, falling to half value in about 1200 years. The period of movement
of the leaf should thus gradually increase with the time, and ultimately
the effect would become too small to observe.

The action of this radium clock is the nearest approach to an apparent
perpetual motion that has so far been observed.

A determination of the amount of the charge carried off by the β rays of
radium has been made by Wien[124]. A small quantity of radium, placed in
a sealed platinum vessel, was hung by an insulating thread inside a
glass cylinder, which was exhausted to a low pressure. A connection
between the platinum vessel and an electrode sealed on to the external
glass cylinder could be made, when required, by tilting the tube. Wien
found that in a good vacuum the platinum vessel became charged to about
100 volts. The rate of escape of negative electricity from the platinum
vessel containing 4 milligrams of radium bromide corresponded to 2·91 ×
10⁻¹² amperes. If the charge on each particle is taken as 1·1 × 10⁻²⁰
electromagnetic units, this corresponds to an escape of 2·66 × 10⁷
particles per second. From 1 gram of radium bromide the corresponding
number would be 6·6 × 10⁹ per second. Since some of the β rays are
absorbed in their passage through the walls of the containing vessel and
through the radium itself, the actual number projected per second from 1
gram of radium bromide must be greater than the above value. This has
been found by the writer to be the case. The method employed reduced the
absorption of the β rays to a minimum, and the total number emitted per
second by 1 gram of radium bromide in radio-active equilibrium was found
to be 4·1 × 10¹⁰, or about six times the number found by Wien. A
detailed account of the method employed cannot be given with advantage
at this stage, but will be found later in Section 253.


=81. Determination of= _e_/_m_. We have seen (Section 50) that, in their
passage between the plates of a condenser, the cathode rays are
deflected towards the positive plate. Shortly after the discovery of the
magnetic deviation of the β rays from radium, Dorn[125] and
Becquerel[126] showed that they also were deflected by an electric
field.

By observing separately the amount of the electric and magnetic
deviation, Becquerel was able to determine the ratio of _e_/_m_ and the
velocity of the projected particles. Two rectangular copper plates, 3·45
cms. high and 1 cm. apart, were placed in a vertical plane and insulated
on paraffin blocks. One plate was charged to a high potential by means
of an influence machine, and the other was connected with earth. The
active matter was placed in a narrow groove cut in a lead plate parallel
to the copper plates and placed midway between them. The photographic
plate, enveloped in black paper, was placed horizontally above the plate
containing the active substance. The large and diffuse pencil of rays
thus obtained was deflected by the electric field, but the deviation
amounted to only a few millimetres and was difficult to measure. The
method finally adopted was to place vertically above the active matter a
thin screen of mica, which cut the field into two equal parts. Thus, in
the absence of an electric field, a narrow rectangular shadow was
produced on the plate.

When the electric field was applied, the rays were deflected and a part
of the pencil of rays was stopped by the mica screen. A shadow was thus
cast on the plate which showed the direction of deviation and
corresponded to the least deviable rays which gave an impression through
the black paper.

If a particle of mass _m_, charge _e_, and velocity _u_, is projected
normally to an electric field of strength _X_, the acceleration α is in
the direction of the field, and is given by

                                    _Xe_
                                α = ----- .
                                     _m_

Since the particle moves with a constant acceleration parallel to the
field, the path of the particle is the same as that of a body projected
horizontally from a height with a constant velocity and acted on by
gravity. The path of the particle is thus a parabola, whose axis is
parallel to the field and whose apex is at the point where the particle
enters the electric field. The linear deviation _d₁_ of the ray parallel
to the field after traversing a distance _l_ is given by

                                     1 _Xe_   _l²_
                         _d₁_ = --   -- ----  ---- .
                                     2  _m_   _u²_

On leaving the electric field, the particle travels in the direction of
the tangent to the path at that point. If θ is the angular deviation of
the path at that point

                                     _eXl_
                             tan θ = -----   .
                                     _mu²_

The photographic plate was at a distance _h_ above the extremity of the
field. Thus the particles struck the plate at a distance _d₂_ from the
original path given by

             $$ d_2 = h \tan \theta + d_1 $$

             $$     = \frac {Xle} {mu^2} (\frac {l} {2} + h) $$

In the experimental arrangement the values were

                             _d₂_ = ·4 cms.;
                             _X_ = 1·02 × 10¹²;
                             _l_ = 3·45 cms.;
                             _h_ = 1·2 cms.

If the radius _R_ of curvature of the path of the same rays is observed
in a magnetic field of strength _H_ perpendicular to the rays,

                                _e_     _V_
                                ---   = ----
                                _m_     _HR_

Combining these two equations we get

        $$ u = \frac {X . l (\frac {l} {2} + h)} {H . R . d_2} $$ .

A difficulty arose in identifying the part of the complex pencil of rays
for which the electric and magnetic deviations were determined.
Becquerel estimated that the value of _HR_ for the rays deflected by the
electric field was about 1600 C.G.S. units. Thus

                     _u_ = 1·6 × 10¹⁰ cms. per second,

                     and
                     _e_
                     ----   = 10⁷.
                     _m_

Thus these rays had a velocity more than half the velocity of light, and
an apparent mass about the same as the cathode ray particles, _i.e._
about ¹⁄₁₀₀₀ of the mass of the hydrogen atom. The β ray is therefore
analogous in all respects to the cathode ray, except that it differs in
velocity. In a vacuum tube the cathode rays generally have a velocity of
about 2 × 10⁹ cms. per sec. In special tubes with strong fields the
velocity may be increased to about 10¹⁰ cms. per sec. These β particles,
then, behave like isolated units of negative electricity, identical with
the electrons set free by an electric discharge in a vacuum tube. The
electrons projected from radium have velocities varying from about
0·2_V_ to at least 0·96_V_, where _V_ is the velocity of light, and thus
have an average speed considerably greater than that of the electrons
produced in a vacuum tube. These moving electrons are able to pass
through much greater thicknesses of matter before they are absorbed than
the slower electrons produced in a vacuum tube, but the difference is
one merely of degree and not of kind. Since electrons are continuously
and spontaneously expelled from radium with enormous velocities, they
must acquire their energy of motion from the matter itself. It is
difficult to avoid the conclusion, that this velocity has not been
suddenly impressed on the electron. Such a sudden gain of velocity would
mean an immense and sudden concentration of energy on a small particle,
and it is more probable that the electron before its expulsion has been
in rapid orbital or oscillatory motion in the atom, and, by some means,
suddenly escapes from its orbit. According to this view, the energy of
the electron is not suddenly created but is only made obvious by its
escape from the system to which it belongs.


=82. Variation of= _e_/_m_ =with the velocity of the electron=. The fact
that radium throws off electrons with rates of speed varying from ⅕ to
⁹⁄₁₀ the velocity of light has been utilised by Kaufmann[127] to examine
whether the ratio _e_/_m_ of the electrons varies with the speed. We
have seen (Section 48) that, according to the electromagnetic theory, a
charge of electricity in motion behaves as if it had apparent mass. For
small speeds, this additional electrical mass is equal to

                                2   _e²_
                                --  ----,
                                3   _a_

where _a_ is the radius of the body, but it increases rapidly as the
speed of light is approached. It is very important to settle whether the
mass of the electron is due partly to mechanical and partly to
electrical mass, or whether it can be explained by virtue of electricity
in motion independently of the usual conception of mass.

Slightly different formulae expressing the variation of mass with speed
have been developed by J. J. Thomson, Heaviside, and Searle. To
interpret his results Kaufmann used a formula developed by M.
Abraham[128].

           Let _m₀_ = mass of electron for slow speeds;
               _m_     = apparent mass of electron at any speed;
               _u_     = velocity of electron;
               _V_     = velocity of light.

           Let β = _u_/_V_; then it can be shown that

$$ \frac {m} {m₀} = \frac {3} {4} \psi (\beta) $$ (1),

where

    $$ \psi (\beta) = \frac {1}{\beta^2} (\frac {1 + \beta^2} {2\beta}
       \log \frac {1 + \beta} {1 − \beta} − 1) $$ (2).

The experimental method employed to determine _e_/_m_ and _u_ is similar
to the method of crossed spectra. Some strongly active radium was placed
at the bottom of a brass box. The rays from this passed between two
brass plates insulated and about 1·2 mm. apart. These rays fell on a
platinum diaphragm, containing a small tube about 0·2 mm. in diameter,
which allowed a narrow bundle of rays to pass. The rays then struck a
photographic plate enveloped in a thin layer of aluminium.

In the experiments the diaphragm was about 2 cms. from the active
material and at the same distance from the photographic plate. When the
whole apparatus was placed in a vacuum, a P.D. of from 2000 to 5000
volts could be applied between the plates without a spark. The rays were
deflected in their passage through the electric field, and produced what
may be termed an electric spectrum on the plate.

[Illustration: Fig. 28.]

If a magnetic field is superimposed parallel to the electric field by
means of an electromagnet, a magnetic spectrum is obtained perpendicular
to the electric spectrum. The combination of the two spectra gives rise
to a curved line on the plate. The double trace obtained on the
photographic plate with reversal of the magnetic field is shown in Fig.
28. Disregarding some small corrections, it can readily be shown that if
_y_ and _z_ are the electric and magnetic deviations respectively,

                                _z_
                         β = κ₁ -----      (3),
                                _y_

                         and

                         _e_      _z²_
                         ---  = κ ----     (4).
                         _m_      _y_

From these two equations, combined with (1), we obtain

    $$ \frac {y} {z^2 \psi (\kappa_1 \frac {z} {y})} = \kappa_2 $$ ...
       (5),

where κ, κ₁, κ₂ are constants.

Equation (5) gives the curve that should be obtained on the plate
according to the electromagnetic theory. This is compared by trial with
the actual curve obtained on the plate.

In this way Kaufmann[129] found that the value of _e_/_m_ decreased with
the speed, showing that, assuming the charge constant, the mass of the
electron increased with the speed.

The following numbers give some of the preliminary results obtained by
this method.

                  Velocity of electron _e_/_m_


                  2·36 × 10¹⁰ cms. per     1·31 × 10⁷
                  sec.

                  2·48        „            1·17 × 10⁷
                  „

                  2·59        „            0·97 × 10⁷
                  „

                  2·72        „            0·77 × 10⁷
                  „

                  2·85        „            0·63 × 10⁷
                  „

For the cathode rays S. Simon[130] obtained a value for _e_/_m_ of 1·86
× 10⁷ for an average speed of about 7 × 10⁹ cms. per second.

In a later paper[131] with some very active radium, more satisfactory
photographs were obtained, which allowed of accurate measurement. The
given equation of the curve was found to agree satisfactorily with
experiment.

The table given below, deduced from the results given by Kaufmann, shows
the agreement between the theoretical and experimental values, _u_ being
the velocity of the electron and _V_ that of light.

The average percentage error between the observed and calculated value
is thus not much more than one per cent. It is remarkable how nearly the
velocity of the electron has to approach the velocity of light before
the value of _m_/_m₀_ becomes large. This is shown in the following
table which gives the calculated values of _m_/_m₀_ for different
velocities of the electron.

                 Value of   Observed value  Percentage
                 _u_/_V_    of _m_/_m₀      difference
                                            from
                                            theoretical
                                            values


                 Small      1

                 ·732       1·34            −1·5 %

                 ·752       1·37            −0·9 „

                 ·777       1·42            −0·6 „

                 ·801       1·47            +0·5 „

                 ·830       1·545           +0·5 „

                 ·860       1·65             0   „

                 ·883       1·73            +2·8 „

                 ·933       2·05            −7·8 „ ?

                 ·949       2·145           −1·2 „

                 ·963       2·42            +0·4 „

      Value of   small    ·1    ·5    ·9   ·99  ·999 ·9999 ·999999
      _u_/_V_

      Calculated 1·00     1·015 1·12  1·81 3·28 4·96 6·68  10·1
      value
      m/m₀

Thus for velocities varying from 0 to ⅒ the velocity of light, the
mass of the electron is practically constant. The increase of mass
becomes appreciable at about half the velocity of light, and increases
steadily as the velocity of light is approached. Theoretically the mass
becomes infinite at the velocity of light, but even when the velocity of
the electron only differs from that of light by one part in a million,
its mass is only 10 times the value for slow speeds.

The above results are therefore in agreement with the view that the mass
of the electron is altogether electrical in origin and can be explained
purely by electricity in motion. The value of _e_/_m₀_, for slow speeds,
deduced from the results was 1·84 × 10⁷, which is in very close
agreement with the value obtained by Simon for the cathode rays, viz.
1·86 × 10⁷.

If the electricity carried by the electron is supposed to be distributed
uniformly over a sphere of radius _a_, for speeds slow compared with the
velocity of light, the apparent mass

                                  2   _e²_
                          _m₀_ = ---  ----
                                  3   _a_

Therefore

                                 2  _e_
                          _a_ = --- ---- . _e_
                                 3  _m₀_

Taking the value of _e_ as 1·13 × 10⁻²⁰, _a_ is 1·4 × 10⁻¹³ cms.

Thus the diameter of an electron is minute compared with the diameter of
an atom.


=83. Distribution of velocity amongst the β particles=. Some interesting
experiments have been recently made by Paschen[132] to determine the
relative number of β particles which are expelled from radium at the
different speeds. The experimental arrangement is shown in Fig. 29.

[Illustration: Fig. 29.]

A small thin silvered glass tube _b_, containing 15 mgrs. of radium
bromide, was placed in the axis of a number of lead vanes arranged round
a cylinder of diameter 2 cms. and length 2·2 cms. When no magnetic field
was acting, the β particles from the radium passed through the openings
and were absorbed in an outer concentric cylinder _aa_ of lead of inner
diameter 3·7 cms. and of thickness 5·5 mms. This outer cylinder was
rigidly connected to the inner cylinder _cc_ by quartz rods _ii_, which
also served to insulate it. The cylinder _c_ and the radium were
connected with earth. A gold-leaf electroscope _E_ was attached to _a_,
and the whole apparatus was enclosed in a glass vessel which was
exhausted to a low vacuum by means of a mercury pump. The glass vessel
was placed in the uniform field of a large electromagnet, so that the
axis of the lead cylinder was parallel to the lines of force.

The outer cylinder gains a negative charge on account of the particles
which are absorbed in it. This negative charge, which is indicated by
the movement of the gold-leaf, tends to be dissipated by the small
ionization produced in the residual gas by the passage of the β rays.
This action of the gas can be eliminated by observing the rate of
movement of the gold leaf when charged alternately to an initial
positive and negative potential. The mean of the two rates is
proportional to the number of β particles which give up their charge to
the lead cylinder. This is evidently the case, since, when the charge is
positive, the ionization of the gas assists the rate of movement of the
gold-leaf, and, when negative, diminishes it to an equal extent.

When a magnetic field is applied, each of the particles describes a
curved path, whose radius of curvature depends on the velocity of the
particle. For weak fields, only the particles of smallest velocity will
be deflected sufficiently not to strike the outer cylinder, but, as the
field is raised, the number will increase until finally all the β
particles fail to reach the outer cylinder. The decrease of the charge
communicated to the outer cylinder with the increase of the strength of
the magnetic field is shown graphically in Fig. 30, Curve I.

The ordinates represent in arbitrary units the charge communicated to
the lead cylinder per second, and thus serve as a measure of the number
of β particles which reach the cylinder. Knowing the dimensions of the
apparatus, and assuming the value _e_/_m_ found by Kaufmann, the
velocity of the particles which just fail to reach the lead cylinder can
be deduced from any strength of the magnetic field. Curve II, Fig. 30 is
the first differential of Curve I, and the ordinates represent the
relative number of β particles which are projected at each velocity.

[Illustration: Fig. 30.]

From the data given by Kaufmann (see section 82) Paschen deduced that
the group of rays examined by the former, which had velocities lying
between 2·12 × 10¹⁰ and 2·90 × 10¹⁰ cms. per second, corresponded to the
group of rays between the points _A_ and _B_, that is, to the group of
rays which were completely deflected from the lead cylinder between the
magnetic fields of strengths of 1875 and 4931 C.G.S. units. Since radium
gives off β particles which require a field of strength over 7000 units
to deflect them, Paschen concluded that β particles are expelled from
radium with still greater velocities than the highest recorded by
Kaufmann.

Paschen considered that the small charge observed in still higher fields
was mainly due to the γ rays. The effect is small and is probably not
due to an actual charge carried by the γ rays but to a secondary effect
produced by them. This question will be discussed in more detail in
section 112.

There is a group of low velocity β particles emitted by radium (see Fig.
30) which have about the same speed as the electrons set free in a
vacuum tube. In consequence of their small velocity, these probably
produce a large proportion of the ionization due to the β rays at short
distances from the radium, for it will be shown (section 103) that the
ionization produced by an electron per unit length of path steadily
decreases with increase of its velocity above a small limiting value.
This observation is confirmed by experiments on the absorption of the β
rays in passing through matter.

In Paschen’s experiments, the glass tube containing the radium was ·5
mms. thick, so that a considerable proportion of the low velocity β
particles must have been stopped by it. This is borne out by some later
experiments of Seitz which will be described in section 85.


=84. Absorption of the β rays by matter=. The β particles produce ions
in their passage through the gas and their energy of motion is
consequently diminished. A similar action takes place also when the β
rays pass through solid and liquid media, and the mechanism of
absorption is probably similar in all cases. Some of the particles in
their passage through matter are completely stopped, while others have
their velocity reduced. In addition, there is a considerable scattering
or diffuse reflection of the rays in traversing matter. The amount of
this scattering depends upon the density of the substance and also upon
the angle of incidence of the rays. This scattering of the rays will be
discussed later in section 111.

There are two general methods of determining the absorption of the β
rays. In the first method, the variation of the ionization current is
observed in a testing vessel when the active matter is covered by
screens differing in material and thickness. This ionization in the
vessel depends upon two quantities, viz. the number of β particles which
pass through the matter and also upon the number of ions produced by
them per unit path. In the absence of any definite information in regard
to the variation of ionization by the electron with its velocity, no
very definite conclusions can be drawn from such experiments.

The advent of pure radium-bromide has made it possible to determine the
actual number of electrons which are absorbed in their passage through a
definite thickness of matter, by measuring the negative charge carried
by the issuing rays. Experiments of this character have been made by
Seitz and will be considered later.

These two methods of determining the absorption of β rays are quite
distinct in principle, and it is not to be expected that the values of
the coefficients of absorption obtained in the two cases should be the
same. The whole question of the absorption of electrons by matter is
very complicated, and the difficulty is still further increased by the
complexity of the β rays emitted by the radio-active substances. Many of
the results obtained by different methods, while pointing to the same
general conclusion, are quantitatively in wide disagreement. Before any
definite advance can be made to a better understanding of the mechanism
of absorption, it will be necessary to determine the variation of the
ionization with the speed of the electron over a very wide range. Some
work has already been done in this direction but not between
sufficiently wide limits.


                           Ionization method.


We shall first consider the results obtained on the absorption of β rays
by measuring the variation of the ionization current, when screens of
different thickness are placed over the active substance. When the
active matter is covered with aluminium foil of thickness ·1 mm., the
current in a testing vessel such as is shown in Fig. 17, is due almost
entirely to the β rays. If a uranium compound is used, it is found that
the saturation current decreases with the thickness of matter traversed
nearly according to an exponential law. Taking the saturation current as
a measure of the intensity of the rays, the intensity _I_ after passing
through a thickness _d_ of matter is given by

                  $$ \frac {I} {I₀} = e^{–λ d} $$,

where λ is the constant of absorption of the rays and _I₀_ is the
initial intensity. For uranium rays, the current is reduced to half its
value after passing through about ·5 mm. of aluminium.

If a compound of thorium or radium is examined in the same way, it is
found that the current does not decrease regularly according to the
above equation. Results of this kind for radium rays have been given by
Meyer and Schweidler[133]. The amount of absorption of the rays by a
certain thickness of matter decreases with the thickness traversed. This
is exactly opposite to what is observed for the α rays. This variation
in the absorption is due to the fact that the β rays are made up of rays
which vary greatly in penetrating power. The rays from uranium are
fairly homogeneous in character, _i.e._ they consist of rays projected
with about the same velocity. The rays from radium and thorium are
complex, _i.e._ they consist of rays projected with a wide range of
velocity and consequently with a wide range of penetrating power. The
electrical examination of the deviable rays thus leads to the same
results as their examination by the photographic method.

Results on the absorption of cathode rays have been given by
Lenard[134], who has shown that the absorption of cathode rays is nearly
proportional to the density of the absorbing matter, and is independent
of its chemical state. If the deviable rays from active bodies are
similar to cathode rays, a similar law of absorption is to be expected.
Strutt[135], working with radium rays, has determined the law of
absorption, and has found it roughly proportional to the density of
matter over a range of densities varying from 0·041 for sulphur dioxide
to 21·5 for platinum. In the case of mica and cardboard, the values of λ
divided by the density were 3·94 and 3·84 respectively, while the value
for platinum was 7·34. In order to deduce the absorption coefficient, he
assumed that the radiation fell off according to an exponential law with
the distance traversed. As the rays from radium are complex, we have
seen that this is only approximately the case.

Since the β rays from uranium are fairly homogeneous, and are at the
same time penetrating in character, they are more suitable for such a
determination than the complex rays of radium. I have in consequence
made some experiments with uranium rays to determine the dependence of
absorption on the density. The results obtained are given in the
following table, where λ is the coefficient of absorption.

               Substance  λ          Density  λ/Density

               Glass      14·0       2·45     5·7
               Mica       14·2       2·78     5·1
               Ebonite     6·5       1·14     5·7
               Wood        2·16       ·40     5·4
               Cardboard   3·7        ·70     5·3
               Iron       44         7·8      5·6
               Aluminium  14·0       2·60     5·4
               Copper     60         8·6      7·0
               Silver     75        10·5      7·1
               Lead      122        11·5     10·8
               Tin        96         7·3     13·2

It will be observed that the value of the absorption constant divided by
the density is very nearly the same for such different substances as
glass, mica, ebonite, wood, iron and aluminium. The divergences from the
law are great, however, for the other metals examined, viz. copper,
silver, lead and tin. In tin the value of λ divided by the density is
2·5 times its value for iron and aluminium. These differences show that
a law for the absorption of the β rays depending only on the density
does not hold for all substances. With an exception in the case of tin,
the value of λ divided by the density for the metals increases in the
same order as their atomic weights.

The absorption of the β rays by matter decreases very rapidly with
increase of speed. For example, the absorption of cathode rays in
Lenard’s experiment (_loc. cit._) is about 500 times as great as for the
uranium β rays. The velocity of the β rays of uranium was found by
Becquerel to be about 1·6 × 10¹⁰ cms. per sec. The velocity of the
cathode rays used in Lenard’s experiment was certainly not less than
⅒ of this, so that, for a decrease of speed of less than 10 times,
the absorption has increased over 500 times.


=85. Number of electrons stopped by matter.= An account will now be
given of the experiments made by Seitz[136], to determine the relative
number of electrons which are stopped in their passage through different
thicknesses of matter. The experimental arrangement is shown in Fig. 31.

[Illustration: Fig. 31.]

The radium was placed outside a glass vessel containing an insulated
brass plate _P_, the connection of which with a wire leading to the
electrometer could be made or broken by a simple electromagnetic device.
The β rays from the radium _R_, after passing through openings in a
brass plate _A_, covered with thin aluminium foil, were absorbed in the
plate _P_. The glass vessel was exhausted, and the charge communicated
to _P_ by the β rays was measured by an electrometer.

In a good vacuum, the magnitude of the current observed is a measure of
the number of β particles absorbed by the upper plate[137]. The
following table shows the results obtained when different thicknesses of
tin foil were placed over the radium. The second table gives the ratio
_I_/_I₀_ where _I₀_ is the rate of discharge observed before the
absorbing screen is introduced. The mean value of the absorption
constant λ was deduced from the equation

                   $$ \frac {I} {I₀} = e^{–λ d} $$

where _d_ is the thickness of matter traversed.

The values included in the brackets have not the same accuracy as the
others. There is thus a wide difference in penetrating power of the β
particles emitted from radium, and some of them are very readily
absorbed.

When a lead screen 3 mms. thick was placed over the radium—a thickness
sufficient to absorb all the readily deflectable β rays—a small negative
charge was still given to the plate, corresponding to ·29 per cent. of
the maximum. This is a very much smaller value than was observed by
Paschen (see Fig. 30).

               Thickness of    _I_/_I₀_         λ
               Tin in mms.


               0·00834         ·869            175

               0·0166          ·802            132·5

               0·0421          ·653            101·5

               0·0818          ·466            93·5

               0·124           ·359            82·5

               0·166           ·289            74·9

               0·205           ·230            71·5

               0·270           ·170            65·4

               0·518           ·065            53

               0·789           ·031            44

               1·585           ·0059           32

               2·16            ·0043           25

This difference may, in part, be due to the fact that, in Paschen’s
experiments, a large proportion of the slow velocity electrons were
absorbed in the glass tube of ·5 mm. thickness containing the radium.

Seitz also determined the relative thickness, compared with tin, of
different substances which reduced the negative charge communicated to
_P_ by a definite amount. A few of the numbers are given below, and
expressed in terms of tin as unity.

                         Substance   Thickness
                                       Tin = 1


                         Lead              ·745

                         Gold              ·83

                         Platinum          ·84

                         Silver           1

                         Steel            1·29

                         Aluminium        1·56

                         Water            1·66

                         Paraffin         1·69

The thickness required to stop a given proportion of the β rays thus
decreases with the density, but not nearly so fast as the density
increases. These results are difficult to reconcile with the density-law
of absorption found by Lenard from the cathode rays, or with the results
of the ionization method already considered. A further experimental
examination of the whole question is very much to be desired.


=86. Variation of the amount of radiation with the thickness of the
layer of radiating material.= The radiations are sent out equally from
all portions of the active mass, but the ionization of the gas which is
measured is due only to the radiations which escape into the air. The
depth from which the radiations can reach the surface depends on the
absorption of the radiation by the active matter itself.

Let λ be the absorption constant of the homogeneous radiation by the
active material. It can readily be shown that the intensity _I_ of the
rays issuing from a layer of active matter, of thickness _d_, is given
by

$$ \frac {I} {I_{0}} = 1 − e^{–λ d} $$,

where _I₀_ is the intensity at the surface due to a very thick layer.

This equation has been confirmed experimentally by observing the current
due to the β rays for different thicknesses of uranium oxide. In this
case _I_ = (½)_I₀_ for a thickness of oxide corresponding to ·11 gr. per
sq. cm. This gives a value of λ divided by density of 6·3. This is a
value slightly greater than that observed for the absorption of the same
rays in aluminium. Such a result shows clearly that the substance which
gives rise to the β rays does not absorb them to a much greater extent
than does ordinary matter of the same density.

The value of λ will vary, not only for the different active substances,
but also for the different compounds of the same substance.




                               PART III.


                              The α Rays.


=87. The α rays=. The magnetic deviation of the β rays was discovered
towards the end of 1899, at a comparatively early stage in the history
of radio-activity, but three years elapsed before the true character of
the α rays was disclosed. It was natural that great prominence should
have been given in the early stages of the subject to the β rays, on
account of their great penetrating power and marked action in causing
phosphorescence in many substances. The α rays were, in comparison, very
little studied, and their importance was not generally recognized. It
will, however, be shown that the α rays play a far more important part
in radio-active processes than the β rays, and that the greater portion
of the energy emitted in the form of ionizing radiations is due to them.


=88. The nature of the α rays=. The nature of the α rays was difficult
to determine, for a magnetic field sufficient to cause considerable
deviation of the β rays produced no appreciable effect on the α rays. It
was suggested by several observers that they were, in reality, secondary
rays set up by the β or cathode rays in the active matter from which
they were produced. Such a view, however, failed to explain the
radio-activity of polonium, which gave out α rays only. Later work also
showed that the matter, which gave rise to the β rays from uranium,
could be chemically separated from the uranium, while the intensity of
the α rays was unaffected. These and other results show that the α and β
rays are produced quite independently of one another. The view that they
are an easily absorbed type of Röntgen rays fails to explain a
characteristic property of the α rays, viz. that the absorption of the
rays in a given thickness of matter, determined by the electrical
method, increases with the thickness of matter previously traversed. It
does not seem probable that such an effect could be produced by a
radiation like X rays, but the result is to be expected if the rays
consist of projected bodies, which fail to ionize the gas when their
velocity is reduced below a certain value. From observations of the
relative ionization produced in gases by the α and β rays, Strutt[138]
suggested in 1901 that the α rays might consist of positively charged
bodies projected with great velocity. Sir William Crookes[139], in 1902,
advanced the same hypothesis. From a study of the α rays of polonium
Mme. Curie[140] in 1900 suggested the probability that these rays
consisted of bodies, projected with great velocity, which lost their
energy by passing through matter.

The writer was led independently to the same view by a mass of indirect
evidence which received an explanation only on the hypothesis that the
rays consisted of matter projected with great velocity. Preliminary
experiments with radium of activity 1000 showed that it was very
difficult to determine the magnetic deviation of the α rays. When the
rays were passed through slits sufficiently narrow to enable a minute
deviation of the rays to be detected, the ionizing effect of the issuing
rays was too small to be measured with certainty. It was not until
radium of activity 19,000 was obtained that it was possible to detect
the deviation of these rays in an intense magnetic field. How small the
magnetic deviation is may be judged from the fact that the α rays,
projected at right angles to a magnetic field of 10,000 C.G.S. units,
describe the arc of a circle of about 39 cms. radius, while under the
same conditions the cathode rays produced in a vacuum tube would
describe a circle of about ·01 cm. radius. It is therefore not
surprising that the α rays were for some time thought to be non-deviable
in a magnetic field.


=89. Magnetic deviation of the α rays=. The general method employed[141]
to detect the magnetic deviation of the α rays was to allow the rays to
pass through narrow slits and to observe whether the rate of discharge
of an electroscope, due to the issuing rays, was altered by the
application of a strong magnetic field. Fig. 32 shows the general
arrangement of the experiment. The rays from a thin layer of radium of
activity 19,000 passed upwards through a number of narrow slits _G_, in
parallel, and then through a thin layer of aluminium foil, ·00034 cm.
thick, into the testing vessel _V_. The ionization produced by the rays
in the testing vessel was measured by the rate of movement of the leaves
of a gold-leaf electroscope _B_. The gold-leaf system was insulated
inside the vessel by a sulphur bead _C_, and could be charged by means
of a movable wire _D_, which was afterwards earthed. The rate of
movement of the gold-leaf was observed through small mica windows in the
testing vessel by means of a microscope provided with a micrometer
eye-piece.

[Illustration: Fig. 32.]

In order to increase the ionization in the testing vessel, the rays
passed through 20 to 25 slits of equal width, placed side by side. This
was arranged by cutting grooves at regular intervals in side-plates into
which brass plates were slipped. The width of the slit varied in
different experiments between ·042 cm. and ·1 cm. The magnetic field was
applied perpendicular to the plane of the paper, and parallel to the
plane of the slits. The rays are thus deflected in a direction
perpendicular to the plane of the slits and a very small amount of
deviation is sufficient to cause the rays to impinge on the sides of the
plate where they are absorbed.

The testing vessel and system of plates were waxed to a lead plate _P_
so that the rays entered the vessel _V_ only through the aluminium foil.
It is necessary in these experiments to have a steady stream of gas
passing downwards between the plates in order to prevent the diffusion
of the emanation from the radium upwards into the testing vessel. The
presence in the testing vessel of a small amount of this emanation,
which is always given out by radium, would produce great ionization and
completely mask the effect to be observed. For this purpose, a steady
current of dry electrolytic hydrogen of about 2 c.c. per second was
passed into the testing vessel; it then streamed through the porous
aluminium foil, and passed between the plates carrying the emanation
with it away from the apparatus. The use of a stream of hydrogen instead
of air greatly simplifies the experiment, for it _increases_ the
ionization current due to the α rays in the testing vessel, and at the
same time greatly _diminishes_ that due to the β and γ rays. This is
caused by the fact that the α rays are much more readily absorbed in air
than in hydrogen, while the rate of production of ions due to the β and
γ rays is much less in hydrogen than in air. The intensity of the α rays
after passing between the plates is consequently greater when hydrogen
is used; and since the rays pass through a sufficient distance of
hydrogen in the testing vessel to be largely absorbed, the total amount
of ionization produced by them is greater with hydrogen than with air.

The following is an example of an observation on the magnetic
deviation:—

    Pole-pieces 1·90 × 2·50 cms.

    Strength of field between pole-pieces 8370 units.

    Apparatus of 25 parallel plates of length 3·70 cms., width ·70 cm.,
       with an average air-space between plates of ·042 cm.

    Distance of radium below plates 1·4 cm.

                                              Rate of discharge of
                                              electroscope in
                                              volts per minute

     (1) Without magnetic field               8·33

     (2) With magnetic field                  1·72

     (3) Radium covered with thin layer of    0·93
     mica to absorb all α rays

     (4) Radium covered with mica and         0·92
     magnetic field applied

The mica plate, ·01 cm. thick, was of sufficient thickness to absorb
completely all the α rays, while it allowed the β rays and γ rays to
pass through without appreciable absorption. The difference between (1)
and (3), 7·40 volts per minute, gives the rate of discharge due to the α
rays alone; the difference between (2) and (3), 0·79 volts per minute,
that due to the α rays not deviated by the magnetic field employed.

The amount of α rays not deviated by the field is thus about 11% of the
total. The small difference between (3) and (4) measures the small
ionization due to the β rays, for they would be completely deviated by
the magnetic field; (4) comprises the effect of the γ rays together with
the natural leak of the electroscope in hydrogen.

In this experiment there was a good deal of stray magnetic field acting
on the rays before they reached the pole-pieces. The diminution of the
rate of discharge due to the α rays was found to be proportional to the
strength of field between the pole-pieces. With a more powerful magnetic
field, the whole of the α rays were deviated, showing that they
consisted _entirely_ of projected charged particles.

In order to determine the _direction_ of deviation of the rays, the rays
were passed through slits one mm. in width, each of which was half
covered with a brass strip. The diminution of the rate of discharge in
the testing vessel for a given magnetic field in such a case depends
upon the _direction_ of the field. In this way it was found that the
rays were deviated in the _opposite sense_ to the cathode rays. Since
the latter consist of negatively charged particles, the α rays must
consist of _positively_ charged particles.

These results were soon after confirmed by Becquerel[142], by the
photographic method, which is very well adapted to determine the
character of the path of the rays acted on by a magnetic field. The
radium was placed in a linear groove cut in a small block of lead. Above
this source, at a distance of about 1 centimetre, was placed a metallic
screen, formed of two plates, leaving between them a narrow opening
parallel to the groove. Above this was placed the photographic plate.
The whole apparatus was placed in a strong magnetic field parallel to
the groove. The strength of the magnetic field was sufficient to deflect
the β rays completely away from the plate. When the plate was parallel
to the opening, there was produced on it an impression, due to the α
rays alone, which became more and more diffuse as the distance from the
opening increased. This distance should not exceed 1 or 2 centimetres on
account of the absorption of the rays in air. If, during the exposure,
the magnetic field is reversed for equal lengths of time, on developing
the plate two images of the α rays are observed which are deflected in
opposite directions. This deviation, even in a strong field, is small
though quite appreciable and is opposite in sense to the deviation
observed for the β or cathodic rays from the same material.

M. Becquerel[143], by the same method, found that the α rays from
polonium were deviated in the same direction as the α rays from radium;
and thus that they also consist of projected positive bodies. In both
cases, the photographic impressions were sharply marked and did not show
the same diffusion which always appears in photographs of the β rays.


=90. Electrostatic deviation of the α rays=. If the rays are charged
bodies, they should be deflected in passing through a strong electric
field. This was found by the writer to be the case, but the electric
deviation is still more difficult to detect than the magnetic deviation,
as the intensity of the electric field must of necessity be less than
that required to produce a spark in the presence of radium. The
apparatus was similar to that employed for the magnetic deviation (Fig.
32) with this exception, that the brass sides which held the plates in
position, were replaced by ebonite. Alternate plates were connected
together and charged to a high potential by means of a battery of small
accumulators. The discharge in the electroscope, due to the α rays, was
found to be diminished by application of the electric field. With plates
·055 cm. apart and 4·5 cms. high, the diminution was only 7% with a P.D.
of 600 volts between the slits. With a special arrangement of plates,
with slits only ·01 cm. apart, the discharge was diminished about 45%
with an electric field corresponding to 10,000 volts per cm.


=91. Determination of the constants of the rays.= If the deviation of
the rays in both an electric and magnetic field is known, the values of
the velocity of the rays, and the ratio _e_/_m_ of the charge of the
particle to its mass can be determined by the method, first used by J.
J. Thomson for the cathode rays, which is described in section 50. From
the equations of a moving charged body, the radius of curvature ρ of the
path of the rays in a magnetic field of strength _H_ perpendicular to
the path of the rays is given by

                                  _m_
                           _H_ρ = ---- _V_ .
                                  _e_

If the particle, after passing through a uniform magnetic field for a
distance _l₁_, is deviated through a small distance _d₁_ from its
original direction,

                    2ρ_d₁_ = _l₁²_

            or
                           _l₁²_   _e_    _H_
                    _d₁_ = -----   ----   ----     (1).
                            2      _m_    _V_


If the rays pass through a uniform electric field of strength _X_ and
length _l₂_ with a deviation _d₂_,

                                1   _Xel₂²_
                        _d₂_ = ---  ------      (2),
                                2   _mV²_

since _Xe_/_m_ is the acceleration of the particle, at right angles to
its direction, and _l₂_/_V_ is the time required to travel through the
electric field.

From equations (1) and (2)

                         _d₁_   _l₂²_   _X_
                   _V_ = ----  -----   -----,
                         _d₂_   _l₁²_   _H_

             and

                 _e_     2_d₁_  _V_
                ----   = ----   ---   .
                 _m_     _l₁²_  _H_

The values of _V_ and _e_/_m_ are thus completely determined from the
combined results of the electric and magnetic deviation. It was found
that

                       _V_ = 2·5 × 10⁹ cms. per sec.
                       _e_/_m_ = 6 × 10³.

On account of the difficulty of obtaining a large electrostatic
deviation, these values are only approximate in character.

The results on the magnetic and electric deviation of the α rays of
radium have been confirmed by Des Coudres[144], by the photographic
method. Some pure radium bromide was used as a source of radiation. The
whole apparatus was enclosed in a vessel which was exhausted to a low
vacuum. In this way, not only was he able to determine the photographic
action of the rays at a much greater distance from the source, but he
was also able to apply a stronger electric field without the passage of
a spark. He found values of the constants given by

                       _V_ = 1·65 × 10⁹ cms. per sec.
                       _e_/_m_ = 6·4 × 10³.

These values are in very good agreement with the numbers found by the
electric method. The α rays from radium are complex, and probably
consist of a stream of positively charged bodies projected at velocities
lying between certain limits. The amount of deviation of the particles
in a magnetic field will thus differ according to the velocity of the
particle. The photographic results of Becquerel seem to indicate that
the velocity of the rays of radium can vary only within fairly narrow
limits, since the trajectory of the rays in a magnetic field is sharply
marked and not nearly as diffuse as in similar experiments with the β
rays. The evidence, however, discussed in the following section, shows
that the velocities of the α particles from a thick layer of radium vary
over a considerable range.


=92.= Becquerel[145] has examined the amount of magnetic deviation of
the α rays at different distances from the source of the rays in a very
simple way. A narrow vertical pencil of the rays, after its passage
through a narrow slit, fell on a photographic plate, which was inclined
at a small angle to the vertical and had its lower edge perpendicular to
the slit. The trajectory of the rays is shown by a fine line traced on
the plate. If a strong magnetic field is applied parallel to the slit,
the trajectory of the rays is displaced to the right or left according
to the direction of the field. If equal times of exposure are given for
the magnetic field in the two directions, on developing the plate two
fine diverging lines are found traced on the plate. The distance between
these lines at any point is a measure of twice the average deviation at
that point, corresponding to the value of the magnetic field. By
measuring the distance between the trajectories at various points,
Becquerel found that the radius of curvature of the path of the rays
_increased_ with the distance from the slit. The product _H_ρ of the
strength of the field and the radius of curvature of the path of the
rays is shown in the following table.

               Distance in mms.     _H_ρ
               from the slit

               1                    2·91 × 10⁵

               3                    2·99   „

               5                    3·06   „

               7                    3·15   „

               8                    3·27   „

               9                    3·41   „

The writer (_loc. cit._) showed that the _maximum_ value of _H_ρ for
complete deviation of the α rays was 390,000. The results are thus in
good agreement. Since

                                     _m_
                             _H_ρ = -----  _V_
                                     _e_

these results show that the values either of _V_ or of _e_/_m_ for the
projected particles vary at different distances from the source.
Becquerel considered that the rays were homogeneous, and, in order to
explain the results, has suggested that the charge on the projected
particles may gradually decrease with the distance traversed, so that
the radius of curvature of the path steadily increases with the distance
from the source. It, however, seems more probable that the rays consist
of particles projected with different velocities, and that the slower
particles are more quickly absorbed in the gas. In consequence of this,
only the swifter particles are present some distance from the source.

This conclusion is borne out by some recent experiments of Bragg and
Kleeman[146] on the nature of the absorption of α particles by matter,
which are discussed in more detail in sections 103 and 104. They found
that the α particles from a thick layer of radium are complex, and have
a wide range of penetrating power and presumably of velocity. This is
due to the fact that the α particles emitted from the radium come from
different depths. Since their velocity is reduced in their transit
through matter, a pencil of α rays will consist of particles which
differ considerably in speed. Those which are just able to emerge from
the radium will be absorbed in a very short depth of air, while those
that come from the surface will be able to pass through several
centimetres of air before they lose their power of ionizing the gas.
Since the α particles have different velocities, they will be unequally
deflected by the magnetic field, the slower moving particles describing
a more curved path than the swifter ones. Consequently, the outer edge
of the trace of the pencil of rays on the photographic plate, as
obtained by Becquerel, will be the locus of the points where the
photographic action of the α particles end. It was found that the α
particles are most efficient as ionizers of the gas just before their
power of ionizing ends. The loss of ionizing power of the α particles
seems to be fairly abrupt, and, for particles of the same velocity, to
occur always after traversing a definite distance in air. On the
assumption that the photographic as well as the ionizing action is most
intense just before the particles are stopped, and ceases fairly
abruptly, Bragg has been able to account numerically for the
measurements (see above table) recorded by Becquerel. Quite apart from
the special assumptions required for such a quantitative comparison of
theory with experiment, there can be little doubt that the increase of
value of _H_ρ with distance can be satisfactorily explained as a
consequence of the complex character of the pencil of rays[147].

Becquerel states that the amount of deviation, in a given magnetic
field, was the same for the α rays of polonium and of radium. This shows
that the value of

                                 _m_
                                 ---- _V_
                                 _e_

is the same for the α rays from the two substances. Since the α rays
from polonium are far more readily absorbed than the α rays from radium,
this result would indicate that the value of _m_/_e_ is greater for the
α particles of polonium than of radium. Further experimental evidence is
required on this important point.


=93. Charge carried by the α rays=. We have seen that the negative
charge carried by the β particles has been readily measured. Since there
is reason to believe (section 229) that four α particles are expelled
from radium for each β particle, it is to be expected that the positive
charge carried by the α particles should be determined still more
readily. All the initial experiments, however, made to detect this
charge, gave negative results; and, before successful results were
obtained, it was found necessary to eliminate some secondary actions,
which at first completely masked the effects to be looked for.

In consequence of the importance of this question, a brief account will
be given of the methods of measurement adopted and the special
experimental difficulties which have arisen.

In the first place, it must be remembered that only a small fraction of
the α rays, emitted from a layer of powdered radium bromide, escape into
the surrounding gas. On account of the ease with which the α rays are
stopped in their passage through matter, only those escape which are
expelled from a superficial layer, and the rest are absorbed by the
radium itself. On the other hand, a much larger proportion of the β rays
escape, on account of their greater power of penetration. In the second
place, the α particle is a far more efficient ionizer of the gas than
the β particle, and, in consequence, if the charge carried by the α rays
is to be determined by methods similar to those employed for the β rays
(see section 80), the pressure of the gas surrounding the conductor to
be charged must be very small in order to eliminate, as far as possible,
the loss of charge resulting from the ionization of the residual gas by
the α rays[148].

The experimental arrangement used by the writer is shown in Fig. 33.

A thin film of radium was obtained on a plate _A_ by evaporation of a
radium solution containing a known weight of radium bromide. Some hours
after evaporation, the activity of the radium, measured by the α rays,
is about 25 per cent. of its maximum value, and the β rays are almost
completely absent. The activity measured by the α and β rays is then
slowly regained, and recovers its original value after about a month’s
interval (see chapter XI.). The experiments were made on the active
plate when its activity was a minimum, in order to avoid complications
due to the presence of β rays. The film of radium was so thin that only
a very small fraction of the α rays was absorbed.

[Illustration: Fig. 33.]

The active plate _A_ was insulated in a metal vessel _D_, and was
connected to one pole of the battery, the other pole being earthed. The
upper electrode, which was insulated and connected with a Dolezalek
electrometer, consisted of a rectangular copper vessel _BC_, the lower
part of which was covered with a thin sheet of aluminium foil. The α
rays passed through the foil, but were stopped by the copper sides of
the vessel. This arrangement was found to reduce the secondary
ionization produced at the surface of the upper plate. The outside
vessel _D_ could be connected with either _A_ or _B_ or with earth. By
means of a mercury pump, the vessel was exhausted to a very low
pressure. If the rays carry a positive charge, the current between the
two plates measured by the electrometer should be greater when _A_ is
charged positively. No certain difference, however, between the currents
in the two directions was observed, even when a very good vacuum was
obtained. In some arrangements, it was found that the current was even
greater when the lower plate was negative than when it was positive. An
unexpected experimental result was also noticed. The current between the
parallel plates at first diminished with the pressure, but soon reached
a limiting value which was not altered however good a vacuum was
produced. For example, in one experiment, the current between the two
parallel plates, placed about 3 mms. apart, was initially 6·5 × 10⁹
amperes and fell off directly as the pressure. The current reached a
limiting value of about 6 × 10⁻¹² amperes, or about ¹⁄₁₀₀₀ of the value
at atmospheric pressure. The magnitude of this limiting current was not
much altered if the air was replaced by hydrogen.

Experiments of a similar character have been made by Strutt[149] and J.
J. Thomson[150]; using an active bismuth plate coated with
radio-tellurium (polonium) after Marckwald’s method. This substance
emits only α rays, and is thus especially suitable for experiments of
this kind. Strutt employed the method used by him to show the charge
carried by the β rays (Fig. 27). He found, however, that, even in the
lowest possible vacuum, the electroscope rapidly lost its charge and at
the same rate whether it was charged positively or negatively. This is
in agreement with the results found by the writer with radium.

In the experiments of J. J. Thomson, the electroscope was attached to a
metal disc placed 3 cms. from the plate of radio-tellurium. A very low
vacuum was produced by Dewar’s method by absorbing the residual gas in
cocoanut charcoal immersed in liquid air. When the electroscope was
charged negatively, an extremely slow rate of leak was observed, but
when charged positively the leak was about 100 times greater. This
showed that the polonium gave out large quantities of negative
electricity, but not enough positive to be detected. By placing the
apparatus in a strong magnetic field, the negative particles were
prevented from reaching the electroscope and the positive leak was
stopped.

These results indicate that these negative particles are not projected
with sufficient velocity to move against the repulsion exerted by the
electrified body, and are bent by a magnetic field. There thus seems
little doubt that a stream of negative particles (electrons) is
projected from the active surface at a very slow speed. Such low
velocity electrons are also projected from uranium and radium. It is
probable that these electrons are a type of secondary radiation, set up
at the surfaces on which the α rays fall. The particles would be
extremely readily absorbed in the gas, and their presence would be
difficult to detect except in low vacua. J. J. Thomson at first obtained
no evidence that the α particles of polonium were charged; but in later
experiments, where the plates were closer together, the electroscope
indicated that the α rays did carry a positive charge.

In order to see whether the positive charge due to the α rays from
radium could be detected when the slow moving ions were prevented from
escaping by a magnetic field, I placed the apparatus of Fig. 33 between
the pole-pieces of a large electromagnet, so that the magnetic field was
parallel to the plane of the plates[151]. A very marked alteration was
observed both on the magnitude of the positive and negative currents. In
a good vacuum, the upper plate received a positive charge, independently
of whether the lower plate was charged positively or negatively or was
connected with earth. After the magnetic field had reached a certain
value, a great increase in its strength had no appreciable effect on the
magnitude of the current.

The following table illustrates the results obtained when the two plates
were 3 mms. apart, and were both coated with thin aluminium foil.

       Potential        Current in arbitrary units
       of lower
       plate

                  Without magnetic     With magnetic field
                  field

       0          —                    +·36

       +2 volts   2·0                  +·46}

                                           }         ·39

       −2   „     2·5                  +·33}


       +4   „     2·8                  +·47}

                                           }         ·41

       −4   „     3·5                  +·35}


       +8   „     3·1                  +·56}

                                           }         ·43

       −8   „     4·0                  +·31}


       +84  „     3·5                  +·77}

                                           }         ·50

       −84  „     5·2                  +·24}

Let _n_ be the number of α particles, carrying a charge _e_, which are
absorbed in the upper plate. Let ι₀ be the current due to the slight
ionization of the residual gas.

If only a small potential is applied to the lower plate, this current
should be equal in magnitude but opposite in sign when the potential is
reversed. Let ι₁ be the charge per sec. communicated to the upper
electrode when the lower plate is charged positively and ι₂ the value
when charged negatively. Then

                                     ι₁ = ι₀ + _ne_,
                                     ι₂ = ι₀ + _ne_;

                     adding we get

                                            ι₁ + ι₂
                                     _ne_ = ------   .
                                             2

Now in the third column of the above table it is seen that (ι₁ + ι₂)/2
has the values ·39, ·41, ·43 for 2, 4, and 8 volts respectively. The
numbers are thus in fairly good agreement. Similar results were obtained
when a brass plate was substituted for the upper electrode shown in the
figure. Taking into consideration that the magnitude of _ne_ is
independent of the strength of the magnetic field above a certain small
value, and the good agreement of the numbers obtained with variation of
voltage, I think that there can be no doubt that the positive charge
communicated to the upper electrode was carried by the α particles. This
positive charge was not small, for using a weight of ·48 mgrs. radium
bromide spread in a thin foil over an area of about 20 sq. cms., the
charge communicated by the particles corresponded to a current 8·8 ×
10⁻¹³ amperes, and, with the Dolezalek electrometer employed, it was
necessary to add a capacity of ·0024 microfarads to the electrometer
system.

In these experiments, the film of radium bromide was so thin, that only
a very small percentage of the α particles was stopped by the radium
itself. Assuming that each α particle carries the same charge as an ion,
viz. 1·1 × 10⁻¹⁹ coulombs, and remembering that half of the α particles
are absorbed in the lower plate, the total number _N_ of α particles
expelled per second from one gram of radium bromide (at its minimum
activity) can be deduced. In two separate experiments where the amount
of radium used was ·194 and ·484 mgrs. respectively, the values of _N_
were in close agreement and equal to 3·6 × 10¹⁰. Now it will be shown
later that in radium there are three other products in radio-active
equilibrium, each of which probably gives out the same number of α
particles as radium itself. If this is the case, the total number of α
particles expelled per second from 1 gram of radium bromide _in
radio-active equilibrium_ is 4_N_ or 1·44 × 10¹¹. Assuming the
composition of radium bromide as RaBr₂, the number per second per gram
of radium is 2·5 × 10¹⁰. This number will be found to be in very good
agreement with that deduced from indirect data (chapter XIII.). The
value of _N_ is of great importance in determining the magnitude of
various quantities in radio-active calculations.


=94. Mass and energy of the α particle.= It has been pointed out that
the α rays from radium and polonium are analogous to the Canal rays of
Goldstein, for both carry a positive charge and are difficult to deflect
by a magnetic field. The experiments of Wien have shown that the
velocity of projection of the canal rays varies with the gas in the tube
and the intensity of the electric field applied, but it is generally
about ⅒ of the velocity of the α particle from radium. The value of
_e_/_m_ is also variable, depending upon the gas in the tube.

It has been shown that for the α rays of radium

                                       _e_
                    V = 2·5 × 10⁹ and  ---- = 6 × 10³.
                                       _m_

Now the value of _e_/_m_ for the hydrogen atom, liberated in the
electrolysis of water, is 10⁴. Assuming the charge carried by the α
particle to be the same as that carried by the hydrogen atom, the mass
of the α particle is about twice that of the hydrogen atom. Taking into
consideration the uncertainty attaching to the experimental value of
_e_/_m_ for the α particle, if the α particle consists of any known kind
of matter, this result indicates that it consists either of projected
helium or hydrogen. Further evidence on this important question is given
in section 260.

The α rays from all the radio-active substances and their products, such
as the radio-active emanations and the matter causing excited activity,
possess the same general properties and do not vary very much in
penetrating power. It is thus probable that in all cases the α rays from
the different radio-active substances consist of positively charged
bodies projected with great velocity. Since the rays from radium are
made up in part of α rays from the emanation stored in the radium, and
from the excited activity which it produces, the α rays from each of
these products must consist of positively charged bodies; for it has
been shown that _all_ the α rays from radium are deviated in a strong
magnetic field.

The kinetic energy of each projected particle is enormous, compared with
its mass. The kinetic energy of each α particle is

                1          1  _m_
               --- _mV²_ = -- ---  _V²e_ = 5·9 × 10⁻⁶ ergs.
                2          2  _e_

Taking the velocity of a rifle bullet as 10⁵ cms. per second, it is seen
that, mass for mass, the energy of motion of the α rays is 6 × 10⁸ times
as great as that of the rifle bullet. In this projection of bodies
atomic in size with great velocity probably lies the principal cause of
the heating effects produced by radium (chapter XII).


=95. Atomic disintegration.= The radio-activity of the radio-elements is
an atomic and not a molecular property. The rate of emission of the
radiations depends only on the amount of the element present and is
independent of its combination with inactive substances. In addition, it
will be shown later that the rate of emission is not affected by wide
variations of temperature, or by the application of any known chemical
or physical forces. Since the power of radiating is a property of the
radio-atoms, and the radiations consist for the most part of positively
and negatively charged masses projected with great velocity, it is
necessary to suppose that the atoms of the radio-elements are undergoing
disintegration, in the course of which parts of the atom escape from the
atomic system. It seems very improbable that the α and β particles can
suddenly acquire their enormous velocity of projection by the action of
forces existing inside or outside the atom. For example, the α particle
would have to travel from rest between two points differing in potential
by 5·2 million volts in order to acquire the kinetic energy with which
it escapes. Thus it seems probable that these particles are not set
suddenly in motion, but that they escape from an atomic system in which
they were already in rapid oscillatory or orbital motion. On this view,
the energy is not communicated to the projected particles, but exists
beforehand in the atoms from which they escape. The idea that the atom
is a complicated structure consisting of charged parts in rapid
oscillatory or orbital motion has been developed by J. J. Thomson,
Larmor and Lorentz. Since the α particle is atomic in size, it is
natural to suppose that the atoms of the radio-active elements consist
not only of the electrons in motion, but also of positively charged
particles whose mass is about the same as that of the hydrogen or helium
atom.

It will be shown later that only a minute fraction of the atoms of the
radio-element need break up per second in order to account for the
radiations even of an enormously active element like radium. The
question of the possible causes which lead to this atomic disintegration
and the consequences which follow from it will be discussed later in
chapter XIII.


=96. Experiments with a zinc sulphide screen.= A screen of Sidot’s
hexagonal blend (phosphorescent crystalline zinc sulphide) lights up
brightly under the action of the α rays of radium and polonium. If the
surface of the screen is examined with a magnifying glass, the light
from the screen is found not to be uniformly distributed but to consist
of a number of scintillating points of light. No two flashes succeed one
another at the same point, but they are scattered over the surface,
coming and going rapidly without any movement of translation. This
remarkable action of the radium and polonium rays on a zinc sulphide
screen was discovered by Sir William Crookes[152], and independently by
Elster and Geitel[153], who observed it with the rays given out from a
wire which has been charged negatively either in the open air or in a
vessel containing the emanation of thorium.

In order to show the scintillations of radium on the screen, Sir William
Crookes has devised a simple apparatus which he has called the
“Spinthariscope.” A small piece of metal, which has been dipped in a
radium solution, is placed several millimetres away from a small zinc
sulphide screen. This screen is fixed at one end of a short brass tube
and is looked at through a lens fixed at the other end of the tube.
Viewed in this way, the surface of the screen is seen as a dark
background, dotted with brilliant points of light which come and go with
great rapidity. The number of points of light per unit area to be seen
at one time falls off rapidly as the distance from the radium increases,
and, at several centimetres distance, only an occasional one is seen.
The experiment is extremely beautiful, and brings vividly before the
observer the idea that the radium is shooting out a stream of
projectiles, the impact of each of which on the screen is marked by a
flash of light.

The scintillating points of light on the screen are the result of the
impact of the α particles on its surface. If the radium is covered with
a layer of foil of sufficient thickness to absorb all the α rays the
scintillations cease. There is still a phosphorescence to be observed on
the screen due to the β and γ rays, but this luminosity is not marked by
scintillations to any appreciable extent. Sir William Crookes showed
that the number of scintillations was about the same in vacuo as in air
at atmospheric pressure. If the screen was kept at a constant
temperature, but the radium cooled down to the temperature of liquid
air, no appreciable difference in the number of scintillations was
observed. If, however, the screen was gradually cooled to the
temperature of liquid air, the scintillations diminished in number and
finally ceased altogether. This is due to the fact that the screen loses
to a large extent its power of phosphorescence at such a low
temperature.

Not only are scintillations produced by radium, actinium, and polonium,
but also by the emanations and other radio-active products which emit α
rays. In addition, F. H. Glew[154] has found that they can be observed
from the metal uranium, thorium compounds and various varieties of
pitchblende. In order to show the scintillations produced by
pitchblende, a flat surface was ground, and a transparent screen, whose
lower surface was coated with zinc sulphide, placed upon it. Glew has
designed a modified and very simple form of spinthariscope. A
transparent screen, coated on one side with a thin layer of zinc
sulphide, is placed in contact with the active material, and the
scintillations observed by a lens in the usual way.

Since there is no absorption in the air, the luminosity is a maximum.
The relative transparency of different substances placed between the
active material and the screen may, in this way, be directly studied.

The production of scintillations appears to be a general property of the
α rays from all radio-active substances. The scintillations are best
shown with a zinc sulphide screen; but are also observed with willemite
(zinc silicate), powdered diamond, and potassium platinocyanide (Glew,
_loc. cit._). If a screen of barium platinocyanide is exposed to the α
rays from radium, the scintillations are difficult to observe, and the
luminosity is far more persistent than for a zinc sulphide screen
exposed under the same conditions. The duration of the phosphorescence
in this case probably accounts for the absence of visible
scintillations.

There can be no doubt that the scintillations result from the continuous
bombardment of the sensitive screen by the α particles. Each of these
particles moves with enormous velocity, and has a considerable energy of
motion. On account of the ease with which these particles are stopped,
most of this energy is given up at the surface of the screen, and a
portion of the energy is in some way transformed into light. Zinc
sulphide is very sensitive to mechanical shocks. Luminosity is observed
if a penknife is drawn across the screen, or if a current of air is
directed on to the screen. The disturbance effected by the impact of the
α particle extends over a distance very large compared with the size of
the impinging particle, so that the spots of light produced have an
appreciable area. Recently Becquerel[155] has made an examination of the
scintillations produced by different substances, and has concluded that
the scintillations are due to irregular cleavages in the crystals
composing the screen, produced by the action of the α rays.
Scintillations can be mechanically produced by crushing a crystal.
Tommasina[156] found that a zinc sulphide screen removed from the action
of the radium rays for several days, showed the scintillations again
when an electrified rod was brought near it.

The number of scintillations produced in zinc sulphide depends upon the
presence of a slight amount of impurity and on its crystalline state. It
can be shown that even with the most sensitive zinc sulphide screens,
the number of scintillations is probably only a small fraction of the
total number of α particles which fall upon it. It would appear that the
crystals are in some way altered by the bombardment of the α particles,
and that some of the crystals occasionally break up with emission of
light[157].

Although the scintillations from a particle of pure radium bromide are
very numerous, they are not too numerous to be counted. Close to the
radium, the luminosity is very bright, but by using a high power
microscope the luminosity can still be shown to consist of
scintillations. Since the number of scintillations probably bears no
close relation to the number of α particles emitted, a determination of
the number of scintillations would have no special physical
significance. The relation between the number of α particles and the
number of scintillations would probably be variable, depending greatly
on the exact chemical composition of the sensitive substance and also
upon its crystalline state.


=97. Absorption of the α rays by matter=. The α rays from the different
radio-active substances can be distinguished from one another by the
relative amounts of their absorption by gases or by thin screens of
solid substances. When examined under the same conditions, the α rays
from the active substances can be arranged in a definite order with
reference to the amount of absorption in a given thickness of matter.

In order to test the amount of absorption of the α rays for different
thicknesses of matter, an apparatus similar to that shown in Fig. 17, p.
98, was employed[158]. A thin layer of the active material was spread
uniformly over an area of about 30 sq. cms., and the saturation current
observed between two plates 3·5 cms. apart. With a thin layer[159] of
active material, the ionization between the plates is due almost
entirely to the α rays. The ionization due to the β and γ rays is
generally less than 1% of the total.

The following table shows the variation of the saturation current
between the plates due to the α rays from radium and polonium, with
successive layers of aluminium foil interposed, each ·00034 cm. in
thickness. In order to get rid of the ionization due to the β rays from
radium, the radium chloride employed was dissolved in water and
evaporated. This renders the active compound, for the time, nearly free
from β rays.

The initial current with 1 layer of aluminium over the active material
is taken as 100. It will be observed that the current due

             _Polonium._                       _Radium._

   Layers of Current     Ratio of    Layers of Current   Ratio of
   aluminium             decrease    aluminium           decrease
                         for each                        for each
                         layer                           layer


   0         100                     0         100
                         ·41                             ·48
   1          41                     1          48
                         ·31                             ·48
   2         12·6                    2          23
                         ·17                             ·60
   3         2·1                     3         13·6
                         ·067                            ·47
   4         ·14                     4         6·4
                                                         ·39
   5         0                       5         2·5
                                                         ·36
                                     6         ·9

                                     7          0

to the radium rays decreases very nearly by half its value for each
additional thickness until the current is reduced to about 6% of the
maximum. It then decays more rapidly to zero. Thus, for radium, over a
wide range, the current decreases approximately according to an
exponential law with the thickness of the screen, or

$$ \frac {i} {i₀} = e^{λ d} $$,

where _i_ is the current for a thickness _d_, and _i₀_ the initial
current. In the case of polonium, the decrease is far more rapid than
would be indicated by the exponential law. By the first layer, the
current is reduced to the ratio ·41. The addition of the third layer
cuts the current down to a ratio of ·17. For most of the active bodies,
the current diminishes slightly faster than the exponential law would
lead one to expect, especially when the radiation is nearly all
absorbed.


=98.= The increase of absorption of the α rays of polonium with the
thickness of matter traversed has been very clearly shown in some
experiments made by Mme Curie. The apparatus employed is shown in Fig.
34.

[Illustration: Fig. 34.]

The saturation current was measured between two parallel plates _PP´_ 3
cms. apart. The polonium _A_ was placed in the metal box _CC_, and the
rays from it, after passing through an opening in the lower plate _P´_,
covered with a layer of thin foil _T_, ionized the gas between the
plates. For a certain distance _AT_, of 4 cms. or more, no appreciable
current was observed between _P_ and _P´_. As the distance _AT_ was
diminished, the current increased in a very sudden manner, so that for a
small variation of the distance _AT_ there was a large increase of
current. With still further decrease of distance the current increases
in a more regular manner. The results are shown in the following table,
where the screen _T_ consisted of one and two layers of aluminium foil
respectively. The current due to the rays, without the aluminium screen,
is in each case taken as 100.

               Distance AT in cms.  3·5 2·5 1·9 1·45 0·5


               For 100 rays         0   0   5   10   25
               transmitted by one
               layer

               For 100 rays         0   0   0   0    0·7
               transmitted by two
               layers

The metallic screen thus cuts off a greater proportion of the rays the
greater the distance of air which the radiations traverse. The effects
are still more marked if the plates _PP´_ are close together. Results
similar but not so marked are found if radium is substituted for the
polonium.

It follows from these experiments that the ionization per unit volume,
due to a large plate uniformly covered with the radio-active matter,
falls off rapidly with the distance from the plate. At a distance of 10
cms. the α rays from uranium, thorium, or radium have been completely
absorbed in the gas, and the small ionization then observed in the gas
is due to the more penetrating β and γ rays. The relative amount of the
ionization observed at a distance from the source will increase with the
thickness of the layer of active matter, but will reach a maximum for a
layer of a certain thickness. The greater proportion of the ionization,
due to unscreened active matter, is thus entirely confined to a shell of
air surrounding it not more than 10 cms. in depth.

[Illustration: Fig. 35.]


=99.= The α rays from different compounds of the same active element,
although differing in amount, have about the same average penetrating
power. Experiments on this point have been made by the writer[160] and
by Owens[161]. Thus in comparing the relative power of penetration of
the α rays from the different radio-elements, it is only necessary to
determine the penetrating power for one compound of each of the
radio-elements. Rutherford and Miss Brooks[162] have determined the
amount of absorption of the α rays from the different active substances
in their passage through successive layers of aluminium foil ·00034 cm.
thick. The curves of absorption are given in Fig. 35. For the purpose of
comparison in each case, the initial current with the bare active
compound was taken as 100. A very thin layer of the active substance was
used, and, in the case of thorium and radium, the emanations given off
were removed by a slow current of air through the testing vessel. A
potential difference of 300 volts was applied between the plates, which
was sufficient to give the maximum current in each case.

Curves for the minerals organite and thorite were very nearly the same
as for thoria.

For comparison, the absorption curves of the excited radiations of
thorium and radium are given, as well as the curve for the
radio-elements uranium, thorium, radium, and polonium. The α radiations
may be arranged in the following order, as regards their power of
penetration, beginning with the most penetrating.

                        Thorium}
                        Radium } excited radiation.
                        Thorium.
                        Radium.
                        Polonium.
                        Uranium.

The same order is observed for all the absorbing substances examined,
viz., aluminium, Dutch metal, tinfoil, paper, and air and other gases.
The differences in the absorption of the α rays from the active bodies
are thus considerable, and must be ascribed either to a difference of
mass or of velocity of the α particles or to a variation in both these
quantities.

Since the α rays differ either in mass or velocity, it follows that they
cannot be ascribed to any single radio-active impurity common to all
radio-active bodies.


=100. Absorption of the α rays by gases=. The α rays from the different
radio-active substances are quickly absorbed in their passage through a
few centimetres of air at atmospheric pressure and temperature. In
consequence of this, the ionization of the air, due to the α rays, is
greatest near the surface of the radiating body and falls off very
rapidly with the distance (see section 98).

[Illustration: Fig. 36.]

A simple method of determining the absorption in gases is shown in Fig.
36. The maximum current is measured between two parallel plates _A_ and
_B_ kept at a _fixed_ distance of 2 cms. apart, and then moved by means
of a screw to different distances from the radio-active surface. The
radiation from this active surface passed through a circular opening in
the plate _A_, covered with thin aluminium foil, and was stopped by the
upper plate. For observations on other gases besides air, and for
examining the effect at different pressures, the apparatus is enclosed
in an air-tight cylinder.

If the radius of the active surface is large compared with the distance
of the plate _A_ from it, the intensity of the radiation is
approximately uniform over the opening in the plate _A_, and falls off
with the distance _x_ traversed according to an exponential law. Thus

$$ \frac {I} {I₀} = e^{–λ x} $$,

where λ is the “absorption constant” of the radiation for the gas under
consideration[163]. Let

            _x_ = distance of lower plate from active material,
            _l_ = distance between the two fixed plates.

The energy of the radiation at the lower plate is then

$$ I₀ e^{–λ x} $$,

and at the upper plate

$$ I₀ e^{–λ (l + x)} $$ .

The total number of ions produced between the parallel plates _A_ and
_B_ is therefore proportional to

$$ e^{–λ x} − e^{–λ (l + x)} = e^{–λ x} (1 -
e^{–λ l}) $$ .

Since the factor

$$ 1 = e^{–λ l} $$

is a constant, the saturation current between _A_ and _B_ varies as

$$ e^{–λ x} $$,

_i.e._ it decreases according to an exponential law with the distance
traversed.

[Illustration: Fig. 37.]

The variation of the current between _A_ and _B_ with the distance from
a thin layer of uranium oxide is shown in Fig. 37 for different gases.
The initial measurements were taken at a distance of about 3·5 mms. from
the active surface. The actual values of this initial current were
different for the different gases, but, for the purposes of comparison,
the value is in each case taken as unity.

It will be seen that the current falls off with the distance
approximately in a geometrical progression, a result which is in
agreement with the simple theory given above. The distance through which
the rays pass before they are absorbed is given below for different
gases.

                    Gas             Distance in
                                    mms. to absorb
                                    half of
                                    radiation

                    Carbonic acid   3

                    Air             4·3

                    Coal-gas        7·5

                    Hydrogen       16

The results for hydrogen are only approximate, as the absorption is
small over the distance examined.

The absorption is least in hydrogen and greatest in carbonic acid, and
follows the same order as the densities of the gases. In the case of air
and carbonic acid, the absorption is proportional to the density, but
this rule is widely departed from in the case of hydrogen. Results for
the relative absorption by air of the α rays from the different active
bodies are shown in Fig. 38.

[Illustration: Fig. 38.]

The initial observation was made about 2 mms. from the active surface,
and the initial current is in each case taken as 100. The current, as in
the case of uranium, falls off at first approximately in geometrical
progression with the distance. The thickness of air, through which the
radiation passes before the intensity is reduced to half value, is given
below.

                                    Distance in mms.

               Uranium              4·3

               Radium               7·5

               Thorium             10

               Excited radiation   16·5
               from Thorium and
               Radium

The order of absorption by air of the radiations from the active
substances is the same as the order of absorption by the metals and
solid substances examined.


=101. Connection between absorption and density.= Since in all cases the
radiations first diminish approximately according to an exponential law
with the distance traversed, the intensity _I_ after passing through a
thickness _x_ is given by

$$ I = I₀ e^{–λ x} $$

where λ is the absorption constant and _I₀_ the initial intensity.

The following table shows the value of λ with different radiations for
air and aluminium.

                   Radiation             λ for  λ for air
                                     aluminium

               Excited radiation           830        ·42

               Thorium                    1250        ·69

               Radium                     1600        ·90

               Uranium                    2750          1·6

Taking the density of air at 20° C. and 760 mms. as 0·00120 compared
with water as unity, the following table shows the value of λ divided by
density for the different radiations.

                   Radiation         Aluminium        Air
               Excited radiation           320        350
               Thorium                     480        550
               Radium                      620        740
               Uranium                    1060       1300

Comparing aluminium and air, the absorption is thus roughly proportional
to the density for all the radiations. The divergence, however, between
the absorption-density numbers is large when two metals like tin and
aluminium are compared. The value of λ for tin is not much greater than
for aluminium, although the density is nearly three times as great.

If the absorption is proportional to the density, the absorption in a
gas should vary directly as the pressure, and this is found to be the
case. Some results on this subject have been given by the writer (_loc.
cit._) for uranium rays between pressures of ¼ and 1 atmosphere. Owens
(_loc. cit._) examined the absorption of the α radiation in air from
thoria between the pressures of 0·5 to 3 atmospheres and found that the
absorption varied directly as the pressure.

The variation of absorption with density for the projected positive
particles is thus very similar to the law for the projected negative
particles and for cathode rays. The absorption, in both cases, depends
mainly on the density, but is not in all cases directly proportional to
it. Since the absorption of the α rays in gases is probably mainly due
to the exhaustion of the energy of the rays by the production of ions in
the gas, it seems probable that the absorption in metals is due to a
similar cause.


=102. Relation between ionization and absorption in gases.= It has been
shown (section 45) that if the α rays are completely absorbed in a gas,
the _total_ ionization produced is about the same for all the gases
examined. Since the rays are unequally absorbed in different gases,
there should be a direct connection between the relative ionization and
the relative absorption. This is seen to be the case if the results of
Strutt (section 45) are compared with the relative absorption constants
(section 100).

          Gas                         Relative        Relative
                                    absorption      ionization

          Air                             1               1

          Hydrogen                         ·27             ·226

          Carbon dioxide                  1·43            1·53

Considering the difficulty of obtaining accurate determinations of the
absorption, the relative ionization in a gas is seen to be directly
proportional to the relative absorption within the limits of
experimental error. This result shows that the energy absorbed in
producing an ion is about the same in air, hydrogen, and carbon dioxide.


=103. Mechanism of the absorption of α rays by matter=. The experiments,
already described, show that the ionization of the gas, due to the α
rays from a large plane surface of radio-active matter, falls off in
most cases approximately according to an exponential law, until most of
the rays are absorbed, whereupon the ionization decreases at a much
faster rate. In the case of polonium, the ionization falls off more
rapidly than is to be expected on the simple exponential law.

The ionization produced in the gas is due to the collision of the
rapidly moving α particles with the molecules of the gas in their path.
On account of its large mass, the α particle is a far more efficient
ionizer than the β particle moving at the same speed. It can be deduced
from the results of experiment that each projected α particle is able to
produce about 100,000 ions in passing through a few centimetres of the
gas before its velocity is reduced to the limiting value, below which it
no longer ionizes the gas in its path.

Energy is required to ionize the gas, and this energy can only be
obtained at the expense of the kinetic energy of the projected α
particle. Thus it is to be expected that the α particle should gradually
lose its velocity and energy of motion in its passage through the gas.

Since the rate of absorption of the α rays in gases is deduced from
measurements of the ionization of the gas at different distances from
the source of radiation, a knowledge of the law of variation of the
ionizing power of the projected α particle with its speed is required in
order to interpret the results. The experimental data on this question
are, however, too incomplete to be applied directly to a solution of
this question. Townsend[164] has shown that a moving electron produces
ions in the gas after a certain limiting velocity is reached. The number
of ions produced per centimetre of its path through the gas then rises
to a maximum, and for still higher speeds continuously decreases. For
example, Townsend found that the number of ions produced by an electron
moving in an electric field was small at first for weak fields, but
increased with the strength of the electric field to a maximum
corresponding to the production of 20 ions per cm. of path in air at a
pressure of 1 mm. of mercury. Durack[165] found that the electrons,
generated in a vacuum tube, moving with a velocity of about 5 × 10⁹
cms. per second produced a pair of ions every 5 cms. of path at 1 mm.
pressure. In a later paper, Durack showed that for the electrons from
radium, which are projected with a velocity greater than half the
velocity of light, a pair of ions was produced every 10 cms. of path.
The high speed electron from radium is thus a very inefficient ionizer
and produces only about ¹⁄₁₀₀ of the ionization per unit path observed
by Townsend for the slow moving electron.


=104.= In the case of the α particle, no direct measurements have been
made upon the variation of the ionization with the velocity of the
particle, so that the law of absorption of the rays cannot be deduced
directly. An indirect attack upon the question has, however, been made
recently by Bragg and Kleeman[166] who have formulated a simple theory
to account for the experimental results which they have obtained upon
the absorption of the α rays. The α particles from each simple type of
radio-active matter are supposed to be projected with the same velocity,
and to pass through a definite distance a in air at atmospheric pressure
and temperature before they are all absorbed. As a first approximation
the ionization per unit path is supposed to be the same over the whole
length traversed before absorption, and to cease fairly suddenly at a
definite distance from the source of radiation. This is in agreement
with the observed fact that the ionization between parallel plates
increases very rapidly when it approaches nearer than a certain distance
to the radiant source. The range _a_ depends upon the initial energy of
motion of the α particle and will thus be different for different kinds
of radio-active matter. If a thick layer of radio-active matter is
employed, only the α particles from the surface have a range _a_. Those
which reach the surface from a depth _d_ have their range diminished by
an amount ρ_d_, where ρ is the density of the radio-active matter
compared with air. This is merely an expression of the fact that the
absorption of the α rays is proportional to the thickness and density of
matter traversed. The rays from a thick layer of active matter will thus
be complex, and will consist of particles of different velocity whose
ranges have all values between 0 and _a_.

Suppose that a narrow pencil of α rays is emitted from a thick layer of
radio-active material, and confined by metal stops as in Fig. 39.

[Illustration: Fig. 39.]

The pencil of rays passes into an ionization vessel _AB_ through a fine
wire gauze _A_. The amount of ionization is to be determined between _A_
and _B_ for different distances _h_ from the source of the rays _R_ to
the plate _A_.

All the particles coming from a depth _x_ of the material given by _h_ =
_a_ − ρ_x_ will enter the ionization vessel. The number of ions produced
in a depth _dh_ of the ionization vessel is equal to _nxdh_, _i.e._ to

                              _a_ − _h_
                          _n_ ---------   _dh_,
                                  ρ

where _n_ is a constant.

If the depth of the ionization vessel be _b_, the total number of ions
produced in the vessel is

$$ \int_h^{h+b} n \frac {a-h} {\rho} dh = \frac {nb} {\rho} (a − h -
\frac {b} {2}) $$ .

This supposes that the stream of particles passes completely across the
vessel. If not, the expression becomes

$$ \int_h^a n \frac {a − h} {\rho} dh = \frac {n (a − h)^2} {2\rho} $$ .

If the ionization in the vessel _AB_ is measured, and a curve plotted
showing its relation to _h_, the curve in the former case should be a
straight line whose slope is _nb_/ρ and in the latter a parabola.

Thus if a thin layer of radio-active material is employed and a shallow
ionization vessel, the ionization would be represented by a curve such
as _APM_ (Fig. 40), where the ordinates represent distances from the
source of radiation, and the abscissae the ionization current between
the plates _AB_.

[Illustration: Fig. 40.]

In this case, _PM_ is the range of the α particles from the lowest layer
of the radio-active matter. The current should be constant for all
distances less than _PM_.

For a thick layer of radio-active matter, the curve should be a straight
line such as _APB_.

Curves of the above character should only be obtained when definite
cones of rays are employed, and where the ionization vessel is shallow
and includes the whole cone of rays. In such a case the inverse square
law need not be taken into account.

In the experiments previously recorded (sections 99 and 100), the
ionization was measured between parallel plates several centimetres
apart for a large area of radio-active material. Such an arrangement was
necessary at the time at which the experiments were made, as only weak
radio-active material was available. Measurable electrical effects could
not then be obtained with narrow cones of rays and shallow ionization
vessels, but this disadvantage is removed by the advent of pure radium
bromide as a source of radiation.

The interesting experiments described by Bragg and Kleeman show that the
theoretical curves are approximately realized in practice. The chief
difficulty experienced in the analysis of the experimental results was
due to the fact that radium is a complex radio-active substance and
contains four radio-active products each of which gives rise to α rays
which have different ranges. The general character of the results
obtained from radium are shown graphically in Fig. 41, curves _A_, _B_,
_C_, _D_.

[Illustration: Fig. 41.]

The ordinates represent the distance between the radium and the gauze of
the testing vessel; the abscissae the current in the ionization vessel
in arbitrary units. Five milligrams of radium bromide were used, and the
depth of the ionization vessel was about 5 mms. Curve _A_ is for a cone
of rays of angle 20°. The initial current at a distance of 7 cms. is due
to the β and γ rays and natural leak. This curve is initially parabolic,
and then is made up of two straight lines. Curve _B_ is for a smaller
cone, and shows the straight line character of the curve to within a
short distance of the radium. Curve _C_ was obtained under the same
condition as curve _A_, but with a layer of gold beater’s skin placed
over the radium. The effect of this is to reduce all the ordinates of
curve _A_ by the same quantity. This is to be expected on the simple
theory already considered. Curve _D_ was obtained when the radium was
heated so as to get rid of the emanation and its products. The α
particles of greatest range are quite absent and the curve is simpler in
character.

[Illustration: Fig. 42.]

The complex character of the radium curves are more clearly brought out
by a careful examination of a portion of the curve at distances between
2 and 5 cms. from the radium, using an ionization vessel of depth only 2
mms. The results are shown in Fig. 42, where the curve is seen to
consist approximately of four straight lines of different slopes
represented by _PQ_, _QR_, _RS_, _ST_.

Such a result is to be expected, for it will be shown later that four
distinct α ray products exist in radium when in radio-active
equilibrium. Each of these products of radium emits an equal number of α
particles per second, but the range of each is different. If _a₁_ is the
range of one stream, _a₂_ of another, the ionization in the vessel _AB_,
when two streams enter the vessel, should be

           _nb_                      _nb_
           ----   (_a₁_-_h_-_b_/2) + ----- (_a₂_ − _h_ − _b_/2),
            ρ                         ρ

_i.e._

                     _nb_
                     ---- (_a₁_ + _a₂_ − 2_h_ − _b_) .
                      ρ

Thus the slope of the curve should in this case be 2_nb_/ρ, while if
only one stream enters, it should be _nb_/ρ. When three reach it, the
slope should be 3_nb_/ρ and for four 4_nb_/ρ. These results are realized
fairly closely in practice. The curve (Fig. 42) consists of four parts,
whose slopes are in the proportion 16, 34, 45, 65, _i.e._ very nearly in
the ratio 1, 2, 3, 4.

Experiments were also made with very thin layers of radium bromide,
when, as we have seen (Fig. 40) a very different shape of curve is to be
expected. An example of the results is shown in Fig. 43, curves I., II.
and III. Curve I. is obtained from radium bromide which has been heated
to drive off the emanation, and curves II. and III. from the same
substance several days later, when the emanation was again accumulating.
The portion _PQ_, which is absent in the first curve, is probably due to
the “excited” activity produced by the emanation. By careful examination
of the successive changes in the curves after the radium has been heated
to drive off the emanation, it is possible to tell the range of the α
rays from each of the different products, and this has been done to some
extent by Bragg and Kleeman.

It will be seen later that the results here obtained support in a novel
way the theory of radio-active changes which has been advanced from data
of quite a different character.

The inward slope of the curve in Fig. 43 due to the radium indicates
that the α particles become more efficient ionizers as their velocity
decreases. This is in agreement with observations on the β rays. In some
cases Bragg also observed that the α particles are the most efficient
ionizers just before they lose their power of ionizing the gas.

[Illustration: Fig. 43.]

Thus we may conclude from these experiments that the α particles from a
simple radio-active substance traverse a definite distance in air, at a
definite pressure and temperature, and that the ionization ends fairly
abruptly. If the rays traverse a sheet of metal, the effective range of
ionization is diminished by a distance corresponding to ρ_d_, where ρ is
the density of the material compared with air and _d_ its thickness. The
α rays from a thick layer of a simple radio-active substance consist of
α particles of different velocities, which have ranges in air lying
between 0 and the maximum range. The ionization of the particles per
unit path is greatest near the end of its range, and decreases somewhat
as we approach the radiant source. A complex source of rays like radium
gives out four types of rays, each of which has a different but distinct
range.

From this theory it is possible to calculate approximately the decrease
of current to be observed when sheets of metal foil are placed over a
large area of radio-active substance. This is the method that has been
employed to obtain the curves of Figs. 35 and 38.

Suppose a very thin layer of simple radio-active matter is employed (for
example a bismuth plate covered with radio-tellurium or a metal plate
made active by exposure to the presence of the thorium or radium
emanations) and that the ionization vessel is of sufficient depth to
absorb the α rays completely.

Let _d_ be the thickness of the metal plate, ρ its density compared with
air. Consider a point _P_ close to the upper side of the plate. The
range of the particles moving from a point, when the path makes an angle
θ with the normal at _P_, is _a_ − ρ_d_ sec θ, where _a_ is the range in
air. The rays coming from points such that the paths make an angle with
the normal greater than

$$ \cos^{−1} \frac {\rho d} {a} $$

will thus be absorbed in the plate. By integrating over the circular
area under the point _P_, it is easy to show that the total ionization
in the vessel is proportional to

$$ \int₀^{\cos^{−1} \frac {\rho d} {a}} 2 \pi \sin \theta \cos \theta (a −
\rho d \sec \theta) d\theta = \frac {\pi (a − \rho d)^2} {a} $$ .

The curves showing the relation between current and distance of metal
traversed should thus be parabolic with respect to _d_. This is
approximately the case for a simple substance like radio-tellurium. The
curve for a thick layer of radium would be difficult to calculate on
account of the complexity of the rays, but we know from experiment that
it is approximately exponential. An account of some recent
investigations made to determine the range of velocity over which the α
particle is able to ionize the gas is given in Appendix A. The results
there given strongly support the theory of absorption of the α rays
discussed above.




                                PART IV.


                    The γ or very penetrating Rays.


=105.= In addition to the α and β rays, the three active substances,
uranium, thorium, and radium, all give out a radiation of an
extraordinarily penetrating character. These γ rays are considerably
more penetrating than the X rays produced in a “hard” vacuum tube. Their
presence can readily be observed for an active substance like radium,
but is difficult to detect for uranium and thorium unless a large
quantity of active material is used. Villard[167], using the
photographic method, first drew attention to the fact that radium gave
out these very penetrating rays, and found that they were non-deviable
by a magnetic field. This result was confirmed by Becquerel[168].

Using a few milligrams of radium bromide, the γ rays can be detected in
a dark room by the luminosity they excite in the mineral willemite or a
screen of platinocyanide of barium. The α and β rays are completely
absorbed by placing a thickness of 1 centimetre of lead over the radium,
and the rays which then pass through the lead consist entirely of γ
rays. The very great penetrating power of these rays is easily observed
by noting the slight diminution of the luminosity of the screen when
plates of metal several centimetres thick are placed between the radium
and the screen. These rays also produce ionization in gases and are best
investigated by the electrical method. The presence of the γ rays from
30 mgrs. of radium bromide can be observed in an electroscope after
passing through 30 cms. of iron.


=106. Absorption of the γ rays=. In an examination of the active
substances by the electrical method, the writer[169] found that both
uranium and thorium gave out γ rays in amount roughly proportional to
their activity. An electroscope of the type shown in Fig. 12 was
employed. This was placed on a large lead plate ·65 cm. thick, the
active substance being placed in a closed vessel beneath.

The discharge due to the natural ionization of the air in the
electroscope was first observed. The additional ionization due to the
active substance must be that produced by rays which have passed through
the lead plate and the walls of the electroscope. The following table
shows that the discharge due to these rays decreases approximately
according to an exponential law with the thickness of lead traversed.

               Thickness of lead       Rate of discharge
               ·62 cms.                              100
               „  +  ·64 cms.                         67
               „  + 2·86  „                           23
               „  + 5·08  „                            8

Using 100 grs. of uranium and thorium, the discharge due to the rays
through 1 cm. of lead was quite appreciable, and readily measured. The
results showed that the amount of γ rays was about the same for equal
weights of thorium and uranium oxides. The penetrating power was also
about the same as for the radium rays.

[Illustration: Fig. 44.]

The writer showed that the absorption of the γ rays from radium was
approximately proportional to the density of the substance traversed. A
more detailed examination of the absorption of these rays in various
substances has been recently made by McClelland[170]. The curve (Fig.
44) shows the decrease of the ionization current in a testing vessel due
to the β and γ rays with successive layers of lead. It is seen that the
β rays are almost completely stopped by 4 mms. of lead; the ionization
is then due entirely to the γ rays.

In order to leave no doubt that all the β rays were absorbed, the radium
was covered with a thickness of 8 mms. of lead, and measurements of the
coefficient of absorption λ were made for additional thicknesses. The
average value of λ was calculated from the usual equation

                  $$ \frac {I} {I₀} = e^{–λ d} $$,

where _d_ is the thickness of matter traversed. The following table
shows the value of λ, (I) for the first 2·5 mms. of matter traversed
(after initially passing through 8 mms. of lead), (II) for the thickness
2·5 to 5 mms., (III) for 5 to 10 mms., (IV) 10 to 15 mms.

TABLE A.

                 Substance            I    II   III    IV

               Platinum           1·167
               Mercury             ·726  ·661  ·538  ·493
               Lead                ·641  ·563  ·480  ·440
               Zinc                ·282  ·266  ·248  ·266
               Aluminium           ·104  ·104  ·104  ·104
               Glass               ·087  ·087  ·087  ·087
               Water               ·034  ·034  ·034  ·034

In the above table, the absorption in aluminium, glass and water was too
small to determine with accuracy the variation of λ with distance
traversed. It will be observed that, for the denser substances, the
coefficient of absorption decreases with the distance through which the
rays have passed. This indicates that the rays are heterogeneous. The
variation of λ is more marked in heavy substances.

Table B gives the values of λ divided by density for the above numbers.
If the absorption were directly proportional to the density, the
quotient would be the same in all cases.

TABLE B.

λ _divided by density_.

                  Substance           I    II   III    IV

               Platinum            ·054
               Mercury             ·053  ·048  ·039  ·036
               Lead                ·056  ·049  ·042  ·037
               Zinc                ·039  ·037  ·034  ·033
               Aluminium           ·038  ·038  ·038  ·038
               Glass               ·034  ·034  ·034  ·034
               Water               ·034  ·034  ·034  ·034

The numbers in column I vary considerably, but the agreement becomes
closer in the succeeding columns, until in column IV the absorption is
very nearly proportional to the density.

It is seen that the absorption of all three types of rays from
radio-active substances is approximately proportional to the density of
the substance traversed—a relation first observed by Lenard for the
cathode rays. This law of absorption thus holds for both positively and
negatively electrified particles projected from the radio-active
substances, and also for the electromagnetic pulses which are believed
to constitute the γ rays; although the absorption of the α rays, for
example, is 10,000 times greater than for the γ rays. We have seen in
section 84 that the value of the absorption constant λ for lead is 122
for the β rays from uranium. The value for the γ rays from radium varies
between ·64 and ·44, showing that the γ rays are more than 200 times as
penetrating as the β rays.

=107. Nature of the rays.= In addition to their great penetrating power,
the γ rays differ from the α and β rays in not being deflected to an
appreciable degree by a magnetic or electric field. In a strong magnetic
field, it can be shown, using the photographic method, that there is an
abrupt discontinuity between the β and γ rays, for the former are bent
completely away from the latter. This indicates that, as regards the
action of a magnetic field, there is no gradual transition of magnetic
properties between the β and γ rays. Paschen[171] has examined the γ
rays in a very intense magnetic field, and, from the absence of
deflection of these rays, has calculated that, if they consist of
electrified particles carrying an ionic charge, and projected with a
velocity approaching that of light, their apparent mass must be at least
45 times greater than that of the hydrogen atom.

It now remains for us to consider whether the γ rays are corpuscular in
character, or whether they are a type of electromagnetic pulse in the
ether similar to Röntgen rays. They resemble Röntgen rays in their great
penetrating power and in their absence of deflection in a magnetic
field. Earlier experiments seemed to indicate an important difference
between the action of γ and X rays. It is well known that ordinary X
rays produce much greater ionization in gases such as sulphuretted
hydrogen and hydrochloric acid gas, than in air, although the
differences in density are not large. For example, exposed to X rays,
sulphuretted hydrogen has six times the conductivity of air, while with
γ rays the conductivity only slightly exceeds that of air. The results
obtained by Strutt, in this connection, have already been given in
section 45. It is there shown that the relative conductivity of gases
exposed to γ rays (and also to α and β rays) is, in most cases, nearly
proportional to their relative densities; but, under X rays, the
relative conductivity for some gases and vapours is very much greater
than for the γ rays. It must be remembered, however, that the results
obtained by Strutt were for “soft X rays,” whose penetrating power was
very much less than that of the γ rays. In order to see if the relative
conductivity of gases produced by X rays depended upon their penetrating
power, A. S. Eve[172] made some experiments with a very “hard” X ray
bulb, which gave an unusually penetrating type of rays.

The results of the measurements are shown in the table below, where the
conductivity for each type of rays is expressed relative to air as
unity. The results obtained for “soft” X rays by Strutt and by Eve for γ
rays are added for comparison.

It is seen that the hard rays show a much closer agreement than the soft
rays with the density law found for the γ rays. The high values
previously obtained for the vapours of chloroform and carbon
tetrachloride are greatly reduced, and are very nearly the same as for
the γ rays. On the other hand, the vapour of methyl iodide is an
exception, and still shows a high conductivity. The γ rays were,
however, forty times as penetrating as the hard X rays, and it is
probable that the value of methyl iodide would be reduced with still
more penetrating X rays.

_Relative conductivities of gases._

             Gas             Relative “Soft” “Hard” γ rays
                              Density X rays X rays


             Hydrogen             ·07    ·11    ·42     ·19

             Air                  1·0    1·0    1·0    1·0

             Sulphuretted         1·2    6       ·9    1·23
             Hydrogen

             Chloroform           4·3   32      4·6    4·8

             Methyl Iodide        5·0   72     13·5    5·6

             Carbon               5·3   45      4·9    5·2
             Tetrachloride

The hard X rays were found to give far more secondary radiation than the
γ rays, but this effect is probably also a function of the penetrating
power of the primary rays. It will be seen later (section 112) that γ
rays give rise to a secondary radiation of the β ray type. This has also
been observed for the X rays.

Considering the experimental evidence as a whole, there is undoubtedly a
very marked similarity between the properties of γ and X rays. The view
that the γ rays are a type of very penetrating X rays, also receives
support from theoretical considerations. We have seen (section 52) that
the X rays are believed to be electromagnetic pulses, akin in some
respects to short light waves, which are set up by the sudden stoppage
of the cathode ray particles. Conversely, it is also to be expected that
X rays will be produced at the sudden starting, as well as at the sudden
stopping, of electrons. Since most of the β particles from radium are
ejected from the radium atom with velocities much greater than the
cathode particles in a vacuum tube, X rays of a very penetrating
character will arise. But the strongest argument in support of this view
is derived from an examination of the origin and connection of the β and
γ rays from radio-active substances. It will be shown later that the α
ray activity observed in radium arises from several disintegration
products, stored up in the radium, while the β and γ rays arise only
from one of these products named radium _C_. It is found, too, that the
activity measured by the γ rays is always proportional to the activity
measured by the β rays, although by separation of the products the
activity of the latter may be made to undergo great variations in value.

Thus the intensity of the γ rays is always proportional to the rate of
expulsion of β particles, and this result indicates that there is a
close connection between the β and γ rays. Such a result is to be
expected if the β particle is the parent of the γ ray, for the expulsion
of each electron from radium will give rise to a narrow spherical pulse
travelling from the point of disturbance with the velocity of light.


=108.= There is another possible hypothesis in regard to the nature of
these rays. It has been shown (sections 48 and 82) that the apparent
mass of an electron increases as the speed of light is approached;
theoretically it should be very great when the velocity of the electron
is exceedingly close to the velocity of light. In such a case, a moving
electron would be difficult to deflect by a magnetic or electric field.

The view that the γ rays are electrons carrying a negative charge and
moving with a velocity nearly equal to that of light has recently been
advocated by Paschen[173]. He concluded from experiment that the γ rays
like the β rays carried a negative charge. We have seen (section 85)
that Seitz also observed that a small negative charge was communicated
to bodies on which the γ rays impinged, but the magnitude of this charge
was much smaller than that observed by Paschen. I do not think that much
weight can be attached to observations that a small positive or negative
charge is communicated to bodies on which the γ rays fall, for it will
be shown later that a strong secondary radiation, consisting in part of
electrons, is set up during the passage of the γ rays through matter. It
is not improbable that the small charge observed is not a direct result
of the charge carried by the γ rays, but is an indirect effect due to
the secondary radiations emitted from the surface of bodies. There is no
doubt that a thick lead vessel, completely enclosing a quantity of
radium, acquires a small positive charge, but this result would follow
whether the γ rays carry a charge or not, since the secondary radiations
from the lead surface consist of projected particles which carry with
them a negative charge.

On this corpuscular theory of the nature of the γ rays, each electron
must have a large apparent mass, or otherwise it would be appreciably
deflected by an intense magnetic field. The energy of motion of the
electron must, in consequence, be very great, and, if the number of the
electrons constituting the γ rays is of the same order of magnitude as
the number of the β particles, a large heating effect is to be expected
when the γ rays are stopped in matter. Paschen[174] made some
experiments on the heat emission of radium due to the γ rays; he
concluded that the γ rays were responsible for more than half of the
total heat emission of radium and carried away energy at the rate of
over 100 gram calories per hour per gram of radium. This result was not
confirmed by later experiments of Rutherford and Barnes[175], who found
that the heating effect of the γ rays could not be more than a few per
cent. of the total heat emission of radium. These results will be
considered later in chapter XII.

The weight of evidence, both experimental and theoretical, at present
supports the view that the γ rays are of the same nature as the X rays
but of a more penetrating type. The theory that the X rays consist of
non-periodic pulses in the ether, set up when the motion of electrons is
arrested, has found most favour, although it is difficult to provide
experimental tests to decide definitely the question. The strongest
evidence in support of the wave nature of the X rays is derived from the
experiments of Barkla[176], who found that the amount of secondary
radiation set up by the X rays on striking a metallic surface depended
on the orientation of the X ray bulb. The rays thus showed evidence of a
one-sidedness or polarization which is only to be expected if the rays
consist of a wave motion in the ether.




                                PART V.


                            Secondary Rays.


=109. Production of secondary rays.= It has long been known that Röntgen
rays, when they impinge on solid obstacles, produce secondary rays of
much less penetrating power than the incident rays. This was first shown
by Perrin and has been investigated in detail by Sagnac, Langevin,
Townsend and others. Thus it is not surprising that similar phenomena
should be observed for the radiation from radio-active substances. By
means of the photographic method, Becquerel[177] has made a close study
of the secondary radiations produced by radio-active substances. In his
earliest observations, he noticed that radiographs of metallic objects
were always surrounded by a diffuse border. This effect is due to the
secondary rays set up by the incident rays at the surface of the screen.

The secondary rays produced by the α rays are very feeble. They are best
shown by polonium, which gives out only α rays, so that the results are
not complicated by the action of the β rays. Strong secondary rays are
set up at the point of impact of the β or cathodic rays. Becquerel found
that the magnitude of this action depended greatly on the velocity of
the rays. The rays of lowest velocity gave the most intense secondary
action, while the penetrating rays gave, in comparison, scarcely any
secondary effect. In consequence of the presence of this secondary
radiation, the photographic impression of a screen pierced with holes is
not clear and distinct. In each case there is a double photographic
impression, due to the primary rays and the secondary rays set up by
them.

These secondary rays are deviable by a magnetic field, and in turn
produce tertiary rays and so on. The secondary rays are in all cases
more readily deviated and absorbed than the primary rays, from which
they arise. The very penetrating γ rays give rise to secondary rays,
which cause intense action on the photographic plate. When some radium
was placed in a cavity inside a deep lead block, rectangular in shape,
besides the impression due to the direct rays through the lead,
Becquerel observed that there was also a strong impression due to the
secondary rays emitted from the surface of the lead. The action of these
secondary rays on the plate is so strong that the effect on the plate
is, in many cases, increased by adding a metal screen between the active
material and the plate.

The comparative photographic action of the primary and secondary rays
cannot be taken as a relative measure of the intensity of their
radiations. For example, only a small portion of the energy of the β
rays is in general absorbed in the sensitive film. Since the secondary
rays are far more easily absorbed than the primary rays, a far greater
proportion of their energy is expended in producing photographic action
than in the case of the primary rays. It is thus not surprising that the
secondary rays set up by the β and γ rays may in some cases produce a
photographic impression comparable with, if not greater than, the effect
of the incident rays.

On account of these secondary rays, radiographs produced by the β rays
of radium in general show a diffuse border round the shadow of the
object. For this reason radiographs of this kind lack the sharpness of
outline of X ray photographs.


=110. Secondary radiation produced by α rays=. Mme Curie[178] has shown
by the electric method that the α rays of polonium produce secondary
rays. The method adopted was to compare the ionization current between
two parallel plates, when two screens of different material, placed over
the polonium, were interchanged.

These results show that the α rays of polonium are modified in passing
through matter, and that the amount of secondary rays set up varies with
screens of different material. Mme Curie, using the same method, was
unable to observe any such effect for the β rays of radium. The
production of secondary rays by the β rays of radium is, however,
readily shown by the photographic method. We have already seen (section
93) that very low velocity electrons accompany the α rays from radium or
radio-tellurium spread on a metal plate. These electrons are probably
liberated when the α rays escape from or impinge upon matter, and the
number emitted depends upon the kind of matter used as a screen. The
differences shown in the above table when the screens were interchanged
are explained simply in this way.

               Screens employed     Thickness  Current
                                    in mms.    observed


               Aluminium            0·01

               Cardboard            0·005      17·9


               Cardboard            0·005

               Aluminium            0·01        6·7


               Aluminium            0·01

               Tin                  0·005     150


               Tin                  0·005

               Aluminium            0·01      126


               Tin                  0·005

               Cardboard            0·005      13·9


               Cardboard            0·005

               Tin                  0·005      4·4

[Illustration: Fig. 45.]


=111. Secondary rays produced by= β =and= γ =rays=. An examination of
the amount and character of the secondary radiation emitted by various
substances, when exposed to the β and γ rays of radium, has recently
been made by A. S. Eve[179]. The general experimental method employed is
shown in Fig. 45.

The electroscope (Fig. 45) was placed behind a lead screen 4·5 cms.
thick, which stopped all the β rays and absorbed the greater proportion
of the γ rays from the radium tube placed at _R_. On bringing near a
plate of matter _M_, the primary rays fell upon it and some of the
secondary rays, emitted in all directions, passed into the side of the
electroscope, which was covered with aluminium foil of thickness ·05 mm.
Before the plate _M_ was placed in position the rate of discharge of the
electroscope was due to the natural leak and the γ rays from _R_, and
the secondary radiation from the air. On bringing the radiator _M_ into
position, the rate of discharge was much increased, and the difference
between the rate of movement of the gold-leaf in the two cases was taken
as a measure of the amount of secondary rays from _M_. The absorption of
the secondary rays was tested by placing an aluminium plate ·85 mm.
thick before the face of the electroscope.

The secondary rays were found to be fairly homogeneous, for the
intensity fell off according to an exponential law with the distance
traversed. The value of the absorption constant λ was determined from
the usual equation

                  $$ \frac {I} {I₀} = e^{–λ d} $$,

where _d_ is the thickness of the screen. The table given below shows
the results obtained when thick plates of different substances of the
same dimensions were placed in a definite position at _M_. The secondary
radiation from fluids was obtained by a slight alteration of the
experimental arrangements.

Thirty milligrammes of radium bromide were used, and the results are
expressed in terms of the number of scale divisions passed over per
second by the gold-leaf.

It will be noticed that the amount of secondary radiation follows in
most cases the same order as the densities, and is greatest for mercury.
The value of (secondary radiation)/density is not a constant, but varies
considerably, being greatest for light substances. The absorption
constant of the secondary rays from different radiators is not very
different, with the exception of substances such as granite, brick, and
cement, which give out secondary rays of nearly twice the penetrating
power of other substances.

β _and_ γ _rays_.

           Radiator      Density Secondary     Sec. Aluminium
                                 Radiation   Rad. / ·085 cm.
                                            Density λ


           Mercury          13·6       147     10·8

           Lead             11·4       141     12·4  18·5

           Copper            8·8        79      9·0  20

           Brass             8·4        81      9·6  21

           Iron              7·8        75      9·6  20
           (wrought)

           Tin               7·4        73      9·9  20·3

           Zinc              7·0        79     11·3

           Granite           2·7        54     20·0  12·4

           Slate             2·6        53     20·4  12·1

           Aluminium         2·6        42     16·1  24

           Glass             2·5        44     17·6  24

           Cement            2·4        47     19·6  13·5

           Brick             2·2        49     22·3  13·0

           Ebonite           1·1        32     29·1  26

           Water             1·0        24     24·0  21

           Ice               ·92        26     28·2

           Paraffin           ·9        17     18·8  21
           solid

           „    liquid       ·85        16     18·8

           Mahogany          ·56      21·4     38·2  23

           Paper             ·4?      21·0     52    22

           Millboard         ·4?      19·4     48    20·5

           Papier-mâché      ...      21·9

           Basswood          ·36      20·7     57    22

           Pine              ·35      21·8     62    21


           X ray screen               75·2           23·6

The secondary radiation not only comes from the surface of the radiator
but from a considerable depth. The amount of secondary rays increases
with the thickness of the radiator, and, in the case of glass and
aluminium, reaches a practical maximum for a plate about 3 mms. thick.

In the above table, the secondary radiation arises from both the β rays
and γ rays together. When the β rays were cut off by a layer of lead 6·3
mms. thick, placed between the radium and the radiator, the effect on
the electroscope was reduced to less than 20 per cent. of its former
value, showing that the β rays supplied more than 80 per cent. of the
secondary radiation. The following table shows the relative amount of
secondary rays from different substances when exposed to β and γ rays
together and to γ rays alone. The amount from lead in each case is taken
as a standard and equal to 100. The amount of secondary radiation found
by Townsend from soft X rays is added for comparison.

_Secondary Radiations._

                               β and     γ Röntgen
                     Radiator      γ  rays
                                rays


                     Lead        100   100     100

                     Copper       57    61     291

                     Brass        58    59     263

                     Zinc         57   ...     282

                     Aluminium    30    30      25

                     Glass        31    35      31

                     Paraffin     12    20     125

It will be observed that the relative amounts are about the same for the
γ rays alone as for the β and γ rays together. On the other hand, the
amount of secondary radiation set up by X rays is very different, lead
for example giving much less than brass or copper. The secondary rays
from the γ rays alone are slightly less penetrating than for the β and γ
rays together, but are far more penetrating than the secondary radiation
from the X rays examined by Townsend.

The amount of secondary radiation set up by the β and γ rays is mainly
independent of the state of the surface of the radiator. About the same
amount is obtained from iron as from iron filings; from liquid as from
solid paraffin; and from ice as from water[180].

Becquerel has shown that the secondary rays set up by the β rays are
deflected by a magnet and consist of negatively charged particles
(electrons). It has been pointed out in section 52 that the cathode
rays are diffusely reflected from the metal on which they fall. These
secondary rays consist in part of electrons moving with about the
same velocity as the primary, and in part of some electrons with a much
slower speed. The secondary rays set up by the β rays of radium have on
an average less penetrating power than the primary rays, and
consequently less velocity than the primary rays. It must be remembered
that the β rays from radium are very complex, and consist of electrons
projected with a considerable range of velocities. The secondary rays
are, on an average, certainly more penetrating than the most easily
absorbed β rays emitted from radium, and probably move with a velocity
of about half that of light.

It is still uncertain whether the secondary rays are produced by the
action of the primary rays on matter, or whether they consist of a
portion of the primary rays whose direction of motion has been deflected
in their passage through matter, so that they emerge again with
diminished velocity from the surface.


=112. Magnetic deflection of secondary rays from γ rays=. It has been
seen that the secondary rays set up by the γ rays alone are very similar
in character to those caused by the β rays. This result was still
further confirmed by Eve, who showed that the secondary rays produced by
the γ rays are readily deflected by a magnetic field. The experimental
arrangement is shown in Fig. 46.

[Illustration: Fig. 46.]

A small electroscope was mounted on one side of a lead platform 1·2 cms.
thick, which rested on a lead cylinder 10 cms. high and 10 cms. in
diameter. The radium was placed at the bottom of a hole reaching to the
centre of the cylinder.

On applying a strong magnetic field, at right angles to the plane of the
paper, so as to bend the secondary rays from the platform towards the
electroscope, the rate of discharge was much increased. On reversing the
field, the effect was much diminished. Since the γ rays are not
themselves deflected by a magnetic field, this result shows that the
secondary radiation is quite different in character from the primary
rays, and consists of electrons projected with a velocity (deduced from
the penetrating power) of about half the velocity of light. We have
already pointed out that the emission of electrons from a substance
traversed by the rays will account sufficiently well for the charge
observed by Paschen, without the necessity of assuming that the γ rays
carry a negative charge of electricity.

The secondary radiation set up by Röntgen rays, like that due to the β
and γ rays, consists in part of electrons projected with considerable
velocity. These three types of rays seem about equally efficient in
causing the expulsion of electrons from the substance through which they
pass. We have seen that the X and γ rays are, in all probability,
electromagnetic pulses set up by the sudden starting or stopping of
electrons, and, since these rays in turn cause the removal of electrons,
the process appears to be reversible. Since the β rays pass through some
thickness of matter before their energy of motion is arrested, theory
would lead us to expect that a type of soft X rays should be generated
in the absorbing matter.




                                PART VI.


=113. Comparison of the ionization produced by the α and β rays=. With
unscreened active material the ionization produced between two parallel
plates, placed as in Fig. 17, is mainly due to the α rays. On account of
the slight penetrating power of the α rays, the current due to them
practically reaches a maximum with a small thickness of radio-active
material. The following saturation currents were observed[181] for
different thicknesses of uranium oxide between parallel plates
sufficiently far apart for all the α rays to be absorbed in the gas
between them.

_Surface of uranium oxide 38 sq. cms._

               Weight of uranium    Saturation current
               oxide in grammes per in amperes per sq.
               sq. cm. of surface   cm. of surface

               ·0036                1·7 × 10⁻¹³

               ·0096                3·2 × 10⁻¹³

               ·0189                4·0 × 10⁻¹³

               ·0350                4·4 × 10⁻¹³

               ·0955                4·7 × 10⁻¹³

The current reached about half its maximum value for a weight of oxide
·0055 gr. per sq. cm. If the α rays are cut off by a metallic screen,
the ionization is then mainly due to the β rays, since the ionization
produced by the γ rays is small in comparison. For the β rays from
uranium oxide it has been shown (section 86) that the current reaches
half its maximum value for a thickness of 0·11 gr. per sq. cm.

Meyer and Schweidler[182] have found that the radiation from a water
solution of uranium nitrate is very nearly proportional to the amount of
uranium present in the solution.

On account of the difference in the penetrating power of the α and β
rays, the ratio of the ionization currents produced by them depends on
the thickness of the radio-active layer under examination. The following
comparative values of the current due to the α and β rays were obtained
for very thin layers of active matter[183]. A weight of ⅒ gramme of
fine powder, consisting of uranium oxide, thorium oxide, or radium
chloride of activity 2000, was spread as uniformly as possible over an
area of 80 sq. cms. The saturation current was observed between parallel
plates 5·7 cms. apart. This distance was sufficient to absorb most of
the α rays from the active substances. A layer of aluminium ·009 cm.
thick absorbed all the α rays.

                           Current    Current   Ratio of
                          due to α   due to β   currents
                              rays       rays        β/α


                Uranium          1          1      ·0074

                Thorium          1        ·27      ·0020

                Radium        2000       1350      ·0033

In the above table the saturation current due to the α and β rays of
uranium is, in each case, taken as unity. The third column gives the
ratio of the currents observed for equal weights of substance. The
results are only approximate in character, for the ionization due to a
given weight of substance depends on its fineness of division. In all
cases, the current due to the β rays is small compared with that due to
the α rays, being greatest for uranium and least for thorium. As the
thickness of layer increases, the ratio of currents β/α steadily
increases to a constant value.


=114. Comparison of the energy radiated by the α and β rays=. It has not
yet been found possible to measure directly the energy of the α and β
rays. A comparison of the energy radiated in the two forms of rays can,
however, be made indirectly by two distinct methods.

If it be assumed that the same amount of energy is required to produce
an ion by either the α or the β ray, and that the same proportion of the
total energy is used up in producing ions, an approximate estimate can
be made of the ratio of the energy radiated by the α and β rays by
measuring the ratio of the total number of ions produced by them. If λ
is the coefficient of absorption of the β rays in air, the rate of
production of ions per unit volume at a distance x from the source is

$$ q₀ e^{–λ x} $$

where _q₀_ is the rate of ionization at the source.

The total number of ions produced by complete absorption of the rays is

$$ \int₀^{\infty} q₀ e^{–λ x} dx = \frac {q₀} {λ} $$,

Now λ is difficult to measure experimentally for air, but an approximate
estimate can be made of its value from the known fact that the
absorption of β rays is approximately proportional to the density of any
given substance.

For β rays from uranium the value of λ for aluminium is about 14, and λ
divided by the density is 5·4. Taking the density of air as ·0012, we
find that for air

λ = ·0065.

The total number of ions produced in air is thus 154_q₀_ when the rays
are completely absorbed.

Now from the above table the ionization due to the β rays is ·0074 of
that produced by α rays, when the β rays passed through a distance of
5·7 cms. of air.

Thus we have approximately

        Total number of ions produced by β rays  ·0074
        ----------------------------------     = ----- × 154 = 0·20.
        Total number of ions produced by α rays   5·7

Therefore about ⅙ of the total energy radiated into air by a thin
layer of uranium is carried by the β rays or electrons. The ratio for
thorium is about ¹⁄₂₂ and for radium about ¹⁄₁₄, assuming the rays to
have about the same average value of λ.

This calculation takes into account only the energy which is radiated
out into the surrounding gas; but on account of the ease with which the
α rays are absorbed, even with a thin layer, the greater proportion of
the radiation is _absorbed by the radio-active substance itself_. This
is seen to be the case when it is recalled that the α radiation of
thorium or radium is reduced to half value after passing through a
thickness of about 0·0005 cm. of aluminium. Taking into consideration
the great density of the radio-active substances, it is probable that
most of the radiation which escapes into the air is due to a thin skin
of the powder not much more than ·0001 cm. in thickness.

                  *       *       *       *       *

An estimate, however, of the relative rate of emission of energy by the
α and β rays from a thick layer of material can be made in the following
way:—For simplicity suppose a thick layer of radio-active substance
spread uniformly over a large plane area. There seems to be no doubt
that the radiations are emitted uniformly from each portion of the mass;
consequently, the radiation, which produces the ionizing action in the
gas above the radio-active layer, is the sum total of all the radiation
which reaches the surface of the layer.

                  *       *       *       *       *

Let λ₁ be the average coefficient of absorption of the α rays in the
radio-active _substance itself_ and σ the specific gravity of the
substance. Let _E₁_ be the _total_ energy radiated per sec. per unit
mass of the substance when the absorption of the rays in the substance
itself is disregarded. The energy per sec. radiated to the upper surface
by a thickness _dx_ of a layer of unit area at a distance _x_ from the
surface is given by

$$ \frac {1} {2} E_1 \sigma e^{–λ_1 x} dx $$ .

The total energy _W₁_ per unit area radiated to the surface per sec. by
a thickness _d_ is given by

    $$ W_1 = \frac {1}{2} \int₀^d E_1 \sigma e^{–λ_1 x} dx $$
    $$     = \frac {E_1 \sigma} {2 λ_1} (1 − e^{–λ_1 d}) $$
    $$     = \frac {E_1 \sigma} {2 λ_1} $$

if λ₁_d_ is large.

                  *       *       *       *       *

In a similar way it may be shown that the energy _W₂_ of the β rays
reaching the surface is given by

$$ W_2 = \frac {E_2 \sigma} {2 λ_2} $$

where _E₂_ and λ₂ are the values for the β rays corresponding to _E₁_
and λ₁ for the α rays. Thus it follows that

                            _E₁_        λ₁_W₁_
                            ----    = ------
                            _E₂_        λ₂_W₂_

λ₁ and λ₂ are difficult to determine directly for the radio-active
substance itself, but it is probable that the ratio λ₁/λ₂ is not very
different from the ratio for the absorption coefficients for another
substance like aluminium. This follows from the general result that the
absorption of both α and β rays is proportional to the density of the
substance; for it has already been shown in the case of the β rays from
uranium that the absorption of the rays in the radio-active material is
about the same as for non-radio-active matter of the same density.

With a thick layer of uranium oxide spread over an area of 22 sq. cms.,
it was found that the saturation current between parallel plates 6·1
cms. apart, due to the α rays, was 12·7 times as great as the current
due to the β rays. Since the α rays were entirely absorbed between the
plates and the total ionization produced by the β rays is 154 times the
value at the surface of the plates,

                  _W₁_         total number of ions due to α rays
                  -----     = ----------------------------------
                  _W₂_         total number of ions due to β rays

                       12·7 × 6·1
                      = ------   = 0·5 approximately.
                         154

Now the value of λ₁ for aluminium is 2740 and of λ₂ for the same metal
14, thus

             _E₁_            λ₁_W₁_
             -----      = ----------        = 100 approximately
             _E₂_            λ₂_W₂_

This shows that the energy radiated from a thick layer of material by
the β rays is only about 1 per cent. of the energy radiated in the form
of α rays.

This estimate is confirmed by calculations based on independent data.
Let _m₁_, _m₂_ be the masses of the α and β particles respectively and
_v₁_, _v₂_ their velocities.

  $$ \frac {Energy of one \alpha particle} {Energy of one \beta particle}
     = \frac {m_1 v_1^2} {m_2 v_2^2} $$
  $$ = \frac {\frac {m_1} {e} v_1^2} {\frac {m^2} {e} v_2^2} $$ .

Now it has been shown that for the α rays of radium

                             _v₁_ = 2·5 × 10⁹,

                             _e_
                              ----   = 6 × 10³.
                             _m₁_

The velocity of the β rays of radium varies between wide limits. Taking
for an average value

                            _v₂_ =  1·5 × 10¹⁰,

                            _e_
                            ----   = 1·8 × 10⁷,
                            _m₂_

it follows that the energy of the α particle from radium is almost 83
times the energy of the β particle. If equal numbers of α and β
particles are projected per second, the total energy radiated in the
form of α rays is about 83 times the amount in the form of β rays.

Evidence will be given later (section 253) to show that the number of α
particles projected is probably four times the number of β particles; so
that a still greater proportion of the energy is emitted in the form of
α rays. These results thus lead to the conclusion that, from the point
of view of the energy emitted, the α rays are far more important than
the β rays. This conclusion is supported by other evidence which is
discussed in chapters XII and XIII, where it will be shown that the α
rays play by far the most important part in the changes occurring in
radio-active bodies, and that the β rays only appear in the latter
stages of the radio-active processes. From data based on the relative
absorption and ionization of the β and γ rays in air, it can be shown
that the γ rays carry off about the same amount of energy as the β rays.
These conclusions are confirmed by direct measurement of the heating
effect of radium, which is discussed in detail in chapter XII.

Footnote 111:

  In an examination of uranium the writer (_Phil. Mag._ p. 116, Jan.
  1899) found that the rays from uranium consist of two kinds, differing
  greatly in penetrating power, which were called the α and β rays.
  Later, it was found that similar types of rays were emitted by thorium
  and radium. On the discovery that very penetrating rays were given out
  by uranium and thorium as well as by radium, the term γ was applied to
  them by the writer. The word “ray” has been retained in this work,
  although it is now settled that the α and β rays consist of particles
  projected with great velocity. The term is thus used in the same sense
  as by Newton, who applied it in the _Principia_ to the stream of
  corpuscles which he believed to be responsible for the phenomenon of
  light. In some recent papers, the α and β rays have been called the α
  and β “emanations.” This nomenclature cannot fail to lead to
  confusion, since the term “radio-active emanation” has already been
  generally adopted in radio-activity as applying to the material
  substance which gradually _diffuses_ from thorium and radium
  compounds, and itself emits rays.

Footnote 112:

  This method of illustration is due to Mme Curie, _Thèse présentée à la
  Faculté des Sciences de Paris_, 1903.

Footnote 113:

  Giesel, _Annal. d. Phys._ 69, p. 834, 1899.

Footnote 114:

  Meyer and Schweidler, _Phys. Zeit._ 1, pp. 90, 113, 1899.

Footnote 115:

  Becquerel, _C. R._ 129, pp. 997, 1205, 1899.

Footnote 116:

  Curie, _C. R._ 130, p. 73, 1900.

Footnote 117:

  Rutherford, _Phil. Mag._ January, 1899.

Footnote 118:

  Rutherford and Grier, _Phil. Mag._ September, 1902.

Footnote 119:

  Becquerel, _C. R._ 130, pp. 206, 372, 810, 979. 1900.

Footnote 120:

  M. and Mme Curie, _C. R._ 130, p. 647, 1900.

Footnote 121:

  The activity of the radium preparation was not stated in the paper.

Footnote 122:

  Dorn, _Phys. Zeit._ 4, No. 18, p. 507, 1903.

Footnote 123:

  Strutt, _Phil. Mag._ Nov. 1903.

Footnote 124:

  Wien, _Phys. Zeit._ 4, No. 23, p. 624, 1903.

Footnote 125:

  Dorn, _C. R._ 130, p. 1129, 1900.

Footnote 126:

  Becquerel, _C. R._ 130, p. 809, 1900.

Footnote 127:

  Kaufmann, _Phys. Zeit._ 4, No. 1 b, p. 54, 1902.

Footnote 128:

  Abraham, _Phys. Zeit._ 4, No. 1 b, p. 57, 1902.

Footnote 129:

  Kaufmann, _Nachrichten d. Ges. d. Wiss. zu Gött._, Nov. 8, 1901.

Footnote 130:

  Simon, _Annal. d. Phys._ p. 589, 1899.

Footnote 131:

  Kaufmann, _Phys. Zeit._ 4, No. 1 b, p. 54, 1902.

Footnote 132:

  Paschen, _Annal. d. Phys._ 14, p. 389, 1904.

Footnote 133:

  Meyer and Schweidler, _Phys. Zeit._ pp. 90, 113, 209, 1900.

Footnote 134:

  Lenard, _Annal. d. Phys._ 56, p. 275, 1895.

Footnote 135:

  Strutt, _Nature_, p. 539, 1900.

Footnote 136:

  Seitz, _Phys. Zeit._ 5, No. 14, p. 395, 1904.

Footnote 137:

  It is presumed that the results were corrected, if necessary, for the
  discharging action due to the ionized gas, although no direct mention
  of this is made in the paper by Seitz.

Footnote 138:

  Strutt, _Phil. Trans._ A, p. 507, 1901.

Footnote 139:

  Crookes, _Proc. Roy. Soc._ 1902. _Chem. News_, 85, p. 109, 1902.

Footnote 140:

  Mme Curie, _C. R._ 130, p. 76, 1900.

Footnote 141:

  Rutherford, _Phil. Mag._ Feb. 1903. _Phys. Zeit._ 4, p. 235, 1902.

Footnote 142:

  Becquerel, _C. R._ 136, p. 199, 1903.

Footnote 143:

  Becquerel, _C. R._ 136, p. 431, 1903.

Footnote 144:

  Des Coudres, _Phys. Zeit._ 4, No. 17, p. 483, 1903.

Footnote 145:

  Becquerel, _C. R._ 136, p. 1517, 1903.

Footnote 146:

  Bragg, _Phil. Mag._ Dec. 1904; Bragg and Kleeman, _Phil. Mag._ Dec.
  1904.

Footnote 147:

  Further experimental results bearing on this important question are
  given in an Appendix to this book.

Footnote 148:

  Bakerian Lecture, _Phil. Trans._ A, p. 169, 1904.

Footnote 149:

  Strutt, _Phil. Mag._ Aug. 1904.

Footnote 150:

  J. J. Thomson, _Proc. Camb. Phil._ Soc. 13, Pt. I. p. 39, 1905.
  _Nature_, Dec. 15, 1904.

Footnote 151:

  Rutherford, _Nature_, March 2, 1905. J. J. Thomson, _Nature_, March 9,
  1905.

Footnote 152:

  Crookes, _Proc. Roy. Soc._ 81, p. 405, 1903.

Footnote 153:

  Elster and Geitel, _Phys. Zeit._ No. 15, p. 437, 1903.

Footnote 154:

  Glew, _Arch. Röntgen Ray_, June 1904.

Footnote 155:

  Becquerel, _C. R._ 137, Oct. 27, 1903.

Footnote 156:

  Tommasina, _C. R._ 137, Nov. 9, 1903.

Footnote 157:

  An interesting side-light is thrown on this question by the
  experiments described in Appendix A of this book.

Footnote 158:

  Rutherford and Miss Brooks, _Phil. Mag._ July 1902.

Footnote 159:

  In order to obtain a thin layer, the compound to be tested is ground
  to a fine powder and then sifted through a fine gauge uniformly over
  the area, so that the plate is only partially covered.

Footnote 160:

  Rutherford, _Phil. Mag._ Jan. 1899.

Footnote 161:

  Owens, _Phil. Mag._ Oct. 1899.

Footnote 162:

  Rutherford and Miss Brooks, _Phil. Mag._ July, 1900.

Footnote 163:

  Since the ionization at any point above the plate is the resultant
  effect of the α particles coming from all points of the large
  radio-active layer, λ is not the same as the coefficient of absorption
  of the rays from a point source. It will however be proportional to
  it. For this reason λ is called the “absorption constant.”

Footnote 164:

  Townsend, _Phil. Mag._ Feb. 1901.

Footnote 165:

  Durack, _Phil. Mag._ July 1902, May 1903.

Footnote 166:

  Bragg and Bragg and Kleeman, _Phil. Mag._ Dec. 1904.

Footnote 167:

  Villard, _C. R._ 130, pp. 1010, 1178, 1900.

Footnote 168:

  Becquerel, _C. R._ 130, p. 1154, 1900.

Footnote 169:

  Rutherford, _Phys. Zeit._ 3, p. 517, 1902.

Footnote 170:

  McClelland, _Phil. Mag._ July 1904.

Footnote 171:

  Paschen, _Phys. Zeit._ 5, No. 18, p. 563, 1904.

Footnote 172:

  A. S. Eve, _Phil. Mag._ Nov. 1904.

Footnote 173:

  Paschen, _Annal. d. Physik_, 14, p. 114, 1904; 14, 2, p. 389, 1904.
  _Phys. Zeit._ 5, No. 18, p. 563, 1904.

Footnote 174:

  Paschen, _Phys. Zeit._ 5, No. 18, p. 563, 1904.

Footnote 175:

  Rutherford and Barnes, _Phil. Mag._ May 1905. _Nature_, p. 151, Dec.
  15, 1904.

Footnote 176:

  Barkla, _Nature_, March 17, 1904.

Footnote 177:

  Becquerel, _C.R._ 132, pp. 371, 734, 1286. 1901.

Footnote 178:

  Mme Curie, _Thèse présentée à la Faculté des Sciences_, Paris 1903, p.
  85.

Footnote 179:

  A. S. Eve, _Phil. Mag._ Dec. 1904.

Footnote 180:

  In a recent paper (_Phil. Mag._ Feb. 1905), McClelland has, in the
  main, confirmed the experimental results obtained by Eve. An
  electrometer was used instead of an electroscope. He finds, in
  addition, that the amount of secondary radiation depends on the angle
  of incidence of the primary rays, and is greatest for an angle of 45°.
  In a letter to _Nature_ (Feb. 23, p. 390, 1905), he states that more
  recent experiments have shown that the amount of secondary radiation
  from different substances is a function of their atomic weights rather
  than of their densities. In every case examined, the amount of
  secondary radiation increases with the atomic weight, but is not
  proportional to it.

Footnote 181:

  Rutherford and McClung, _Phil. Trans._ A. p. 25, 1901.

Footnote 182:

  Meyer and Schweidler, _Wien Ber._ 113, July, 1904.

Footnote 183:

  Rutherford and Grier, _Phil. Mag._ Sept. 1902.




                               CHAPTER V.
                     PROPERTIES OF THE RADIATIONS.


=115.= Besides their power of acting on a photographic plate, and of
ionizing gases, the radiations from active bodies are able to produce
marked chemical and physical actions in various substances. Most of
these effects are due either to the α or β rays. The γ rays produce
little effect in comparison. Since the β rays are similar in all
respects to high velocity cathode rays, it is to be expected that they
will produce effects similar in character to those produced by the
cathode rays in a vacuum tube.


                         Phosphorescent action.


Becquerel[184] has studied the action of radium rays in producing
phosphorescence in various bodies. The substance to be tested was placed
above the radium in the form of powder on a very thin mica plate.
Examination was made of the sulphides of calcium and strontium, ruby,
diamond, varieties of spar, phosphorus and hexagonal blende. Substances
like the ruby and spar, which phosphoresce under luminous rays, did not
phosphoresce under the radium rays. On the other hand, those which were
made luminous by ultra-violet light were also luminous under the action
of radium rays. The radium rays show distinct differences from X rays.
For example, a diamond which was very luminous with radium rays was
unaffected by X rays. The double sulphate of uranium and potassium is
more luminous than hexagonal blende under X rays, but the reverse is
true for radium rays; under the influence of these rays, sulphide of
calcium gave a blue luminosity but was hardly affected by X rays.

The following table shows the relative phosphorescence excited in
various bodies.

          Substance            Without screen.   Across screen
                                     Intensity of black paper


          Hexagonal blende               13·36             ·04

          Platino-cyanide of              1·99             ·05
          barium

          Diamond                         1·14             ·01

          Double sulphate of              1·00             ·31
          Uranium and
          Potassium

          Calcium Fluoride                 ·30             ·02

In the last column the intensity without the screen is in each case
taken as unity. The great diminution of intensity after the rays have
passed through black paper shows that most of the phosphorescence
developed without the screen is, in the majority of cases, due to the α
rays.

Bary[185] has made a very complete examination of the class of
substances which become luminous under radium rays. He found that the
great majority of substances belong to the alkali metals and alkaline
earths. All these substances were also phosphorescent under the action
of X rays.

Crystalline zinc sulphide (Sidot’s blende) phosphoresces very brightly
under the influence of the rays from radium and other very active
substances. This was observed by Curie and Debierne in their study of
the radium emanation and the excited activity produced by it. It has
also been largely used by Giesel as an optical means of detecting the
presence of emanations from very active substances. It is an especially
sensitive means of detecting the presence of α rays, when it exhibits
the “scintillating” property already discussed in section 96. In order
to show the luminosity due to the α rays, the screen should be held
close to the active substance, as the rays are absorbed in their passage
through a few centimetres of air. Zinc sulphide is also luminous under
the action of the β rays, but the phosphorescence is far more persistent
than when produced by the α rays.

Very beautiful luminous effects are produced by large crystals of the
platinocyanides exposed to the radium rays. Those containing lithium
give a brilliant pink colour. The calcium and barium salts fluoresce
with a deep green light, and the sodium compound with a lemon yellow.
The mineral willemite (zinc silicate) was recently found by Kunz to be
an even more sensitive means of detecting the presence of the radiations
than platinocyanide of barium. It fluoresces showing a beautiful
greenish colour, and a piece of mineral exposed to the action of the
rays appears quite translucent. The crystals of the platinocyanides of
barium and lithium are especially suited for showing the action of the γ
rays, and, in this respect, are superior to willemite.

A very striking effect is shown by the mineral kunzite—a new variety of
spodumene discovered by Kunz[186]. This is a transparent gem like
crystal, often of very large size, which glows with a beautiful reddish
colour under the action of the β or γ rays, but does not appear to be
sensitive to the α rays. The luminosity extends throughout the crystal,
but is not so marked as in the platinocyanides or willemite. The mineral
sparteite[187], a form of calcite containing a few per cent. of
manganese, has been found by Ambrecht to fluoresce with a very deep
orange light under the β and γ rays. The colour appears to depend on the
intensity of the rays, and is deeper close to the radium than at some
distance away.

If kunzite and sparteite are exposed to the action of the cathode rays
in a vacuum tube, the colour is different from that produced by the
radium rays. The former appears a deep yellow, instead of the deep red
observed with the radium rays.

The different actions of the radium rays on these fluorescent substances
can be illustrated very simply and beautifully by the following
experiment. A small U tube is filled with fragments of the fluorescent
substance arranged in layers. The U tube is immersed in liquid air and
the emanation from about 30 mgrs. of radium bromide is condensed in the
tube. On closing the tube and removing it from the liquid air, the
emanation distributes itself uniformly in the tube. The shades of colour
produced in the different substances are clearly seen.

It is observed that all the crystals increase in luminosity for several
hours, on account of the excited activity produced by the emanation.
This effect is especially observed in kunzite, which at first hardly
responds to the rays, since the β and γ rays, which causes it to
fluoresce, are not given out by the emanation itself but by one of its
later products. The intensity of the β and γ rays is, in consequence,
small at first but rises to a maximum after several hours; the
luminosity observed varies in a corresponding manner.

Sir William Crookes[188] has made an examination of the effect of
continued exposure of a diamond to the radium rays. An “off-colour”
diamond, of a pale yellow colour, was placed inside a tube with radium
bromide. After 78 days’ exposure, the diamond had darkened and become
bluish green in tint; when heated at 50° in a mixture of potassium
chlorate for ten days, the diamond lost its dull surface colour and was
bright and transparent, and its tint had changed to a pale bluish green.
The rays have thus a double action on the diamond; the less penetrating
β rays produce a superficial darkening due to the change of the surface
into graphite, while the more penetrating β rays and the γ rays produce
a change of colour throughout its mass. The diamond phosphoresced
brightly during the whole course of its exposure to the rays. Crookes
also observed that the diamond still retained enough activity to affect
a photographic plate 35 days after removal, although, during the period
of 10 days, it was heated in a mixture sufficiently powerful to remove
the outer skin of graphite. This residual activity may possibly be due
to a slow transformation product of the emanation which is deposited on
the surface of bodies (see chapter XI).

Marckwald observed that the α rays from radio-tellurium produced marked
phosphorescence on some kinds of diamonds. An account of the various
luminous effects produced on different gems by exposure to the radium
and actinium rays has been given by Kunz and Baskerville[189].

Both zinc sulphide and platinocyanide of barium diminish in luminosity
after exposure for some time to the action of the rays. To regenerate a
screen of the latter, exposure to solar light is necessary. A similar
phenomenon has been observed by Villard for a screen exposed to Röntgen
rays. Giesel made a screen of platinocyanide of radio-active barium.
The screen, very luminous at first, gradually turned brown in colour,
and at the same time the crystals became dichroic. In this condition the
luminosity was much less, although the active substance had increased in
activity after preparation. Many of the substances which are luminous
under the rays from active substances lose this property to a large
extent at low temperatures[190].


=116. Luminosity of radium compounds.= All radium compounds are
spontaneously luminous. This luminosity is especially brilliant in the
dry haloid salts, and persists for long intervals of time. In damp air
the salts lose a large amount of their luminosity, but they recover it
on drying. With very active radium chloride, the Curies have observed
that the light changes in colour and intensity with time. The original
luminosity is recovered if the salt is dissolved and dried. Many
inactive preparations of radiferous barium are strongly luminous. The
writer has seen a preparation of impure radium bromide which gave out a
light sufficient to read by in a dark room. The luminosity of radium
persists over a wide range of temperature and is as bright at the
temperature of liquid air as at ordinary temperatures. A slight
luminosity is observed in a solution of radium, and if crystals are
being formed in the solution, they can be clearly distinguished in the
liquid by their greater luminosity.


=117. Spectrum of the phosphorescent light of radium and actinium.=
Compounds of radium, with a large admixture of barium, are usually
strongly self-luminous. This luminosity decreases with increasing
purity, and pure radium bromide is only very feebly self-luminous. A
spectroscopic examination of the slight phosphorescent light of pure
radium bromide has been made by Sir William and Lady Huggins[191]. On
viewing the light with a direct vision spectroscope, there were faint
indications of a variation of luminosity at different points along the
spectrum. In order to get a photograph of the spectrum within a
reasonable time, they made use of a quartz spectroscope of special
design which had been previously employed in a spectroscopic examination
of faint celestial objects. After three days’ exposure with a slit of
¹⁄₄₅₀ of an inch in width, a negative was obtained which showed a number
of bright lines. The magnified spectrum is shown in Fig. 46 A. The lines
of this spectrum were found to agree not only in position but also in
relative intensity with the band spectrum of nitrogen. The band spectrum
of nitrogen and also the spark spectrum[192] of radium are shown in the
same figure.

Some time afterwards Sir William Crookes and Prof. Dewar showed that
this spectrum of nitrogen was not obtained if the radium was contained
in a highly exhausted tube. Thus it appears that the spectrum is due to
the action of the radium rays either on occluded nitrogen or the
nitrogen in the atmosphere surrounding the radium.

It is very remarkable that a phosphorescent light, like that of radium
bromide, should show a bright line spectrum of nitrogen. It shows that
radium at ordinary temperatures is able to set up radiations which are
produced only by the electric discharge under special conditions.

Sir William and Lady Huggins were led to examine the spectrum of the
natural phosphorescent light of radium with the hope that some
indications might be obtained thereby of the processes occurring in the
radium atom. Since the main radiation from radium consists of positively
charged atoms projected with great velocity, radiations must be set up
both in the expelled body and in the system from which it escapes.

[Illustration: Fig. 46a.]

Giesel[193] observed that the spectrum of the phosphorescent light of
actinium consists of three bright lines. Measurements of the wave length
were made by Hartmann[194]. The luminosity was very slight and a long
exposure was required. The lines observed were in the red, blue and
green. The wave length λ and velocity are shown below.

                 Line      Intensity    λ

                 1             10   4885·4 ± 0·1
                                    Ångström units

                 2              6   5300   ±   6       „

                 3              1   5909   ±  10       „

The line 4885 was very broad; the other two lines were so feeble that it
was difficult to determine their wave length with accuracy. Hartmann
suggests that these lines may be found in the spectrum of the new stars.
The lines observed have no connection with radium or its emanation[195].


=118. Thermo-luminescence.= E. Wiedemann and Schmidt[196] have shown
that certain bodies after exposure to the cathode rays or the
electric spark become luminous when they are heated to a temperature
much below that required to cause incandescence. This property of
thermo-luminescence is most strikingly exhibited in certain cases
where two salts, one of which is much in excess of the other, are
precipitated together. It is to be expected that such bodies would
also acquire the property when exposed to the β or cathodic rays of
radium. This has been found to be the case by Wiedemann[197].
Becquerel showed that fluor-spar, exposed to the radium rays, was
luminous when heated. The glass tubes in which radium is kept are
rapidly blackened. On heating the tube, a strong luminosity is
observed, and the coloration to a large extent disappears. The
peculiarity of many of these bodies lies in the fact that the
property of becoming luminous when heated is retained for a long
interval of time after the body is removed from the influence of the
exciting cause. It appears probable that the rays cause chemical
changes in these bodies, which are permanent until heat is applied.
A portion of the chemical energy is then released in the form of
visible light.


                           Physical actions.


=119. Some electric effects.= Radium rays have the same effect as
ultra-violet light and Röntgen rays in increasing the facility with
which a spark passes between electrodes. Elster and Geitel[198] showed
that if two electrodes were separated by a distance such that the spark
just refused to pass, on bringing near a specimen of radium the spark at
once passes. This effect is best shown with short sparks from a small
induction coil. The Curies have observed that radium completely
enveloped by a lead screen 1 cm. thick produces a similar action. The
effect in that case is due to the γ rays alone. This action of the rays
can be very simply illustrated by connecting two spark-gaps with the
induction coil in parallel. The spark-gap of one circuit is adjusted so
that the discharge just refuses to pass across it, but passes by the
other. When some radium is brought near the silent spark-gap, the spark
at once passes and ceases in the other[199].

Hemptinne[200] found that the electrodeless discharge in a vacuum tube
began at a higher pressure when a strong preparation of radium was
brought near the tube. In one experiment the discharge without the rays
began at 51 mms. but with the radium rays at 68 mms. The colour of the
discharge was also altered.

Himstedt[201] found that the resistance of selenium was diminished by
the action of radium rays in the same way as by ordinary light.

F. Henning[202] examined the electrical resistance of a barium chloride
solution containing radium of activity 1000, but could observe no
appreciable difference between it and a similar pure solution of barium
chloride. This experiment shows that the action of the rays from the
radium does not produce any appreciable change in the conductivity of
the barium solution.

Kohlrausch and Henning[203] have recently made a detailed examination of
the conductivity of pure radium bromide solutions, and have obtained
results very similar to those for the corresponding barium solutions.
Kohlrausch[204] found that the conductivity of water exposed to the
radiations from radium increased more rapidly than water which had not
been exposed. This increase of conductivity may have been due to an
increase of the conductivity of the water itself, or to an increased
rate of solution of the glass of the containing vessel.

Specimens of strongly active material have been employed to obtain the
potential at any point of the atmosphere. The ionization due to the
active substance is so intense that the body to which it is attached
rapidly takes up the potential of the air surrounding the active
substance. In this respect it is more convenient and rapid in its action
than the ordinary taper or water dropper, but on account of the
disturbance of the electric field by the strong ionization produced, it
is probably not so accurate a method as that of the water dropper.


=120. Effect on liquid and solid dielectrics.= P. Curie[205] made the
very important observation that liquid dielectrics became partial
conductors under the influence of radium rays. In these experiments the
radium, contained in a glass tube, was placed in an inner thin cylinder
of copper. This was surrounded by a concentric copper cylinder, and the
liquid to be examined filled the space between. A strong electric field
was applied, and the current through the liquid measured by means of an
electrometer.

The following numbers illustrate the results obtained:

               Substance                 Conductivity in
                                      megohms per 1 cm.³


               Carbon bisulphide              20 × 10⁻¹⁴

               Petroleum ether                   15    „

               Amyline                           14    „

               Carbon chloride                    8    „

               Benzene                            4    „

               Liquid air                         1·3  „

               Vaseline oil                       1·6  „

Liquid air, vaseline oil, petroleum ether, amyline, are normally nearly
perfect insulators. The conductivity of amyline and petroleum ether due
to the rays at −17° C. was only ⅒ of its value at 0° C. There is thus
a marked action of temperature on the conductivity. For very active
material the current was proportional to the voltage. With material of
only ¹⁄₅₀₀ of the activity, it was found that Ohm’s law was not obeyed.

The following numbers were obtained:

                              Volts    Current
                                 50        109
                                100        185
                                200        255
                                400        335

For an increase of voltage of 8 times, the current only increases about
3 times. The current in the liquid thus tends to become “saturated” as
does the ordinary ionization current through a gas. These results have
an important bearing on the ionization theory, and show that the
radiation probably produces ions in the liquid as well as in the gas. It
was also found that X rays increased the conductivity to about the same
extent as the radium rays.

Becquerel[206] has recently shown that solid paraffin exposed to the β
and γ rays of radium acquires the property of conducting electricity to
a slight extent. After removal of the radium the conductivity diminishes
with time according to the same law as for an ionized gas. These results
show that a solid as well as a liquid and gaseous dielectric is ionized
under the influence of radium rays.


=121. Effect of temperature on the radiations.= Becquerel[207], by the
electric method, determined the activity of uranium at the temperature
of liquid air, and found that it did not differ more than 1 per cent.
from the activity at ordinary temperatures. In his experiments, the α
rays from the uranium were absorbed before reaching the testing vessel,
and the electric current measured was due to the β rays alone. P.
Curie[208] found that the luminosity of radium and its power of exciting
fluorescence in bodies were retained at the temperature of liquid air.
Observations by the electric method showed that the activity of radium
was unaltered at the temperature of liquid air. If a radium compound is
heated in an open vessel, it is found that the activity, measured by the
α rays, falls to about 25 per cent. of its original value. This is
however not due to a change in the radio-activity, but to the release of
the radio-active emanation, which is stored in the radium. No alteration
is observed if the radium is heated in a closed vessel from which none
of the radio-active products are able to escape.


=122. Motion of radium in an electric field.= Joly[209] found that a
disc, one side of which is coated with a few milligrams of radium
bromide, exhibits, when an electrified body is brought near it, motions
very different to those observed in the case of an inactive substance.
The electrified body, whether positive or negative, repels the suspended
body if brought up to it on the side coated with radium, but attracts it
if presented to the naked side.

This effect is very simply shown by constructing a small apparatus like
a radiometer. Two covered glasses are attached to the end of a glass
fibre about 6 cms. long, the surfaces lying in the same plane. The
apparatus is free to rotate on a pivot. The two vanes are coated on
alternate faces with radium bromide, and the whole apparatus contained
within a glass receiver. If an electrified rod of ebonite or sealing wax
is brought up close to the receiver, a rotation is communicated to the
vane which increases as the pressure of the air is lowered to 5 or 6
cms. of mercury. By placing the apparatus between parallel plates
connected with the terminals of a Wimshurst machine, a steady rotation
is communicated to the vanes. The rotation is always in such a direction
that the radium coated surface is repelled from the electrified body.

This action was examined still further by attaching the vanes to the
glass beam of a Coulomb’s balance. A metal sphere, which could be
charged from without, was fixed facing the side coated with radium. A
repulsion was always observed except when the charge was very strong and
the vane near the sphere. If, however, the two vanes were connected by a
light wire and a similar sphere placed exactly opposite the other, an
attraction was observed if one sphere was charged, but a repulsion if
both were charged. These effects were observed whether the vanes were of
aluminium or glass.

Joly found that the effect could not be explained by any direct action
due to the movement of the ions in an electric field. The recoil, due to
the expulsion of α particles from one side of the vane, is far too small
to account for the movement observed.

This effect can, I think, be simply accounted for by taking into
consideration the difference in conductivity of the gas on the two sides
of the radium coated vane. If a small vane, coated uniformly with radium
on both sides, and mounted on an insulating support, be brought near a
charged body kept at a constant potential, it acts like a water dropper
and rapidly acquires very nearly the average potential which existed at
that point before the vane was brought up. The mechanical force acting
on the vane will, in consequence, be small. If, however, the vane is
only coated with radium on the side near the charged body, the
ionization and consequently the conductivity of the gas is much greater
between the vane and the charged body than on the opposite side.
Suppose, for simplicity, the body is charged to a positive potential. On
account of the greater conductivity of the gas on the side facing the
charged body, it will rapidly acquire a positive charge, and the
potential of the vane will reach a higher value than existed at that
place before the vane was introduced. This will result in a repulsion of
the vane. This also accounts for the attraction observed in the
experiment with the Coulomb’s balance already referred to. Suppose that
one sphere is positively charged and the other earthed, and the two
vanes metallically connected together. The vane next to the charged body
will become charged positively, but this charge will be dissipated
rapidly on account of the ionization of the gas close to the opposite
vane, and, in most conditions, this loss of charge will be so rapid that
the potential of the vane is unable to reach the value which would exist
at that place in the field, if the vane were removed. There will, in
consequence, be an attracting force acting on the vane towards the
sphere.

The repulsion observed by Joly is thus only an indirect result of the
ionization in the gas produced by the radium, and should be shown under
conditions where similar unequal distribution of ionization is produced
by any other sources.

Since radium gives out heat at a fairly rapid rate, a radiometer in
which the vanes were coated on one side with radium instead of
lampblack, should rotate at low pressure of the gas, even if no source
of light is brought near it. This should evidently be the case, since
the face coated with radium should reach a slightly higher temperature
than the other. This experiment has been tried, but the effect seems too
small to produce rotation of the vanes.


                           Chemical actions.


=123.= Rays from active radium preparations change oxygen into
ozone[210]. Its presence can be detected by the smell or by the action
on iodide of potassium paper. This effect is due to the α and β rays
from the radium, and not to the luminous rays from it. Since energy is
required to produce ozone from oxygen, this must be derived from the
energy of the radiations.

The Curies found that radium compounds rapidly produced coloration in
glass. For moderately active material the colour is violet, for more
active material it is yellow. Long continued action blackens the glass,
although the glass may have no lead in its composition. This coloration
gradually extends through the glass, and is dependent to some extent on
the kind of glass used.

Giesel[211] found that he could obtain as much coloration in rock-salt
and fluor-spar by radium rays, as by exposure to the action of cathode
rays in a vacuum tube. The coloration, however, extended much deeper
than that produced by the cathode rays. This is to be expected, since
the radium rays have a higher velocity, and consequently greater
penetrating power, than the cathode rays produced in an ordinary vacuum
tube. Goldstein observed that the coloration is far more intense and
rapid when the salts are melted or heated to a red heat. Melted
potassium sulphate, under the action of a very active preparation of
radium, was rapidly coloured a strong greenish blue which gradually
changed into a dark green. Salomonsen and Dreyer[212] found that plates
of quartz were coloured by exposure to radium rays. When examined
minutely, plates cut perpendicular to the optic axis showed the presence
of lines and striae, parallel to the binary axes. Adjacent portions of
the striated system differed considerably in intensity of coloration and
clearly revealed the heterogeneity of structures of the crystal.

The cause of these colorations by cathode and radium rays has been the
subject of much discussion. Elster and Geitel[213] observed that a
specimen of potassium sulphate, coloured green by radium rays, showed a
strong photo-electric action, _i.e._ it rapidly lost a negative charge
of electricity when exposed to the action of ultra-violet light. All
substances coloured by cathode rays show a strong photo-electric action,
and, since the metals sodium and potassium themselves show
photo-electric action to a very remarkable degree, Elster and Geitel
have suggested that the colorations are caused by a solid solution of
the metal in the salt.

Although the coloration due to radium rays extends deeper than that due
to the cathode rays, when exposed to light the colour fades away at
about the same rate in the two cases.

Becquerel[214] found that white phosphorus is changed into the red
variety by the action of radium rays. This action was shown to be due
mainly to the β rays. The secondary radiation set up by the primary rays
also produced a marked effect. Radium rays, like ordinary light rays,
also caused a precipitate of calomel in the presence of oxalic acid.

Hardy and Miss Willcock[215] found that a solution of iodoform in
chloroform turned purple after exposure for 5 minutes to the rays from 5
milligrams of radium bromide. This action is due to the liberation of
iodine. By testing the effect of screens of different thicknesses, over
the radium, this action was found to be mainly due to the β rays from
the radium. Röntgen rays produce a similar coloration.

Hardy[216] also observed an action of the radium rays on the coagulation
of globulin. Two solutions of globulin from ox serum were used, one made
electro-positive by adding acetic acid, and the other electro-negative
by adding ammonia. When the globulin was exposed close to the radium in
naked drops, the opalescence of the electro-positive solution rapidly
diminished, showing that the solution became more complete. The
electro-negative solution was rapidly turned to a jelly and became
opaque. These actions were found to be due to the α rays of radium
alone.

This is further evidence in favour of the view that the α rays consist
of projected positively charged bodies of atomic dimensions, for a
similar coagulation effect is produced by the metallic ions of liquid
electrolytes, and has been shown by W. C. D. Whetham[217] to be due to
the electric charges carried by the ions.


=124. Gases evolved from radium.= Curie and Debierne[218] observed that
radium preparations placed in a vacuum tube continually lowered the
vacuum. The gas evolved was always accompanied by the emanation, but no
new lines were observed in its spectrum. Giesel[219] has observed a
similar evolution of gas from solutions of radium bromide. Giesel
forwarded some active material to Runge and Bödlander, in order that
they might test the gas spectroscopically. From 1 gram of a 5 per cent.
radium preparation they obtained 3·5 c.c. of gas in 16 days. This gas
was found, however, to be mainly hydrogen, with 12 per cent. of oxygen.
In later experiments Ramsay and Soddy[220] found that 50 milligrams of
radium bromide evolved gases at the rate of about 0·5 c.c. per day. This
is a rate of evolution about twice that observed by Runge and Bödlander.
On analysing the gases about 28·9 per cent. consisted of oxygen, and the
rest hydrogen. The slight excess of hydrogen over that attained in the
decomposition of water, they consider to be due to the action of oxygen
on the grease of the stop-cocks. The radio-active emanation from radium
has a strong oxidizing action and rapidly produces carbon dioxide, if
carbonaceous matter is present. The production of gas is probably due to
the action of the radiations in decomposing water. The amount of energy
required to produce the rate of decomposition observed by Ramsay and
Soddy—about 10 c.c. per day for 1 gram of radium bromide—corresponds to
about 30 gram-calories per day. This amount of energy is about two per
cent. of the total energy emitted in the form of heat.

Ramsay and Soddy (_loc. cit._) have also observed the presence of helium
in the gases evolved by solution of radium bromide. This important
result is considered in detail in section 267.


                         Physiological actions.


=125.= Walkhoff first observed that radium rays produce burns of much
the same character as those caused by Röntgen rays. Experiments in this
direction have been made by Giesel, Curie and Becquerel, and others,
with very similar results. There is at first a painful irritation, then
inflammation sets in, which lasts from 10 to 20 days. This effect is
produced by all preparations of radium, and appears to be due mainly to
the α and β rays.

Care has to be taken in handling radium on account of the painful
inflammation set up by the rays. If a finger is held for some minutes at
the base of a capsule containing a radium preparation, the skin becomes
inflamed for about 15 days and then peels off. The painful feeling does
not disappear for two months.

Danysz[221] found that this action is mainly confined to the skin, and
does not extend to the underlying tissue. Caterpillars subjected to the
action of the rays lost their power of motion in several days and
finally died.

Radium rays have been found beneficial in certain cases of cancer. The
effect is apparently similar to that produced by Röntgen rays, but the
use of radium possesses the great advantage that the radiating source
can be enclosed in a fine tube and introduced at the particular point at
which the action of the rays is required. The rays have also been found
to hinder or stop the development of microbes[222].

It would be out of place here to give an account of the numerous
experiments that have been made by physicists and physiologists on the
action of the rays of radium and of other radio-active substances on
different organisms, such as caterpillars, mice and guinea-pigs. In some
cases, the experiments have been carried out by placing the organisms in
an atmosphere impregnated with the radium emanation. The effect of an
exposure under such conditions for several days or weeks has been found
generally harmful and in many cases fatal. The literature in this new
department of study is already large and is increasing rapidly.

Another interesting action of the radium rays has been observed by
Giesel. On bringing up a radium preparation to the closed eye, in a dark
room, a sensation of diffuse light is observed. This effect has been
examined by Himstedt and Nagel[223] who have shown that it is due to a
fluorescence produced by the rays in the eye itself. The blind are able
to perceive this luminosity if the retina is intact, but not if the
retina is diseased. Hardy and Anderson[224] have examined this effect in
some detail. The sensation of light is produced both by the β and γ
rays. The eyelid practically absorbs all the β rays, so that the
luminosity observed with a closed eye is due to the γ rays alone. The
lens and retina of the eye are strongly phosphorescent under the action
of the β and γ rays. Hardy and Anderson consider that the luminosity
observed in a dark room with the open eye (the phosphorescent light of
the radium itself being stopped by black paper) is to a large extent due
to the phosphorescence set up in the eyeball. The γ rays, for the most
part, produce the sensation of light when they strike the retina.

Tommasina stated that the air exhaled by man contained a larger
proportion of ions than ordinary air, and, in consequence, caused an
increased rate of discharge of an electroscope. The experiment was
repeated by Elster and Geitel but with negative results. On the other
hand, they found that the breath of Dr Giesel, of Braunschweig, who had
been engaged continuously in the chemical separation of the radio-active
bodies, caused a rapid loss of charge of an electroscope. This increased
rate of discharge was probably mainly due to the radium emanation, with
which his system had become impregnated by inhaling the emanation-laden
air of the laboratory.

Footnote 184:

  Becquerel, _C. R._ 129, p. 912, 1899.

Footnote 185:

  Bary, _C. R._ 130, p. 776, 1900.

Footnote 186:

  Kunz and Baskerville, _Amer. Journ. Science_ XVI. p. 335, 1903.

Footnote 187:

  See _Nature_, p. 523, March 31, 1904.

Footnote 188:

  Crookes, _Proc. Roy. Soc._ 74, p. 47, 1904.

Footnote 189:

  Kunz and Baskerville, _Science_ XVIII, p. 769, Dec. 18, 1903.

Footnote 190:

  Beilby in a recent communication to the Royal Society (Feb. 9 and 23,
  1905) has examined in some detail the production of phosphorescence by
  the β and γ rays of radium and has put forward a theory to account for
  the different actions observed.

Footnote 191:

  Huggins, _Proc. Roy. Soc._ 72, pp. 196 and 409, 1903.

Footnote 192:

  The spark spectrum of the radium bromide showed the _H_ and _K_ lines
  of calcium and also faintly some of the strong lines of barium. The
  characteristic lines of radium of wave-lengths 3814·59, 3649·7, 4340·6
  and 2708·6, as shown by Demarçay and others are clearly shown in the
  figure. The strong line of wave-length about 2814 is due to radium.

Footnote 193:

  Giesel, _Ber. d. D. Chem. Ges._ 37, p. 1696, 1904.

Footnote 194:

  Hartmann, _Phys. Zeit._ 5, No. 18, p. 570, 1904.

Footnote 195:

  In a recent paper, Giesel (_Ber. d. D. Chem. Ges._ No. 3, p. 775,
  1905) has shown that the bright lines are due to didymium, which is
  present as an impurity. Exposure of didymium to the radium rays also
  causes the appearance of the lines.

Footnote 196:

  Wiedemann and Schmidt, _Wied. Annal._ 59, p. 604, 1895.

Footnote 197:

  Wiedemann, _Phys. Zeit._ 2, p. 269, 1901.

Footnote 198:

  Elster and Geitel, _Annal. d. Phys._ 69, p. 673, 1899.

Footnote 199:

  Willows and Peck (_Phil. Mag._ March, 1905) found that under some
  conditions, especially for long sparks, the rays of radium hindered
  the passage of the spark.

Footnote 200:

  Hemptinne, _C. R._ 133, p. 934, 1901.

Footnote 201:

  Himstedt, _Phys. Zeit._ p. 476, 1900.

Footnote 202:

  Henning, _Annal. d. Phys._ p. 562, 1902.

Footnote 203:

  Kohlrausch and Henning, _Verh. Deutsch. Phys. Ges._ 6, p. 144, 1904.

Footnote 204:

  Kohlrausch, _Verh. Deutsch. Phys. Ges._ 5, p. 261, 1904.

Footnote 205:

  P. Curie, _C. R._ 134, p. 420, 1902.

Footnote 206:

  Becquerel, _C. R._ 136, p. 1173, 1903.

Footnote 207:

  Becquerel, _C. R._ 133, p. 199, 1901.

Footnote 208:

  P. Curie, Société de Physique, March 2, 1900.

Footnote 209:

  Joly, _Phil. Mag._ March, 1904.

Footnote 210:

  S. and P. Curie, _C. R._ 129, p. 823, 1899.

Footnote 211:

  Giesel, _Verhandlg. d. D. Phys. Ges._ Jan. 5, 1900.

Footnote 212:

  Salomonsen and Dreyer, _C. R._ 139, p. 533, 1904.

Footnote 213:

  Elster and Geitel, _Phys. Zeit._ p. 113, No. 3, 1902.

Footnote 214:

  Becquerel, _C. R._ 133, p. 709, 1901.

Footnote 215:

  Hardy and Miss Willcock, _Proc. Roy. Soc._ 72, p. 200, 1903.

Footnote 216:

  Hardy, _Proc. Physiolog. Soc._ May 16, 1903.

Footnote 217:

  Whetham, _Phil. Mag._ Nov. 1899; _Theory of Solution_, Camb. 1902, p.
  396.

Footnote 218:

  Curie and Debierne, _C. R._ 132, p. 768, 1901.

Footnote 219:

  Giesel, _Ber. D. d. Chem. Ges._ 35, p. 3605, 1902.

Footnote 220:

  Ramsay and Soddy, _Proc. Roy. Soc._ 72, p. 204, 1903.

Footnote 221:

  Danysz, _C. R._ 136, p. 461, 1903.

Footnote 222:

  Aschkinass and Caspari, _Arch. d. Ges. Physiologie_, 86, p. 603, 1901.

Footnote 223:

  Himstedt and Nagel, _Drude’s Annal._ 4, p. 537, 1901.

Footnote 224:

  Hardy and Anderson, _Proc. Roy. Soc._ 72, p. 393, 1903.




                              CHAPTER VI.
             CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER.


=126.= An account will now be given of some experiments which have
thrown much light, not only on the nature of the processes which serve
to maintain the radio-activity of the radio-active bodies, but also on
the source of the energy continuously emitted by those bodies. In this
chapter, for simplicity, the radio-activity of uranium and thorium will
alone be considered, for it will be seen later that the changes taking
place in these two substances are typical of those which occur in all
radio-active substances.

We have seen (section 23) that there is some doubt whether the
radio-activity of thorium is due to that element itself, or to an
unknown radio-active constituent associated with it. This uncertainty,
however, will present no serious difficulty when we are discussing the
radio-activity of thorium, for the general conclusions are, for the most
part, independent of whether thorium is the primary radio-active
constituent or not. For simplicity, however, it will be assumed for the
present that the radio-activity is due to thorium itself. If future
research should definitely show that the radio-activity, ordinarily
observed in thorium, is due to a new radio-active element mixed with it,
the radio-active processes considered will refer to this new element.


=127. Uranium X.= The experiments of Mme Curie show that the
radio-activity of uranium and radium is an atomic phenomenon. The
activity of any uranium compound depends only on the amount of that
element present, and is unaffected by its chemical combination with
other substances, and is not appreciably affected by wide variations of
temperature. It would thus seem probable, since the activity of uranium
is a specific property of the element, that the activity could not be
separated from it by chemical agencies.

In 1900, however, Sir William Crookes[225] showed that, by a single
chemical operation, uranium could be obtained photographically inactive
while the whole of the activity could be concentrated in a small residue
free from uranium. This residue, to which he gave the name of Ur X, was
many hundred times more active photographically, weight for weight, than
the uranium from which it had been separated. The method employed for
this separation was to precipitate a solution of the uranium with
ammonium carbonate. On dissolving the precipitate in an excess of the
reagent, a light precipitate remained behind. This was filtered, and
constituted the Ur X. The active substance Ur X was probably present in
very small quantity, mixed with impurities derived from the uranium. No
new lines were observed in its spectrum. A partial separation of the
activity of uranium was also effected by another method. Crystallized
uranium nitrate was dissolved in ether, when it was found that the
uranium divided itself between the ether and water present in two
unequal fractions. The small part dissolved in the water layer was found
to contain practically all the activity when examined by the
photographic method, while the other fraction was almost inactive. These
results, taken by themselves, pointed very strongly to the conclusion
that the activity of uranium was not due to the element itself, but to
some other substance, associated with it, which had distinct chemical
properties.

Results of a similar character were observed by Becquerel[226]. It was
found that barium could be made photographically very active by adding
barium chloride to the uranium solution and precipitating the barium as
sulphate. By a succession of precipitations the uranium was rendered
photographically almost inactive, while the barium was strongly active.

The inactive uranium and the active barium were laid aside; but, on
examining them a year later, it was found _that the uranium had
completely regained its activity, while that of the barium had
completely disappeared_. The loss of activity of uranium was thus only
temporary in character.

In the above experiments, the activity of uranium was examined by the
photographic method. The photographic action produced by uranium is due
almost entirely to the β rays. The α rays, in comparison, have little if
any effect. Now the radiation from Ur X consists entirely of β rays, and
is consequently photographically very active. If the activity of uranium
had been measured electrically without any screen over it, the current
observed would have been due very largely to the α rays, and little
change would have been observed after the removal of Ur X, since only
the constituent responsible for the β rays was removed. This important
point is discussed in more detail in section 205.


=128. Thorium X.= Rutherford and Soddy[227], working with thorium
compounds, found that an intensely active constituent could be separated
from thorium by a single chemical operation. If ammonia is added to a
thorium solution, the thorium is precipitated, but a large amount of the
activity is left behind in the filtrate, which is chemically free from
thorium. This filtrate was evaporated to dryness, and the ammonium salts
driven off by ignition. A small residue was obtained which, weight for
weight, was in some cases several thousand times more active than the
thorium from which it was obtained, while the activity of the
precipitated thorium was reduced to less than one half of its original
value. This active constituent was named Th X from analogy to Crookes’
Ur X.

The active residue was found to consist mainly of impurities from the
thorium; the Th X could not be examined chemically, and probably was
present only in minute quantity. It was also found that an active
constituent could be partly separated from thorium oxide by shaking it
with water for some time. On filtering the water, and evaporating down,
a very active residue was obtained which was analogous in all respects
to Th X.

On examining the products a month later, it was found that the _Th X was
no longer active, while the thorium had completely regained its
activity_. A long series of measurements was then undertaken to examine
the time-rate of these processes of decay and recovery of activity.

[Illustration: Fig. 47.]

The results are shown graphically in Fig. 47, where the final activity
of the thorium and the initial activity of the Th X are in each case
taken as 100. The ordinates represent the activities determined by means
of the ionization current, and the abscissae represent the time in days.
It will be observed that both curves are irregular for the first two
days. The activity of the Th X increased at first, while the activity of
the thorium diminished. Disregarding these initial irregularities of the
curves, which will be explained in detail in section 208, it will be
seen that, after the first two days, the time taken for the thorium to
recover half its lost activity is about equal to the time taken by the
Th X to lose half its activity. This time in each case is about four
days. The percentage proportion of the activity regained by the thorium,
over any given interval, is approximately equal to the percentage
proportion of the activity lost by the Th X during the same interval.

[Illustration: Fig. 48.]

If the recovery curve is produced backwards to meet the vertical axis,
it does so at a minimum of 25 per cent., and the above conclusions hold
more accurately, if the recovery is assumed to start from this minimum.
This is clearly shown by Fig. 48, where the percentages of activity
recovered, reckoned from the 25 per cent. minimum, are plotted as
ordinates. In the same figure the decay curve, after the second day, is
shown on the same scale. The activity of the Th X decays with the time
according to an exponential law, falling to half value in about four
days. If _I₀_ is the initial activity and _I{t}_ is the activity after
a time _t_, then

$$ \frac {I₀} {I_t} = e^{–λt} $$, where λ is a constant and _e_
the natural base of logarithms. The experimental curve of the rise of
activity from a minimum to a maximum value is therefore expressed by the
equation

$$ \frac {I_t} {I₀} = 1 − e^{–λt} $$,

where _I₀_ is the amount of activity recovered when the state of
constant activity is reached, _I_{t}_ the activity recovered after a
time _t_, and λ is the _same constant_ as before.


=129. Uranium X.= Similar results were obtained when uranium was
examined. The Ur X was separated by Becquerel’s method of successive
precipitations with barium. The decay of the separated activity and the
recovery of the lost activity are shown graphically in Fig. 49. A more
detailed discussion of this experiment is given in section 205.

[Illustration: Fig. 49.]

The curves of decay and recovery exhibit the same peculiarities and can
be expressed by the same equations as in the case of thorium. The
time-rate of decay and recovery is, however, much slower than for
thorium, the activity of the Ur X falling to half its value in about 22
days.

A large number of results of a similar character have been obtained from
other radio-active products, separated from the radio-elements, but the
cases of thorium and uranium will suffice for the present to form a
basis for the discussion of the processes that are taking place in
radio-active bodies.


=130. Theory of the phenomena.= These processes of decay and recovery go
on at exactly the same rate if the substances are removed from the
neighbourhood of one another, or enclosed in lead, or placed in a vacuum
tube. It is at first sight a remarkable phenomenon that the processes of
decay and recovery should be so intimately connected, although there is
no possibility of mutual interaction between them. These results,
however, receive a complete explanation on the following hypotheses:

    (1) That there is a constant rate of production of fresh
    radio-active matter by the radio-active body;

    (2) That the activity of the matter so formed decreases according to
    an exponential law with the time from the moment of its formation.

Suppose that _q₀_ particles of new matter are produced per second from a
given mass of matter. The rate of emission of energy due to the
particles produced in the time _dt_, is, at the moment of their
formation, equal to _Kq₀__dt_, where _K_ is a constant.

It is required to find the activity due to the whole matter produced
after the process has continued for a time _T_.

The activity _dI_, due to the matter produced during the time _dt_ at
the time _t_, decays according to an exponential law during the time _T_ −
_t_ that elapses before its activity is estimated, and in consequence
is given by

$$ dI = Kq₀e^{–λ (T-t)} dt $$,

where λ is the constant of decay of activity of the active matter. The
activity _I_{T}_ due to the whole matter produced in the time _T_ is
thus given by

         $$ I_t = \int₀^T Kq₀e^{–λ (T-t)} dt $$

         $$     = \frac {Kq₀} {λ} (1 − e^{–λ T}) $$ .


The activity reaches a maximum value _I₀_ when _T_ is very great, and is
then given by

                                   _Kq₀_
                            _I₀_ = -----
                                    λ

thus

$$ \frac {I_T} {I₀} = 1 − e^{–λ T} $$ .

This equation agrees with the experimental results for the recovery of
lost activity. Another method for obtaining this equation is given later
in section 133.

A state of equilibrium is reached when the rate of loss of activity of
the matter already produced is balanced by the activity supplied by the
production of new active matter. According to this view, the
radio-active bodies are undergoing change, but the activity remains
constant owing to the action of two opposing processes. Now, if this
active matter can at any time be separated from the substance in which
it is produced, the decay of its activity, as a whole, should follow an
exponential law with the time, since each portion of the matter
decreases in activity according to an exponential law with the time,
whatever its age may be. If _I₀_ is the initial activity of the
separated product, the activity _I_{t}_ after an interval _t_ is given
by

$$ \frac {I_T} {I₀} = e^{–λt} $$ .

Thus, the two assumptions—of uniform production of active matter and of
the decay of its activity in an exponential law from the moment of its
formation—satisfactorily explain the relation between the curves of
decay and recovery of activity.


=131. Experimental evidence.= It now remains to consider further
experimental evidence in support of these hypotheses. The primary
conception is that the radio-active bodies are able to produce from
themselves matter of chemical properties different from those of the
parent substance, and that this process goes on at a constant rate. This
new matter initially possesses the property of activity, and loses it
according to a definite law. The fact that a proportion of the activity
of radium and thorium can be concentrated in small amounts of active
matter like Th X or Ur X does not, of itself, prove directly that a
material constituent responsible for the activity has been chemically
separated. For example, in the case of the separation of Th X from
thorium, it might be supposed that the non-thorium part of the solution
is rendered temporarily active by its association with thorium, and that
this property is retained through the processes of precipitation,
evaporation, and ignition, and finally manifests itself in the residue
remaining. According to this view it is to be expected that any
precipitate capable of removing the thorium completely from its solution
should yield active residues similar to those obtained from ammonia. No
such case has, however, been observed. For example, when thorium nitrate
is precipitated by sodium or ammonium carbonate, the residue from the
filtrate after evaporation and ignition is free from activity and the
thorium carbonate obtained has the normal amount of activity. In fact,
ammonia is the only reagent yet found capable of completely separating
Th X from thorium. A partial separation of the Th X can be made by
shaking thorium oxide with water owing to the greater solubility of Th X
in water.

Thorium and uranium behave quite differently with regard to the action
of ammonia and ammonium carbonate. Ur X is completely precipitated with
the uranium in an ammonia solution and the filtrate is inactive. Ur X is
separated by ammonium carbonate, while Th X under the same conditions is
completely precipitated with the thorium. The Ur X and the Th X thus
behave like distinct types of matter with well-marked chemical
properties quite distinct from those of the substances in which they are
produced. The removal of Ur X by the precipitation of barium is probably
not directly connected with the chemical properties of Ur X. The
separation is probably due to the dragging down of the Ur X with the
dense barium precipitate. Sir William Crookes found that the Ur X was
dragged down by precipitates when no question of insolubility was
involved, and such a result is to be expected if the Ur X exists in
extremely minute quantity. It must be borne in mind that the actual
amount of the active constituents Th X and Ur X, separated from thorium
and uranium, is probably infinitesimal, and that the greater proportion
of the residues is due to impurities present in the salt and the
reagents, a very small amount of active matter being mixed with them.


=132. Rate of production of Th X.= If the recovery of the activity of
uranium or thorium is due to the continuous production of new active
matter, it should be possible to obtain experimental evidence of the
process. As the case of thorium has been most fully investigated, a
brief account will be given of some experiments made by Rutherford and
Soddy[228] to show that Th X is produced continuously at a constant
rate. Preliminary experiments showed that three successive
precipitations were sufficient to remove the Th X almost completely from
the thorium. The general method employed was to precipitate a solution
of 5 grams of thorium-nitrate with ammonia. The precipitate was then
redissolved in nitric acid and the thorium again precipitated as before,
as rapidly as possible, so that the Th X produced in the time between
successive precipitations should not appreciably affect the results. The
removal of the Th X was followed by measurements of the activity of the
residues obtained from successive filtrates. In three successive
precipitations the activities of the residues were proportional to 100,
8, 1·6 respectively. Thus two precipitations are nearly sufficient to
free the thorium from Th X.

The thorium freed from Th X was then allowed to stand for a definite
time, and the amount of Th X formed during that time found by
precipitating it, and measuring its radio-activity. According to the
theory, the activity _I_{t}_ of the thorium formed in the time _t_ is
given by

$$ \frac {I_t} {I₀} = 1 − e^{–λt} $$,

where _I₀_ is the total activity of Th X, when there is radio-active
equilibrium.

If λ_t_ is small,

                              _I_{t}_
                              ------  = λ_t_.
                              _I₀_

Since the activity of Th X falls to half value in 4 days, the value of λ
expressed in hours = ·0072. After standing a period of 1 hour about
¹⁄₁₄₀, after 1 day ⅙, after 4 days ½ of the maximum should be
obtained. The experimental results obtained showed an agreement, as good
as could be expected, with the equation expressing the result that the
Th X was being produced at a constant rate.

The thorium-nitrate which had been freed from Th X was allowed to stand
for one month, and then it was again subjected to the same process. The
activity of the Th X was found to be the same as that obtained from an
equal amount of the original thorium-nitrate. In one month, therefore,
the Th X had been regenerated, and had reached a maximum value. By
leaving the thorium time to recover fully its activity, this process can
be repeated indefinitely, and equal amounts of Th X are obtained at each
precipitation. Ordinary commercial thorium-nitrate and the purest
nitrate obtainable showed exactly the same action, and equal amounts of
Th X could be obtained from equal weights. These processes thus appear
to be independent of the chemical purity of the substance[229].

The process of the production of Th X is continuous, and no alteration
has been observed in the amount produced in the given time after
repeated separations. After 23 precipitations extending over 9 days, the
amount produced in a given interval was about the same as at the
beginning of the process.

These results are all in agreement with the view that the Th X is being
continuously produced from the thorium compound at a constant rate. The
amount of active matter produced from 1 gram of thorium is probably
extremely minute, but the electrical effects due to its activity are so
large that the process of production can be followed after extremely
short intervals. With a sensitive electrometer the amount of Th X
produced per minute in 10 grams of thorium-nitrate gives a rapid
movement to the electrometer needle. For larger intervals it is
necessary to add additional capacity to the system to bring the effects
within range of the instrument.


=133. Rate of decay of activity.= It has been shown that the activity of
Ur X and Th X decays according to an exponential law with the time.
This, we shall see later, is the general law of decay of activity in any
type of active matter, obtained by itself, and freed from any secondary
active products which it may, itself, produce. In any case, when this
law is not fulfilled, it can be shown that the activity is due to the
superposition of two or more effects, each of which decays in an
exponential law with the time. The physical interpretation of this law
still remains to be discussed.

It has been shown that in uranium and thorium compounds there is a
continuous production of active matter which keeps the compound in
radio-active equilibrium. The changes by which the active matter is
produced must be chemical in nature, since the products of the action
are different in chemical properties from the matter in which the
changes take place. The activity of the products has afforded the means
of following the changes occurring in them. It now remains to consider
the connection between the activity at any time, and the amount of
chemical change taking place at that time.

In the first place, it is found experimentally that the saturation
ionization current _i_{t}_, after the active product has been allowed to
decay for a time _t_, is given by

$$ \frac {i_t} {i₀} = e^{–λt} $$,

where _i₀_ is the initial saturation current and λ the constant of
decay.

Now the saturation current is a measure of the total number of ions
produced per second in the testing vessel. It has already been shown
that the α rays, which produce the greater proportion of ionization in
the gas, consist of positively charged particles projected with great
velocity. Suppose for simplicity that each atom of active matter, in the
course of its change, gives rise to one projected α particle. Each α
particle will produce a certain average number of ions in its path
before it strikes the boundaries or is absorbed in the gas. Since the
number of projected particles per second is equal to the number of atoms
changing per second, the number of atoms _n_{t}_ which change per second
at the time _t_ is given by

$$ \frac {n_t} {n₀} = e^{–λt} $$,

where _n₀_ is the initial number which change per second. On this view,
then, the law of decay expresses the result that the number of atoms
changing in unit time, diminishes according to an exponential law with
the time. The number of atoms _N_{t}_ which remain _unchanged_ after an
interval _t_ is given by

             $$ N_t = \int_t^{\infty} n_t dt $$

             $$    = \frac {n₀} {λ} e^{–λt} $$ .

If _N₀_ is the number of atoms at the beginning,

$$ N₀ = \frac {n₀} {λ} $$,

Thus

$$ \frac {N_t} {N_₀} = e^{–λt} $$ (1).

or the law of decay expresses the fact that the _activity of a product
at any time is proportional to the number of atoms which remain
unchanged at that time_.

This is the same as the law of monomolecular change in chemistry, and
expresses the fact that there is only one changing system. If the change
depended on the mutual action of two systems, the law of decay would be
different, since the rate of decay in that case would depend on the
relative concentration of the two reacting substances. This is not so,
for not a single case has yet been observed in which the law of decay
was affected by the amount of active matter present.

From the above equation (1)

                           _dN_{t}_
                           -------   =  –λ_N_{t}_,
                            _dt_

or the number of systems changing in unit time is proportional to the
number unchanged at that time.

In the case of recovery of activity, after an active product has been
removed, the number of systems changing in unit time, when radio-active
equilibrium is produced, is equal to λ_N₀_. This must be equal to the
number _q₀_ of new systems applied in unit time, or

                             _q₀_ = λ_N₀_,

                                       _q₀_
                             and  λ = ------;
                                       _N₀_

λ has thus a distinct physical meaning, and may be defined as the
proportion of the total number of systems present which change per
second. It has different values for different types of active matter,
but is invariable for any particular type of matter. For this reason, λ
will be termed the “_radio-active constant_” of the product.

We are now in a position to discuss with more physical definiteness the
gradual growth of Th X in thorium, after the Th X has been completely
removed from it. Let _q₀_ particles of Th X be produced per second by
the thorium, and let _N_ be the number of particles of Th X present at
any time _t_ after the original Th X was removed. The number of
particles of Th X which change every second is λ_N_, where λ is the
radio-active constant of Th X. Now, at any time during the process of
recovery, the rate of increase of the number of particles of Th X = the
rate of production − the rate of change; that is

                          _dN_
                          ------   = _q₀_ − λ_N_.
                          _dt_

The solution of this equation is of the form

$$ N = ae^{–λt} + b $$,

where _a_ and _b_ are constants.

Now when _t_ is very great, the number of particles of Th X present
reach a maximum value _N₀_.

                Thus, since _N_ = _N₀_ when _t_ = infinity,
                                            _b_ = _N₀_;

                since _N_ = 0 when _t_ = 0,

                          _a_ + _b_ = 0;
                hence     _b_ = -_a_ = _N₀_,

and the equation becomes

$$ \frac {N} {N₀} = 1 − e^{–λt} $$ .

This is equivalent to the equation already obtained in section 130,
since the intensity of the radiation is always proportional to the
number of particles present.


=134. Influence of conditions on the rate of decay.= Since the activity
of any product, at any time, may be taken as a measure of the rate at
which chemical change takes place, it may be used as a means of
determining the effect of conditions on the changes occurring in
radio-active matter. If the rate of change should be accelerated or
retarded, it is to be expected that the value of the radio-active
constant λ will be increased or decreased, _i.e._ that the decay curve
will be different under different conditions.

No such effect, however, has yet been observed in any case of
radio-active change, where none of the active products produced are
allowed to escape from the system. The rate of decay is unaltered by any
chemical or physical agency, and in this respect the changes in
radio-active matter are sharply distinguished from ordinary chemical
changes. For example, the rate of decay of activity from any product
takes place at the same rate when the substance is exposed to light as
when it is kept in the dark, and at the same rate in a vacuum as in air
or any other gas at atmospheric pressure. Its rate of decay is unaltered
by surrounding the active matter by a thick layer of lead under
conditions where no ordinary radiation from outside can affect it. The
activity of the matter is unaffected by ignition or chemical treatment.
The material giving rise to the activity can be dissolved in acid and
re-obtained by evaporation of the solution without altering the
activity. The rate of decay is the same whether the active matter is
retained in the solid state or kept in solution. When a product has lost
its activity, resolution or heat does not regenerate it, and as we shall
see later, the rate of decay of the active products, so far examined, is
the same at a red heat as at the temperature of liquid air. In fact, no
variation of physical or chemical conditions has led to any observable
difference in the decay of activity of any of the numerous types of
active matter which have been examined.


=135. Effect of conditions on the rate of recovery of activity.= The
recovery of the activity of a radio-element with time, when an active
product is separated from it, is governed by the rate of production of
fresh active matter and by the decay of activity of that already
produced. Since the rate of decay of the activity of the separated
product is independent of conditions, the rate of recovery of activity
can be modified only by a change of the rate of production of fresh
active matter. As far as experiments have gone, the rate of production,
like the rate of decay, is independent of chemical or physical
conditions. There are indeed certain cases which are apparent exceptions
to this rule. For example, the escape of the radio-active emanations
from thorium and radium is readily affected by heat, moisture and
solution. A more thorough investigation, however, shows that the
exception is only apparent and not real. These cases will be discussed
more in detail in chapter VII, but it may be stated here that the
differences observed are due to differences in the rate of escape of the
emanations into the surrounding gas, and not to differences in the rate
of production. For this reason it is difficult to test the question at
issue in the case of the thorium compounds, which in most cases readily
allow the emanation produced by them to escape into the air.

In order to show that the rate of production is independent of molecular
state, temperature, etc., it is necessary in such a case to undertake a
long series of measurements extending over the whole time of recovery.
It is impossible to make accurate relative comparisons to see if the
activity is altered by the conversion of one compound into another. The
relative activity in such a case, when measured by spreading a definite
weight of material uniformly on a metal plate, varies greatly with the
physical conditions of the precipitate, although the total activity of
two compounds may be the same.

The following method[230] offers an accurate and simple means of
studying whether the rate of production of active matter is influenced
by molecular state. The substance is chemically converted into any
compound required, care being taken that active products are recovered
during the process. The new compound is then spread on a metal plate and
compared with a standard sample of uranium for several days or weeks as
required. If the rate of production of active matter is altered by the
conversion, there should be an increase or decrease of activity to a new
steady value, where the production of active matter is again balanced by
the rate of decay. This method has the great advantage of being
independent of the physical condition of the precipitate. It can be
applied satisfactorily to a compound of thorium like the nitrate and the
oxide which has been heated to a white heat, after which treatment only
a slight amount of emanation escapes. The nitrate was converted into the
oxide in a platinum crucible by treatment with sulphuric acid and
ignition to a white heat. The oxide so obtained was spread on a plate,
but no change of its activity was observed with time, showing that in
this case the rate of production was independent of molecular state.
This method, which is limited in the case of thorium, may be applied
generally to the uranium compounds where the results are not complicated
by the presence of an emanation.

No differences have yet been observed in the recovery curves of
different thorium compounds after the removal of Th X. For example, the
rate of recovery is the same whether the precipitated hydroxide is
converted into the oxide or into the sulphate.


=136. Disintegration hypothesis.= In the discussion of the changes in
radio-active bodies, only the active products Ur X and Th X have been
considered. It will, however, be shown later that these two products are
only examples of many other types of active matter which are produced by
the radio-elements, and that each of these types of active matter has
definite chemical as well as radio-active properties, which distinguish
it, not only from the other active products, but also from the substance
from which it is produced.

The full investigation of these changes will be shown to verify in every
particular the hypothesis that radio-activity is the accompaniment of
chemical changes of a special kind occurring in matter, and that the
constant activity of the radio-elements is due to an equilibrium
process, in which the rate of production of fresh active matter balances
the rate of change of that already formed.

The nature of the process taking place in the radio-elements, in order
to give rise to the production at a constant rate of new kinds of active
matter, will now be considered. Since in thorium or uranium compounds
there is a continuous production of radio-active matter, which differs
in chemical properties from the parent substance, some kind of change
must be taking place in the radio-element. This change, by which new
matter is produced, is very different in character from the molecular
changes dealt with in chemistry, for no chemical change is known which
proceeds at the same rate at the temperatures corresponding to a red
heat and to liquid air, and is independent of all physical and chemical
actions. If, however, the production of active matter is supposed to be
the result of changes, not in the molecule, but in the _atom itself_, it
is not to be expected that the temperature would exert much influence.
The general experience of chemistry in failing to transform the elements
by the action of temperature is itself strong evidence that wide ranges
of temperature have not much effect in altering the stability of the
chemical atom.

The view that the atoms of the radio-elements are undergoing spontaneous
disintegration was put forward by Rutherford and Soddy as a result of
evidence of this character. The discovery of the _material_ nature of
the α rays added strong confirmation to the hypothesis; for it has been
pointed out (section 95) that the expulsion of α particles must be the
result of a disintegration of the atoms of the radio-element. Taking the
case of thorium as an example, the processes occurring in the atom may
be pictured in the following way. It must be supposed that the thorium
atoms are not permanently stable systems, but, on an average, a constant
small proportion of them—about one atom in every 10¹⁶ will suffice—break
up per second. The disintegration consists in the expulsion from the
atom of one or more α particles with great velocity. For simplicity, it
will be supposed that each atom expels _one_ α particle. It has been
shown that the α particle of radium has a mass about twice that of the
hydrogen atom. From the similarity of the α rays from thorium and
radium, it is probable that the α particle of thorium does not differ
much in mass from that of radium, and may be equal to it. The α
particles expelled from the thorium atoms as they break up constitute
what is known as the “non-separable activity” of thorium. This activity,
measured by the α rays, is about 25 per cent. of the maximum. After the
escape of an α particle, the part of the atom left behind, which has a
mass slightly less than that of the thorium atom, tends to rearrange its
components to form a temporarily stable system. It is to be expected
that it will differ in chemical properties from the thorium atom from
which it was derived. The atom of the substance Th X is, on this view,
the thorium atom minus one α particle. The atoms of Th X are far more
unstable than the atoms of thorium, and one after the other they break
up, each atom expelling one α particle as before. These projected α
particles give rise to the _radiation_ from the Th X. Since the activity
of Th X falls to half its original value in about four days, on an
average half of the atoms of Th X break up in four days, the number
breaking up per second being always proportional to the number present.
After an atom of Th X has expelled an α particle, the mass of the system
is again reduced, and its chemical properties are changed. It will be
shown (section 154) that the Th X produces the thorium emanation, which
exists as a radio-active gas, and that this in turn is transformed into
matter which is deposited on solid bodies and gives rise to the
phenomena of excited activity. The first few successive changes
occurring in thorium are shown diagrammatically below (Fig. 50).

[Illustration: Fig. 50.]

Thus as a result of the disintegration of the thorium atom, a series of
chemical substances is produced, each of which has distinctive chemical
properties. Each of these products is radio-active, and loses its
activity according to a definite law. Since thorium has an atomic weight
of 237, and the weight of the α particle is about 2, it is evident that,
if only _one_ α particle is expelled at each change, the process of
disintegration could pass through a number of successive stages and yet
leave behind, at the end of the process, a mass comparable with that of
the parent atom.

It will be shown later that a process of disintegration, very similar to
that already described for thorium, must be supposed to take place also
in uranium, actinium and radium. The full discussion of this subject
cannot be given with advantage until two of the most important products
of the three substances thorium, radium and actinium, viz. the
radio-active emanations and the matter which causes excited activity,
have been considered in detail.


=137. Magnitude of the changes.= It can be calculated by several
independent methods (see section 246) that, in order to account for the
radio-activity observed in thorium, about 3 × 10⁴ atoms in each gram of
thorium suffer disintegration per second. It is well known (section 39)
that 1 cubic centimetre of hydrogen at atmospheric pressure and
temperature contains about 3·6 × 10¹⁹ molecules. From this it follows
that one gram of thorium contains 3·6 × 10²¹ atoms. The fraction which
breaks up per second is thus about 10¹⁷. This is an extremely small
ratio, and it is evident that the process could continue for long
intervals of time, before the amount of matter changed would be capable
of detection by the spectroscope or by the balance. With the
electroscope it is possible to detect the radiation from 10⁻⁵ gram of
thorium, _i.e._ the electroscope is capable of detecting the ionization
which accompanies the disintegration of a single thorium atom per
second. The electroscope is thus an extraordinarily delicate means for
detection of minute changes in matter, which are accompanied, as in the
case of the radio-elements, by the expulsion of charged particles with
great velocity. It is possible to detect by its radiation the amount of
Th X produced in a second from 1 gram of thorium, although the process
would probably need to continue thousands of years before it could be
detected by the balance or the spectroscope. It is thus evident that the
changes occurring in thorium are of an order of magnitude quite
different from that of ordinary chemical changes, and it is not
surprising that they have never been observed by direct chemical
methods.

Footnote 225:

  Crookes, _Proc. Roy. Soc._ 66, p. 409, 1900.

Footnote 226:

  Becquerel, _C. R._ 131, p. 137, 1900; 133, p. 977, 1901.

Footnote 227:

  Rutherford and Soddy, _Phil. Mag._ Sept. and Nov. 1902. _Trans. Chem.
  Soc._ 81, pp. 321 and 837, 1902.

Footnote 228:

  Rutherford and Soddy, _Phil. Mag._ Sept. 1902.

Footnote 229:

  The general method of regarding the subject would be unchanged, even
  if it were proved that the radio-activity of thorium is not due to
  thorium at all but to a small constant amount of a radio-active
  impurity mixed with it.

Footnote 230:

  Rutherford and Soddy, _Phil. Mag._ Sept. 1902.




                              CHAPTER VII.
                        RADIO-ACTIVE EMANATIONS.


=138. Introduction.= A most important and striking property possessed by
radium, thorium, and actinium, but not by uranium or polonium, is the
power of continuously emitting into the surrounding space a material
emanation, which has all the properties of a radio-active gas. This
emanation is able to diffuse rapidly through gases and through porous
substances, and may be separated from the gas with which it is mixed by
condensation by the action of extreme cold. This emanation forms a
connecting link between the activity of the radio-elements themselves
and their power of exciting activity on surrounding objects, and has
been studied more closely than the other active products on account of
its existence in the gaseous state. The emanations from the three active
bodies all possess similar radio-active properties, but the effects are
more marked in the case of the emanation from radium, on account of the
very great activity of that element.


                           Thorium Emanation.


=139. Discovery of the emanation.= In the course of examination of the
radiations of thorium, several observers had noted that some of the
thorium compounds, and especially the oxide, were very inconstant
sources of radiation, when examined in open vessels by the electrical
method. Owens[231] found that this inconstancy was due to the presence
of air currents. When a closed vessel was used, the current, immediately
after the introduction of the active matter, increased with the time,
and finally reached a constant value. By drawing a steady stream of air
through the vessel the value of the current was much reduced. It was
also observed that the radiations could apparently pass through large
thicknesses of paper, which completely absorbed the ordinary α
radiation.

In an investigation of these peculiar properties of thorium compounds,
the writer[232] found that the effects were due to an emission of
radio-active particles of some kind from the thorium compounds. This
“emanation,” as it was termed for convenience, possesses the properties
of ionizing the gas and acting on a photographic plate, and is able to
diffuse rapidly through porous substances like paper and thin metal
foil.

The emanation, like a gas, is completely prevented from escaping by
covering the active matter with a thin plate of mica. The emanation can
be carried away by a current of air; it passes through a plug of
cotton-wool and can be bubbled through solutions without any loss of
activity. In these respects, it behaves very differently from the ions
produced in the gas by the rays from active substances, for these give
up their charges completely under the same conditions.

Since the emanation passes readily through large thicknesses of
cardboard, and through filters of tightly packed cotton-wool, it does
not seem likely that the emanation consists of particles of dust given
off by the active matter. This point was tested still further by the
method used by Aitken and Wilson, for detecting the presence of dust
particles in the air. The oxide, enclosed in a paper cylinder, was
placed in a glass vessel, and the dust was removed by repeated small
expansions of the air over a water surface. The dust particles act as
nuclei for the formation of small drops and are then removed from the
air by the action of gravity. After repeated expansions, no cloud was
formed, and the dust was considered to be removed. After waiting for
some time to allow the thorium emanation to collect, further expansions
were made but no cloud resulted, showing that for the small expansions
used, the particles were too small to become centres of condensation.
The emanation then could not be regarded as dust emitted from thorium.

Since the power of diffusing rapidly through porous substances, and
acting on a photographic plate, is also possessed by a chemical
substance like hydrogen peroxide, some experiments were made to see if
the emanation could be an agent of that character. It was found,
however, that hydrogen peroxide is not radio-active, and that its action
on the plate is a purely chemical one, while it is the _radiation_ from
the emanation and not the _emanation_ itself that produces ionizing and
photographic effects.


=140. Experimental arrangements.= The emanation from thorium is given
off in minute quantity. No appreciable lowering of the vacuum is
observed when an emanating compound is placed in a vacuum tube and no
new spectrum lines are observed.

For an examination of the emanation, an apparatus similar in principle
to that shown in Fig. 51 is convenient.

[Illustration: Fig. 51.]

The thorium compound, either bare or enclosed in a paper envelope, was
placed in a glass tube _C_. A current of air from a gasometer, after
passing through a tube containing cotton-wool to remove dust particles,
bubbled through sulphuric acid in the vessel _A_. It then passed through
a bulb containing tightly packed cotton-wool to prevent any spray being
carried over. The emanation, mixed with air, was carried from the vessel
_C_ through a plug of cotton-wool _D_, which removed completely all the
ions carried with the emanation. The latter then passed into a long
brass cylinder, 75 cm. in length and 6 cm. in diameter. The insulated
cylinder was connected with a battery in the usual way. Three insulated
electrodes, _E_, _F_, _H_, of equal lengths, were placed along the axis
of the cylinder, supported by brass rods passing through ebonite corks
in the side of the cylinder. The current through the gas, due to the
presence of the emanation, was measured by means of an electrometer. An
insulating key was arranged so that any one of the electrodes _E_, _F_,
_H_ could be rapidly connected with one pair of quadrants of the
electrometer, the other two being always connected with earth. The
current observed in the testing cylinder vessel was due entirely to the
ions produced by the emanation carried into the vessel by the current of
air. On substituting a uranium compound for the thorium, not the
slightest current was observed. After a constant flow has passed for
about 10 minutes, the current due to the emanation reaches a constant
value.

The variation of the ionization current with the voltage is similar to
that observed for the gas ionized by the radiations from the active
bodies. The current at first increases with the voltage, but finally
reaches a saturation value.


=141. Duration of the activity of the emanation.= The emanation rapidly
loses its activity with time. This is very readily shown with the
apparatus of Fig. 51. The current is found to diminish progressively
along the cylinder, and the variation from electrode to electrode
depends on the velocity of the flow of air.

If the velocity of the air current is known, the decay of activity of
the emanation with time can be deduced. If the flow of air is stopped,
and the openings of the cylinder closed, the current steadily diminishes
with time. The following numbers illustrate the variation with time of
the saturation current, due to the emanation in a closed vessel. The
observations were taken successively, and as rapidly as possible after
the current of air was stopped.

                       Time in seconds       Current
                                     0        100
                                    28         69
                                    62         51
                                   118         25
                                   155         14
                                   210          6·7
                                   272          4·1
                                   360          1·8

Curve _A_, Fig. 52, shows the relation existing between the current
through the gas and the time. The current just before the flow of air
was stopped is taken as unity. The current through the gas, which is a
measure of the activity of the emanation, diminishes according to an
exponential law with the time like the activity of the products Ur X and
Th X. The rate of decay is, however, much more rapid, the activity of
the emanation decreasing to half value in about one minute. According to
the view developed in section 136, this implies that half of the
emanation particles have undergone change in one minute. After an
interval of 10 minutes the current due to the emanation is very small,
showing that practically all the emanation particles present have
undergone change.

[Illustration: Fig. 52.]

The rate of decay has been more accurately determined by Rossignol and
Gimingham[233] who found that the activity fell to half value in about
51 seconds. Bronson[234], using the steady deflection method described
in section 69, found the corresponding time 54 seconds.

The decrease of the current with the time is an actual measure of the
decrease of the activity of the emanation, and is not in any way
influenced by the time that the ions produced take to reach the
electrodes. If the ions had been produced from a uranium compound the
duration of the conductivity for a saturation voltage would only have
been a fraction of a second.

The rate of decay of the activity of the emanation is independent of the
electromotive force acting on the gas. This shows that the radio-active
particles are not destroyed by the electric field. The current through
the gas at any particular instant, after stoppage of the flow of air,
was found to be the same whether the electromotive force had been acting
the whole time or had been just applied for the time of the test.

The emanation itself is unaffected by a strong electric field and so
cannot be charged. By testing its activity after passing it through long
concentric cylinders, charged to a high potential, it was found that the
emanation certainly did not move with a velocity greater than ·00001 cm.
per second, for a gradient of 1 volt per cm., and there was no evidence
to show that it moved at all. This conclusion has been confirmed by the
experiments of McClelland[235].

The rate at which the emanation is produced is independent of the gas
surrounding the active matter. If in the apparatus of Fig. 51 air is
replaced by hydrogen, oxygen, or carbonic acid, similar results are
obtained, though the current observed in the testing vessel varies for
the different gases on account of the unequal absorption by them of the
radiation from the emanation.

If a thorium compound, enclosed in paper to absorb the α radiation, is
placed in a closed vessel, the saturation current due to the emanation
is found to vary directly as the pressure. Since the rate of ionization
is proportional to the pressure for a constant source of radiation, this
experiment shows that the rate of emission of the emanation is
independent of the pressure of the gas. The effect of pressure on the
rate of production of the emanation is discussed in more detail later in
section 157.


=142. Effect of thickness of layer.= The amount of emanation emitted by
a given area of thorium compound depends on the thickness of the layer.
With a very thin layer, the current between two parallel plates, placed
in a closed vessel as in Fig. 17, is due very largely to the α rays.
Since the α radiation is very readily absorbed, the current due to it
practically reaches a maximum when the surface of the plate is
completely covered by a thin layer of the active material. On the other
hand the current produced by the emanation increases until the layer is
several millimetres in thickness, and then is not much altered by adding
fresh active matter. This falling off of the current after a certain
thickness has been reached is to be expected, since the emanation, which
takes several minutes to diffuse through the layer above it, has already
lost a large proportion of its activity.

With a thick layer of thorium oxide in a closed vessel, the current
between the plates is largely due to the radiation from the emanation
lying between the plates. The following tables illustrate the way in
which the current varies with the thickness of paper for both a thin and
a thick layer.


TABLE I. _Thin Layer._

Thickness of sheets of paper ·0027.

                             No. of    Current
                          layers of
                              paper


                                  0          1

                                  1        ·37

                                  2        ·16

                                  3        ·08


TABLE II. _Thick Layer._

Thickness of paper ·008 cm.

                             No. of    Current
                          layers of
                              paper


                                  0          1

                                  1        ·74

                                  2        ·74

                                  5        ·72

                                 10        ·67

                                 20        ·55

The initial current with the unscreened compound is taken as unity. In
Table I, for a thin layer of thorium oxide, the current diminished
rapidly with additional layers of thin paper. In this case the current
is due almost entirely to the α rays. In Table II the current falls to
·74 for the first layer. In this case about 26% of the current is due to
the α rays, which are practically absorbed by the layer ·008 cm. in
thickness. The slow decrease with additional layers shows that the
emanation diffuses so rapidly through a few layers of paper that there
is little loss of activity during the passage. The time taken to diffuse
through 20 layers is however appreciable, and the current consequently
has decreased. After passing through a layer of cardboard 1·6 mms. in
thickness the current is reduced to about one-fifth of its original
value. In closed vessels the proportion of the total current, due to the
emanation, varies with the distance between the plates as well as with
the thickness of the layer of active material. It also varies greatly
with the compound examined. In the nitrate, which gives off only a small
amount of emanation, the proportion is very much smaller than in the
hydroxide, which gives off a large amount of emanation.


=143. Increase of current with time.= The current due to the emanation
does not reach its final value for some time after the active matter has
been introduced into the closed vessel. The variation with time is shown
in the following table. The saturation current due to thorium oxide,
covered with paper, was observed between concentric cylinders of 5·5
cms. and ·8 cm. diameter.

Immediately before observations on the current were made, a rapid stream
of air was blown through the apparatus. This removed most of the
emanation. However, the current due to the ionization of the gas by the
emanation, as it was carried along by the current of air, was still
appreciable. The current consequently does not start from zero.

                            Time in    Current
                            seconds

                                  0          9

                                 23         25

                                 53         49

                                 96         67

                                125         76

                                194         88

                                244         98

                                304         99

                                484        100

The results are shown graphically in Fig. 52, curve _B_. The decay of
the activity of the emanation with time, and the rate of increase of the
activity due to the emanation in a closed space, are connected in the
same way as the decay and recovery curves of Th X and Ur X.

With the previous notation, the decay curve is given by

$$ \frac {I_T} {I₀} = e^{–λt} $$ .

and the recovery curve by

$$ \frac {I_t} {I₀} = 1 − e^{–λt} $$,

where λ is the radio-active constant of the emanation.

This relation is to be expected, since the decay and recovery curves of
the emanation are determined by exactly the same conditions as the decay
and recovery curves of Ur X and Th X. In both cases there is:

    (1) A supply of fresh radio-active particles produced at a constant
    rate.

    (2) A loss of activity of the particles following an exponential law
    with the time.

In the case of Ur X and Th X, the active matter produced manifests its
activity in the position in which it is formed; in this new phenomenon,
a proportion of the active matter in the form of the emanation escapes
into the surrounding gas. The activity of the emanation, due to a
thorium compound kept in a closed vessel, thus reaches a maximum when
the rate of supply of fresh emanation particles from the compound is
balanced by the rate of change of those already present. The time for
recovery of half the final activity is about 1 minute, the same as the
time taken for the emanation, when left to itself, to lose half its
activity.

If _q₀_ is the number of emanation particles escaping into the gas per
second, and _N₀_ the final number when radio-active equilibrium is
reached, then (section 133),

                               _q₀_ = λ_N₀_.

Since the activity of the emanation falls to half value in 1 minute

                                 λ = ¹⁄₈₇,

and _N₀_ = 87_q₀_, or the number of emanation particles present when a
steady state is reached is 87 times the number produced per second.


                           Radium Emanation.


=144. Discovery of the emanation.= Shortly after the discovery of the
thorium emanation, Dorn[236] repeated the results, and, in addition,
showed that radium compounds also gave off radio-active emanations, and
that the amount given off was much increased by heating the compound.
The radium emanation differs from the thorium emanation in the rate at
which it loses its activity. It decays far more slowly, but in other
respects the emanations of thorium and radium have much the same
properties. Both emanations ionize the gas with which they are mixed,
and affect a photographic plate. Both diffuse readily through porous
substances but are unable to pass through a thin plate of mica; both
behave like a temporarily radio-active gas, mixed in minute quantity
with the air or other gas in which they are conveyed.


=145. Decay of activity of the emanation.= Very little emanation escapes
from radium chloride in the solid state, but the amount is largely
increased by heating, or by dissolving the compound in water. By
bubbling air through a radium chloride solution, or passing air over a
heated radium compound, a large amount of emanation may be obtained
which can be collected, mixed with air, in a suitable vessel.

Experiments to determine accurately the rate of decay of activity of the
emanation have been made by P. Curie[237], and Rutherford and
Soddy[238]. In the experiments of the latter, the emanation mixed with
air was stored over mercury in an ordinary gas-holder. From time to
time, equal quantities of air mixed with the emanation were measured off
by a gas pipette and delivered into a testing vessel. The latter
consisted of an air-tight brass cylinder carrying a central insulated
electrode. A saturation voltage was applied to the cylinder, and the
inner electrode was connected to the electrometer with a suitable
capacity in parallel. The saturation current was observed _immediately_
after the introduction of the active gas into the testing vessel, and
was taken as a measure of the activity of the emanation present. The
current increased rapidly with the time owing to the production of
excited activity on the walls of the containing vessel. This effect is
described in detail in chapter VIII.

The measurements were made at suitable intervals over a period of 33
days. The following table expresses the results, the initial activity
being taken as 100.

                            Time in   Relative
                              hours   Activity

                                  0        100

                               20·8         85·7

                              187·6         24·0

                              354·9          6·9

                              521·9          1·5

                              786·9          0·19

The activity falls off according to an exponential law with the time,
and decays to half value in 3·71 days. With the usual notation

$$ \frac {I_T} {I₀} = e^{–λt} $$ .

the mean value of λ deduced from the results is given by

                        λ =  2·16 × 10⁻⁶ = ¹⁄₄₆₃₀₀₀.

P. Curie determined the rate of decay of activity of the emanation by
another method. The active matter was placed at one end of a sealed
tube. After sufficient time had elapsed the portion of the tube
containing the radium compound was removed. The loss of activity of the
emanation, stored in the other part, was tested at regular intervals by
observing the ionization current due to the rays which passed through
the walls of the glass vessel. The testing apparatus and the connections
are shown clearly in Fig. 53. The ionization current is observed between
the vessels _BB_ and _CC_. The glass tube _A_ contains the emanation.

[Illustration: Fig. 53.]

Now it will be shown later that the emanation itself gives off only α
rays, and these rays are completely absorbed by the glass envelope,
unless it is made extremely thin. The rays producing ionization in the
testing vessel were thus not due to the α rays from the emanation at
all, but to the β and γ rays due to the excited activity produced on the
walls of the glass tube by the emanation inside it. What was actually
measured was thus the decay of the excited activity derived from the
emanation, and not the decay of activity of the emanation itself. Since,
however, when a steady state is reached, the amount of excited activity
is nearly proportional at any time to the activity of the emanation, the
rate of decay of the excited activity on the walls of the vessel
indirectly furnishes a measure of the rate of decay of the emanation
itself. This is only true if the emanation is placed for four or five
hours in the tube before observations begin, in order to allow the
excited activity time to reach a maximum value.

Using this method P. Curie obtained results similar to those obtained by
Rutherford and Soddy by the direct method. The activity decayed
according to an exponential law with the time, falling to half value in
3·99 days.

The experiments were performed under the most varied conditions but the
rate of decay was found to remain unaltered. The rate of decay did not
depend on the material of the vessel containing the emanation or on the
nature or pressure of the gas with which the emanation was mixed. It was
unaffected by the amount of emanation present, or by the time of
exposure to the radium, provided sufficient time had elapsed to allow
the excited activity to reach a maximum value before the observations
were begun. P. Curie[239] found that the rate of decay of activity was
not altered by exposing the vessel containing the emanation to different
temperatures, ranging from +450° to −180° C.

In this respect the emanations of thorium and radium are quite
analogous. The rate of decay seems to be unaffected by any physical or
chemical agency, and the emanations behave in exactly the same way as
the radio-active products Th X and Ur X, already referred to. The
radio-active constant λ is thus a fixed and unalterable quantity for
both emanations, although in one case its value is about 5000 times
greater than in the other.


                       Emanations from Actinium.


=146.= Debierne[240] found that actinium gives out an emanation similar
to the emanation of thorium and radium. The loss of activity of the
emanation is even more rapid than for the thorium emanation, for its
activity falls to half value in 3·9 seconds. In consequence of the rapid
decay of activity, the emanation is able to diffuse through the air only
a short distance from the active matter before it loses the greater
proportion of its activity. Giesel early observed that the radio-active
substance separated by him, which we have seen (section 18) is identical
in radio-active properties with actinium, gave off a large amount of
emanation. It was in consequence of this property, that he gave it the
name of the “emanating substance” and later “emanium.” The impure
preparations of this substance emit the emanation very freely and in
this respect differ from most of the thorium compounds. The emanation
from actinium like those from thorium and radium possesses the property
of exciting activity on inactive bodies, but it has not yet been studied
so completely as the better known emanations of thorium and radium.


          Experiments with large amounts of Radium Emanation.


=147.= With very active specimens of radium a large amount of emanation
can be obtained, and the electrical, photographic, and fluorescent
effects are correspondingly intense. On account of the small activity of
thorium and the rapid decay of its emanation the effects due to it are
weak, and can be studied only for a few minutes after its production.
The emanation from radium, on the other hand, in consequence of the slow
decay of its activity, may be stored mixed with air in an ordinary
gas-holder, and its photographic and electrical actions may be examined
several days or even weeks after, quite apart from those of the radium
from which it was obtained.

It is, in general, difficult to study the radiation due to the emanation
alone, on account of the fact that the emanation is continually
producing a secondary type of activity on the surface of the vessel in
which the emanation is enclosed. This excited activity reaches a maximum
value several hours after the introduction of the emanation, and, as
long as it is kept in the vessel, this excited activity on the walls
decays at the same rate as the emanation itself, _i.e._ it falls to half
its initial value in about 4 days. If, however, the emanation is blown
out, the excited activity remains behind on the surface, but rapidly
loses its activity in the course of a few hours. After several hours the
intensity of the residual radiation is very small.

These effects and their connection with the emanation are discussed more
fully in chapter VIII. Giesel[241] has recorded some interesting
observations of the effect of the radium emanation on a screen of
phosphorescent zinc sulphide. When a few centigrams of moist radium
bromide were placed on a screen any slight motion of the air caused the
luminosity to move to and fro on the screen. The direction of
phosphorescence could be altered at will by a slow current of air. The
effect was still further increased by placing the active material in a
tube and blowing the air through it towards the screen. A screen of
barium platinocyanide or of Balmain’s paint failed to give any visible
light under the same conditions. The luminosity was not altered by a
magnetic field, but it was affected by an electric field. If the screen
were charged the luminosity was more marked when it was negative than
when it was positive.

Giesel states that the luminosity was not equally distributed, but was
concentrated in a peculiar ring-shaped manner over the surface of the
screen. The concentration of luminosity on the negative, rather than on
the positive, electrode is probably due to the excited activity, caused
by the emanation, and not to the emanation itself, for this excited
activity is concentrated chiefly on the negative electrode in an
electric field (see chapter VIII).

An experiment to illustrate the phosphorescence produced in some
substances by the rays from a large amount of emanation is described in
section 165.


=148.= Curie and Debierne[242] have investigated the emanation from
radium, and the excited activity produced by it. Some experiments were
made on the amount of emanation given off from radium under very low
pressures. The tube containing the emanation was exhausted to a good
vacuum by a mercury pump. It was observed that a gas was given off from
the radium which produced excited activity on the glass walls. This gas
was extremely active, and rapidly affected a photographic plate through
the glass. It caused fluorescence on the surface of the glass and
rapidly blackened it, and was still active after standing ten days. When
spectroscopically examined, this gas did not show any new lines, but
generally those of the spectra of carbonic acid, hydrogen, and mercury.
In the light of the results described in section 124 the gas, given off
by the radium, was probably the non-active gases hydrogen and oxygen, in
which the active emanation was mixed in minute quantity. It will be
shown later (section 242) that the energy radiated from the emanation is
enormous compared with the amount of matter involved, and that the
effects observed, in most cases, are produced by an almost infinitesimal
amount of the emanation.

In further experiments, Curie and Debierne[243] found that many
substances were phosphorescent under the action of the emanation and the
excited activity produced by it. In their experiments, two glass bulbs
_A_ and _B_ (Fig. 54) were connected with a glass tube. The active
material was placed in the bulb _A_ and the substance to be examined in
the other.

[Illustration: Fig. 54.]

They found that, in general, substances that were phosphorescent in
ordinary light became luminous. The sulphide of zinc was especially
brilliant and became as luminous as if exposed to a strong light. After
sufficient time had elapsed the luminosity reached a constant value. The
phosphorescence is partly due to the excited activity produced by the
emanation on its surface, and partly to the direct radiation from the
emanation.

Phosphorescence was also produced in glass. Thuringian glass showed the
most marked effects. The luminosity of the glass was found to be about
the same in the two bulbs, but was more marked in the connecting tube.
The effect in the two bulbs was the same even if connected by a very
narrow tube.

Some experiments were also made with a series of phosphorescent plates
placed in the vessel at varying distances apart. With the plates 1 mm.
apart the effect was very feeble, but increased directly as the distance
and was large for a distance of 3 cms.

These effects receive a general explanation on the views already put
forward. When the radium is placed in the closed vessel, the emanation
is given off at a constant rate and gradually diffuses throughout the
enclosure. Since the time taken for diffusion of the emanation through
tubes of ordinary size is small compared with the time required for the
activity to be appreciably reduced, the emanation, and also the excited
activity due to it, will be nearly equally distributed throughout the
vessel.

The luminosity due to it should thus be equal at each end of the tube.
Even with a capillary tube connecting the two bulbs, the gas
continuously given off by the radium will always carry the emanation
with it and cause a practically uniform distribution.

The gradual increase of the amount of emanation throughout the tube will
be given by the equation

$$ \frac {N_t} {N₀} = 1 − e^{–λt} $$ .

where _N_{t}_ is the number of emanation particles present at the time
_t_, _N₀_ the number present when radio-active equilibrium is reached,
and λ is the radio-active constant of the emanation. The phosphorescent
action, which is due partly to the radiations from the emanation and
partly to the excited activity on the walls, should thus reach half the
maximum value in four days and should practically reach its limit after
three weeks’ interval.

The variation of luminosity with different distances between the screens
is to be expected. The amount of excited activity deposited on the
boundaries is proportional to the amount of emanation present. Since the
emanation is equally distributed, the amount of excited activity
deposited on the screens, due to the emanation between them, varies
directly as the distance, provided the distance between the screens is
small compared with their dimensions. Such a result would also follow if
the phosphorescence were due to the radiation from the emanation itself,
provided that the pressure of the gas was low enough to prevent
absorption of the radiation from the emanation in the gas itself between
the screens.


                    Measurements of Emanating Power.


=149. Emanating power.= The compounds of thorium in the solid state vary
very widely in the amount of emanation they emit under ordinary
conditions. It is convenient to use the term _emanating power_ to
express the amount of emanation given off per second by one gram of the
compound. Since, however, we have no means of determining absolutely the
amount of emanation present, all measurements of emanating power are of
necessity comparative. In most cases, it is convenient to take a given
weight of a thorium compound, kept under conditions as nearly as
possible constant, and to compare the amount of emanation of the
compound to be examined with this standard.

In this way comparisons of the emanating power of thorium compounds have
been made by Rutherford and Soddy[244], using an apparatus similar to
that shown in Fig. 51 on page 240.

A known weight of the substance to be tested was spread on a shallow
dish, placed in the glass tube _C_. A stream of dry dust-free air, kept
constant during all the experiments, was passed over the compound and
carried the emanation into the testing vessel. After ten minutes
interval, the current due to the emanation in the testing vessel reached
a constant value. The compound was then removed, and the standard
comparison sample of equal weight substituted; the saturation current
was observed when a steady state was again reached. The ratio of these
two currents gives the ratio of the emanating power of the two samples.

It was found experimentally that, for the velocities of air current
employed, the saturation current in the testing vessel was directly
proportional to the weight of thorium, for weights up to 20 grams. This
is explained by the supposition that the emanation is removed by the
current of air from the mass of the compound, as fast as it is formed.

    Let _i₁_ = saturation current due to a weight ω₁ of the standard,
        _i₂_ = „ „ „ „ ω₂ of the sample to be tested.

           (emanating power of specimen)   _i₂_   ω₁
    Then   ---------------------------- =  ----  ---
           (emanating power of standard)   _i₁_   ω₂

By means of this relation the emanating power of compounds which are not
of equal weight can be compared.

It was found that thorium compounds varied enormously in emanating
power, although the percentage proportion of thorium present in the
compound was not very different. For example, the emanating power of
thorium hydroxide was generally 3 to 4 times greater than that of
ordinary thoria, obtained from the manufacturer. Thorium nitrate, in the
solid state, had only ¹⁄₂₀₀ of the emanating power of ordinary thoria,
while preparations of the carbonate were found to vary widely among
themselves in emanating power, which depended upon slight variations in
the method of preparation.


=150. Effect of conditions on emanating power.= The emanating power of
different compounds of thorium and radium is much affected by the
alteration of chemical and physical conditions. In this respect the
emanating power, which is a measure of the rate of escape of the
emanation into the surrounding gas, must not be confused with the rate
of decay of the activity of the emanations themselves, which has already
been shown to be unaffected by external conditions.

Dorn (_loc. cit._) first observed that the emanating power of thorium
and radium compounds was much affected by moisture. In a fuller
investigation of this point by Rutherford and Soddy, it was found that
the emanating power of thoria is from two to three times greater in a
moist than in a dry gas. Continued desiccation of the thoria in a glass
tube, containing phosphorus pentoxide, did not reduce the emanating
power much below that observed in ordinary dry air. In the same way
radium chloride in the solid state gives off very little emanation when
in a dry gas, but the amount is much increased in a moist gas.

The rate of escape of emanation is much increased by solution of the
compound. For example, thorium nitrate, which has an emanating power of
only ¹⁄₂₀₀ that of thoria in the solid state, has in solution an
emanating power of 3 to 4 times that of thoria. P. Curie and Debierne
observed that the emanating power of radium was also much increased by
solution.

Temperature has a very marked effect on the emanating power. The
writer[245] showed that the emanating power of ordinary thoria was
increased three to four times by heating the substance to a dull red
heat in a platinum tube. If the temperature was kept constant the
emanation continued to escape at the increased rate, but returned to its
original value on cooling. If, however, the compound was heated to a
white heat, the emanating power was greatly reduced, and it returned on
cooling to about 10% of the original value. Such a compound is said to
be _de-emanated_. The emanating power of radium compounds varies in a
still more striking manner with rise of temperature. The rate of escape
of the emanation is momentarily increased even 10,000 times by heating
to a dull red heat. This effect does not continue, for the large escape
of the emanation by heating is in reality due to the release of the
emanation stored up in the radium compound. Like thoria, when the
compound has once been heated to a very high temperature, it loses its
emanating power and does not regain it. It regains its power of
emanating, however, after solution and re-separation.

A further examination of the effect of temperature was made by
Rutherford and Soddy[246]. The emanating power of thoria decreases very
rapidly with lowering of temperature, and at the temperature of solid
carbonic acid it is only about 10% of its ordinary value. It rapidly
returns to its original value when the cooling agent is removed.

Increase of temperature from 80° C. to a dull red heat of platinum thus
increases the emanating power about 40 times, and the effects can be
repeated again and again, with the same compound, provided the
temperature is not raised to the temperature at which de-emanation
begins. De-emanation sets in above a red heat, and the emanating power
is then permanently diminished, but even long-continued heating at a
white heat never entirely destroys the emanating power.


=151. Regeneration of emanating power.= An interesting question arises
whether the de-emanation of thorium and radium is due to a removal or
alteration of the substance which produces the emanation, or whether
intense ignition merely changes the rate of escape of the emanation from
the solid into the surrounding atmosphere.

It is evident that the physical properties of the thoria are much
altered by intense ignition. The compound changes in colour from white
to pink; it becomes denser and also far less readily soluble in acids.
In order to test if the emanating power could be regenerated by a cyclic
chemical process, the de-emanated thoria was dissolved, precipitated as
hydroxide and again converted into oxide. At the same time a specimen of
the ordinary oxide was subjected to an exactly parallel process. The
emanating power of both these compounds was the same, and was from two
to three times greater than that of ordinary thoria.

Thus de-emanation does not permanently destroy the power of thorium of
giving out an emanation, but merely produces an alteration of the amount
of the emanation which escapes from the compound.


=152. Rate of production of the emanation.= The emanating power of
thorium compounds, then, is a very variable quantity, much affected by
moisture, heat, and solution. Speaking generally, increased temperatures
and solution greatly increase the emanating power of both thorium and
radium.

The wide differences between the emanating powers of these substances in
the solid state and in solution pointed to the conclusion that the
differences were probably due to the rate of escape of the emanation
into the surrounding gas, and not to a variation of the rate of reaction
which gave rise to the emanation. It is obvious that a very slight
retardation in the rate of escape of the thorium emanation from the
compound into the gas, will, on account of the rapid decay of activity
of the emanation, produce great changes in emanating power. The
regeneration of the emanating power of de-emanated thoria and radium by
solution and chemical treatment made it evident that the original power
of thorium and radium of producing the emanation still persisted in an
unaltered degree.

The question whether the emanation was produced at the same rate in
emanating as in non-emanating compounds can be put to a sharp
quantitative test. If the rate of production of emanation goes on at the
same rate in the solid compound where very little escapes, as in the
solution where probably all escapes, the emanation must be _occluded_ in
the compound, and consequently there must be a sudden release of this
emanation on solution of the compound. On account of the very slow decay
of the activity of the emanation of radium, the effects should be far
more marked in that compound than in thorium.

From the point of view developed in section 133, the exponential law of
decay of the emanation expresses the result that _N_{t}_ the number of
particles remaining unchanged at the time _t_ is given by

$$ \frac {N_t} {N₀} = e^{–λt} $$ (1).

where _N₀_ is the initial number of particles present. When a steady
state is reached, the rate of production _q₀_ of fresh emanation
particles is exactly balanced by the rate of change of the particles
_N₀_ already present, _i.e._

                               _q₀_ = λ_N₀_,

_N₀_ in this case represents the amount of emanation “occluded” in the
compound. Substituting the value of λ found for the radium emanation in
section 145,

                         _N₀_
                         -----    = 1/λ = 463,000.
                         _q₀_

The amount of emanation stored in a non-emanating radium compound should
therefore be nearly 500,000 times the amount produced per second by the
compound. This result was tested in the following way[247].

A weight of ·03 gr. of radium chloride of activity 1000 times that of
uranium was placed in a Drechsel bottle and a sufficient amount of water
drawn in to dissolve it. The released emanation was swept out by a
current of air into a small gas holder and then into a testing cylinder.
The initial saturation current was proportional to _N₀_. A rapid current
of air was then passed through the radium solution for some time in
order to remove any slight amount of emanation which had not been
removed initially. The Drechsel bottle was closed air-tight, and allowed
to stand undisturbed for a definite time _t_. The accumulated emanation
was then swept out as before into the testing vessel. The new ionization
current represents the value of _N_{t}_ the amount of emanation formed
in the compound during the interval _t_.

                 In the experiment      _t_ = 105 minutes,
                 and the observed value
                 _N__{t}
                  -----     = ·0131.
                 _N₀_

Assuming that there is no decay during the interval,

                       _N_{t}_ = 105 × 60 × _q₀_.

                                _N₀_
                       Thus      ---  = 480,000.
                                _q₀_

Making the small correction for the decay of activity during the
interval,

                            _N₀_
                            -----    = 477,000.
                            _q₀_

We have previously shown that from the theory

                       _N₀_          1
                       -----   =   ---- = 463,000.
                       _q₀_          λ

The agreement between theory and experiment is thus as close as could be
expected from the nature of the experiments. This experiment proves
conclusively that the rate of production of emanation in the solid
compound is the same as in the solution. In the former case it is
occluded, in the latter it escapes as fast as it is produced.

It is remarkable how little emanation, compared with the amount stored
up in the compound, escapes from solid radium chloride in a dry
atmosphere. One experiment showed that the emanating power in the dry
solid state was less than ½% of the emanating power of the solution.
Since nearly 500,000 times as much emanation is stored up in the solid
compound as is produced per second, this result showed that the amount
of emanation which escaped per second was less than 10⁻⁸ of that
occluded in the compound.

If a solid radium chloride compound is kept in a moist atmosphere, the
emanating power becomes comparable with the amount produced per second
in the solution. In such a case, since the rate of escape is continuous,
the amount occluded will be much less than the amount for the
non-emanating material.

The phenomenon of occlusion of the radium emanation is probably not
connected in any way with its radio-activity, although this property has
here served to measure it. The occlusion of helium by minerals presents
almost a complete analogy to the occlusion of the radium emanation. Part
of the helium is given off by fergusonite, for example, when it is
heated and all of it when the mineral is dissolved.


=153.= Similar results hold for thorium, but, on account of the rapid
loss of activity of the emanation, the amount of emanation occluded in a
non-emanating compound is very small compared with that observed for
radium. If the production of the thorium emanation proceeds at the same
rate under all conditions, the solution of a solid non-emanating
compound should be accompanied by a rush of emanation greater than that
subsequently produced. With the same notation as before we have for the
thorium emanation,

                           _N₀_       1
                           -----  = ----- = 87.
                           _q₀_       λ

This result was tested as follows: a quantity of finely powdered thorium
nitrate, of emanating power ¹⁄₂₀₀ of ordinary thoria, was dropped into a
Drechsel bottle containing hot water and the emanation rapidly swept out
into the testing vessel by a current of air. The ionization current rose
quickly to a maximum, but soon fell again to a steady value; showing
that the amount of emanation released when the nitrate dissolves, is
greater than the subsequent amount produced from the solution.

The rapid loss of the activity of the thorium emanation makes a
quantitative comparison like that for radium very difficult. By slightly
altering the conditions of the experiment, however, a definite proof was
obtained that the rate of production of emanation is the same in the
solid compound as in the solution. After dropping in the nitrate, a
rapid air stream was blown through the solution for 25 seconds into the
testing vessel. The air stream was stopped and the ionization current
immediately measured. The solution was then allowed to stand undisturbed
for 10 minutes. In that time the accumulation of the emanation again
attained a practical maximum and again represented a steady state. The
stream of air was blown through, as before, for 25 seconds, stopped and
the current again measured. In both cases, the electrometer recorded a
movement of 14·6 divisions per second. By blowing the same stream of air
continuously through the solution the final current corresponded to 7·9
divisions per second or about one-half of that observed after the first
rush.

Thus the rate of production of emanation is the same in the solid
nitrate as in the solution, although the emanating power, _i.e._ the
rate of escape of the emanation, is over 600 times greater in the
solution than in the solid.

It seems probable that the rate of production of emanation by thorium,
like the rate of production of Ur X and Th X, is independent of
conditions. The changes of emanating power of the various compounds by
moisture, heat, and solution must therefore be ascribed solely to an
alteration in the rate of escape of the emanation into the surrounding
gas and not to an alteration in the rate of its production in the
compound.

On this view, it is easy to see that slight changes in the mode of
preparation of a thorium compound may produce large changes in emanating
power. Such effects have been often observed, and must be ascribed to
slight physical changes in the precipitate. The fact that the rate of
production of the emanation is independent of the physical or chemical
conditions of the thorium, in which it is produced, is thus in harmony
with what had previously been observed for the radio-active products Ur
X and Th X.


                    Source of the Thorium Emanation.


=154.= Some experiments of Rutherford and Soddy[248] will now be
considered, which show that the thorium emanation is produced, not
directly by the thorium itself, but by the active product Th X.

When the Th X, by precipitation with ammonia, is removed from a quantity
of thorium nitrate, the precipitated thorium hydroxide does not at first
possess appreciable emanating power. This loss of emanating power is not
due, as in the case of the de-emanated oxide, to a retardation in the
rate of escape of the emanation produced; for the hydroxide, when
dissolved in acid, still gives off no emanation. On the other hand, the
solution, containing the Th X, possesses emanating power to a marked
degree. When the precipitated hydroxide and the Th X is left for some
time, it is found that the Th X decreases in emanating power, while the
hydroxide gradually regains its emanating power. After about a month’s
interval, the emanating power of the hydroxide has nearly reached a
maximum, while the emanating power of the Th X has almost disappeared.

The curves of decay and recovery of emanating power with time are found
to be exactly the same as the curves of decay and recovery of activity
of Th X and the precipitated hydroxide respectively, shown in Fig. 47.
The emanating power of Th X, as well as its activity, falls to half
value in four days, while the hydroxide regains half its final emanating
power as well as half its lost activity in the same interval.

It follows from these results that the emanating power of Th X is
directly proportional to its activity, _i.e._ that the rate of
production of emanating particles is always proportional to the number
of α particles, projected from the Th X per second. _The radiation from
Th X thus accompanies the change of the Th X into the emanation._ Since
the emanation has chemical properties distinct from those of the Th X,
and also a distinctive rate of decay, it cannot be regarded as a vapour
of Th X, but it is a distinct chemical substance, produced by the
changes occurring in Th X. On the view advanced in section 136, the atom
of the emanation consists of the part of the atom of Th X left behind
after the expulsion of one or more α particles. The atoms of the
emanation are unstable, and in turn expel α particles. This projection
of α particles constitutes the radiation from the emanation, which
serves as a measure of the amount of emanation present. Since the
activity of the emanation falls to half value in _one_ minute while that
of Th X falls to half value in four days, the emanation consists of
atoms which disintegrate at intervals nearly 6000 times shorter than
those of the atoms of Th X.


              Source of the Radium and Actinium Emanation.


=155.= No intermediate stage—Radium X—between radium and its emanation,
corresponding to the Th X for thorium, has so far been observed. The
emanation from radium is probably produced directly from that element.
In this respect, the radium emanation holds the same position in regard
to radium as Th X does to thorium, and its production from radium can be
explained on exactly similar lines. It will be shown later in chapter X,
that the emanation of actinium, like that of thorium, does not arise
directly from the parent element but from an intermediate product
actinium X, which is very analogous in physical and chemical properties
to Th X.


                    Radiations from the Emanations.


=156.= Special methods are necessary to examine the nature of the
radiation from the emanations, for the radiations arise from the volume
of the gas in which the emanations are distributed. Some experiments to
examine the radiations from the thorium emanation were made by the
writer in the following way.

[Illustration: Fig. 55.]

A highly emanating thorium compound wrapped in paper was placed inside a
lead box _B_ about 1 cm. deep, shown in Fig. 55. An opening was cut in
the top of the box, over which a very thin sheet of mica was waxed. The
emanation rapidly diffused through the paper into the vessel, and after
ten minutes reached a state of radio-active equilibrium. The penetrating
power of the radiation from the emanation which passed through the thin
mica window was examined by the electrical method in the usual way by
adding screens of thin aluminium foil. The results are expressed in the
following table:

                   Thickness of mica window    ·0015 cm.
                   Thickness of aluminium foil ·00034 cm.

                          Layers of    Current
                               foil

                                  0        100

                                  1         59

                                  2         30

                                  3         10

                                  4          3·2

The greater proportion of the conductivity is thus due to α rays, as in
the case of the radio-active elements. The amount of absorption of these
α rays by aluminium foil is about the same as that of the rays from the
active bodies. No direct comparison can be made, for the α rays from the
emanation show the characteristic property of increased rate of
absorption with thickness of matter traversed. Before testing, the rays
have been largely absorbed by the mica window, and the penetrating power
has consequently decreased.

No alteration in the radiation from the emanation was observed on
placing an insulated wire inside the emanation vessel, and charging it
to a high positive or negative potential. When a stream of air through
the vessel carried away the emanation as fast as it was produced, the
intensity of the radiation fell to a small fraction of its former value.

No evidence of any β rays in the radiations was found in these
experiments, although a very small effect would have been detected.
After standing some hours, however, β rays began to appear. These were
due to the excited activity deposited on the walls of the vessel from
the emanation, and not directly to the emanation itself.

The radium emanation, like that of thorium, only gives rise to α rays.
This was tested in the following way[249]:

A large amount of emanation was introduced into a cylinder made of sheet
copper ·005 cm. thick, which absorbed all the α rays but allowed the β
and γ rays, if present, to pass through with but little loss. The
external radiation from the cylinder was determined at intervals,
commencing about two minutes after the introduction of the emanation.
The amount observed at first was extremely small, but increased rapidly
and practically reached a maximum in three or four hours. Thus the
radium emanation only gives out α rays, the β rays appearing as the
excited activity is produced on the walls of the vessel. On sweeping out
the emanation by a current of air, there was no immediately appreciable
decrease of the radiation. This is another proof that the emanation does
not emit any β rays. In a similar way it can be shown that the emanation
does not give out γ rays; these rays always make their appearance at the
same time as the β rays.

The method of examination of the radiations from the emanations has been
given in some detail, as the results are of considerable importance in
the discussion, which will be given later in chapters X and XI, of the
connection between the changes occurring in radio-active products and
the radiations they emit. There is no doubt that the emanations, apart
from the excited activity to which they give rise, only give out α rays,
consisting most probably of positively charged bodies projected with
great velocity.


     Effect of pressure on the rate of production of the Emanation.


=157.= It has already been mentioned that the conductivity due to the
thorium emanation is proportional to the pressure of the gas, pointing
to the conclusion that the rate of production of the emanation is
independent of the pressure, as well as of the nature of the surrounding
gas. This result was directly confirmed with the apparatus of Fig. 55.
When the pressure of the gas under the vessel was slowly reduced, the
radiation, tested outside the window, increased to a limit, and then
remained constant over a wide range of pressure. This increase, which
was far more marked in air than in hydrogen, is due to the fact that the
α rays from the emanation were partially absorbed in the gas inside the
vessel when at atmospheric pressure. At pressures of the order of 1
millimetre of mercury the external radiation decreased, but experiment
showed that this must be ascribed to a removal of the emanation by the
pump, and not to a change in the rate of production. The thorium
compounds very readily absorb water-vapour, which is slowly given off at
low pressures, and in consequence some of the emanation is carried out
of the vessel with the water-vapour.

Curie and Debierne[250] found that both the amount of excited activity
produced in a closed vessel containing active samples of radium, and
also the time taken to reach a maximum value, were independent of the
pressure and nature of the gas. This was true in the case of a solution
down to the pressure of the saturated vapour, and in the case of solid
salts to very low pressures. When the pump was kept going at pressures
of the order of ·001 mm. of mercury, the amount of excited activity was
much diminished. This was probably not due to any alteration of the rate
of escape of the emanation, but to the removal of the emanation by the
action of the pump as fast as it was formed.

Since the amount of excited activity, when in a state of radio-active
equilibrium, is a measure of the amount of emanation producing it, these
results show that the amount of emanation present when the rate of
production balances the rate of decay is independent of the pressure and
nature of the gas. It was also found that the time taken to reach the
point of radio-active equilibrium was independent of the size of the
vessel or the amount of active matter present. This proves that the
state of equilibrium cannot in any way be ascribed to the possession by
the emanation of any appreciable vapour pressure; for if such were the
case, the time taken to reach the equilibrium value should depend on the
size of the vessel and the amount of active matter present. The results
are, however, in agreement with the view that the emanation is present
in minute quantity in the tube, and that the equilibrium is governed
purely by the radio-active constant λ, the constant of decay of activity
of the emanation. This has been seen to be the same under all conditions
of concentration, pressure and temperature, and, provided the rate of
supply of the emanation from the active compound is not changed, the
time-rate of increase of activity to the equilibrium value will always
be the same, whatever the size of the vessel or the nature and pressure
of the surrounding gas.


                   Chemical Nature of the Emanations.


=158.= We shall now consider some experiments on the physical and
chemical properties of the emanations themselves, without reference to
the material producing them, in order to see if they possess any
properties which connect them with any known kind of matter.

It was soon observed that the thorium emanation passed unchanged through
acid solutions, and later the same result was shown to hold true in the
case of both emanations for every reagent that was tried. Preliminary
observations[251] showed that the thorium emanation, obtained in the
usual way by passing air over thoria, passed unchanged in amount through
a platinum tube heated electrically to the highest temperature
obtainable. The tube was then filled with platinum-black, and the
emanation passed through it in the cold, and with gradually increasing
temperatures, until the limit was reached. In another experiment, the
emanation was passed through a layer of red-hot lead-chromate in a glass
tube. The current of air was replaced by a current of hydrogen, and the
emanation was sent through red-hot magnesium-powder and red-hot
palladium-black, and, by using a current of carbon dioxide, through
red-hot zinc-dust. In every case the emanation passed through without
sensible change in the amount. If anything, a slight increase occurred,
owing to the time taken for the gas-current to pass through the tubes
when hot being slightly less than when cold, the decay _en route_ being
consequently less. The only known gases capable of passing in unchanged
amount through all the reagents employed are the recently discovered
members of the argon family.

But another possible interpretation might be put upon the results. If
the emanation were the manifestation of a type of excited radio-activity
on the surrounding atmosphere, then, since from the nature of the
experiments it was necessary to employ in each case as the atmosphere, a
gas not acted on by the reagent employed, the result obtained might be
expected. Red-hot magnesium would not retain an emanation consisting of
radio-active hydrogen, nor red-hot zinc-dust an emanation consisting of
radio-active carbon dioxide. The incorrectness of this explanation was
shown in the following way. Carbon dioxide was passed over thoria, then
through a =T=-tube, where a current of air met and mixed with it, both
passing on to the testing-cylinder. But between this and the =T=-tube a
large soda-lime tube was introduced, and the current of gas was thus
freed from its admixed carbon dioxide, before being tested in the
cylinder for the emanation. The amount of emanation found was quite
unchanged, whether carbon dioxide was sent over thoria in the manner
described, or whether, keeping the other arrangements as before, an
equally rapid current of air was substituted for it. The theory that the
emanation is an effect of the excited activity on the surrounding medium
is thus excluded.

Experiments of a similar kind on the radium emanation were made later. A
steady stream of gas was passed through a radium chloride solution and
then through the reagent to be employed, into a testing-vessel of small
volume, so that any change in the amount of emanation passing through
could readily be detected. The radium emanation, like that of thorium,
passed unchanged in amount through every reagent used.

In later experiments by Sir William Ramsay and Mr Soddy[252], the
emanation from radium was exposed to still more drastic treatment. The
emanation in a glass tube was sparked for several hours with oxygen over
alkali. The oxygen was then removed by ignited phosphorus and no visible
residue was left. When, however, another gas was introduced, mixed with
the minute amount of emanation in the tube and withdrawn, the activity
of emanation was found to be unaltered. In another experiment, the
emanation was introduced into a magnesium lime tube, which was heated
for three hours at a red heat. The emanation was then removed and
tested, but no diminution in its discharging power was observed.

The emanations of thorium and radium thus withstand chemical treatment
in a manner hitherto unobserved except in gases of the argon family.


=159.= Ramsay and Soddy (_loc. cit._) record an interesting experiment
to illustrate the gaseous nature of the emanation. A large amount of the
radium emanation was collected in a small glass tube. This tube
phosphoresced brightly under the influence of the rays from the
emanation. The passage of the emanation from point to point was observed
in a darkened room by the luminosity excited in the glass. On opening
the stop-cock connecting with the Töpler pump, the slow flow through the
capillary tube was noticed, the rapid passage along the wider tubes, the
delay in passing through a plug of phosphorous pentoxide, and the rapid
expansion into the reservoir of the pump. When compressed, the
luminosity of the emanation increased, and became very bright as the
small bubble containing the emanation was expelled through the fine
capillary tube.


                      Diffusion of the Emanations.


=160.= It has been shown that the emanations of thorium and radium
behave like radio-active gases, distributed in minute amount in the air
or other gas in which they are tested. With the small quantities of
active material so far investigated, the emanations have not yet been
collected in sufficient amount to determine their density. Although the
molecular weight of the emanations cannot yet be obtained by direct
chemical methods, an indirect estimate of it can be made by determining
the rate of their inter-diffusion into air or other gases. The
coefficients of inter-diffusion of various gases have long been known,
and the results show that the coefficient of diffusion of one gas into
another is, for the simpler gases, approximately inversely proportional
to the square root of the product of their molecular weights. If,
therefore, the coefficient of diffusion of the emanation into air is
found to have a value, lying between that of two known gases _A_ and
_B_, it is probable that the molecular weight of the emanation lies
between that of _A_ and _B_.

Although the volume of the emanation given off from radium is very
small, the electrical conductivity produced by the emanation in the gas,
with which it is mixed, is often very large, and offers a ready means of
measuring the emanation present.

Some experiments have been made by Miss Brooks and the writer[253] to
determine the rate of the diffusion of the radium emanation into air, by
a method similar to that employed by Loschmidt[254] in 1871, in his
investigations of the coefficient of inter-diffusion of gases.

[Illustration: Fig. 56.]

Fig. 56 shows the general arrangement. A long brass cylinder _AB_, of
length 73 cms., and diameter 6 cms., was divided into two equal parts by
a movable metal slide _S_. The ends of the cylinder were closed with
ebonite stoppers. Two insulated brass rods, _a_ and _b_, each half the
length of the tube, passed through the ebonite stoppers and were
supported centrally in the tube. The cylinder was insulated and
connected with one pole of a battery of 300 volts, the other pole of
which was earthed. The central rods could be connected with a sensitive
quadrant electrometer. The cylinder was covered with a thick layer of
felt, and placed inside a metal box filled with cotton wool in order to
keep temperature conditions as steady as possible.

In order to convey a sufficient quantity of emanation into the
half-cylinder _A_, it was necessary to heat the radium slightly. The
slide _S_ was closed and the side tubes opened. A slow current of dry
air from a gasometer was passed through a platinum tube, in which a
small quantity of radium compound was placed. The emanation was carried
with the air into the cylinder _A_. When a sufficient quantity had been
introduced, the stream of air was stopped. The side tubes were closed by
fine capillary tubes. These prevented any appreciable loss of gas due to
the diffusion, but served to keep the pressure of the gas inside _A_ at
the pressure of the outside air. The three entrance tubes into the
cylinder, shown in the figure, were for the purpose of initially mixing
the emanation and gas as uniformly as possible.

After standing several hours to make temperature conditions steady, the
slide was opened, and the emanation began to diffuse into the tube _B_.
The current through the tubes _A_ and _B_ was measured at regular
intervals by an electrometer, with a suitable capacity in parallel.
Initially there is no current in _B_, but after the opening of the
slide, the amount in _A_ decreased and the amount in _B_ steadily
increased. After several hours the amount in each half is nearly the
same, showing that the emanation is nearly uniformly diffused throughout
the cylinder.

It can readily be shown[255] that if

        _K_ = coefficient of diffusion of the emanation into air,
        _t_ = duration of diffusion experiments in secs.,
        _a_ = total length of cylinder,
        _S₁_ = partial pressure of emanation in tube _A_ at end of
       diffusion,
        _S₂_ = partial pressure of emanation in tube _B_ at end of
       diffusion,

then

$$ \frac {S_1 − S_2} {S_1 + S_2} = \frac {8} {\pi^2} (e^{\frac {\pi^2
Kt} {a^2}} + \frac {1}{9}e^{-\frac {9 \pi^2 Kt} {a^2}} + ...) $$ .

Now the values of _S₁_ and _S₂_ are proportional to the saturation
ionization currents due to the emanations in the two halves of the
cylinder. From this equation _K_ can be determined, if the relative
values of _S₁_ and _S₂_ are observed after diffusion has been in
progress for a definite interval _t_.

The determination of _S₁_ and _S₂_ is complicated by the excited
activity produced on the walls of the vessel. The ionization due to this
must be subtracted from the total ionization observed in each half of
the cylinder, for the excited activity is produced from the material
composing the emanation, and is removed to the electrodes in an electric
field. The ratio of the current due to excited activity to the current
due to the emanation depends on the time of exposure to the emanation,
and is only proportional to it for exposures of several hours.

The method generally adopted in the experiments was to open the slide
for a definite interval, ranging in the experiments from 15 to 120
minutes. The slide was then closed and the currents in each half
determined at once. The central rods, which had been kept negatively
charged during the experiments, had most of the excited activity
concentrated on their surfaces. These were removed, new rods substituted
and the current immediately determined. The ratio of the currents in the
half cylinders under these conditions was proportional to _S₁_ and _S₂_,
the amounts of emanation present in the two halves of the cylinder.

The values of _K_, deduced from different values of _t_, were found to
be in good agreement. In the earlier experiments the values of _K_ were
found to vary between ·08 and ·12. In some later experiments, where
great care was taken to ensure that temperature conditions were very
constant, the values of _K_ were found to vary between ·07 and ·09. The
lower value ·07 is most likely nearer the true value, as temperature
disturbances tend to give too large a value of _K_. No certain
differences were observed in the value of _K_ whether the air was dry or
damp, or whether an electric field was acting or not.


=161.= Some experiments on the rate of diffusion of the radium emanation
into air were made at a later date by P. Curie and Danne[256]. If the
emanation is contained in a closed reservoir, it has been shown that its
activity, which is a measure of the amount of emanation present,
decreases according to an exponential law with the time. If the
reservoir is put in communication with the outside air through a
capillary tube, the emanation slowly diffuses out, and the amount of
emanation in the reservoir is found to decrease according to the same
law as before, but at a faster rate. Using tubes of different lengths
and diameters, the rate of diffusion was found to obey the same laws as
a gas. The value of _K_ was found to be 0·100. This is a slightly
greater value of _K_ than the lowest value 0·07 found by Rutherford and
Miss Brooks. No mention is made by Curie and Danne of having taken any
special precautions against temperature disturbances, and this may
account for the higher value of _K_ obtained by them.

They also found that the emanation, like a gas, always divided itself
between two reservoirs, put in connection with one another, in the
proportion of their volumes. In one experiment one reservoir was kept at
a temperature of 10° C. and the other at 350° C. The emanation divided
itself between the two reservoirs in the same proportion as would a gas
under the same conditions.


=162.= For the purpose of comparison, a few of the coefficients of
inter-diffusion of gases, compiled from Landolt and Bernstein’s tables,
are given below.

              Gas or vapour    Coefficient of     Molecular
                               diffusion into      weight
                               air


               Water vapour    0·198                   18

               Carbonic acid   0·142                   44
               gas

               Alcohol vapour  0·101                   46

               Ether vapour    0·077                   74

               Radium          0·07                     ?
               emanation

The tables, although not very satisfactory for the purpose of
comparison, show that the coefficient of inter-diffusion follows the
inverse order of the molecular weights. The value of _K_ for the radium
emanation is slightly less than for ether vapour, of which the molecular
weight is 74. We may thus conclude that the emanation is of greater
molecular weight than 74. It seems likely that the emanation has a
molecular weight somewhere in the neighbourhood of 100, and is probably
greater than this, for the vapours of ether and alcohol have higher
diffusion coefficients compared with carbonic acid than the theory would
lead us to anticipate. Comparing the diffusion coefficients of the
emanation and carbonic acid into air, the value of the molecular weight
of the emanation should be about 176 if the result observed for the
simple gases, viz. that the coefficient of diffusion is inversely
proportional to the square root of the molecular weights, holds true in
the present case. Bumstead and Wheeler[257] compared the rates of
diffusion of the radium emanation and of carbon dioxide through a porous
plate, and concluded that the molecular weight of the emanation was
about 180. On the disintegration theory, the atom of the emanation is
derived from the radium atom by the expulsion of one α particle. Thus,
it is to be expected that its molecular weight would be over 200.

It is of interest to compare the value of _K_ = ·07 with the value of
_K_ determined by Townsend (section 37) for the gaseous ions produced in
air at ordinary pressure and temperature, by Röntgen rays or by the
radiations from active substances. Townsend found that the value of _K_
in dry air was ·028 for the positive ions and ·043 for the negative
ions. The radium emanation thus diffuses more rapidly than the ions
produced by its radiation in the gas, and behaves as if its mass were
smaller than that of the ions produced in air, but considerably greater
than that of the air molecules with which it is mixed.

It is not possible to regard the emanation as a temporarily modified
condition of the gas originally in contact with the active body. Under
such conditions a much larger value of _K_ would be expected. The
evidence derived from the experiments on diffusion strongly supports the
view that the emanation is a gas of heavy molecular weight.

Makower[258] has recently attacked the question of the molecular weight
of the radium emanation by another method. The rate of diffusion of the
emanation through a porous plug of plaster-of-Paris was compared with
that of the gases oxygen, carbon dioxide, and sulphur dioxide. It was
found that Graham’s law, viz. that the coefficient of diffusion _K_ is
inversely proportional to the square root of its molecular weight _M_,
was not strictly applicable. The value of _K_ √_M_ was not found to be
constant for these gases, but decreased with increase of molecular
weight of the gas. If, however, a curve was plotted with _K_ √_M_ as
ordinate and _K_ as abscissa, the points corresponding to the values of
O, CO₂ and SO₂ were found to lie on a straight line. By linear
extrapolation, the molecular weight of the emanation was estimated. The
value obtained from experiments on three different porous plugs was
85·5, 97, and 99 respectively. This method indicates that the molecular
weight of the radium emanation is about 100; but in all the experiments
on diffusion, it must be remembered that the emanation, whose rate of
inter-diffusion is being examined, exists in minute quantity mixed with
the gas, and is compared with the rate of inter-diffusion of gases which
are present in large quantity. For this reason, deductions of the
molecular weight of the emanation may be subject to comparatively large
errors, for which it is difficult to make correction.


                  Diffusion of the Thorium Emanation.


=163.= On account of the rapid decay of the activity of the thorium
emanation, it is not possible to determine the value of _K_ its
coefficient of diffusion into air by the methods employed for the radium
emanation. The value of _K_ has been determined by the writer in the
following way. A plate _C_, Fig. 57, covered with thorium hydroxide, was
placed horizontally near the base of a long vertical brass cylinder _P_.
The emanation released from the thorium compound diffuses upwards in the
cylinder.

[Illustration: Fig. 57.]

Let _p_ be the partial pressure of the emanation at a distance _x_ from
the source _C_. This will be approximately uniform over the cross
section of the cylinder. From the general principles of diffusion we get
the equation

                              _d²p_     _dp_
                          _K_ ----- = − ----  .
                              _dx²_     _dt_

The emanation is continuously breaking up and expelling α particles. The
emanation-residue gains a positive charge, and, in an electric field, is
removed at once from the gas to the negative electrode.

Since the activity of the emanation at any time is always proportional
to the number of particles which have not broken up, and since the
activity decays with the time according to an exponential law,

$$ p = p_1 e^{–λt} $$,

where _p₁_ is the value of _p_ when _t_ = 0 and λ is the _radio-active
constant_ of the emanation.

                              Then

                              _dp_
                              ----  =  -λ_p_,
                              _dt_

                              and

                               _d²p_
                             K -----  = λ_p_.
                               _dx²_

Thus

$$ p = Ae^{-\sqrt {\frac {λ} {K} x}} + Be^{\sqrt {\frac {λ}
{K} x}} $$ .

Since _p_ = 0 when _x_ = infinity, _B_ = 0. If _p_ = _p₀_ when _x_ = 0,
_A_ = _p₀_.

Thus

$$ p = p₀ e^{-\sqrt {\frac {λ} {K} x}} $$ .

It was not found convenient in the experiments to determine the activity
of the emanation along the cylinder, but an equivalent method was used
which depends upon measuring the distribution of “excited activity,”
produced along a central rod _AB_, which is charged negatively.

It will be shown later (section 177) that the amount of excited activity
at any point is always proportional to the amount of emanation at that
point. The distribution of “excited activity” along the central rod from
the plate _C_ upwards thus gives the variation of _p_ for the emanation
along the tube.

In the experiments, the cylinder was filled with dry air at atmospheric
pressure and was kept at a constant temperature. The central rod was
charged negatively and exposed from one to two days in the presence of
the emanation. The rod was then removed, and the distribution of the
excited activity along it determined by the electric method. It was
found that the amount of excited activity fell off with the distance _x_
according to an exponential law, falling to half value in about 1·9 cms.
This is in agreement with the above theory.

Since the activity of the emanation falls to half value in 1 minute, λ =
·0115. The value _K_ = ·09 was deduced from the average of a number of
experiments. This is a slightly greater value than _K_ = ·07, obtained
for the radium emanation, but the results show that the two emanations
do not differ much from one another in molecular weight.

Makower (_loc. cit._) compared the rates of diffusion of the thorium and
radium emanation through a porous plate, and concluded that the two
emanations were of about the same molecular weight, thus confirming the
results obtained by the above method.


                Diffusion of the Emanation into Liquids.


=164.= Experiments have been made by Wallstabe[259] on the coefficient
of diffusion of the radium emanation into various liquids. The radium
emanation was allowed to diffuse into a closed reservoir, containing a
cylinder of the liquid under observation. The cylinder was provided with
a tube and a stop-cock extending beyond the closed vessel, so that
different layers of the liquid could be removed. The liquid was then
placed in a closed testing vessel, where the ionization current due to
the escape of the emanation from the liquid was observed to rise to a
maximum after several hours, and then to decay. This maximum value of
the current was taken as a measure of the amount of emanation absorbed
in the liquid.

The coefficient of diffusion _K_ of the emanation into the liquid can be
obtained from the same equation used to determine the diffusion of the
thorium emanation into air,

$$ p = p₀ e^{-\sqrt {\frac {λ} {K} x}} $$ .

where λ is the constant of decay of activity of the radium emanation and
_x_ the depth of the layer of water from the surface.

Putting

$$ a = \sqrt {\frac {λ} {K}} $$,

it was found that

                            for water  α = 1·6,
                            for toluol α = ·75.

The value of λ expressed in terms of a day as the unit of time is about
·17.

Thus the value of _K_ for the diffusion of the radium emanation into
water = ·066 cm.² / day.

The value of _K_ found by Stefan[260] for the diffusion of carbon
dioxide into water was 1·36 cm.²/day. These results are thus in harmony
with the conclusion drawn from the diffusion of the radium emanation
into air, and show that the radium emanation behaves as a gas of high
molecular weight.


                    Condensation of the Emanations.


=165. Condensation of the emanations.= During an investigation of the
effect of physical and chemical agencies on the thorium emanation,
Rutherford and Soddy[261] found that the emanation passed unchanged in
amount through a white-hot platinum tube and through a tube cooled to
the temperature of solid carbon dioxide. In later experiments the
effects of still lower temperatures were examined, and it was then found
that at the temperature of liquid air both emanations were
condensed[262].

If either emanation is conveyed by a slow stream of hydrogen, oxygen, or
air through a metal spiral immersed in liquid air, and placed in
connection with a testing vessel as in Fig. 51, no trace of emanation
escapes in the issuing gas. When the liquid air is removed and the
spiral plunged into cotton-wool, several minutes elapse before any
deflection of the electrometer needle is observed, and then the
condensed emanation volatilizes rapidly, and the movement of the
electrometer needle is very sudden, especially in the case of radium.
With a fairly large amount of radium emanation, under the conditions
mentioned, a very few seconds elapse after the first sign of movement
before the electrometer needle indicates a deflection of several hundred
divisions per second. It is not necessary in either case that the
emanating compound should be retained in the gas stream. After the
emanation is condensed in the spiral, the thorium or radium compound may
be removed and the gas stream sent directly into the spiral. But in the
case of thorium, under these conditions, the effects observed are
naturally small owing to the rapid loss of the activity of the emanation
with time, which proceeds at the same rate at the temperature of liquid
air as at ordinary temperatures.

If a large amount of radium emanation is condensed in a glass =U= tube,
the progress of the condensation can be followed by the eye, by means of
the phosphorescence which the radiations excite in the glass. If the
ends of the tube are sealed and the temperature allowed to rise, the
glow diffuses uniformly throughout the tube, and can be concentrated at
any point to some extent by local cooling of the tube with liquid air.


=166. Experimental arrangements.= A simple experimental arrangement to
illustrate the condensation and volatilization of the emanation and some
of its characteristic properties is shown in Fig. 58. The emanation
obtained from a few milligrams of radium bromide by solution or heating
is condensed in the glass =U= tube _T_ immersed in liquid air. This =U=
tube is then put into connection with a larger glass tube _V_, in the
upper part of which is placed a piece of zinc sulphide screen _Z_, and
in the lower part of the tube a piece of the mineral willemite. The
stop-cock _A_ is closed and the =U= tube and the vessel _V_ are
partially exhausted by a pump through the stop-cock _B_. This lowering
of the pressure causes a more rapid diffusion of the emanation when
released. The emanation does not escape if the tube _T_ is kept immersed
in liquid air. The stop-cock _B_ is then closed, and the liquid air
removed. No luminosity of the screen or the willemite in the tube _V_ is
observed for several minutes, until the temperature of _T_ rises above
the point of volatilization of the emanation. The emanation is then
rapidly carried into the vessel _V_, partly by expansion of the gas in
the tube _T_ with rising temperature, and partly by the process of
diffusion. The screen _Z_ and the willemite _W_ are caused to
phosphoresce brilliantly under the influence of the rays from the
emanation surrounding them.

[Illustration: Fig. 58.]

If the end of the vessel _V_ is then plunged into liquid air, the
emanation is again condensed in the lower end of the tube, and the
willemite phosphoresces much more brightly than before. This is not due
to an increase of the phosphorescence of willemite at the temperature of
the liquid air, but to the effect of the rays from the emanation
condensed around it. At the same time the luminosity of the zinc
sulphide gradually diminishes, and practically disappears after several
hours if the end of the tube is kept in the liquid air. If the tube is
removed from the liquid air, the emanation again volatilizes and lights
up the screen _Z_. The luminosity of the willemite returns to its
original value after the lapse of several hours. This slow change of the
luminosity of the zinc sulphide screen and of the willemite is due to
the gradual decay of the “excited activity” produced by the emanation on
the surface of all bodies exposed to its action (chapter VIII). The
luminosity of the screen is thus due partly to the radiation from the
emanation and partly to the excited radiation caused by it. As soon as
the emanation is removed from the upper to the lower part of the tube,
the “excited” radiation gradually diminishes in the upper and increases
in the lower part of the tube.

The luminosity of the screen gradually diminishes with the time as the
enclosed emanation loses its activity, but is still appreciable after an
interval of several weeks.

An apparatus of a similar character to illustrate the condensation of
the radium emanation has been described by P. Curie[263].

[Illustration: Fig. 59.]


=167. Determination of the temperature of condensation.= A detailed
investigation was made by Rutherford and Soddy (_loc. cit._) of the
temperatures at which condensation and volatilization commenced for the
two emanations. The experimental arrangement of the first method is
shown clearly in Fig. 59. A slow constant stream of gas, entering at
_A_, was passed through a copper spiral _S_, over 3 metres in length,
immersed in a bath of liquid ethylene. The copper spiral was made to act
as its own thermometer by determining its electrical resistance. The
resistance temperature curve was obtained by observation of the
resistances at 0°, the boiling point of liquid ethylene −103·5°, the
solidification point of ethylene −169° and in liquid air. The
temperature of the liquid air was deduced from the tables given by Baly
for the boiling point of liquid air for different percentages of oxygen.
The resistance-temperature curve, for the particular spiral employed,
was found to be nearly a straight line between 0° and −192°C., cutting
the temperature axis if produced nearly at the absolute zero. The
resistance of the spiral, deduced from readings on an accurately
calibrated Weston millivoltmeter, with a constant current through the
spiral, was thus very approximately proportional to the absolute
temperature. The liquid ethylene was kept vigorously stirred by an
electric motor, and was cooled to any desired temperature by surrounding
the vessel with liquid air.

The general method employed for the radium emanation was to pass a
suitable amount of emanation, mixed with the gas to be used, from the
gas holder _B_ into the spiral, cooled below the temperature of
condensation. After the emanation was condensed in the spiral, a current
of electrolytic hydrogen or oxygen was passed through the spiral. The
temperature was allowed to rise gradually, and was noted at the instant
when a deflection of the electrometer, due to the presence of emanation
in the testing vessel _T_, was observed. The resistance, subject to a
slight correction due to the time taken for the emanation to be carried
into the testing vessel, gave the temperature at which some of the
emanation commenced to volatilize. The ionization current in the testing
vessel rose rapidly to a maximum value, showing that, for a small
increase of temperature, the whole of the radium emanation was
volatilized. The following table gives an illustration of the results
obtained for a current of hydrogen of 1·38 cubic centimetres per second.

                        Temperature   Divisions per
                                      second of the
                                       electrometer


                              −160°               0

                              −156°               0

                              −154°·3             1

                              −153°·8            21

                              −152°·5            24

The following table shows the results obtained for different currents of
hydrogen and oxygen.

                         Current of Gas _T₁_    _T₂_

          Hydrogen    ·25 c.c. per sec. −151·3     −150
          „             ·32     „     „ −153·7     −151
          „             ·92     „     „ −152       −151
          „            1·38     „     „ −154       −153
          „            2·3      „     „ −162·5     −162
          Oxygen        ·34     „     „ −152·5     −151·5
          „             ·58     „     „ −155       −153

The temperature _T₁_ in the above table gives the temperature of initial
volatilization, _T₂_ the temperature for which half of the condensed
emanation had been released. For slow currents of hydrogen and oxygen,
the values of _T₁_ and _T₂_ are in good agreement. For a stream of gas
as rapid as 2·3 cubic centimetres per second the value of _T₁_ is much
lower. Such a result is to be expected; for, in too rapid a stream, the
gas is not cooled to the temperature of the spiral, and, in consequence,
the inside surface of the spiral is above the mean temperature, and some
of the emanation escapes at a temperature apparently much lower. In the
case of oxygen, this effect appears for a gas stream of 0·58 cubic
centimetres per second.

In the experiments on the thorium emanation, on account of the rapid
loss of activity, a slightly different method was necessary. The steady
stream of gas was passed over the thorium compound, and the temperature
was observed at the instant when an appreciable movement of the
electrometer appeared. This gave the temperature at which a small
fraction of the thorium emanation escaped condensation, and not the
value _T₁_ observed for the radium emanation, which gave the temperature
for which a small fraction of the previously condensed emanation was
volatilized.

The following table illustrates the results obtained.

                           Current of Gas  Temperature


                Hydrogen   ·71 c.c. per    −155° C.
                           sec.

                „          1·38  „      „  −159° C.

                Oxygen     ·58  „      „   −155° C.

On comparing these results with the values obtained for the radium
emanation, it will be observed that with equal gas streams the
temperatures are nearly the same.

A closer examination of the thorium emanation showed, however, that this
apparent agreement was only accidental, and that there was, in reality,
a very marked difference in the effect of temperature on the two
emanations. It was found experimentally that the radium emanation was
condensed very near the temperature at which volatilization commenced,
and that the points of condensation and volatilization were defined
fairly sharply.

[Illustration: Fig. 60.]

On the other hand, the thorium emanation required a range of over 30° C.
after condensation had started in order to ensure complete condensation.
Fig. 60 is an example of the results obtained with a steady gas stream
of 1·38 c.c. per sec. of oxygen. The ordinates represent the percentage
proportion of the emanation uncondensed at different temperatures. It
will be observed that condensation commences about −120°, and that very
little of the emanation escapes condensation at −155° C.

To investigate this difference of behaviour in the two emanations, a
static method was employed, which allowed an examination of the two
emanations to be made under comparable conditions. The emanation, mixed
with a small amount of the gas to be used, was introduced into the cool
spiral, which had been exhausted previously by means of a mercury pump.
The amount of emanation remaining uncondensed after definite intervals
was rapidly removed by means of the pump, and was carried with a
constant auxiliary stream of gas into the testing vessel.

Tested in this way, it was found that the volatilization point of the
radium emanation was very nearly the same as that obtained by the
blowing method, viz. −150° C. With thorium, on the other hand, the
condensation started at about −120° C., and, as in the blowing method,
continued over a range of about 30° C. The proportion of the emanation
condensed at any temperature was found to depend on a variety of
conditions, although the point at which condensation commenced, viz.
−120° C., was about the same in each case. It depended on the pressure
and nature of the gas, on the concentration of the emanation, and on the
time for which it was left in the spiral. For a given temperature a
greater proportion of the emanation was condensed, the lower the
pressure and the longer the time it was left in the spiral. Under the
same conditions, the emanation was condensed more rapidly in hydrogen
than in oxygen.


=168.= Thus there is no doubt that the thorium emanation begins to
condense at a temperature higher than that at which the radium emanation
condenses. The explanation of the peculiar behaviour of the thorium
emanation is clear when the small number of emanation particles present
in the gas are taken into consideration. It has been shown that both
emanations give out only α rays. It is probable that the α particles
from the two emanations are similar in character and produce about the
same number of ions in their passage through the gas. The number of ions
produced by each α particle before its energy is dissipated is probably
about 70,000. (See section 252.)

Now, in the experiment, the electrometer readily measured a current of
10⁻³ electrostatic units. Taking the charge on an ion as 3·4 × 10⁻¹⁰
electrostatic units, this corresponds to a production in the testing
vessel of about 3 × 10⁶ ions per sec., which would be produced by about
40 expelled α particles per second. Each radiating particle cannot expel
less than one α particle and may expel more, but it is likely that the
number expelled by an atom of the thorium emanation is not greatly
different from that expelled by an atom of the radium emanation.

In section 133 it has been shown that, according to the law of decay,
λ_N_ particles change per second when _N_ are present. Thus, to produce
40 α particles, λ_N_ cannot be greater than 40. Since for the thorium
emanation λ is ¹⁄₈₇, it follows that _N_ cannot be greater than 3500.
The electrometer thus detected the presence of 3500 particles of the
thorium emanation, and since in the static method the volume of the
condensing spiral was about 15 c.c., this corresponded to a
concentration of about 230 particles per c.c. An ordinary gas at
atmospheric pressure and temperature probably contains about 3·6 × 10¹⁹
molecules per c.c. Thus the emanation would have been detected on the
spiral if it had possessed a partial pressure of less than 10⁻¹⁷ of an
atmosphere.

It is not surprising then that the condensation point of the thorium
emanation is not sharply defined. It is rather a matter of remark that
condensation should occur so readily with so sparse a distribution of
emanation particles in the gas; for, in order that condensation may take
place, it is probable that the particles must approach within one
another’s sphere of influence.

Now in the case of the radium emanation, the rate of decay is about 5000
times slower than that of the thorium emanation, and consequently the
actual number of particles that must be present to produce the same
ionization per second in the two cases must be about 5000 times greater
in the case of radium than in the case of thorium. This conclusion
involves only the assumption that the same number of rays is produced by
a particle of emanation in each case, and that the expelled particles
produce in their passage through the gas the same number of ions. The
number of particles present, in order to be detected by the
electrometer, in this experiment, must therefore have been about 5000 ×
3500, _i.e._ about 2 × 10⁷. The difference of behaviour in the two cases
is well explained by the view that, _for equal electrical effects_, the
number of radium emanation particles must be far larger than the number
of thorium emanation particles. The probability of the particles coming
into each other’s sphere of influence will increase very rapidly as the
concentration of the particles increases, and, in the case of the radium
emanation, once the temperature of condensation is attained, all but a
small proportion of the total number of particles present will condense
in a very short time. In the case of the thorium emanation, however, the
temperature might be far below that of condensation, and yet a
considerable portion remain uncondensed for comparatively long
intervals. On this view the experimental results obtained might
reasonably be expected. A greater proportion of emanation condenses the
longer the time allowed for condensation under the same conditions. The
condensation occurs more rapidly in hydrogen than in oxygen, as the
diffusion is greater in the former gas. For the same reason the
condensation occurs faster the lower the pressure of the gas present.
Finally, when the emanation is carried by a steady stream of gas, a
smaller proportion condenses than in the other cases, because the
concentration of emanation particles per unit volume of gas is less
under these conditions.

It is possible that the condensation of the emanations may not occur in
the gas itself but at the surface of the containing vessel. Accurate
observations of the temperature of condensation have so far only been
made in a copper spiral, but condensation certainly occurs in tubes of
lead or glass at about the same temperature as in tubes of copper.


=169.= In experiments that were made by the static method with a very
large quantity of radium emanation, a slight amount of escape of the
condensed emanation was observed several degrees below the temperature
at which most of the emanation was released. This is to be expected,
since, under such conditions, the electrometer is able to detect a very
minute proportion of the whole quantity of the emanation condensed.

Special experiments, with a large quantity of emanation, that were made
with the spiral immersed in a bath of rapidly boiling nitric oxide,
showed this effect very clearly. For example, the condensed emanation
began to volatilize at −155° C. In 4 minutes the temperature had risen
to −153·5°, and the amount volatilized was four times as great as at
−155°. In the next 5½ minutes the temperature had increased to −152·3°
and practically the whole quantity, which was at least fifty times the
amount at the temperature of −153·5°, had volatilized.

It thus seems probable that, if the temperature were kept steady at the
point at which volatilization was first observed, and the released
emanation removed at intervals, the whole of the emanation would in
course of time be liberated at that temperature. Curie and Dewar and
Ramsay have observed that the emanation condensed in a =U= tube,
immersed in liquid air, slowly escapes if the pump is kept steadily
working. These results point to the probability that the condensed
emanation possesses a true vapour pressure, but great refinements in
experimental methods would be necessary before such a conclusion could
be definitely established.

The true temperature of condensation of the thorium emanation is
probably about −120° C., and that of radium about −150° C. Thus there is
no doubt that the two emanations are quite distinct from each other in
this respect, and also with regard to their radio-activity, although
they both possess the property of chemical inertness. These results on
the temperatures of condensation do not allow us to make any comparison
of the condensation points of the emanations with those of known gases,
since the lowering of the condensation points of gases with diminution
of pressure has not been studied at such extremely minute pressures.


=170.= It has been found[264] that the activity of the thorium
emanation, when condensed in the spiral at the temperature of liquid
air, decayed at the same rate as at ordinary temperatures. This is in
accord with results of a similar kind obtained by P. Curie for the
radium emanation (section 145), and shows that the value of the
radio-active constant is unaffected by wide variations of temperature.


              Amount of Emanation from Radium and Thorium.


=171.= It has been shown in section 93 from experimental data that 1
gram of radium bromide at its minimum activity emits about 3·6 × 10¹⁰ α
particles per second. Since the activity due to the emanation stored up
in radium, when in a state of radio-active equilibrium, is about one
quarter of the whole and about equal to the minimum activity, the number
of α particles projected per second by the emanation from 1 gram of
radium bromide is about 3·6 × 10¹⁰. It has been shown in section 152
that 463,000 times the amount of emanation produced per second is stored
up in the radium. But, in a state of radio-active equilibrium, the
number of emanation particles breaking up per second is equal to the
number produced per second. Assuming that each emanation particle in
breaking up expels one α particle, it follows that the number of
emanation particles present in 1 gram of radium bromide in radio-active
equilibrium is 463,000 × 3·6 × 10¹⁰, _i.e._ 1·7 × 10¹⁶. Taking the
number of hydrogen molecules in 1 c.c. of gas at atmospheric pressure
and temperature as 3·6 × 10¹⁹ (section 39), the volume of the emanation
from 1 gram of radium bromide is 4·6 × 10⁻⁴ cubic centimetres at
atmospheric pressure and temperature. Assuming the composition of radium
bromide as RaBr₂, the amount from 1 gram of radium in radio-active
equilibrium is 0·82 cubic millimetres. Quite independently of any method
of calculation it was early evident that the volume of the emanation was
very small, for all the earlier attempts made to detect its presence by
its volume were unsuccessful. It will be seen, however, that, when
larger quantities of radium were available for experiment, the emanation
has been collected in volume sufficiently large to measure.

In the case of thorium, the maximum quantity of emanation to be obtained
from 1 gram of the solid is very minute, both on account of the small
activity of thorium and of the rapid break up of the emanation after its
production. Since the amount of emanation, stored in a non-emanating
thorium compound, is only 87 times the rate of production, while in
radium it is 463,000 times, and the rate of production of the emanation
by radium is about 1 million times faster than by thorium, it follows
that the amount of emanation to be obtained from 1 gram of thorium is
not greater than 10⁻¹⁰ of the amount from an equal weight of radium,
_i.e._ its volume is not greater than 10⁻¹³ c.c. at the ordinary
pressure and temperature. Even with large quantities of thorium, the
amount of emanation is too small ever to be detected by its volume.


=172. Volume of the emanation from radium.= The evidence already
considered points very strongly to the conclusion that the emanation
possesses all the properties of a chemically inert gas of high molecular
weight.

Since the emanation continuously breaks up, and is transformed into a
solid type of matter, which is deposited on the surface of bodies, the
volume of the emanation, when separated from radium, should contract at
the same rate as it loses its activity, _i.e._ it should decrease to
half value in about four days. The amount of emanation to be obtained
from a given quantity of radium is a maximum when the rate of production
of new emanation balances its rate of change. This condition is
practically attained when the emanation has been allowed to collect for
an interval of one month. The probable volume of the emanation to be
obtained from 1 gram of radium was early calculated on certain
assumptions, and from data then available the writer[265] deduced that
the volume of the emanation from 1 gram of radium lay between ·06 and ·6
cubic millimetre at atmospheric pressure and temperature, and was
probably nearer the latter value. The volume to be expected on the
latest data has been discussed in the preceding section and shown to be
about ·82 cubic mm. The volume of the emanation is thus very small, but
not too small to be detected if several centigrams of radium are
available. This has been proved to be the case by Ramsay and Soddy[266]
who, by very careful experiment, finally succeeded in isolating a small
quantity of the emanation and in determining its volume. The
experimental method employed by them will now be briefly described.

[Illustration: Fig. 61.]

The emanation from 60 milligrams of radium bromide in solution was
allowed to collect for 8 days and then drawn off through the inverted
siphon _E_ (Fig. 61) into the explosion burette _F_. This gas consisted
for the most part of hydrogen and oxygen, produced by the action of the
radiations on the water of the solution. After explosion, the excess of
hydrogen mixed with emanation was left some time in contact with caustic
soda, placed in the upper part of the burette, in order to remove all
trace of carbon dioxide. In the meantime the upper part of the apparatus
had been completely evacuated. The connection _C_ to the pump was
closed, and the hydrogen and emanation were allowed to enter the
apparatus, passing over a phosphorous pentoxide tube _D_. The emanation
was condensed in the lower part of the capillary tube _A_, by
surrounding it with the tube _B_ filled with liquid air. The process of
condensation was rendered manifest by the brilliant luminosity of the
lower part of the tube. The mercury from the burette was then allowed to
run to _G_, and the apparatus again completely evacuated. The connection
of the pump was again closed, the liquid air was removed and the
volatilized emanation forced into the fine capillary tube _A_.
Observations were then made, from day to day, of the volume of the
emanation. The results are given in the table below.

            Time   Volume        Time       Volume

           Start   0·124 cub.    7 days     0·0050 cub. mm.
                   mm.

           1 day   0·027   „     9  „       0·0041   „

           3  „    0·011   „     11  „      0·0020   „

           4  „    0·0095  „     12  „      0·0011   „

           6  „    0·0063  „     28  „      0·0004   „

The volume contracted with the time, and was very small after a month’s
interval, but the minute bubble of the emanation still retained its
luminosity to the last. The tube became deep purple in colour, which
rendered readings difficult except with a strong light. There was a
sudden decrease in the first day, which may have been due to the mercury
sticking in the capillary tube.

The experiments were repeated with another capillary tube and the volume
of gas observed at normal pressure was 0·0254 c. mm. The gas obtained
was found to obey Boyle’s law within the limit of experimental error
over a considerable range of pressure. But, unlike in the first
experiment, the gas did not contract but expanded rapidly during the
first few hours, and then more slowly, finally reaching a volume after
23 days of 0·262 c. mm. or about 10 times the initial volume. The
measurements were complicated by the appearance of bubbles of gas in the
top of the mercury column. The differences observed in these two
experiments are difficult to account for. We shall see, later, that the
emanation always produces helium, and, in the first experiment, the
decrease of the volume to zero indicates that the helium was buried or
absorbed in the walls of the tube. In the second case, probably owing to
some difference in the glass of the capillary tube, the helium may have
been released. This suggestion is confirmed by the observation that the
volume of gas, after the experiment ended, gave a brilliant spectrum of
helium.

We shall see later that there is considerable evidence that the α
particles expelled from radio-active substances consist of helium atoms.
Since the particles are projected with great velocity, they will first
be buried in the walls of the tube, and then may gradually diffuse out
into the gas again under conditions probably depending on the kind of
glass employed. Since α particles are projected from the emanation and
also from two of the rapidly changing products which arise from it, the
volume of helium should, on this view, be three times the initial volume
of the emanation. If the helium produced escaped from the walls of the
tube into the gas, the apparent volume of the gas in the capillary
should increase to three times the initial volume in a month’s interval,
for during that time the emanation itself has been transformed into a
solid type of matter deposited on the walls of the tube.

Ramsay and Soddy concluded from their experiments that the maximum
volume of emanation to be obtained from 1 gram of radium was about 1
cubic millimetre at standard pressure and temperature, and that the
emanation was produced from 1 gram of radium at the rate of 3 × 10⁻⁶ c.
mm. per second. This amount is in very good agreement with the
calculated value, and is a strong indication of the general correctness
of the theory on which the calculations are based.


=173. Spectrum of the emanation.= After the separation of the emanation
and the determination of its volume, Ramsay and Soddy made numerous
attempts to obtain its spectrum. In some of the earlier experiments
several bright lines were seen for a short time, but these lines were
soon masked by the appearance of the hydrogen lines. In later
experiments Ramsay and Collie[267] succeeded in obtaining a spectrum of
the emanation, which persisted for a short time, during which a rapid
determination of the wave-lengths was made. They state that the spectrum
was very brilliant, consisting of very bright lines, the spaces between
being perfectly dark. The spectrum bore a striking resemblance in
general character to the spectrum of the gases of the argon family.

The spectrum soon faded, and the spectrum of hydrogen began to appear.
The following table shows the wave-length of the lines observed in the
spectrum. The degree of coincidence of the lines of known wave-lengths
shows that the error is probably less than five Ångström units.

     Wave-length Remarks

     6567        Hydrogen C; true wave-length, 6563; observed each
                 time.

     6307        Observed only at first; evanescent.

     5975        „              „          „

     5955        „              „          „

     5805        Observed each time; persistent.

     5790        Mercury; true wave-length, 5790.

     5768        „             „         5769.

     5725        Observed only at first; evanescent.

     5595        Observed each time; persistent and strong.

     5465        Mercury; true wave-length, 5461.

     5105        Not observed at first; appeared after some
                 seconds; persisted and was visible
                 during the second examination.

     4985        Observed each time; persistent and strong.

     4865        Hydrogen F; true wave-length, 4861.

     4690        Observed only at first.

     4650        Not observed when the emanation was examined
                 again.

     4630        „               „                „

     4360        Mercury: true wave-length, 4359.

The experiments were repeated with a new supply of emanation, and some
of the stronger lines were observed again, while some new lines made
their appearance. Ramsay and Collie suggest that the strong line 5595
may be identical with a line which was observed by Pickering[268] in the
spectrum of lightning, and was not identified with the spectrum of any
known gas.

Until large quantities of radium are available for the experimenter it
would appear difficult to make sure how many of these lines must be
ascribed to the spectrum of the emanation or to measure the wave-lengths
with accuracy.

The results are of great interest, as showing that the emanation has a
definite and new spectrum of the same general character as the argon
group of gases to which, as we have seen, it is chemically allied.


                          Summary of Results.


=174.= The investigations into the nature of the radio-active emanations
have thus led to the following conclusions:—The radio-elements thorium,
radium and actinium continuously produce from themselves radio-active
emanations at a rate which is constant under all conditions. In some
cases, the emanations continuously diffuse from the radio-active
compounds into the surrounding gas; in other cases, the emanations are
unable to escape from the material in which they are produced, but are
occluded, and can only be released by solution or by the action of heat.

The emanations possess all the properties of radio-active gases. They
diffuse through gases, liquids, and porous substances, and can be
occluded in some solids. Under varying conditions of pressure, volume,
and temperature, the emanations distribute themselves in the same way
and according to the same laws as does a gas.

The emanations possess the important property of condensation under the
influence of extreme cold, and by that means can be separated from the
gases with which they are mixed. The radiation from the emanation is
material in nature, and consists of a stream of positively charged
particles projected with great velocity.

The emanations possess the property of chemical inertness, and in this
respect resemble the gases of the argon family. The emanations are
produced in minute amount; but a sufficient quantity of the radium
emanation has been obtained to determine its volume and its spectrum.
With regard to their rates of diffusion, the emanations of both thorium
and radium behave like gases of high molecular weight.

These emanations have been detected and their properties investigated by
the property they possess of emitting radiations of a special character.
These radiations consist entirely of α rays, _i.e._ particles, projected
with great velocity, which carry a positive charge and have a mass about
twice that of the hydrogen atom. The emanations do not possess the
property of permanently radiating, but the intensity of the radiations
diminishes according to an exponential law with the time, falling to
half value, from actinium in 4 seconds, from thorium in one minute, and
from radium in about four days. The law of decay of activity does not
seem to be influenced by any physical or chemical agency.

The emanation particles gradually break up, each particle as it breaks
up expelling a charged body. The emanation after it has radiated ceases
to exist as such, but is transformed into a new kind of matter, which is
deposited on the surface of bodies and gives rise to the phenomena of
excited activity. This last property, and the connection of the
emanation with it, are discussed in detail in the next chapter.

Footnote 231:

  Owens, _Phil. Mag._ p. 360, Oct. 1899.

Footnote 232:

  Rutherford, _Phil. Mag._ p. 1, Jan. 1900.

Footnote 233:

  Rossignol and Gimingham, _Phil. Mag._ July, 1904.

Footnote 234:

  Bronson, _Amer. Journ. Science_, Feb. 1905.

Footnote 235:

  _Phil. Mag._ April, 1904.

Footnote 236:

  Dorn, _Abh. der. Naturforsch. Ges. für Halle-a-S._, 1900.

Footnote 237:

  P. Curie, _C. R._ 135, p. 857, 1902.

Footnote 238:

  Rutherford and Soddy, _Phil. Mag._ April, 1903.

Footnote 239:

  P. Curie, _C. R._ 136, p. 223, 1903.

Footnote 240:

  Debierne, _C. R._ 136, p. 146, 1903.

Footnote 241:

  Giesel, _Ber. D. deutsch. Chem. Ges._ p. 3608, 1902.

Footnote 242:

  Curie and Debierne, _C. R._ 132, pp. 548 and 768, 1901.

Footnote 243:

  Curie and Debierne, _C. R._ 133, p. 931, 1901.

Footnote 244:

  Rutherford and Soddy, _Trans. Chem. Soc._ p. 321, 1902. _Phil. Mag._
  Sept. 1902.

Footnote 245:

  Rutherford, _Phys. Zeit._ 2, p. 429, 1901.

Footnote 246:

  Rutherford and Soddy, _Phil. Mag._ Nov. 1902.

Footnote 247:

  Rutherford and Soddy, _Phil. Mag._ April, 1903.

Footnote 248:

  Rutherford and Soddy, _Phil. Mag._ Nov. 1902.

Footnote 249:

  Rutherford and Soddy, _Phil. Mag._ April, 1903.

Footnote 250:

  Curie and Debierne, _C. R._ 133, p. 931, 1901.

Footnote 251:

  Rutherford and Soddy, _Phil. Mag._ Nov. 1902.

Footnote 252:

  Ramsay and Soddy, _Proc. Roy. Soc._ 72, p. 204, 1903.

Footnote 253:

  Rutherford and Miss Brooks, _Trans. Roy. Soc. Canada 1901_, _Chem.
  News 1902_.

Footnote 254:

  Loschmidt, _Sitzungsber. d. Wien. Akad._ 61, II. p. 367, 1871.

Footnote 255:

  See Stefan, _Sitzungsber. d. Wien. Akad._ 63, II. p. 82, 1871.

Footnote 256:

  P. Curie and Danne, _C. R._ 136, p. 1314, 1903.

Footnote 257:

  Bumstead and Wheeler, _Amer. Jour. Science_, Feb. 1904.

Footnote 258:

  Makower, _Phil. Mag._ Jan. 1905.

Footnote 259:

  Wallstabe, _Phys. Zeit._ 4, p. 721, 1903.

Footnote 260:

  Stefan, _Wien. Ber._ 2, p. 371, 1878.

Footnote 261:

  Rutherford and Soddy, _Phil. Mag._ Nov. 1902.

Footnote 262:

  _Phil. Mag._ May, 1903.

Footnote 263:

  P. Curie, Société de Physique, 1903.

Footnote 264:

  Rutherford and Soddy, _Phil. Mag._ May, 1903.

Footnote 265:

  _Nature_, Aug. 20, 1903.

Footnote 266:

  _Proc. Roy. Soc._ 73, No. 494, p. 346, 1904.

Footnote 267:

  _Proc. Roy. Soc._ 73, No. 495, p. 470, 1904.

Footnote 268:

  Pickering, _Astrophys. Journ._ Vol. 14, p. 368, 1901.




                             CHAPTER VIII.
                        EXCITED RADIO-ACTIVITY.


=175. Excited radio-activity.= One of the most interesting and
remarkable properties of thorium, radium, and actinium, is their power
of “exciting” or “inducing” temporary activity on all bodies in their
neighbourhood. A substance which has been exposed for some time in the
presence of radium or thorium behaves as if its surface were covered
with an invisible deposit of intensely radio-active material. The
“excited” body emits radiations capable of affecting a photographic
plate and of ionizing a gas. Unlike the radio-elements themselves,
however, the activity of the body does not remain constant after it has
been removed from the influence of the exciting active material, but
decays with the time. The activity lasts for several hours when due to
radium and several days when due to thorium.

This property was first observed by M. and Mme. Curie[269] for radium,
and independently by the writer[270] for thorium[271].

If any solid body is placed inside a closed vessel containing an
emanating compound of thorium or radium, its surface becomes
radio-active. For thorium compounds the amount of excited activity on a
body is in general greater the nearer it is to the active material. In
the case of radium, however, provided the body has been exposed for
several hours, the amount of excited activity is to a large extent
independent of the position of the body in the vessel containing the
active material. Bodies are made active whether exposed directly to the
action of the radio-active substance or screened from the action of the
direct rays. This has been clearly shown in some experiments of P.
Curie. A small open vessel _a_ (Fig. 62) containing a solution of radium
is placed inside a larger closed vessel _V_.

[Illustration: Fig. 62.]

Plates _A_, _B_, _C_, _D_, _E_ are placed in various positions in the
enclosure. After exposure for a day, the plates after removal are found
to be radio-active even in positions completely shielded from the action
of the direct rays. For example, the plate _D_ shielded from the direct
radiation by the lead plate _P_ is as active as the plate _E_, exposed
to the direct radiation. The amount of activity produced in a given time
on a plate of given area in a definite position is independent of the
material of the plate. Plates of mica, copper, cardboard, ebonite, all
show equal amounts of activity. The amount of activity depends on the
area of the plate and on the amount of free space in its neighbourhood.
Excited radio-activity is also produced in water if exposed to the
action of an emanating compound.


=176. Concentration of excited radio-activity on the negative
electrode.= When thorium or radium is placed in a closed vessel, the
whole interior surface becomes strongly active. In a strong electric
field, on the other hand, the writer found that the activity was
confined entirely to the negative electrode. By suitable arrangements,
the whole of the excited activity, which was previously distributed over
the surface of the vessel, can be concentrated on a small negative
electrode placed inside the vessel. An experimental arrangement for this
purpose is shown in Fig. 63.

[Illustration: Fig. 63.]

The metal vessel _V_ containing a large amount of thoria is connected
with the positive pole of a battery of about 300 volts. The wire _AB_ to
be made active is fastened to a stouter rod _BC_, passing through an
ebonite cork inside a short cylinder _D_, fixed in the side of the
vessel. This rod is connected with the negative pole of the battery. In
this way the wire _AB_ is the only conductor exposed in the field with a
negative charge, and it is found that the whole of the excited activity
is concentrated upon it.

In this way it is possible to make a short thin metal wire over 10,000
times as active per unit surface as the thoria from which the excited
activity is derived. In the same way, the excited activity due to radium
can be concentrated mainly on the negative electrode. In the case of
thorium, if the central wire be charged positively, it shows no
appreciable activity. With radium, however, a positively charged body
becomes slightly active. In most cases, the amount of activity produced
on the positive electrode is not more than 5% of the corresponding
amount when the body is negatively charged. For both thorium and radium,
the amount of excited activity on electrodes of the same size is
independent of their material.

All metals are made active to equal extents for equal times of exposure.
When no electric field is acting, the same amount of activity is
produced on insulators like mica and glass as on conductors of equal
dimensions.


=177. Connection between the emanations and excited activity.= An
examination of the conditions under which excited activity is produced
shows that there is a very close connection between the emanation and
the excited activity. If a thorium compound is covered with several
sheets of paper, which cut off the α rays but allow the emanation to
pass through, excited activity is still produced in the space above it.
If a thin sheet of mica is waxed down over the active material, thus
preventing the escape of the emanation, no excited activity is produced
outside it. Uranium and polonium which do not give off an emanation are
not able to produce excited activity on bodies. Not only is the presence
of the emanation necessary to cause excited activity, but the amount of
excited activity is always proportional to the amount of emanation
present. For example, de-emanated thoria produces very little excited
activity compared with ordinary thoria. In all cases the amount of
excited activity produced is proportional to the emanating power. When
passing through an electric field the emanation loses its property of
exciting activity at the same rate as the radiating power diminishes.
This was shown by the following experiment.

A slow constant current of air from a gasometer, freed from dust by its
passage through cotton-wool, passed through a rectangular wooden tube 70
cms. long. Four equal insulated metal plates _A_, _B_, _C_, _D_, were
placed at regular intervals along the tube. The positive pole of a
battery of 300 volts was connected with a metal plate placed in the
bottom of the tube, while the negative pole was connected with the four
plates. A mass of thoria was placed in the bottom of the tube under the
plate _A_, and the current due to the emanation determined at each of
the four plates. After passing a current of air of 0·2 cm. per second
for 7 hours along the tube, the plates were removed and the amount of
excited activity produced on them was tested by the electric method. The
following results were obtained.

                               Relative     Relative
                               current due  excited
                               to emanation activity


               Plate _A_       1            1

               „   _B_       ·55          ·43

               „   _C_       ·18          ·16

               „   _D_          ·072         ·061

Within the errors of measurement, the amount of excited activity is thus
proportional to the radiation from the emanation, _i.e._ to the amount
of emanation present. The same considerations hold for the radium
emanation. The emanation in this case, on account of the slow loss of
its activity, can be stored mixed with air for long periods in a
gasometer, and its effects tested quite independently of the active
matter from which it is produced. The ionization current due to the
excited activity produced by the emanation is always proportional to the
current due to the emanation for the period of one month or more that
its activity is large enough to be measured conveniently by an
electrometer.

If, at any time during the interval, some of the emanation is removed
and introduced into a new testing vessel, the ionization current will
immediately commence to increase, rising in the course of four or five
hours to about twice its original value. This increase of the current is
due to the excited activity produced on the walls of the containing
vessel. On blowing out the emanation, the excited activity is left
behind, and at once begins to decay. Whatever its age, the emanation
still possesses the property of causing excited activity, and in amount
always proportional to its activity, _i.e._ to the amount of emanation
present.

These results show that the power of exciting activity on inactive
substances is a property of the radio-active emanations, and is
proportional to the amount of emanation present.

The phenomenon of excited activity cannot be ascribed to a type of
phosphorescence produced by the rays from the emanation on bodies; for
it has been shown that the activity can be concentrated on the negative
electrode in a strong electric field, even if the electrode is shielded
from the direct radiation from the active substance which gives off the
emanation. The amount of excited activity does not seem in any way
connected with the ionization produced by the emanation in the gas with
which it is mixed. For example, if a closed vessel is constructed with
two large parallel insulated metal plates on the lower of which a layer
of thoria is spread, the amount of the excited activity on the upper
plate when charged negatively, is independent of the distance between
the plates when that distance is varied from 1 millimetre to 2
centimetres. This experiment shows that the amount of excited activity
depends only on the amount of emanation emitted from the thoria; for the
ionization produced with a distance of 2 centimetres between the plates
is about ten times as great as with a distance of 1 millimetre.


=178.= If a platinum wire be made active by exposure to the emanation of
thoria, its activity can be removed by treating the wire with certain
acids[272]. For example, the activity is not much altered by immersing
the wire in hot or cold water or nitric acid, but more than 80% of it is
removed by dilute or concentrated solutions of sulphuric or hydrochloric
acid. The activity has not been destroyed by this treatment but is
manifested in the solution. If the solution be evaporated, the activity
remains behind on the dish.

These results show that the excited activity is due to a deposit on the
surface of bodies of _radio-active matter_ which has definite properties
as regards solution in acids. This active matter is dissolved in some
acids, but, when the solvent is evaporated, the active matter is left
behind. This active matter is deposited on the surface of bodies, for it
can be partly removed by rubbing the body with a cloth, and almost
completely by scouring the plate with sand or emery paper. If a
negatively charged wire is placed in the presence of a large quantity of
radium emanation, it becomes intensely active. If the wire, after
removal, is drawn across a screen of zinc sulphide, or willemite, a
portion of the active matter is rubbed off, and a luminous trail is left
behind on the screen. The amount of active matter deposited is extremely
small, for no difference of weight has been detected in a platinum wire
when made extremely active. On examining the wire under a microscope, no
trace of foreign matter is observed. It follows from these results that
the matter which causes excited activity is many thousand times more
active, weight for weight, than radium itself.

It is convenient to have a definite name for this radio-active matter,
for the term “excited activity” only refers to the radiation from the
active matter and not to the matter itself. The term “active deposit”
will be generally applied to this matter. The active deposit from the
three substances thorium, radium, and actinium is, in each case, derived
from its respective emanation, and possesses the same general property
of concentration on the negative electrode in an electric field and of
acting as a non-volatile type of matter which is deposited from the gas
on to the surface of bodies. These active deposits, while all soluble in
strong acids, are chemically distinct from each other.

The term “active deposit” can, however, only be used when the matter is
spoken of as a whole; for it will be shown later that the matter, under
ordinary conditions, is complex and contains several constituents which
have distinctive physical and chemical properties and also a distinctive
rate of change. According to the theory advanced in section 136, we may
suppose that the emanation of thorium, radium, and actinium is unstable
and breaks up with the expulsion of an α particle. The residue of the
atom of the emanation diffuses to the sides of the vessel or is removed
to the negative electrode in an electric field. This active deposit is
in turn unstable and breaks up in several successive stages.

The “excited activity” proper is the radiation set up by the active
deposit in consequence of the changes occurring in it. On this view, the
emanation is the parent of the active deposit in the same way that Th X
is the parent of the emanation. The proportionality which always exists
between the activity of the emanation and the excited activity to which
it gives rise, is at once explained, if one substance be the parent of
the other.


=179. Decay of the excited activity produced by thorium.= The excited
activity produced in a body after a _long_ exposure to the emanations of
thorium, decays in an exponential law with the time, falling to half
value in about 11 hours. The following table shows the rate of decay of
the excited activity produced on a brass rod.

                            Time in   Current
                              hours

                                  0   100

                                7·9    64

                               11·8    47·4

                               23·4    19·6

                               29·2    13·8

                               32·6    10·3

                               49·2     3·7

                               62·1     1·86

                               71·4     0·86

The results are shown graphically in Fig. 64, Curve _A_.

[Illustration: Fig. 64.]

The intensity of the radiation _I_ after any time _t_ is given by

$$ \frac {I} {I₀} = e^{–λt} $$,

where λ is the radio-active constant.

The rate of decay of excited activity, like that of the activity of
other radio-active products, is not appreciably affected by change of
conditions. The rate of decay is independent of the concentration of the
excited activity, and of the material of the body on which it is
produced. It is independent also of the nature and pressure of the gas
in which it decays. The rate of decay is unchanged whether the excited
activity is produced on the body with or without an electric field.

The amount of excited activity produced on a body increases at first
with the time, but reaches a maximum after an exposure of several days.
An example of the results is given in the following table. In this
experiment a rod was made the cathode in a closed vessel containing
thoria. It was removed at intervals for the short time necessary to test
its activity and then replaced.

                            Time in    Current
                              hours

                               1·58        6·3

                               3·25       10·5

                               5·83       29

                               9·83       40

                              14·00       59

                              23·41       77

                              29·83       83

                              47·00       90

                              72·50       95

                              96·00      100

These results are shown graphically in Curve _B_, Fig. 64. It is seen
that the decay and recovery curves may be represented approximately by
the following equations.

For the decay curve _A_,

$$ \frac {I} {I₀} = e^{–λt} $$ .

For the recovery curve _B_,

$$ \frac {I} {I₀} = 1 − e^{–λt} $$ .

The two curves are thus complementary to one another; they are connected
in the same way as the decay and recovery curves of Ur X, and are
susceptible of a similar explanation.

The amount of excited radio-activity reaches a maximum value when the
rate of supply of fresh radio-active particles balances the rate of
change of those already deposited.


=180. Excited radio-activity produced by a short exposure.= The initial
portion of the recovery curve _B_, Fig. 64, is not accurately
represented by the above equation. The activity for the first few hours
increases more slowly than would be expected from the equation. This
result, however, is completely explained in the light of later results.
The writer[273] found that, for a _short exposure_ of a body to the
thorium emanation, the excited activity upon it after removal, instead
of at once decaying at the normal rate, _increased_ for several hours.
In some cases the activity of the body increased to three or four times
its original value in the course of a few hours and then decayed with
the time at the normal rate.

For an exposure of 41 minutes to the emanation the excited activity
after removal rose to three times its initial value in about 3 hours and
then fell again at about the normal rate to half value in 11 hours.

With a longer time of exposure to the emanation, the ratio of the
increase after removal is much less marked. For a day’s exposure, the
activity after removal begins at once to diminish. In this case, the
increase of activity of the matter deposited in the last few hours does
not compensate for the decrease of activity of the active matter as a
whole, and consequently the activity at once commences to decay. This
increase of activity with time explains the initial irregularity in the
recovery curve, for the active matter deposited during the first few
hours takes some time to reach its maximum activity, and the initial
activity is, in consequence, smaller than would be expected from the
equation.

The increase of activity on a rod exposed for a short interval in the
presence of the thorium emanation has been further investigated by Miss
Brooks. The curve _C_ in Fig. 65 shows the variation with time of the
activity of a brass rod exposed for 10 minutes in the emanation vessel
filled with dust-free air. The excited activity after removal increased
in the course of 3·7 hours to five times its initial value, and
afterwards decayed at the normal rate. The dotted line curve _D_
represents the variation of activity to be expected if the activity
decayed exponentially with the time. The explanation of this remarkable
action is considered in detail in section 207.

[Illustration: Fig. 65.]


=181. Effect of dust on the distribution of excited activity.= Miss
Brooks[274], working in the Cavendish Laboratory, observed that the
excited activity due to the thorium emanation appeared in some cases on
the anode in an electric field, and that the distribution of excited
activity varied in an apparently capricious manner. This effect was
finally traced to the presence of dust in the air of the emanation
vessel. For example, with an exposure of 5 minutes the amount of excited
activity to be observed on a rod depended on the time that the air had
been allowed to remain undisturbed in the emanation vessel beforehand.
The effect increased with the time of standing, and was a maximum after
about 18 hours. The amount of excited activity obtained on the rod was
then about 20 times as great as the amount observed for air freshly
introduced. The activity of this rod did not increase after removal, but
with fresh air, the excited activity, for an exposure of 5 minutes,
increased to five or six times its initial value.

This anomalous behaviour was found to be due to the presence of dust
particles in the air of the vessel, in which the bodies were made
radio-active. These particles of dust, when shut up in the presence of
the emanation, become radio-active. When a negatively charged rod is
introduced into the vessel, a part of the radio-active dust is
concentrated on the rod and its activity is added to the normal activity
produced on the wire. After the air in the vessel has been left
undisturbed for an interval sufficiently long to allow each of the
particles of dust to reach a state of radio-active equilibrium, on the
application of an electric field, all the positively charged dust
particles will at once be carried to the negative electrode. The
activity of the electrode at once commences to decay, since the decay of
the activity of the dust particles on the wire quite masks the initial
rise of the normal activity produced on the wire.

Part of the radio-active dust is also carried to the anode, and the
proportion increases with the length of time during which the air has
been undisturbed. The greatest amount obtained on the anode was about
60% of that on the cathode.

These anomalous effects were found to disappear if the air was made
dust-free by passing through a plug of glass wool, or by application for
some time of a strong electric field.


=182. Decay of excited activity from radium.= The excited activity
produced on bodies by exposure to the radium emanation decays much more
rapidly than the thorium excited activity. For short times of
exposure[275] to the emanation the decay curve is very irregular. This
is shown in Fig. 66.

It was found that the intensity of the radiation measured by the α rays
decreased rapidly for the first 10 minutes after removal, but about 15
minutes after removal reached a value which remained nearly constant for
an interval of about 20 minutes. It then decayed to zero, finally
following an exponential law, the intensity falling to half value in
about 28 minutes. With longer times of exposure, the irregularities in
the curve are not so marked.

[Illustration: Fig. 66.]

Miss Brooks has recently determined the decay curves of the excited
activity of radium for different times of exposure, measured by the α
rays. The results are shown in Fig. 67, where the initial ordinates
represent the activity communicated to the body from different times of
exposure to a constant supply of emanation. It will be observed that in
all cases there is a sudden initial drop of activity, which becomes less
marked with increasing time of exposure. The activity, several hours
after removal, decreases exponentially in all cases, falling to half
value in about 28 minutes.

Not only do the curves of variation of the excited activity after
removal depend upon the time of exposure to the emanation, but they also
depend upon whether the α or β and γ rays are used as a means of
measurement. The curves obtained for the γ rays are identical with those
from the β rays, showing that these two types of rays always occur
together and in the same proportion. The curves measured by the β rays
are very different, especially for the case of a short exposure to the
emanation. This is clearly shown in Fig. 68, which gives the β and γ ray
curves for exposures of 10 minutes, 40 minutes, and 1 hour, and also the
limiting case of an exposure of 24 hours.

[Illustration: Fig. 67.]

[Illustration: Fig. 68.]

About 25 minutes after removal, the activity decays approximately at the
same rate in each case. For convenience of representation, the ordinates
of the curves were adjusted so that they all passed through a common
point. We shall see later (chapter XI) that the rates of decay are not
identically the same until several hours after removal; but, in the
above figure, it is difficult to represent the slight variations. It
will be observed that for the short exposure of 10 minutes the activity
measured by the β rays is small at first but rises to a maximum in about
22 minutes, and then dies away with the time. The curve of decay of
activity, measured by the β rays for a long exposure, does not show the
rapid initial drop which occurs in all the α ray curves. Curie and
Danne[276] made an investigation of the curves of decay of excited
activity for different times of exposure to the radium emanation, but
apparently did not take into account the fact that measurements made by
the α and β rays give quite different curves of decay. Some of the
family of curves, given in their paper, refer to the α rays and others
to the β rays. They showed, however, the important fact that the curve
of decay obtained by them for a long exposure (which is identical with
the β ray curve) could be empirically expressed by an equation of the
form

$$ \frac {I_t} {I₀} = ae^{–λ_1 t} − (a − 1) e^{–λ_2 t} $$,

where _I₀_ is the initial intensity and _I__{_t_} the intensity after
any time _t_; λ₁ = ¹⁄₂₄₂₀, λ₂ = ¹⁄₁₈₆₀. The numerical constant _a_ =
4·20. After an interval of 2·5 hours, the logarithmic decay curve is
nearly a straight line, that is, the activity falls off according to an
exponential law with the time, decreasing to half value in about 28
minutes.

The full explanation of this equation, and of the peculiarities of the
various decay curves of the excited activity of radium, will be
discussed in detail in chapter XI.

As in the case of the excited activity from thorium, the rate of decay
of the excited activity from radium is for the most part independent of
the nature of the body made active. Curie and Danne (_loc. cit._)
observed that the active bodies gave off an emanation itself capable of
exciting activity in neighbouring bodies. This property rapidly
disappeared, and was inappreciable 2 hours after removal. In certain
substances like celluloid and caoutchouc, the decay of activity is very
much slower than for the metals. This effect becomes more marked with
increase of time of exposure to the emanation. A similar effect is
exhibited by lead, but to a less marked degree. During the time the
activity lasts, these substances continue to give off an emanation.

It is probable that these divergencies from the general law are not due
to an actual change in the rate of decay of the true excited activity
but to an occlusion of the emanation by these substances during the
interval of exposure. After exposure the emanation gradually diffuses
out, and thus the activity due to this occluded emanation and the
excited activity produced by it decays very slowly with the time.


=183. Active deposit of very slow decay.= M. and Mme Curie[277] have
observed that bodies which have been exposed for a long interval in the
presence of the radium emanation do not lose all their activity. The
excited activity at first decays rapidly at the normal rate, falling to
half value in about 28 minutes, but a residual activity, which they
state is of the order of ½0,000 of the initial activity, always
remains. A similar effect was observed by Giesel. The writer has
examined the variation of this residual activity, and has found that it
increases for several years. The results are discussed in detail in
chapter XI. It will there be shown that this active deposit of slow
transformation contains the radio-active constituents present in
polonium, radio-tellurium and radio-lead.

[Illustration: Fig. 69.]


=184. The excited activity from actinium.= The emanation of actinium,
like that of thorium and radium, produces excited activity on bodies,
which is concentrated on the negative electrode in an electric field.
Debierne[278] found that the excited activity decays approximately
according to an exponential law, falling to half value in 41 minutes.
Giesel[279] examined the rate of decay of the excited activity of
“emanium”—which, we have seen, probably contains the same radio-active
constituents as actinium—and found that it decayed to half value in 34
minutes. Miss Brooks[280] found that the curves of decay of the excited
activity from Giesel’s emanium varied with the time of exposure to the
emanation. The results are shown graphically in Fig. 69, for time
exposures of 1, 2, 5, 10 and 30 minutes, and also for a long exposure of
21 hours. After 10 minutes the curves have approximately the same rate
of decay. For convenience, the ordinates of the curves are adjusted to
pass through a common point. For a very short exposure, the activity is
small at first, but reaches a maximum about 9 minutes later and finally
decays exponentially to zero.

The curve of variation of activity for a very short exposure has been
determined accurately by Bronson; it is shown later in Fig. 83. He found
that the decay of activity is finally exponential, falling to half value
in 36 minutes.

The explanation of these curves is discussed in detail in chapter X,
section 212.


=185. Physical and chemical properties of the active deposit.= On
account of the slow decay of the activity of the active deposit from the
thorium emanation, its physical and chemical properties have been more
closely examined than the corresponding deposit from radium. It has
already been mentioned that the active deposit of thorium is soluble in
some acids. The writer[281] found that the active matter was dissolved
off the wire by strong or dilute solutions of sulphuric, hydrochloric
and hydrofluoric acids, but was only slightly soluble in water or nitric
acid. The active matter was left behind when the solvent was evaporated.
The rate of decay of activity was unaltered by dissolving the active
matter in sulphuric acid, and allowing it to decay in the solution. In
the experiment, the active matter was dissolved off an active platinum
wire; then equal portions of the solutions were taken at definite
intervals, evaporated down in a platinum dish, and the activity of the
residue tested by the electric method. The rate of decay was found to be
exactly the same as if the active matter had been left on the wire. In
another experiment, an active platinum wire was made the cathode in a
copper sulphate solution, and a thin film of copper deposited on it. The
rate of decay of the activity was unchanged by the process.

A detailed examination of the physical and chemical properties of the
active deposit of thorium has been made by F. von Lerch[282] and some
important and interesting results have been obtained. A solution of the
active deposit was prepared by dissolving the metal which had been
exposed for some time in the presence of the thorium emanation. In most
cases the active matter was precipitated with the metal. For example, an
active copper wire was dissolved in nitric acid and then precipitated by
caustic potash. The precipitate was strongly active. An active magnesium
wire, dissolved in hydrochloric acid and then precipitated as phosphate,
also gave an active precipitate. The activity of the precipitates
decayed at the normal rate, _i.e._ the activity fell to half value in
about 11 hours.

Experiments were also made on the solubility of the active deposit in
different substances. A platinum plate was made active and then placed
in different solutions, and the decrease of the activity observed. In
addition to the acids already mentioned, a large number of substances
were found to dissolve the active deposit to some extent. The active
matter was however not dissolved to an appreciable extent in ether or
alcohol. Many substances became active if added to the active solution
and then precipitated. For example, an active solution of hydrochloric
acid was obtained by dissolving the deposit on an active platinum wire.
Barium chloride was then added and precipitated as sulphate. The
precipitate was strongly active, thus suggesting that the active matter
was carried down by the barium.


=186. Electrolysis of solutions.= Dorn showed that, if solutions of
radiferous barium chloride were electrolysed, both electrodes became
temporarily active, but the anode to a greater degree than the cathode.
F. von Lerch has made a detailed examination of the action of
electrolysis on a solution of the active deposit of thorium. The matter
was dissolved off an active platinum plate by hydrochloric acid, and
then electrolysed between platinum electrodes. The cathode was very
active, but there was no trace of activity on the anode. The cathode
lost its activity at a rate much _faster_ than the normal. With an
amalgamated zinc cathode on the other hand, the rate of decay was
normal. When an active solution of hydrochloric acid was electrolysed
with an electromotive force smaller than that required to decompose
water, the platinum became active. The activity decayed to half value in
4·75 hours while the normal fall is to half value in 11 hours. These
results point to the conclusion that the active matter is complex and
consists of two parts which have different rates of decay of activity,
and can be separated by electrolysis.

Under special conditions it was found possible to make the anode active.
This was the case if the anion attached itself to the anode. For
example, if an active hydrochloric solution was electrolysed with a
silver anode, the chloride of silver formed was strongly active and its
activity decayed at a normal rate. The amount of activity obtained by
placing different metals in active solutions for equal times varied
greatly with the metal. For example, it was found that if a zinc plate
and an amalgamated zinc plate, which show equal potential differences
with regard to hydrochloric acid, were dipped for equal times in two
solutions of equal activity, the zinc plate was seven times as active as
the other. The activity was almost removed from the solution in a few
minutes by dipping a zinc plate into it. Some metals became active when
dipped into an active solution while others did not. Platinum,
palladium, and silver remained inactive, while copper, tin, lead,
nickel, iron, zinc, cadmium, magnesium, and aluminium became active.
These results strongly confirm the view that excited activity is due to
a deposit of active matter which has distinctive chemical behaviour.

G. B. Pegram[283] has made a detailed study of the active deposits
obtained by electrolysis of pure and commercial thorium salts. The
commercial thorium nitrate obtained from P. de Haen gave, when
electrolysed, a deposit of lead peroxide on the anode. This deposit was
radio-active, and its activity decayed at the normal rate of the excited
activity due to thorium. From solutions of pure thorium nitrate, no
visible deposit was obtained on the anode, but it was, however, found to
be radio-active. The activity decayed rapidly, falling to half value in
about one hour. Some experiments were also made on the effect of adding
metallic salts to thorium solutions and then electrolysing them. Anode
and cathode deposits of the oxides or metals obtained in this way were
found to be radio-active, but the activity fell to half value in a few
minutes. The gases produced by electrolysis were radio-active, but this
was due to the presence of the thorium emanation. The explanation of the
results obtained by Pegram and von Lerch will be considered later in
section 207. It will be shown that the active deposit of thorium
contains two distinct substances which have different rates of
transformation.


=187. Effect of temperature.= The activity of a platinum wire which has
been exposed in the presence of the thorium emanation is almost
completely lost by heating the wire to a white heat. Miss F. Gates[284]
found that the activity was not destroyed by the intense heat, but
manifested itself on neighbouring bodies. When the active wire was
heated electrically in a closed cylinder, the activity was transferred
from the wire to the interior surface of the cylinder in unaltered
amount. The rate of decay of the activity was not altered by the
process. By blowing a current of air through the cylinder during the
heating, a part of the active matter was removed from the cylinder.
Similar results were found for the excited activity due to radium.

F. von Lerch (_loc. cit._) determined the amount of activity removed at
different temperatures. The results are shown in the following table for
a platinum wire excited by the thorium emanation[285].


                                Temperature  Percentage of
                                             activity
                                             removed


           Heated 2 minutes      800° C.      0

           then   „    ½ minute 1020° C.     16
           more

           „     „    ½   „     1260° C.     52
           „

           „     „    ½   „     1460° C.     99
           „

The effect of heat on the volatilization of the active deposit of radium
has been examined in detail by Curie and Danne. The interesting and
important results obtained by them will be discussed in chapter XI,
section 226.


=188. Effect of variation of E.M.F. on amount of excited activity from
thorium.= It has been shown that the excited activity is confined to
the cathode in a strong electric field. In weaker fields the activity
is divided between the cathode and the walls of the vessel. This was
tested in an apparatus[286] shown in Fig. 70.

[Illustration: Fig. 70.]

_A_ is a cylindrical vessel of 5·5 cms. diameter, _B_ the negative
electrode passing through insulating ends _C_, _D_. For a potential
difference of 50 volts, most of the excited activity was deposited on
the electrode _B_. For about 3 volts, half of the total excited activity
was produced on the rod _B_, and half on the walls of the vessel.
Whatever the voltage applied, the sum of the activities on the central
rod and the walls of the cylinder was found to be a constant when a
steady state was reached.

When no voltage was applied, diffusion alone was operative, and in that
case about 13 per cent. of the total activity was on the rod _B_. The
application of an electric field has thus no influence on the sum total
of excited activity, but merely controls the proportion concentrated on
the negative electrode.

A more detailed examination of the variation with strength of field of
the amount on the negative electrode was made in a similar manner by F.
Henning[287]. He found that in a strong electric field the amount of
excited activity was practically independent of the diameter of the rod
_B_, although the diameter varied between ·59 mm. and 6·0 mms. With a
small voltage, the amount on the negative electrode varied with its
diameter. The curves showing the relation between the amount of excited
activity and voltage are very similar in character to those obtained for
the variation of the current through an ionized gas with the voltage
applied.

The amount of excited activity reaches a maximum when all the active
matter is removed from the gas as rapidly as it is formed. With weaker
fields, a portion diffuses to the sides of the vessel, and produces
excited activity on the positive electrode.


=189. Effect of pressure on distribution of excited activity.= In a
strong electric field, the amount of excited activity produced on the
cathode is independent of the pressure down to a pressure of about 10
mms. of mercury. In some experiments made by the writer[288], the
emanating thorium compound was placed inside a closed cylinder about 4
cms. in diameter, through which passed an insulated central rod. The
central rod was connected to the negative pole of a battery of 50 volts.
When the pressure was reduced below 10 mms. of mercury, the amount of
excited activity produced on the negative electrode diminished, and was
a very small fraction of its original value at a pressure of ⅒ mm.
Some excited activity was in this case found to be distributed over the
interior surface of the cylinder. It may thus be concluded that at low
pressures the excited activity appears on both anode and cathode, even
in a strong electric field. The probable explanation of this effect is
given in the next section.

Curie and Debierne[289] observed that when a vessel containing an
emanating radium compound was kept pumped down to a low pressure, the
amount of excited activity produced on the vessel was much reduced. In
this case the emanation given off by the radium was removed by the pump
with the other gases continuously evolved from the radium compound. On
account of the very slow decay of activity of the emanation, the amount
of excited activity produced on the walls of the vessel, in the passage
of the emanation through it, was only a minute fraction of the amount
produced when none of the emanation given off was allowed to escape.


=190. Transmission of excited activity.= The characteristic property of
excited radio-activity is that it can be confined to the cathode in a
strong electric field. Since the activity is due to a deposit of
radio-active matter on the electrified surface, the matter must be
transported by positively charged carriers. The experiments of
Fehrle[290] showed that the carriers of excited activity travel along
the lines of force in an electric field. For example, when a small
negatively charged metal plate was placed in the centre of a metal
vessel containing an emanating thorium compound, more excited activity
was produced on the sides and corners of the plate than at the central
part.

A difficulty however arises in connection with the positive charge of
the carrier. According to the view developed in section 136 and later
in chapters X and XI, the active matter which is deposited on bodies
and gives rise to excited activity, is itself derived from the
emanation. The emanations of thorium and radium emit only α rays, _i.e._
positively charged particles. After the expulsion of an α particle, the
residue, which is supposed to constitute the primary matter of the
active deposit, should retain a negative charge, and be carried to the
anode in an electric field. The exact opposite however is observed to be
the case. The experimental evidence does not support the view that the
positively charged α particles, expelled from the emanation, are
directly responsible for the phenomena of excited activity; for no
excited activity is produced in a body exposed to the α rays of the
emanation, provided the emanation itself does not come in contact with
it.

There has been a tendency to attach undue importance to this apparent
discrepancy between theory and experiment. The difficulty is not so much
to offer a probable explanation of the results as to select from a
number of possible causes. While there can be little doubt that the main
factor in the disintegration of the atom consists in the expulsion of an
α particle carrying a positive charge, a complicated series of processes
probably occurs before the residue of the atom is carried to the
negative electrode. The experimental evidence suggests that one or more
negative electrons of slow velocity escape from the atom at the same
time as the particle. This is borne out by the recent discovery that the
particle expelled from radium, freed from the ordinary β rays, and also
from polonium, is accompanied by a number of slowly moving and
consequently easily absorbed electrons. If two negative electrons
escaped at the same time as the α particle, the residue would be left
with a positive charge and would be carried to the negative electrode.
There is also another experimental point which is of importance in this
connection. In the absence of an electric field, the carriers remain in
the gas for a considerable time and undergo their transformation _in
situ_. There is also some evidence (section 227) that, even in an
electric field, the carriers of the active deposit are not swept to the
electrode immediately after the break up of the emanation, but remain
some time in the gas before they gain a positive charge. It must be
remembered that the atoms of the active deposit do not exist as a gas
and by the process of diffusion would tend to collect together to form
aggregates. These aggregates would act as small metallic particles, and,
if they were electro-positive in regard to the gas, would gain a
positive charge from the gas.

There can be little doubt that the processes occurring between the break
up of the emanation and the deposit of the residue in the cathode in an
electric field are complicated, and further careful experiment is
required to elucidate the sequence of the phenomena.

Whatever view is taken of the process by which these carriers obtain a
positive charge, there can be little doubt that the expulsion of an α
particle with great velocity from the atom of the emanation must set the
residue in motion. On account of the comparatively large mass of this
residue, the velocity acquired will be small compared with that of the
expelled α particle, and the moving mass will rapidly be brought to rest
at atmospheric pressure by collision with the gas molecules in its path.
At low pressures, however, the collisions will be so few that it will
not be brought to rest until it strikes the boundaries of the vessel. A
strong electric field would have very little effect in controlling the
motion of such a heavy mass, unless it has been initially brought to
rest by collision with the gas molecules. This would explain why the
active matter is not deposited on the cathode at low pressures in an
electric field. Some direct evidence of a process of this character,
obtained by Debierne on examination of the excited activity produced by
actinium, is discussed in section 192.


=191.= The following method has been employed by the writer[291] to
determine the velocity of the positive carriers of excited activity of
radium and thorium in an electric field. Suppose _A_ and _B_ (Fig. 71)
are two parallel plates exposed to the influence of the emanation, which
is uniformly distributed between them. If an alternating E.M.F. _E₀_ is
applied between the plates, the same amount of excited activity is
produced on each electrode. If, in series with the source of the
alternating E.M.F., a battery of E.M.F. _E₁_ less than _E₀_ is placed,
the positive carrier moves in a stronger electric field in one half
alternation than in the other. A carrier consequently moves over unequal
distances during the two half alternations, since the velocity of the
carrier is proportional to the strength of the electric field in which
it moves. The excited activity will in consequence be unequally
distributed over the two electrodes. If the frequency of alternation is
sufficiently great, only the positive carriers within a certain small
distance of one plate can be conveyed to it, and the rest, in the course
of several succeeding alternations, are carried to the other plate.

[Illustration: Fig. 71.]

When the plate _B_ is negatively charged, the E.M.F. between the plates
is _E₀_ − _E₁_, when _B_ is positive the E.M.F. is _E₀_ + _E₁_.

    Let _d_ = distance between the plates,
        _T_ = time of a half alternation,
        ρ = ratio of the excited radio-activity on the plate _B_ to the
                     sum of the radio-activities on the plates _A_ and
                        _B_,
        _K_ = velocity of the positive carriers for a potential-gradient
                     of 1 volt per centimetre.

On the assumption that the electric field between the plates is uniform,
and that the velocity of the carrier is proportional to the electric
field, the velocity of the positive carrier towards _B_ is

                      _E₀_ − _E₁_
                      ----------  _K_
                         _d_

and, in the course of the next half alternation,

                      _E₀_ + _E₁_
                      ----------  _K_
                         _d_

towards the plate _A_.

If _x₁_ is less than _d_, the greatest distances _x₁_, _x₂_ passed over
by the positive carrier during two succeeding half alternations is thus
given by

                          _E₀_ − _E₁_
                  _x₁_ =  ----------  _KT_
                             _d_

                  and

                          _E₀_ + _E₁_
                  _x₂_ =  ----------  _KT_
                             _d_

Suppose that the positive carriers are produced at a uniform rate of _q_
per second for unit distance between the plates. The number of positive
carriers which reach _B_ during a half alternation consists of two
parts:

(1) One half of those carriers which are produced within the distance
_x₁_ of the plate _B_. This number is equal to

                                1
                               --- _x₁_ _qT_
                                2

(2) All the carriers which are left within the distance _x₁_ from _B_ at
the end of the previous half alternation. The number of these can
readily be shown to be

                             1        _x₁_
                            --- _x₁_  ----  _qT_
                             2        _x₂_

The remainder of the carriers, produced between _A_ and _B_ during a
complete alternation, will reach the other plate _A_ in the course of
succeeding alternations, provided no appreciable recombination takes
place. This must obviously be the case, since the positive carriers
travel further in a half alternation towards _A_ than they return
towards _B_ during the next half alternation. The carriers thus move
backwards and forwards in the changing electric field, but on the whole
move towards the plate _A_.

The total number of positive carriers produced between the plates during
a complete alternation is 2_dqT_. The ratio ρ of the number which reach
_B_ to the total number produced is thus given by

$$ \rho = \frac {\frac {1}{2} x_1qT + \frac {1}{2} x_1 \frac {x_1}{x_2}
qT} {2dqT} = \frac {1}{4} \frac {x_1}{d} \frac {x_1 + x_2} {x_2} $$ .

Substituting the values of _x₁_ and _x₂_, we find that

$$ K = \frac {2 (E₀ + E_1) d^2} {E₀ (E₀ − E_1) T} \rho $$ .

In the experiments, the values of _E₀_, _E₁_, _d_, and _T_ were varied,
and the results obtained were in general agreement with the above
equation.

The following were the results for thorium:

_Plates 1·30 cms. apart._

             _E₀_ +  _E₀_ −   Alternations      ρ    _K_
              _E₁_    _E₁_     per second


               152     101             57    ·27   1·25

               225     150             57    ·38   1·17

               300     200             57    ·44   1·24

_Plates 2 cms. apart._

            _E₀_ +  _E₀_ −   Alternations      ρ    _K_
              _E₁_    _E₁_     per second


               273     207             44    ·37   1·47

               300     200             53   ·286   1·45

The average mobility _K_ deduced from a large number of experiments was
1·3 cms. per sec. per volt per cm. for atmospheric pressure and
temperature. This velocity is about the same as the velocity of the
positive ion produced by Röntgen rays in air, viz. 1·37 cms. per sec.
The results obtained with the radium emanation were more uncertain than
those for thorium on account of the distribution of some excited
activity on the positive electrode. The values of the velocities of the
carriers were however found to be roughly the same for radium as for
thorium.

These results show that the carriers of the active deposit travel in the
gas with about the same velocity as the positive or negative ions
produced by the radiations in the gas. This indicates either that the
active matter becomes attached to positive ions, or that the active
matter itself, acquiring in some way a positive charge, collects a
cluster of neutral molecules which travel with it.


=192. Carriers of the excited activity from actinium and “emanium.”=
Giesel[292] observed that “emanium” gave off a large quantity of
emanation, and that this emanation gave rise to a type of radiation
which he termed the _E_ rays. A narrow metal cylinder containing the
active substance was placed with the open end downwards, about 5 cms.
above the surface of a zinc sulphide screen. The screen was charged
negatively to a high potential by an electric machine, and the cylinder
connected with earth. A luminous spot of light was observed on the
screen, which was brighter at the edge than at the centre. A conductor,
connected with earth, brought near the luminous spot apparently repelled
it. An insulator did not show such a marked effect. On removal of the
active substance, the luminosity of the screen persisted for some time.
This was probably due to the excited activity produced on the screen.

The results obtained by Giesel support the view that the carriers of
excited activity of “emanium” have a positive charge. In a strong
electric field the carriers travel along the lines of force to the
cathode, and there cause excited activity on the screen. The movement of
the luminous zone on the approach of a conductor is due to the
disturbance of the electric field. Debierne[293] found that actinium
also gave off a large amount of emanation, the activity of which decayed
very rapidly with the time, falling to half value in 3·9 seconds.

This emanation produces excited activity on surrounding objects, and at
diminished pressure the emanation produces a uniform distribution of
excited activity in the enclosure containing the emanation. The excited
activity falls to half value in 41 minutes.

Debierne observed that the distribution of excited activity was altered
by a strong magnetic field. The experimental arrangement is shown in
Fig. 71A. The active matter was placed at _M_, and two plates _A_ and
_B_ were placed symmetrically with regard to the source. On the
application of a strong magnetic field normal to the plane of the paper,
the excited activity was unequally distributed between the plates _A_
and _B_. The results showed that the carriers of excited activity were
deviated by a magnetic field in the opposite sense to the cathode rays,
_i.e._ the carriers were positively charged. In some cases, however, the
opposite effect was obtained. Debierne considers that the excited
activity of actinium is due to “ions activants,” the motion of which is
altered by a magnetic field. Other experiments showed that the magnetic
field acted on the “ions activants” and not on the emanation.

[Illustration: Fig. 71A.]

The results of Debierne thus lead to the conclusion that the carriers of
excited activity are derived from the emanation and are projected with
considerable velocity. This result supports the view, advanced in
section 190, that the expulsion of α particles from the emanation must
set the part of the system left behind in rapid motion. A close
examination of the mode of transference of the excited activity by
actinium and the emanation substance is likely to throw further light on
the processes which give rise to the deposit of active matter on the
electrodes.

Footnote 269:

  M. and Mme. Curie, _C. R._ 129, p. 714, 1899.

Footnote 270:

  Rutherford, _Phil. Mag._ Jan. and Feb. 1900.

Footnote 271:

  As regards date of publication, the priority of the discovery of
  “excited activity” belongs to M. and Mme. Curie. A short paper on this
  subject, entitled “Sur la radioactivité provoquée par les rayons de
  Becquerel,” was communicated by them to the _Comptes Rendus_, Nov. 6,
  1899. A short note was added to the paper by Becquerel in which the
  phenomena of excited activity were ascribed to a type of
  phosphorescence. On my part, I had simultaneously discovered the
  emission of an emanation from thorium compounds and the excited
  activity produced by it, in July, 1899. I, however, delayed
  publication in order to work out in some detail the properties of the
  emanation and of the excited activity and the connection between them.
  The results were published in two papers in the _Philosophical
  Magazine_ (Jan. and Feb. 1900) entitled “A radio-active substance
  emitted from thorium compounds,” and “Radio-activity produced in
  substances by the action of thorium compounds.”

Footnote 272:

  Rutherford, _Phil. Mag._ Feb. 1900.

Footnote 273:

  Rutherford, _Phys. Zeit._ 3, No. 12, p. 254, 1902. _Phil. Mag._ Jan.
  1903.

Footnote 274:

  Miss Brooks, _Phil. Mag._ Sept. 1904.

Footnote 275:

  Rutherford and Miss Brooks, _Phil. Mag._ July, 1902.

Footnote 276:

  Curie and Danne, _C. R._ 136, p. 364, 1903.

Footnote 277:

  Mme Curie, _Thèse_, Paris, 1903, p. 116.

Footnote 278:

  Debierne, _C. R._ 138, p. 411, 1904.

Footnote 279:

  Giesel, _Ber. d. D. Chem. Ges._ No. 3, p. 775, 1905.

Footnote 280:

  Miss Brooks, _Phil. Mag._ Sept. 1904.

Footnote 281:

  Rutherford, _Phys. Zeit._ 3, No. 12, p. 254, 1902.

Footnote 282:

  F. von Lerch, _Annal. d. Phys._ 12, p. 745, 1903.

Footnote 283:

  Pegram, _Phys. Review_, p. 424, Dec. 1903.

Footnote 284:

  Miss Gates, _Phys. Review_, p. 300, 1903.

Footnote 285:

  A more complete examination of the effect of temperature on the
  excited activity of thorium has been made by Miss Slater (section
  207).

Footnote 286:

  Rutherford, _Phil. Mag._ Feb. 1900.

Footnote 287:

  Henning, _Annal. d. Phys._ 7, p. 562, 1902.

Footnote 288:

  Rutherford, _Phil. Mag._ Feb. 1900.

Footnote 289:

  Curie and Debierne, _C. R._ 132, p. 768, 1901.

Footnote 290:

  Fehrle, _Phys. Zeit._ 3, No. 7, p. 130, 1902.

Footnote 291:

  Rutherford, _Phil. Mag._ Jan. 1903.

Footnote 292:

  Giesel, _Ber. d. D. Chem. Ges._ 36, p. 342, 1903.

Footnote 293:

  Debierne, _C. R._ 136, pp. 446 and 671, 1903; 138, p. 411, 1904.




                              CHAPTER IX.
                     THEORY OF SUCCESSIVE CHANGES.

=193. Introduction.= We have seen in previous chapters that the
radio-activity of the radio-elements is always accompanied by the
production of a series of new substances with some distinctive physical
and chemical properties. For example, thorium produces from itself an
intensely radio-active substance, Th X, which can be separated from the
thorium in consequence of its solubility in ammonia. In addition,
thorium gives rise to a gaseous product, the thorium emanation, and also
to another substance which is deposited on the surface of bodies in the
neighbourhood of the thorium, where its presence is indicated by the
phenomenon known as “excited activity.”

A close examination of the origin of these products shows that they are
not produced simultaneously, but arise in consequence of a succession of
changes originating in the radio-element. Thorium first of all gives
rise to the product Th X. The Th X produces from itself the thorium
emanation, and this in turn is transformed into a non-volatile
substance. A similar series of changes is observed in radium, with the
exception that there is no product in radium corresponding to the Th X
in the case of thorium. Radium first of all produces an emanation,
which, like thorium, is transformed into a non-volatile substance. In
uranium only one product, Ur X, has been observed, for uranium does not
give off an emanation and in consequence does not produce excited
activity on bodies.

As a typical example of the evidence, from which it is deduced that one
substance is the parent of another, we will consider the connection of
the two products Th X and the thorium emanation. It has been shown
(section 154) that after the separation of Th X from a thorium solution,
by precipitation with ammonia, the precipitated thorium hydroxide has
lost to a large extent its power of emanating. This cannot be ascribed
to a prevention of escape of the emanation produced in it, for very
little emanation is observed when a current of air is drawn through the
hydroxide in a state of solution, when most of the emanation present
would be carried off. On the other hand, the solution containing the Th
X gives off a large quantity of emanation, showing that the power of
giving off an emanation belongs to the product Th X. Now it is found
that the quantity of emanation given off by the separated Th X decreases
according to an exponential law with the time, falling to half value in
four days. The rate of production of emanation thus falls off according
to the same law and at the same rate as the activity of the Th X
measured in the ordinary manner by the α rays. Now this is exactly the
result to be expected if the Th X is the parent of the emanation, for
the activity of Th X at any time is proportional to its rate of change,
_i.e._, to the rate of production of the secondary type of matter by the
emanation in consequence of a change in it. Since the rate of change of
the emanation (half transformed in 1 minute) is very rapid compared with
the rate of change of Th X, the amount of emanation present will be
practically proportional to the activity of the Th X at any instant,
_i.e._, to the amount of unchanged Th X present. The observed fact that
the hydroxide regains its power of emanating in the course of time is
due to the production of fresh Th X by the thorium, which in turn
produces the emanation.

In a similar way, excited activity is produced on bodies over which the
emanation is passed, and in amount proportional to the activity of the
emanation, _i.e._, to the amount of the emanation present. This shows
that the active deposit, which gives rise to the phenomenon of excited
activity, is itself a product of the emanation. The evidence thus seems
to be conclusive that Th X is the parent of the emanation and that the
emanation is the parent of the deposited matter.


=194. Chemical and Physical properties of the active products.= Each of
these radio-active products is marked by some distinctive chemical and
physical properties which differentiate it from the preceding and
succeeding products. For example, Th X behaves as a solid. It is soluble
in ammonia, while thorium is not. The thorium emanation behaves as a
chemically inert gas and condenses at a temperature of −120° C. The
active deposit from the emanation behaves as a solid and is readily
soluble in sulphuric and hydrochloric acids and is only slightly soluble
in ammonia.

The striking dissimilarity which exists in many cases between the
chemical and the physical properties of the parent matter and the
product to which it gives rise is very well illustrated by the case of
radium and the radium emanation. Radium is an element so closely allied
in chemical properties to barium that, apart from a slight difference in
the solubility of the chlorides and bromides, it is difficult to
distinguish chemically between them. It has a definite spectrum of
bright lines similar in many respects to the spectra of the alkaline
earths. Like barium, it is non-volatile at ordinary temperature. On the
other hand, the emanation which is continually produced from radium is a
radio-active and chemically inert gas, which is condensed at a
temperature of −150° C. Both in its spectrum and in the absence of
definite chemical properties, it resembles the argon-helium group of
inert gases, but differs from these gases in certain marked features.

The emanation must be considered to be an unstable gas which breaks down
into a non-volatile type of matter, the disintegration being accompanied
by the expulsion of heavy atoms of matter (α particles) projected with
great velocity. This rate of breaking up is not affected by temperature
over the considerable range which has been examined. After a month’s
interval, the volume of the emanation has shrunk to a small portion of
its initial value. But the most striking property of the emanation,
which, as we shall see later (chapter XII), is a direct consequence of
its radio-activity, is the enormous amount of energy emitted from it.
The emanation in breaking up through its successive stages emits about 3
million times as much energy as is given out by the explosion of an
equal volume of hydrogen and oxygen, mixed in the proper proportions to
form water; and yet, in this latter chemical reaction more heat is
emitted than in any other known chemical change.

We have seen that the two emanations and the products Ur X, Th X lose
their activity with the time according to a simple exponential law, and
at a rate that is independent—as far as observation has gone—of the
chemical and physical agents at our disposal. The time taken for each of
these products to fall to half its value is thus a definite physical
constant which serves to distinguish it from all other products.

On the other hand, the variation of the excited activity produced by
these emanations does not even approximately obey such a law. The rate
of decay depends not only on the time of exposure to the respective
emanations, but also, in the case of radium, on the type of radiation
which is used as a means of comparative measurement. It will be shown,
in succeeding chapters, that the complexity of the decay is due to the
fact that the matter in the active deposits undergoes several successive
transformations, and that the peculiarities of the curves of decay,
obtained under different conditions, can be explained completely on the
assumption that two changes occur in the active deposit from both
thorium and actinium and six in the active deposit from radium.


=195. Nomenclature.= The nomenclature to be applied to the numerous
radio-active products is a question of great importance and also one of
considerable difficulty. Since there are at least seven distinct
substances produced from radium, and probably five from thorium and
actinium, it is neither advisable nor convenient to give each a special
name such as is applied to the parent elements. At the same time, it is
becoming more and more necessary that each product should be labelled in
such a way as to indicate its place in the succession of changes. This
difficulty is especially felt in discussing the numerous changes in the
active deposits from the different emanations. Many of the names
attached to the products were given at the time of their discovery,
before their position in the scheme of changes was understood. In this
way the names Ur X, Th X were applied to the active residues obtained by
chemical treatment of uranium and thorium. Since, in all probability,
these substances are the first products of the two elements, it may be
advisable to retain these names, which certainly have the advantage of
brevity. The name “emanation” was originally given to the radio-active
gas from thorium, and has since been applied to the similar gaseous
products of radium and actinium.

Finding the name “radium emanation” somewhat long and clumsy, Sir
William Ramsay[294] has recently suggested “ex-radio” as an equivalent.
This name is certainly brief and is also suggestive of its origin; but
at least six other ex-radios, whose parentage is as certain as that of
the emanation, remain unnamed. A difficulty arises in applying the
corresponding names ex-thorio, ex-actinio to the other gaseous products,
for, unlike radium, the emanations of thorium and actinium are probably
the second, not the first, disintegration product of the radio-elements
in question. Another name thus has to be applied to the first product in
these cases. It may be advisable to give a special name to the
emanation, since it has been the product most investigated and was the
first to be isolated chemically; but, on the other hand, the name
“radium emanation” is historically interesting, and suggests a type of
volatile or gaseous matter. Since the term “excited” or “induced”
activity refers only to the radiations from the active body, a name is
required for the radiating matter itself. The writer in the first
edition of this book suggested the name “emanation X.”[295] This title
was given from analogy to the names Ur X and Th X, to indicate that the
active matter was product of the emanation. The name, however, is not
very suitable, and, in addition, can only be applied to the initial
product deposited, and not to the further products of its decomposition.
It is very convenient in discussing mathematically the theory of
successive changes to suppose that the deposited matter called _A_ is
changed into _B_, _B_ into _C_, _C_ into _D_, and so on. I have
therefore discarded the name emanation _X_, and have used the terms
radium _A_, radium _B_, and so on, to signify the successive products of
the decomposition of the emanation of radium. A similar nomenclature is
applied to thorium and actinium. This system of notation is elastic and
simple, and I have found it of great convenience in the discussion of
successive products. In speaking generally of the active matter, which
causes excited activity, without regard to its constituents, I have used
the term “active deposit.” The scheme of nomenclature employed in this
book is clearly shown below:—

              Radium     Thorium    Uranium    Actinium

              Radium     Th X       Ur X       Actinium X
              emanation

              Radium _A_ Thorium    Final      Actinium
              (Active)   emanation  product    emanation

              Radium _B_ Thorium               Actinium
              (Active)   _A_                   _A_
                         (Active)              (Active)

              Radium _C_ Thorium               Actinium
              (Active)   _B_                   _B_
                         (Active)              (Active)

              Radium _D_ Thorium               Actinium
              (Active)   _C_                   _C_
                         (final)               (final)

              &c.

Each product on this scheme is the parent of the product below it. Since
only two products have been observed in the active deposit of thorium
and actinium, thorium _C_ and actinium _C_ respectively refer to their
final inactive products. It will be shown in the next chapter that, as
in the case of thorium, an intermediate product exists between actinium
and its emanation. From analogy to the products Th X and Ur X, this
substance is termed “actinium X.”


=196. Theory of Successive Changes.= Before considering the evidence
from which these changes are deduced, the general theory of successive
changes of radio-active matter will be considered. It is supposed that
the matter _A_ changes into _B_, _B_ into _C_, _C_ into _D_, and so on.

Each of these changes is supposed to take place according to the same
law as a monomolecular change in chemistry, _i.e._, the number _N_ of
particles unchanged after a time _t_ is given by

$$ M = N₀ e^{–λt} $$,

where _N₀_ is the initial number and λ the constant of the change.

Since _dN_/_dt_ = -λ_N_, the rate of change at any time is always
proportional to the amount of matter unchanged. It has previously been
pointed out that this law of decay of the activity of the radio-active
products is an expression of the fact that the change is of the same
type as a monomolecular chemical change.

Suppose that _P_, _Q_, _R_ represent the number of particles of the
matter _A_, _B_, and _C_ respectively at any time _t_. Let λ₁, λ₂, λ₃ be
the constants of change of the matter _A_, _B_, and _C_ respectively.

Each atom of the matter _A_ is supposed to give rise to one atom of the
matter _B_, one atom of _B_ to one of _C_, and so on.

The expelled “rays” or particles are non-radio-active, and so do not
enter into the theory.

It is not difficult to deduce mathematically the number of atoms of _P_,
_Q_, _R_, ... of the matter _A_, _B_, _C_, ... existing at any time _t_
after this matter is set aside, if the initial values of _P_, _Q_, _R_, ...are given. In practice, however, it is generally only necessary to
employ three special cases of the theory which correspond, for example,
to the changes in the active deposit, produced on a wire exposed to a
constant amount of radium emanation and then removed, (1) when the time
of exposure is extremely short compared with the period of the changes,
(2) when the time of exposure is so long that the amount of each of the
products has reached a steady limiting value, and (3) for any time of
exposure.

There is also another case of importance which is practically a converse
of Case 3, viz. when the matter _A_ is supplied at a constant rate from
a primary source and the amounts of _A_, _B_, _C_ are required at any
subsequent time. The solution of this can, however, be deduced
immediately from Case 3 without analysis.


=197.= CASE 1. _Suppose that the matter initially considered is all of
one kind A. It is required to find the number of particles P, Q, R of
the matter A, B, C respectively present after any time t._

Then

$$ P = ne^{–λ_1 t} $$,

if _n_ is the number of particles of _A_ initially present. Now _dQ_,
the increase of the number of particles of the matter _B_ per unit time,
is the number supplied by the change in the matter _A_, less the number
due to the change of _B_ into _C_, thus

                            _dQ_/_dt_ = λ₁_P_ − λ₂_Q_      (1).
             Similarly      _dR_/_dt_ = λ₂_Q_ − λ₃_R_      (2).

Substituting in (1) the value of _P_ in terms of _n_,

$$ \frac {dQ} {dt} = λ_1 ne^{–λ_1 t} − λ_2 Q $$ .

The solution of this equation is of the form

$$ Q = n (ae^{–λ_1 t} + be^{–λ_2 t}) $$ ......(3).

By substitution it is found that _a_ = λ₁/(λ₂ − λ₁).

Since _Q_ = 0 when _t_ = 0, _b_ = –λ₁(λ₂ − λ₁).

Thus

$$ Q = \frac {nλ_1} {λ_1 − λ_2} (e^{–λ_2 t} -
e^{–λ_1 t}) $$ .... (4).

Substituting this value of _Q_ in (2), it can readily be shown that

$$ R = n (ae^{–λ_1 t} + be^{–λ_2 t} + ce^{–λ_3 t}) $$ ..... (5).

where

$$ a = \frac {λ_1 λ_2} {(λ_1 − λ_2) (λ_1 -
λ_3)} $$,

$$ b = \frac {- λ_1 λ_2} {(λ_1 − λ_2) (λ_2 −
λ_3)} $$,

$$ c = \frac {λ_1 λ_2} {(λ_1 − λ_3) (λ_2 -
λ_3)} $$,

[Illustration: Fig. 72.]

The variation of the values of _P_, _Q_, _R_ with the time _t_, after
removal of the source, is shown graphically in Fig. 72, curves _A_, _B_,
and _C_ respectively. In order to draw the curves for the practical case
which will be considered later corresponding to the first three changes
in radium _A_, the values of λ₁, λ₂, λ₃ were taken as 3·85 × 10⁻³, 5·38
× 10⁻⁴, 4·13 × 10⁻⁴ respectively, _i.e._, the times required for each
successive type of matter to be half transformed are about 3, 21, and 28
minutes respectively.

The ordinates of the curves represent the relative number of atoms of
the matter _A_, _B_, and _C_ existing at any time, and the value of _n_,
the original number of atoms of the matter _A_ deposited, is taken as
100. The amount of matter _B_ is initially zero, and in this particular
case, passes through a maximum about 10 minutes later, and then
diminishes with the time. In a similar way, the amount of _C_ passes
through a maximum about 37 minutes after removal. After an interval of
several hours the amount of both _B_ and _C_ diminishes very
approximately according to an exponential law with the time, falling to
half value after intervals of 21 and 28 minutes respectively.


=198.= CASE 2. _A primary source supplies the matter A at a constant
rate and the process has continued so long that the amount of the
products A, B, C, ... has reached a steady limiting value. The primary
source is then suddenly removed. It is required to find the amounts of
A, B, C, ... remaining at any subsequent time t._

In this case, the number _n₀_ of particles of _A_, deposited per second
from the source, is equal to the number of particles of _A_ which change
into _B_ per second, and of _B_ into _C_, and so on. This requires the
relation

                 _n₀_ = λ₁_P₀_ = λ₂_Q₀_ = λ₃_R₀_      (6),

where _P₀_, _Q₀_, _R₀_ are the maximum numbers of particles of the
matter _A_, _B_, and _C_ when a steady state is reached.

The values of _P_, _Q_, _R_ at any time _t_ after removal of the source
are given by equations of the same form as (3) and (5) for a short
exposure. Remembering the condition that initially

                           _P_ = _P₀_ = _n₀_/λ₁,
                           _Q_ = _Q₀_ = _n₀_/λ₂,
                           _R_ = _R₀_ = _n₀_/λ₃,

it can readily be shown that

    $$ P = \frac {n₀} {λ_1} e^{–λ_1 t} $$ .... (7),

    $$ Q = \frac {n₀} {λ_1 − λ_2} (\frac {λ_1}
       {λ_2} e^{–λ_2 t} − e^{–λ_1 t}) $$ .... (8),

    $$ R = n₀ (ae^{–λ_1 t} + be^{–λ_2 t} + ce^{–λ_3
       t}) $$ .... (9),

where

$$ a = \frac {λ_2} {(λ_1 − λ_2) (λ_1 -
λ_3)} $$,

$$ b = \frac {–λ_1} {(λ_1 − λ_2) (λ_2 -
λ_3)} $$,

$$ c = \frac {λ_1 λ_2} {λ_3 (λ_1 − λ_3)
(λ_2 − λ_3)} $$ .

[Illustration: Fig. 73.]

The relative numbers of atoms of _P_, _Q_, _R_ existing at any time are
shown graphically in Fig. 73, curves _A_, _B_, _C_ respectively. The
number of atoms _R₀_ is taken as 100 for comparison, and the values of
λ₁, λ₂, λ₃ are taken corresponding to the 3, 21, and 28-minute changes
in the active deposit of radium. A comparison with Fig. 72 for a short
exposure brings out very clearly the variation in the relative amounts
of _P_, _Q_, _R_ in the two cases. Initially the amount of _R_ decreases
very slowly. This is a result of the fact that the supply of _C_ due to
the breaking up of _B_ at first, nearly compensates for the breaking up
of _C_. The values of _Q_ and _R_ after several hours decrease
exponentially, falling to half value in 28 minutes.


=199.= CASE 3. _Suppose that a primary source has supplied the matter A
at a constant rate for any time T and is then suddenly removed. Required
the amounts of A, B, C at any subsequent time._

Suppose that _n₀_ particles of the matter _A_ are deposited each second.
After a time of exposure _T_, the number of particles _P_{T}_ of the
matter _A_ present is given by

$$ P_T = n₀ \int₀^T e^{–λ_1 t} dt = \frac {n₀} {λ_1} (1 -
e^{–λ_1 T}) $$ .

At any time _t_, after removal of the source, the number of particles
_P_ of the matter _A_ is given by

$$ P = P_T e^{–λ_1 t} = \frac {n₀} {λ_1} (1 − e^{–λ_1
T}) e^{–λ_1 t} $$ .

Consider the number of particles _n₀dt_ of the matter _A_ produced
during the interval _dt_. At any later time _t_, the number of particles
_dQ_ of the matter _B_, which result from the change in _A_, is given
(see equation 4) by

$$ dQ = \frac {n₀ λ_1} {λ_1 − λ_2} (e^{–λ_2 t} −
e^{–λ_1 t}) dt = n₀ f(t) dt $$ .... (10).

After a time of exposure _T_, the number of particles _Q_{T}_ of the
matter _B_ present is readily seen to be given by

$$ Q_T = n₀ (f(T) dt + f(T − dt) dt + ... + f(0) dt) = n₀ \int₀^T
f(t) dt $$ .

If the body is removed from the emanation after an exposure _T_, at any
later time _t_ the number of particles of _B_ is in the same way given
by

$$ Q = n₀ \int_t^{T+t} f(t) dt $$ .

It will be noted that the method of deduction of _Q_{T}_ and _Q_ is
independent of the particular form of the function _f_(_t_).

Substituting the particular value of _f_(_t_) given in equation (10) and
integrating, it can readily be deduced that

$$ \frac {Q} {Q_T} = \frac {ae^{–λ_2 t} − be^{–λ_1 t}} {a -
b} $$ .... (11),

where

$$ a = \frac {(1 − e^{–λ_2 T} {λ_2} $$,

$$ b = \frac {(1 − e^{–λ_1 T} {λ_1} $$,

In a similar way, the number of particles _R_ of the matter _C_ present
at any time can be deduced by substitution of the value of _f_(_t_) in
equation (5). These equations are, however, too complicated in form for
simple application to experiment, and will not be considered here.


=200.= CASE 4. _The matter A is supplied at a constant rate from a
primary source. Required to find the number of particles of A, B, C at
any subsequent time t, when initially A, B, C are absent._

The solution can be simply obtained in the following way. Suppose that
the conditions of Case 2 are fulfilled. The products _A_, _B_, _C_ are
in radio-active equilibrium and let _P₀_, _Q₀_, _R₀_ be the number of
particles of each present. Suppose the source is removed. The values of
_P_, _Q_, _R_ at any subsequent time are given by equations (7), (8) and
(9) respectively. Now suppose the source, which has been removed, still
continues to supply _A_ at the same constant rate and let _P₁_, _Q₁_,
_R₁_ be the number of particles of _A_, _B_, _C_ again present with the
source at any subsequent time. Now we have seen, that the rate of change
of any individual product, considered by itself, is independent of
conditions and is the same whether the matter is mixed with the parent
substance or removed from it. Since the values of _P₀_, _Q₀_, _R₀_
represent a steady state where the rate of supply of each kind of matter
is equal to its rate of change, the sum of the number of particles _A_,
_B_, _C_ present at any time with the source, and in the matter from
which it was removed, must at all times be equal to _P₀_, _Q₀_, _R₀_, ...,
that is

                             _P₁_ + _P_ = _P₀_,
                             _Q₁_ + _Q_ = _Q₀_,
                             _R₁_ + _R_ = _R₀_.

This must obviously be the case, for otherwise there would be a
destruction or creation of matter by the mere process of separation of
the source from its products; but, by hypothesis, neither the rate of
supply from the source, nor the law of change of the products, has been
in any way altered by removal.

Substituting the values of _P_, _Q_, _R_ from equations (7), (8), and
(9), we obtain

$$ \frac {P_1} {P₀} = 1 − e^{–λ_1 t} $$,

$$ \frac {Q_1} {Q₀} = 1 − \frac {(λ_1 e^{–λ_2 t} -
λ_2e^{–λ_1 t})} {λ_1 − λ_2} $$,

$$ \frac {R_1} {R₀} = 1 − λ_3 (ae^{–λ_1 t} + be^{–λ_2
t} + ce^{–λ_3 t}) $$,

where _a_, _b_, and _c_ have the values given after equation (9). The
curves representing the increase of _P_, _Q_, _R_, are thus, in all
cases, complementary to the curves shown in Fig. 73. The sum of the
ordinates of the two curves of rise and decay at any time is equal to
100. We have already seen examples of this in the case of the decay and
recovery curves of Ur X and Th X.


=201. Activity of a mixture of products.= In the previous calculations
we have seen how the number of particles of each of the successive
products varies with the time under different conditions. It is now
necessary to consider how this number is connected with the activity of
the mixture of products.

If _N_ is the number of particles of a product, the number of particles
breaking up per second is λ_N_, where λ is the constant of change. If
each particle of each product, in breaking up, emits one α particle, we
see that the number of α particles expelled per second from the mixture
of products at any time is equal to λ₁_P_ + λ₂_Q_ + λ₃_R_ + ...,
where _P_, _Q_, _R_, ... are the numbers of particles of the successive
products _A_, _B_, _C_, .... Substituting the values of _P_, _Q_, _R_
already found from any one of the four cases previously considered, the
variation of the number of α particles expelled per second with the time
can be determined.

The ideal method of measuring the activity of any mixture of
radio-active products would be to determine the number of α or β
particles expelled from it per second. In practice, however, this is
inconvenient and also very difficult experimentally.

Certain practical difficulties arise in endeavouring to compare the
activity of one product with another. We shall see later that, in many
cases, all of the successive products do not emit α rays. Some give out
β and γ rays alone, while there are several “rayless” products, that is,
products which do not emit either α, β, or γ rays. In the case of
radium, for example, radium _A_ gives out only α rays, radium _B_ no
rays at all, while radium _C_ gives out α, β, and γ rays.

In practice, the relative activity of any individual product at any time
is usually determined by relative measurements of the saturation
ionization current produced between the electrodes of a suitable testing
vessel.

Let us consider, for example, the case of a product which gives out only
α rays. The passage of the α particles through the gas produces a large
number of ions in its path. Since the α particles from any individual
product are projected with the same average velocity under all
conditions, the relative amount of the ionization produced per second in
the testing vessel serves as an accurate means of determining the
variation of its activity. No two products, however, emit α particles
with the same average velocity. We have seen that the rays from some
products are more readily stopped in the gas than others. Thus the
relative saturation current, due to two different products in a testing
vessel, does not serve as an accurate method of comparing the relative
number of α particles expelled per second. The ratio of the currents
will in general depend upon the distance between the plates of the
testing vessel, and, unless the relative ionization due to the average α
particle from the two products is known from other data, the comparison
of the currents can, at best, be only an approximate guide to the
relative number of α particles escaping into the gas.


=202.= Some examples will now be considered to show how the factors,
above considered, influence the character of the curves of activity
obtained under different experimental conditions. For the purpose of
illustration, we shall consider the variation after removal of the
excited activity on a body exposed for different times to a constant
supply of the radium emanation. The active deposit on removal consists
in general of a mixture of the products radium _A_, _B_, and _C_. The
nature of the rays from each product, the time for each product to be
transformed, and the value of λ are tabulated below for convenience:—

                Product  Rays       T.         λ (sec⁻¹)

              Radium _A_ α rays      3 min.    3·85 ×
                                               10⁻³

              Radium _B_ no rays    21 min.    5·38 ×
                                               10⁻⁴

              Radium _C_ α, β, γ    28 min.    4·13 ×
                         rays                  10⁻⁴

Since only the product _C_ gives rise to β and γ rays, the activity
measured by either of these types of rays will be proportional to the
amount of _C_ present at any time, _i.e._ to the value of _R_ at any
time. For a long exposure, the variation of activity with time measured
by the β and γ rays will thus be represented by the upper curve _CC_ of
Fig. 73, where the ordinates represent activity. This curve will be seen
to be very similar in shape to the experimental curve for a long
exposure which is given in Fig. 68.

Since radium _B_ does not give out rays, the number of α particles
expelled from the active deposit per second is proportional to λ₁_P_ +
λ₃_R_. The activity measured by the α rays, using the electrical method,
is thus proportional at any time to λ₁_P_ + _K_λ₃_R_, where _K_ is a
constant which represents the ratio of the number of ions, produced in
the testing vessel, by an α particle from _C_ compared with that from an
α particle emitted by _A_.

It will be seen later that, for this particular case, _K_ is nearly
unity. Taking _K_ = 1, the activity at any time after removal is
proportional to λ₁_P_ + λ₃_R_.

CASE 1. We shall first consider the activity curve for a short exposure
to the radium emanation. The relative values of _P_, _Q_, and _R_ at any
time corresponding to this case are graphically shown in Fig. 74. The
activity measured by the α rays at any time will be the sum of the
activities due to _A_ and _C_ separately.

Let curve _AA_ (Fig. 74) represent the activity due to _A_. This
decreases exponentially, falling to half value in 3 minutes. In order to
show the small activity due to _C_ clearly in the Figure, the activity
due to _A_ is plotted after an interval of 6 minutes, when the activity
has been reduced to 25 per cent. of its maximum value. The activity due
to _C_ is proportional to λ₃_R_, and in order to represent the activity
due to _C_ to the same scale as _A_, it is necessary to reduce the scale
of the ordinates of curve _CC_ in Fig. 72 in the ratio λ₃/λ₁.

[Illustration: Fig. 74.]

The activity due to _C_ is thus represented by the curve _CCC_, Fig. 74.
The total activity is thus represented by a curve _A_ + _C_ whose
ordinates are the sum of the ordinates of _A_ and _C_.

This theoretical activity curve is seen to be very similar in its
general features to the experimental curve shown in Fig. 66, where the
activity from a very short exposure is measured by the α rays.

CASE 2. The activity curve for a long exposure to the emanation will now
be considered. The activity after removal of _A_ and _C_ is proportional
to λ₁_P_ + λ₃_R_, where the values of _P_ and _R_ are graphically shown
in Fig. 75 by the curves _AA_, _CC_. Initially after removal, λ₁_P₀_ =
λ₃_R₀_, since _A_ and _C_ are in radio-active equilibrium, and the same
number of particles of each product break up per second. The activity
due to _A_ alone is shown in curve _AA_, Fig. 75. The activity decreases
exponentially, falling to half value in 3 minutes. The activity due to
_C_ at any time is proportional to _R_, and is initially equal to that
of _A_. The activity curve due to _C_ is thus represented by the curve
_CC_, which is the same curve as the upper curve _CC_ of Fig. 73. The
activity of _A_ and _C_ together is represented by the upper curve _A_ +
_C_ (Fig. 75), where the ordinates are equal to the sum of the ordinates
of the curves _A_ and _C_. This theoretical curve is seen to be very
similar in shape to the experimental curve (Fig. 67) showing the decay
of activity of the active deposit from a long exposure measured by the α
rays.

[Illustration: Fig. 75.]


=203. Effect of a rayless change on the activity curves.= Certain
important cases occur in the analysis of radio-active changes, when one
of the products does not give rise to rays and so cannot be detected
directly. The presence of this rayless change can, however, be readily
observed by the variations which occur in the activity of the succeeding
product.

Let us consider, for example, the case where the inactive matter _A_,
initially all of one kind, changes into the matter _B_ which gives out
rays. The inactive matter _A_ is supposed to be transformed according to
the same law as the radio-active products. Let λ₁, λ₂ be the constants
of the change of _A_ and _B_ respectively. If _n_ is the number of
particles of _A_, initially present, we see from the equation (4),
section 197, that the number of particles of the matter _B_ present at
any time is given by

$$ Q = \frac {nλ_1} {λ_1 − λ_2} (e^{–λ_2 t} -
e^{λ_1 t}) $$ .

Differentiating and equating to zero, it is seen that the value of _Q_
passes through a maximum at a time _T_ given by the equation

$$ λ_2 e^{–λ_2 T} = λ_1 e^{–λ_1 T} $$ .

For the sake of illustration, we shall consider the variation of the
activity of the active deposit of thorium, due to a very short exposure
to the emanation. Thorium _A_ gives out no rays, and thorium _B_ gives
out α, β, and γ rays, while thorium _C_ is inactive.

The matter _A_ is half transformed in 11 hours, and _B_ is half
transformed in 55 minutes. The value of λ₁ = 1·75 x 10⁻⁵(sec.)⁻¹ and λ₂
= 2·08 x 10⁻⁴(sec.)⁻¹.

The activity of the mixture of products _A_ + _B_ is due to _B_ alone,
and will, in consequence, be always proportional to the amount of _B_
present, that is, to the value of _Q_.

[Illustration: Fig. 76.]

The variation of activity with time is shown graphically in Fig. 76. The
activity rises from zero to a maximum in 220 minutes and then decays,
finally decreasing, according to an exponential law, with the time,
falling to half value in 11 hours.

This theoretical curve is seen to agree closely in shape with the
experimental curve (Fig. 65), which shows the variation of the activity
of the active deposit of thorium, produced by a short exposure in
presence of the emanation.

There are several points of interest in connection with an activity
curve of this character. The activity, some hours after removal, decays
according to an exponential law, not at the rate of the product _B_,
from which the activity rises, but at the same rate as the first rayless
transformation. This will also be the case if the rayless product has a
slower rate of change than the succeeding active product. Given an
activity curve of the character of Fig. 76, we can deduce from it that
the first change is not accompanied by rays and also the period of the
two changes in question. We are, however, unable to determine from the
curve which of the periods of change refers to the rayless product. It
is seen that the activity curve is unaltered if the values of λ₁, λ₂,
that is, if the periods of the products are interchanged, for the
equation is symmetrical in λ₁, λ₂. For example, in the case of the
active deposit of thorium, without further data it is impossible to
decide whether the period of the first change has a value of 55 minutes
or 11 hours. In such cases the question can only be settled by using
some physical or chemical means in order to separate the product _A_
from _B_, and then testing the rate of decay of their activity
separately. In practice, this can often be effected by electrolysis or
by utilizing the difference in volatility of the two products. If now a
product is separated from the mixture of _A_ and _B_ which loses its
activity according to an exponential law, falling to half value in 55
minutes (and such is experimentally observed), we can at once conclude
that the active product _B_ has the period of 55 minutes.

The characteristic features of the activity curve shown in Fig. 76
becomes less marked with increase of the time of exposure of a body to
the emanation, that is, when more and more of _B_ is mixed with _A_ at
the time of removal. For a long time of exposure, when the products _A_
and _B_ are in radio-active equilibrium, the activity after removal is
proportional to _Q_, where

$$ Q = \frac {n₀} {λ_1 − λ_2} (\frac {λ_1}
{λ_2} e^{–λ_2 t} − e^{λ_1 t}) $$,

(see equation 8, section 198). The value of _Q_, in this case, does not
increase after removal, but at once commences to diminish. The activity,
in consequence, decreases from the moment of removal, but more slowly
than would be given by an exponential law. The activity finally decays
exponentially, as in the previous case, falling to half value in 11
hours.

In the previous case we have discussed the activity curve obtained when
both the active and inactive product have comparatively rapid rates of
transformation. In certain cases which arise in the analysis of the
changes in actinium and radium, the rayless product has a rate of change
extremely slow compared with that of the active product. This
corresponds to the case where the active matter _B_ is supplied from _A_
at a constant rate. The activity curve will thus be identical in form
with the recovery curves of Th X and Ur X, that is, the activity _I_ at
any time _t_ will be represented by the equation

$$ \frac {I_t} {I₀} = 1 − e^{–λ_2 t} $$,

where _I₀_ is the maximum value of the activity and λ₂ the constant of
change of _B_.


=204.= In this chapter we have considered the variation with time, under
different conditions, of the number of atoms of the successive products,
when the period and number of the changes are given. It has been seen
that the activity curves to be expected under various conditions can be
readily deduced from the simple theory. In practice, however, the
investigator has been faced with the much more difficult inverse problem
of deducing the period, number, and character of the products, by
analysis of the activity curves obtained under various conditions.

In the case of radium, where at least seven distinct changes occur, the
problem has been one of considerable difficulty, and a solution has only
been possible by devising special physical and chemical methods of
isolation of some of the products.

We shall see later that two rayless changes occur in radium and actinium
and one in thorium. It is at first sight a very striking fact that the
presence of a substance which does not emit rays can be detected, and
its properties investigated. This is only possible when the rayless
product is transformed into another substance which emits rays; for the
variation of the activity of the latter may be such as to determine not
only the period but also the physical and chemical properties of the
parent product. In the two following chapters the application of the
theory of successive changes will be shown to account satisfactorily for
the complicated processes occurring in the radio-elements.

Footnote 294:

  Ramsay, _Proc. Roy. Soc._ p. 470, June, 1904; _C. R._ 138, June 6,
  1904.

Footnote 295:

  _Phil. Mag._ February, 1904.




                               CHAPTER X.
       TRANSFORMATION PRODUCTS OF URANIUM, THORIUM, AND ACTINIUM.


=205.= In the last chapter the mathematical theory of successive changes
has been considered. The results there obtained will now be applied to
explain the radio-active phenomena observed with uranium, thorium,
actinium, radium, and their products.


                  Transformation products of Uranium.


It has been shown in sections 127 and 129 that a radio-active
constituent Ur X can be separated from uranium by several different
processes. The activity of the separated Ur X decays with the time,
falling to half value in about 22 days. At the same time the uranium,
from which the Ur X has been separated, gradually regains its lost
activity. The laws of decay of Ur X and of the recovery of the lost
activity of the uranium are expressed by the equations

$$ \frac {I} {I₀} = e^{–λt} $$,

and

$$ \frac {I_t} {I₀} = 1 − e^{–λt} $$,

where λ is the radio-active constant of Ur X. The substance Ur X is
produced from uranium at a constant rate, and the constant
radio-activity observed in uranium represents a state of equilibrium,
where the rate of production of new active matter is balanced by the
rate of change of the Ur X already produced.

The radio-active processes occurring in uranium present several points
of difference from the processes occurring in thorium and radium. In the
first place, uranium does not give off an emanation, and in consequence
does not produce any excited activity on bodies. So far only one active
product Ur X has been observed in uranium. This active product Ur X
differs from Th X and the emanations, inasmuch as the radiation from it
consists almost entirely of β rays. This peculiarity of the radiations
from Ur X initially led to some confusion in the interpretation of
observations on Ur X and the uranium from which it had been separated.
When examined by the photographic method, the uranium freed from Ur X
showed no activity, while the Ur X possessed it to an intense degree.
With the electric method, on the other hand, the results obtained were
exactly the reverse. The uranium freed from Ur X showed very little loss
of activity, while the activity of the Ur X was very small. The
explanation of these results was given by Soddy[296] and by Rutherford
and Grier[297]. The α rays of uranium are photographically almost
inactive, but produce most of the ionization in the gas. The β rays, on
the other hand, produce a strong photographic action, but very little
ionization compared with the α rays. When the Ur X is separated from the
uranium, the uranium does not at first give out any β rays. In the
course of time fresh Ur X is produced from the uranium, and β rays begin
to appear, gradually increasing in intensity until they reach the
original value shown before the separation of the Ur X.

In order to determine the recovery curves of uranium after the
separation of Ur X, it was thus necessary to measure the rate of
increase of the β rays. This was done by covering the uranium with a
layer of aluminium of sufficient thickness to absorb all the α rays, and
then measuring the ionization due to the rays in an apparatus similar to
Fig. 17.

Uranium has not yet been obtained inactive when tested by the electric
method. Becquerel[298] has stated that he was able to obtain inactive
uranium, but in his experiments the uranium was covered with a layer of
black paper, which would entirely absorb the α rays. There is no
evidence that the α radiation of uranium has been altered either in
character or amount by any chemical treatment. The α rays appear to be
inseparable from the uranium, and it will be shown later that thorium
and radium as well as uranium also possess a non-separable activity
consisting entirely of α rays. The changes occurring in uranium must
then be considered to be of two kinds, (1) the change which gives rise
to the α rays and the product Ur X, (2) the change which gives rise to
the β rays from Ur X.

The possibility of separating the Ur X, which gives rise to the β rays
of uranium, shows that the α and β rays are produced quite independently
of one another, and by matter of different chemical properties.

Following the general considerations discussed in section 136 we may
suppose that every second some of the atoms of uranium—a very minute
fraction of the total number present will suffice—become unstable and
break up, expelling an α particle with great velocity. The uranium atom,
minus one α particle, becomes the atom of the new substance, Ur X. This
in turn is unstable and breaks up with the expulsion of the β particle
and the appearance of a γ ray.

The changes occurring in uranium are graphically shown in Fig. 77.

[Illustration: Fig. 77.]

On this view the α ray activity of uranium should be an inherent
property of the uranium, and should be non-separable from it by physical
or chemical means. The β and γ ray activity of uranium is a property of
Ur X, which differs in chemical properties from the parent substance and
can at any time be completely removed from it. The final product, after
the decay of Ur X, is so slightly active that its activity has not yet
been observed. We shall see later (chapter XIII.) that there is some
reason to believe that the changes in uranium do not end at this point
but continue through one or more stages, finally giving rise to radium,
or in other words that radium is a product of the disintegration of the
uranium atom. Meyer and Schweidler[299], in a recent paper, state that
the activity due to uranium preparations increases somewhat in a closed
vessel. On removing the uranium no residual activity, however, was
observed. They consider that this effect may be due to a very
short-lived emanation emitted by uranium.


=206. Effect of crystallization on the activity of uranium.= Meyer and
Schweidler[300] recently observed that uranium nitrate, after certain
methods of treatment, showed remarkable variations of its activity,
measured by the β rays. The α ray activity, on the other hand, was
unaltered. Some uranium nitrate was dissolved in water and then shaken
up with ether, and the ether fraction drawn off. The early experiments
of Crookes showed that, by this method, the uranium in the ether portion
was photographically inactive. This is simply explained by supposing
that the uranium X is insoluble in ether, and consequently remained
behind in the water fraction. The ether fraction gradually regained its
β ray activity at the normal rate to be expected if Ur X was produced by
the uranium at a constant rate, for it recovered half its final activity
in about 22 days. Some of the uranium in the water fraction was
crystallized and placed under an electroscope. The β ray activity fell
rapidly at first to half its value in the course of four days. The
activity then remained constant, and no further change was observed over
an interval of one month. Other experiments were made with crystals of
uranium nitrate, which had not been treated with ether. The nitrate was
dissolved in water and a layer of crystals separated. The β ray activity
of these crystals fell rapidly at first, the rate varying somewhat in
different experiments, but reached a minimum value after about five
days. The β ray activity then rose again at a slow rate for several
months.

The rapid drop of activity of the crystals seemed, at first sight, to
indicate that crystallization was able in some way to alter the activity
of uranium.

Dr Godlewski, working in the laboratory of the writer, repeated the work
of Meyer and Schweidler, and obtained results of a similar character,
but the initial drop of activity was found to vary both in rate and
amount in different experiments. These results were at first very
puzzling and difficult to explain, for the mother liquor, left behind
after removal of the crystals, did not show the corresponding initial
rise, which would be expected if the variation of activity were due to
the partial separation of some new product of uranium.

The cause of this effect was, however, rendered very evident by a few
well-considered experiments made by Godlewski. The uranium nitrate was
dissolved in hot water in a flat dish, and allowed to crystallize under
the electroscope. Up to the moment of crystallization the β ray activity
remained constant, but as soon as the crystals commenced to form at the
bottom of the solution the β ray activity rapidly rose in the course of
a few minutes to five times the initial value. After reaching a maximum,
the activity very gradually decreased again to the normal value. If,
however, the plate of crystals was reversed, the β ray activity was
found at first to be much smaller than the normal, but increased as fast
as that of the other side diminished.

The explanation of this effect is simple. Ur X is very soluble in water
and, at first, does not crystallize with the uranium, but remains in the
solution, and, consequently, when the crystallization commences at the
bottom of the vessel the upper layer of liquid becomes richer in uranium
X. Since the β rays arise only from the product Ur X and not from the
uranium itself, and the Ur X is mostly confined to the upper layer, a
much greater proportion of the β rays escape than if the Ur X were
uniformly distributed throughout the thick layer of uranium. When the
amount of water added is just sufficient to supply the water of
crystallization, the Ur X in the upper layer of crystals gradually
diffuses back through the mass and, in consequence, the activity of the
upper surface diminishes and of the lower surface rises. A similar
explanation applies to the effects observed by Meyer and Schweidler. The
water fraction, left behind after treatment with ether, contained all
the Ur X. The first layer of crystals formed in it contained some Ur X,
and this was for the most part confined to the top layer of crystals.
The amount of β rays at first diminished owing to the gradual diffusion
of the Ur X from the surface. In the first experiment, the amount of Ur
X present was in radio-active equilibrium with the uranium, and, after
the initial drop, the β ray activity remained constant. In the second
experiment, the gradual rise is due to the fact that the crystals of
uranium first formed contained less than the equilibrium amount of Ur X.
After falling to a minimum, the β ray activity, in consequence, slowly
rose again to the equilibrium value.

These effects exhibited by uranium are of great interest, and illustrate
in a striking manner the difference in properties of Ur X and the
uranium. The gradual diffusion of the Ur X throughout the mass of
crystals is noteworthy. By measurements of the variation with time of
the β ray activity, it should be possible to deduce its rate of
diffusion into the crystallized mass.


                  Transformation products of Thorium.


=207. Analysis of the active deposit.= The radio-active processes
occurring in thorium are far more complicated than those in uranium. It
has already been shown in chapter vi that a radio-active product Th X is
continuously produced from the thorium. This Th X breaks up, giving rise
to the radio-active emanation. The emanation produces from itself a type
of active matter which is deposited on the surface of bodies, where it
gives rise to the phenomena of excited or induced activity. This active
deposit possesses some distinctive chemical and physical properties
which distinguish it from the emanation and the Th X. We have seen
(section 180) that the rate at which the active deposit loses its
activity depends upon the time of exposure of the body made active to
the emanation. The explanation of the activity curves for different time
of exposure will now be considered.

The curve of variation of activity for a short exposure of 10 minutes
has already been given in Fig. 65. The activity is small at first but
increases rapidly with the time; it passes through a maximum about 4
hours later, and finally decays exponentially with the time, falling to
half value in 11 hours. This remarkable effect can be explained
completely[301] if it be supposed that the active deposit consists of
two distinct substances. The matter initially deposited from the
emanation, which will be called thorium _A_, is supposed to be changed
into thorium _B_. Thorium _A_ is transformed according to the ordinary
exponential law, but the change is not accompanied by any ionizing rays.
In other words, the change from _A_ to _B_ is a “rayless” change. On the
other hand, _B_ breaks up into _C_ with the accompaniment of all three
kinds of rays. On this view the activity of the active deposit at any
time represents the amount of the substance _B_ present, since _C_ is
inactive or active to a very minute extent.

If the variation of the activity imparted to a body exposed for a short
interval in the presence of the thorium emanation, is due to the fact
that there are two successive changes in the deposited matter _A_, the
first of which is a “rayless” change, the activity _I_{t}_ at any time
_t_ after removal should be proportional to the number _Q_{t}_ of
particles of the matter _B_ present at that time. Now, from equation (4)
section 197, it has been shown that

$$ Q = \frac {nλ_1} {λ_1 − λ_2} (e^{–λ_2 t} -
e^{–λ_1 t}) $$ .... (4).

The value of _Q_{t}_ passes through a maximum _Q_{T}_ at the time _T_
when

$$ \frac {λ_2} {λ_1} − e^{-(λ_1–λ_2) T} $$ .

The maximum activity _I_{T}_ is proportional to _Q_{T}_ and

$$ \frac {I_t} {I_T} = \frac {Q_t} {Q_T} = \frac {e^{–λ_2 t} -
e^{–λ_1 t}} {e^{–λ_2 T} − e^{–λ_1 T}} $$ .

It will be shown later that the variation with time of the activity,
imparted to a body by a short exposure, is expressed by an equation of
the above form. It thus remains to fix the values of λ₁, λ₂. Since the
above equation is symmetrical with regard to λ₁, λ₂, it is not possible
to settle from the agreement of the theoretical and experimental curve
which value of λ refers to the first change. The curve of variation of
activity with time is unaltered if the values of λ₁ and λ₂ are
interchanged.

It is found experimentally that the activity 5 or 6 hours after removal
decays very approximately according to an exponential law with the time,
falling to half value in 11 hours. This is the normal rate of decay of
thorium for all times of exposure, provided measurements are not begun
until several hours after the removal of the active body from the
emanation.

This fixes the value of the constants of one of the changes. Let us
assume for the moment that this gives the value of λ₁.

                    Then      λ₁ = 1·75 × 10⁻⁵ (sec)⁻¹.

Since the maximum activity is reached after an interval _T_ = 220
minutes (see Fig. 65), substituting the values of λ₁ and _T_ in the
equation, the value of λ₂ comes out to be

                         λ₂ = 2·08 × 10⁻⁴ (sec)⁻¹.

This value of λ₂ corresponds to a change in which half the matter is
transformed in 55 minutes.

Substituting now the values of λ₁, λ₂, _T_, the equation reduces to

$$ \frac {I_t} {I_T} = 1\cdot37 (e^{–λ_2 t} − e^{–λ_2 t}) $$ .

The agreement between the results of the theoretical equation and the
observed values is shown in the following table:

                  Time in     Theoretical  Observed value
                  minutes        value of              of
                          _I_{t}_/_I_{T}_ _I_{t}_/_I_{T}_


                       15             ·22             ·23

                       30             ·38             ·37

                       60             ·64             ·63

                      120             ·90             ·91

                      220            1·00            1·00

                      305             ·97             ·96

After 5 hours the activity decreased nearly exponentially with the time,
falling to half value in 11 hours.

It is thus seen that the curve of rise of activity for a short exposure
is explained very satisfactorily on the supposition that two changes
occur in the deposited matter, of which the first is a rayless change.

Further data are required in order to fix which of the time constants of
the changes refers to the first change. In order to settle this point,
it is necessary to isolate one of the products of the changes and to
examine the variation of its activity with time. If, for example, a
product can be separated whose activity decays to half value in 55
minutes, it would show that the second change is the more rapid of the
two. Now Pegram[302] has examined the radio-active products obtained by
electrolysis of thorium solutions. The rates of decay of the active
products depended upon conditions, but he found that, in several cases,
rapidly decaying products were obtained whose activity fell to half
value in about 1 hour. Allowing for the probability that the product
examined was not completely isolated by the electrolysis, but contained
also a trace of the other product, this result would indicate that the
last change which gives rise to rays is the more rapid of the two.

This point is very clearly brought out by some recent experiments of
Miss Slater[303], who has made a detailed examination of the effect of
temperature on the active deposit of thorium.

A platinum wire was made active by exposure for a long interval to the
thorium emanation, and then heated for a few minutes to any desired
temperature by means of the electric current. The wire, while being
heated, was surrounded by a lead cylinder in order that any matter
driven off from it should be collected on its surface. The decay of
activity both of the wire and of the lead cylinder was then tested
separately. After heating to a dull red heat, no sensible diminution of
the activity was observed at first, but the rate of decay of the
activity on the wire was found to be more rapid than the normal. The
activity of the lead cylinder was small at first but increased to a
maximum after about 4 hours and then decayed at the normal rate with the
time.

These results are to be expected if some thorium A is volatilized from
the wire; for the rise of activity on the lead cylinder is very similar
to that observed on a wire exposed for a short time in the presence of
the thorium emanation, _i.e._, under the condition that only thorium A
is initially present.

On heating the wire above 700° C. the activity was found to be reduced,
showing that some thorium B had also been removed. By heating for a few
minutes at about 1000° C. nearly all the thorium A was driven off. The
activity on the wire then decayed exponentially with the time, falling
to half value in about 1 hour. After heating for a minute at about 1200°
C. all the activity was removed. These results show that thorium A is
more volatile than B, and that the product which gives out rays, viz.
thorium B, has a period of about 55 minutes.

Another series of experiments was made, in which an active aluminium
disc was placed in an exhausted tube, and exposed to the cathode ray
discharge. Under these conditions, a part of the activity of the disc
was removed. When the disc was made the anode, the loss of activity was
usually 20 to 60 per cent. for half-an-hour’s exposure. If the disc was
made the cathode, the loss was much greater, amounting to about 90 per
cent. in 10 minutes. Part of the active matter removed from the disc was
collected on a second disc placed near it. This second disc on removal
lost its activity at a far more rapid rate than the normal. The rate of
decay on the first disc was also altered, the activity sometimes even
increasing after removal. These results indicate that, in this case, the
apparent volatility of the products is reversed. Thorium B is driven off
from the disc more readily than thorium A. The rates of decay obtained
under different conditions were satisfactorily explained by supposing
that the surfaces of the discs after exposure to the discharge were
coated with different proportions of thorium A and B.

The escape of thorium B from the disc under the influence of the
discharge seems rather to be the result of an action similar to the
well-known “sputtering” of electrodes than to a direct influence of
temperature.

The results obtained by von Lerch[304] on the electrolysis of a solution
of the active deposit also admit of a similar interpretation. Products
were obtained on the electrodes of different rates of decay, losing half
their activity in times varying from about 1 hour to 5 hours. This
variation is due to the admixture of the two products in different
proportions. The evidence, as a whole, thus strongly supports the
conclusion that the active deposit from thorium undergoes two successive
transformations as follows:

(1) A “rayless” change for which λ₁ = 1·75 × 10⁻⁵, _i.e._, in which half
the matter is transformed in 11 hours;

(2) A second change giving rise to α, β and γ rays, for which λ₂ = 2·08
× 10⁻⁴, _i.e._, in which half the matter is transformed in 55
minutes[305].

It is, at first sight, a somewhat unexpected result that the final rate
of decay of the active deposit from thorium gives the rate of change not
of the last product itself, but of the preceding product, which does not
give rise to rays at all.

A similar peculiarity is observed in the decay of the excited activity
of actinium, which is discussed in section 212.

For a long exposure in the presence of a constant supply of thorium
emanation, the equation expressing the variation of activity with time
is found from equation (8), section 198,

  $$ \frac {I_t} {I₀} = \frac {Q} {Q₀} = \frac {λ_2} {λ_2 -
     λ_1} e^{–λ_1 t} − \frac {λ_1} {λ_1 − λ_2}
     e^{–λ_2 t} $$

  $$ = \frac {λ_2 e^{–λ_1 t}} {λ_2 -
     λ_1} (1 − \cdot083 e^{−1\cdot90 × 10^{−4} t} ) $$ .

About 5 hours after removal the second term in the brackets becomes very
small, and the activity after that time will decay nearly according to
an exponential law with the time, falling to half value in 11 hours. For
any time of exposure _T_, the activity at time _t_ after the removal
(see equation 11, section 199) is given by

$$ \frac {I_t} {I₀} = \frac {Q} {Q_T} = \frac {ae^{–λ_2 t} -
be^{–λ_1 t}} {a − b} $$,

where _I₀_ is the initial value of the activity, immediately after
removal, and

            $$ a = \frac {1 − e^{–λ_2 T}} {λ_2} $$,

            $$ b = \frac {1 − e^{–λ_1 T}} {λ_1} $$,

By variation of _T_ the curves of variation of activity for any time of
exposure can be accurately deduced from the equation, when the values of
the two constants λ₁, λ₂ are substituted. Miss Brooks[306] has examined
the decay curves of excited activity for thorium for different times of
exposure and has observed a substantial agreement between experiment and
theory.

[Illustration: Fig. 78.]

The results are shown graphically in Fig. 78. The maximum value of the
activity is, for each time of exposure, taken as 100. The theoretical
and observed values are shown in the Figure.


=208. Analysis of the decay and recovery curves of Th X.= The
peculiarities of the initial portions of the decay and recovery curves
of Th X and thorium respectively (Curves _A_ and _B_, Fig. 47, p. 221),
will now be considered. It was shown that when the Th X was removed from
the thorium by precipitation with ammonia, the radiation increased about
15 per cent. during the first day, passed through a maximum, and then
fell off according to an exponential law, decreasing to half value in
four days. At the same time the activity of the separated hydroxide
decreased for the first day, passed through a minimum, and then slowly
increased again, rising to its original value after the lapse of about
one month.

When a thorium compound is in a state of radio-active equilibrium, the
series of changes in which Th X, the emanation, and thorium A and B are
produced, go on simultaneously. Since a state of equilibrium has been
reached for each of these products, the amount of each product changing
in unit time is equal to the amount of that product supplied from the
preceding change in unit time. Now the matter Th X is soluble in
ammonia, while thorium A and B are not. The Th X is thus removed from
the thorium by precipitation with ammonia, but A and B are left behind
with the thorium. Since the active deposit is produced from the
emanation, which in turn arises from Th X, on the removal of the parent
matter Th X, the radiation due to this active deposit will decay, since
the rate of production of fresh matter no longer balances its own rate
of change. Disregarding the initial irregularity in the decay curve of
the active deposit, its activity will have decayed to half value in
about 11 hours, and to one quarter value at the end of 22 hours. As
soon, however, as the Th X has been separated, new Th X is produced in
the thorium compound. The activity of this new Th X is not, however,
sufficient to compensate at first for the loss of activity due to the
change in the active deposit, so that, as a whole, the activity will at
first _decrease_, then pass through a minimum, then increase again.

The correctness of this point of view has been tested by Rutherford and
Soddy[307] as follows: If the precipitated thorium hydroxide after the
removal of Th X is put through a series of precipitations with ammonia
at short intervals, the Th X is removed almost as fast as it is formed,
and, at the same time, the activity of thorium B in the thorium decays.

The following table indicates the results obtained. A portion of the
precipitated hydroxide was removed after each series of precipitations
and its activity tested in the usual way.

                                                Activity
                                                      of
                                               hydroxide
                                               per cent.

               After 1 precipitation                  46

               After 3 precipitations at              39
               intervals of 24 hours

               After 3 more precipitations at         22
               intervals of 24 hours and 3 at
               intervals of 8 hours

               After 3 more each of 8 hours           24

               After 6 more each of 4 hours           25

[Illustration: Fig. 79.]

The differences in the last three numbers are not significant, for it is
difficult to make accurate comparisons of the activity of thorium
compounds which have been precipitated under slightly different
conditions. It is thus seen that as a result of successive
precipitations, the activity is reduced to a minimum of about 25 per
cent. The recovery curve of the activity of this 23 times precipitated
hydroxide is shown in Fig. 79. The initial drop in the curve is quite
absent, and the curve, starting from the minimum, is practically
identical with the curve shown in Fig. 48, which gives the recovery
curve of thorium hydroxide after the first two days. This residual
activity—about 25 per cent. of the maximum—is non-separable from the
thorium by any chemical process that has been tried.

The initial rise of activity of Th X, after it has been separated, will
now be considered. In all cases it was found that the activity of the
separated Th X had increased about 15 per cent. at the end of 24 hours,
and then steadily decayed, falling to half value in about four days.

This peculiarity of the Th X curve follows, of necessity, from the
considerations already advanced to explain the drop in the recovery
curve. As soon as the Th X is separated, it at once produces from itself
the emanation, and this in turn produces thorium A and B. The activity
due to B at first more than compensates for the decay of activity of the
Th X itself. The total activity thus increases to a maximum, and then
slowly decays to zero according to an exponential law with the time. The
curve expressing the variation of the activity of the separated Th X
with time can be deduced from the theory of successive changes already
considered in chapter IX. In the present case there are four successive
changes occurring at the same time, viz. the change of Th X into the
emanation, of the emanation into thorium A, of A into B, and of B into
an inactive product. Since, however, the change of the emanation into
thorium A (about half changed in one minute) is far more rapid than the
changes occurring in Th X or thorium A and B, for the purposes of
calculation it may be assumed without serious error that the Th X
changes at once into the active deposit. The 55 minute change will also
be disregarded for the same reason.

Let λ₁ and λ₂ be the constants of decay of activity of Th X and of
thorium A respectively. Since the activity of Th X and of thorium A
falls to half value in 4 days and 11 hours respectively, the value of λ₁
= ·0072 and of λ₂ = ·063, where 1 hour is taken as the unit of time.

The problem reduces to the following: _Given the matter A (thorium X)
all of one kind, which changes into B (thorium B), find the activity of
A and B together at any subsequent time._ This corresponds to Case I.
(section 197). The amount _Q_ of B at any time _T_ is given by

$$ Q = \frac {λ_1 n₀} {λ_1 − λ_2} (e^{–λ_2 t} -
e^{–λ_1 t}) $$,

and the activity _I_ at any time of the two together is proportional to
λ₁_P_ + _K_λ₂_Q_, where _K_ is the ratio of the ionization of B compared
with that of A.

Then

  $$ \frac {I_t} {I₀} = \frac {λ_1 P + Kλ_2 Q} {λ_1
     n₀} $$

  $$ = e^{–λ_1 t} (1 + \frac {Kλ_2} {λ_2 − λ_1}
     (1 − e^{-(λ_2 − λ_1) t})) $$,

where _I₀_ is the initial activity due to _n₀_ particles of Th X.

By comparison of this equation with the curve of variation of the
activity of Th X with time, shown in Fig. 47, it is found that _K_ is
almost ·44. It must be remembered that the activity of the emanation and
Th X are included together, so that the activity of thorium B is about
half of the activity of the two preceding products.

The calculated values of _I_{t}_/_I₀_ for different values of _t_ are
shown in the second column of the following table, and the observed
values in the third column.

                         Time Theoretical Observed
                              value       value


                            0 1·00        1·00

                     ·25 days 1·09        —

                       ·5   „ 1·16        —

                      1     „ 1·15        1·17

                      1·5   „ 1·11        —

                      2     „ 1·04        —

                      3     „  ·875        ·88

                      4     „  ·75         ·72

                      6     „  ·53         ·53

                      9     „  ·315        ·295

                     13     „  ·157        ·152

[Illustration: Fig. 80.]

The theoretical and observed values thus agree within the limit of error
in the measurements. The theoretical curve is shown in Curve _A_, Fig.
80 (with the observed points marked, for comparison). The curve _B_
shows the theoretical curve of the decay of the activity of Th X and the
emanation, supposing there is no further change into the active deposit.
Curve _C_ shows the difference curve between the curves _A_ and _B_,
_i.e._ the proportion of the activity at different times due to the
active deposit. The activity due to the latter thus rises to a maximum
about two days after removal of the Th X, and then decays with the time
at the same rate as the Th X itself, _i.e._ the activity falls to half
value every four days. When _t_ exceeds four days, the term

$$ e^{-(λ_2 − λ_1) t} $$

in the theoretical equation is very small.

The equation of decay after this time is therefore expressed by

$$ \frac {I_t} {I₀} = (1 + \frac {Kλ_2} {λ_2 − λ_1})
e^{–λ_1 t} $$,

_i.e._ the activity decays according to an exponential law with the
time.


=209. Radiations from Thorium products.= It has been shown in the last
section that the activity of thorium, by successive precipitations with
ammonia, is reduced to a limiting value of almost 25 per cent. of the
initial activity. This “non-separable activity” consists of α rays, the
β and γ rays being altogether absent. According to the disintegration
theory, this is an expression of the fact that the initial break-up of
the thorium atom is accompanied only by the expulsion of α particles. We
have seen in section 156 that the thorium emanation also gives out only
α rays. In the active deposit, thorium A gives out no rays, while
thorium B emits all three types of rays.

Some hours after separation, Th X gives out α, β, and γ rays, but the
appearance of β and γ rays is probably due to the thorium B associated
with it. The β and γ ray activity of Th X is much reduced if a current
of air is continuously aspirated through a solution of Th X to remove
the emanation. It seems likely that if the emanation could be removed as
fast as it was formed, so as to prevent the formation of thorium B in
its mass, Th X itself would give out only α rays: but, on account of the
rapid rate of change of the thorium emanation, it is difficult to
realize this experimentally.


=210. Transformation products of Thorium.= The transformation products
of thorium and the rays emitted by them are graphically shown below
(Fig. 81).

[Illustration: Fig. 81.]

A table of the transformation products of thorium is shown below, with
some of their physical and chemical properties.

     Product       Time to be    λ (sec)⁻¹   Radiations Physical and
                         half                               chemical
                  transformed                             properties


    Thorium                                      α rays Insoluble in
                                                             ammonia

    Th. X              4 days       2·00 ×       α rays   Soluble in
                                   10⁻⁶                    ammonia

    Emanation        54 secs.       1·28 ×       α rays   Inert gas,
                                   10⁻²                    condenses
                                                            −120° C.

    Thorium A        11 hours       1·75 ×      no rays   Soluble in
                                   10⁻⁵                 strong acids.
                                                         Volatile at
                                                       a white heat.
                                                            B can be
                                                           separated
                                                           from A by
                                                        electrolysis
                                                              and by
                                                          difference
                                                      of volatility.

    Thorium B        55 mins.        2·1 × α, β, γ rays         Same
                                   10⁻⁴

    ?                       —            —            —            -


=211. Transformation products of Actinium.= It has previously been
pointed out (sections 17 and 18) that the actinium of Debierne and the
emanium of Giesel contain the same radio-active constituent. Both give
out a short-lived emanation which imparts activity to the surface of
bodies. Recently, thanks to Dr Giesel of Braunschweig, preparations of
“emanium” have been placed on the market, and most of the investigations
that are described later have been made with this substance.

_Actinium X._ Actinium and thorium are very closely allied in
radio-active properties. Both emit an emanation which is rapidly
transformed, but the rate of change of the actinium emanation is still
more rapid than that of thorium, the activity decreasing to half value
in 3·7 seconds. Miss Brooks[308] has analysed the active deposit from
the emanation of actinium, and has shown that two successive changes
occur in it, very similar in character to those observed in the active
deposit of thorium. It thus seemed probable, from analogy, that an
intermediate product, corresponding to Th X in thorium, would be found
in actinium[309]. Recent work has verified this supposition. Giesel[310]
and Godlewski[311] independently observed that a very active substance
could be separated from “emanium,” very similar in chemical and physical
properties to Th X in thorium. This product will, from analogy, be
called “actinium X.” The same method, which was used by Rutherford and
Soddy to separate Th X from thorium, is also effective in separating
actinium X from actinium. After precipitation of the active solution
with ammonia, actinium X is left behind in the filtrate. After
evaporation and ignition, a very active residue remains. At the same
time, the precipitated actinium loses a large proportion of its
activity.

Giesel observed the separation of an active product, using a fluorescent
screen to detect the radiations. A very complete examination of the
product actinium X has been made by Godlewski in the laboratory of the
writer.

After separation of actinium X, the activity, whether measured by the α
or β rays, increases about 15 per cent. during the first day, and
afterwards decays exponentially with the time, falling to half value in
10·2 days. The activity of the separated actinium was small at first but
steadily increased with the time, reaching a practical maximum after an
interval of sixty days. After the first day, the decay and recovery
curves of activity are complementary to one another. The curves of rise
and decay are shown graphically in Fig. 82, curves I and II
respectively.

Godlewski observed that a solution of actinium, freed from actinium X,
gave out very little emanation, while a solution of actinium X gave off
the emanation in large quantity. The amount of emanation from the
solution was measured by observing the activity produced in a testing
vessel, similar to that shown in Fig. 51, when a constant current of air
was passed through the solution. The emanating power of actinium X
decreased exponentially with the time at the same rate as that at which
the actinium X lost its activity. At the same time the actinium solution
increased in emanating power, reaching its original value after about 60
days. The behaviour of actinium and thorium is thus quite analogous, and
the explanation advanced to explain the decay and recovery curves of
thorium applies equally well to the corresponding curves of actinium.

[Illustration: Fig. 82.]

The actinium X is produced at a constant rate from the parent matter
actinium, and is transformed according to an exponential law with the
time. The constant of change λ = ·068 (day)⁻¹, and this value is
characteristic of the product actinium X. As in the case of thorium, the
above experiments show that the emanation does not arise from actinium
itself but from actinium X. The emanation in turn breaks up and gives
rise to an active deposit on the surface of bodies.


=212. Analysis of the active deposit from the emanation.= Debierne[312]
observed that the excited activity produced by actinium decayed to half
value in about 41 minutes. Miss Brooks[313] showed that the curves of
decay of the excited activity after removal depended upon the duration
of exposure to the emanation. The curves for different times of exposure
have already been shown in Fig. 69.

Bronson, using the direct deflection method described in section 69,
accurately determined the activity curve corresponding to a short
exposure to the actinium emanation. The curve obtained is shown in Fig.
83.

[Illustration: Fig. 83.]

This curve is similar in shape to the corresponding curve obtained for
the active deposit from thorium, and is explained in a similar way. The
activity _I_{t}_ at any time _t_ is given by

$$ \frac {I_t} {I_T} = \frac {e^{–λ_2 t} − e^{–λ_1 t}}
{e^{–λ_2 T} − e^{–λ_1 T}} $$,

where λ₁ and λ₂ are two constants, and _I{_T_} the maximum activity
reached after an interval _T_. After 20 minutes the activity decreased
exponentially with the time, falling to half value in 35·7 minutes. This
gives the value λ₁ = ·0194 (min.)⁻¹. By comparison with the curve, the
value of λ₂ was found to be ·317 (min.)⁻¹. This corresponds to a change
in which half the matter is transformed in 2·15 minutes. Exactly as in
the analogous curve for thorium, it can be shown that the matter
initially deposited undergoes two changes, the first of which is a
rayless one. The same difficulty arises in fixing which of the values of
λ refers to the first change. An experiment made by Miss Brooks (_loc.
cit._) shows that the rayless product has the slower period of
transformation. The active deposit of actinium was dissolved off a
platinum wire and then electrolysed. The anode was found to be active,
and the activity fell off exponentially with the time, decreasing to
half value in about 1·5 minutes. Allowing for the difficulty of
accurately measuring such a rapid rate of decay, this result indicates
that the product which gives out rays has the rapid period of 2·15
minutes. The analysis of the active deposit of actinium thus leads to
the following conclusions:

(1) The matter initially deposited from the emanation, called actinium
A, does not give out rays, and is half transformed in 35·7 minutes.

(2) A changes into B, which is half transformed in 2·15 minutes, and
gives out both α and β (and probably γ) rays.

Godlewski found that the active deposit of actinium was very easily
volatilized. Heating for several minutes at a temperature of 100° C. was
sufficient to drive off most of the active matter. The active deposit is
readily soluble in ammonia and in strong acids.


=213. Radiations from actinium and its products.= Actinium in
radio-active equilibrium gives out α, β, and γ rays. Godlewski found
several points of distinction between the β and γ rays of actinium and
of radium. The β rays of actinium appear to be homogeneous, for the
activity measured by an electroscope was found to fall off accurately
according to an exponential law with the thickness of matter traversed.
The β rays were half absorbed in a thickness of 0·21 mm. of aluminium.
This indicates that the β particles are all projected from actinium with
the same velocity. In this respect actinium behaves very differently
from radium, for the latter gives out β particles whose velocities vary
over a wide range.

After the β rays were absorbed, another type of more penetrating rays
was observed, which probably corresponds to the γ rays from the other
radio-elements. The γ rays of actinium were, however, far less
penetrating than those from radium. The activity due to these rays was
reduced to one-half after passing through 1·9 mms. of lead, while the
thickness of lead required in order to absorb half the γ rays of radium
is about 9 mms.

The active deposit gave out α and β (and probably γ) rays. It was
difficult to decide definitely whether actinium X gave out β as well as
α rays. When the actinium X was heated to a red heat, the β activity was
temporarily reduced to about half its initial value. This decrease was
probably due to the removal of the active deposit, which, we have seen,
is readily volatilized by heat. If the β ray activity cannot be further
reduced, this would point to the conclusion that actinium X, as well as
actinium B, gives out β rays, but the evidence so far obtained is not
conclusive.

The ease with which the active deposit is volatilized by heat offers a
very simple explanation of the initial peculiarities of the decay and
recovery curves (Fig. 82) of actinium X. The activity of actinium X
rises at first, but there is no corresponding decrease in the activity
of the actinium left behind. It has been shown that the active deposit
is soluble in ammonia, and, in consequence, is removed with the actinium
X. The products actinium A and B and actinium X, immediately after
separation, are in radio-active equilibrium and we should not therefore
expect to find any increase of activity after removal, such as is
observed in the case of thorium, where thorium A and B are not removed
with thorium X. However, in heating the actinium X to drive off the
ammonium salts, some of the active deposit is volatilized. After
cooling, the amount of the active deposit increases to nearly its old
value and there is a corresponding increase of the activity.

[Illustration: Fig. 84.]


=214. Products of Actinium.= There is one very interesting point of
distinction between the radio-active behaviour of thorium and actinium.
The latter after removal of actinium X, shows only about 5 per cent. of
the original activity, while thorium, after removal of Th X, always
shows a residual activity of about 25 per cent. of the maximum value.
This very small residual activity indicates that actinium, if completely
freed from all its products, would not give out rays at all, in other
words, the first change in actinium is a rayless one.

The radio-active products of actinium are shown graphically in Fig. 84.
Some of their chemical and physical properties are tabulated below.

        Products   Time to be  Rays         Some Physical and
                   half                     Chemical properties
                   transformed


       Actinium    ?           No rays      Insoluble in ammonia

       Actinium X  10·2 days   α, (β and γ) Soluble in ammonia

       Emanation   3·9 secs.   α rays       Behaves as a gas

       Actinium A  35·7 mins.  No rays      Soluble in ammonia
                                            and strong acids.

       Actinium B  2·15 mins.  α, β and γ   Volatilized at
                                            100°C.  B can be
                                            separated from A by
                                            electrolysis

Footnote 296:

  Soddy, _Trans. Chem. Soc._ 81, p. 460, 1902.

Footnote 297:

  Rutherford and Grier, _Phil. Mag._ Sept. 1902.

Footnote 298:

  Becquerel, _C. R._ 131, p. 137, 1900.

Footnote 299:

  Meyer and Schweidler, _Wien Ber._ Dec. 1, 1904.

Footnote 300:

  Meyer and Schweidler, _Wien Ber._ 113, July, 1904.

Footnote 301:

  Rutherford, _Phil. Trans._ A. 204, pp. 169–219, 1904.

Footnote 302:

  Pegram, _Phys. Rev._ p. 424, December, 1903.

Footnote 303:

  Miss Slater, _Phil. Mag._ 1905.

Footnote 304:

  von Lerch, _Ann. de d.] Phys._ November, 1903.

Footnote 305:

  The ‘rayless change’ certainly does not give out α rays, and special
  experiments showed that no appreciable amount of β rays were present.
  On the other hand, the second change gives out all three types of
  rays.

Footnote 306:

  Miss Brooks, _Phil. Mag._ Sept. 1904.

Footnote 307:

  Rutherford and Soddy, _Trans. Chem. Soc._ 81, p. 837, 1902. _Phil.
  Mag._ Nov. 1902.

Footnote 308:

  Miss Brooks, _Phil. Mag._ Sept. 1904.

Footnote 309:

  Rutherford, _Phil. Trans._ A. p. 169, 1904.

Footnote 310:

  Giesel, _Ber. d. D. Chem. Ges._ p. 775, 1905.

Footnote 311:

  Godlewski, _Nature_, p. 294, Jan. 19, 1905.

Footnote 312:

  Debierne, _C. R._ 138, p. 411, 1904.

Footnote 313:

  Miss Brooks, _Phil. Mag._ Sept. 1904.




                              CHAPTER XI.
                   TRANSFORMATION PRODUCTS OF RADIUM.


=215. Radio-activity of radium.= Notwithstanding the enormous difference
in their relative activities, the radio-activity of radium presents many
close analogies to that of thorium and actinium. Both substances give
rise to emanations which in turn produce “excited activity” on bodies in
their neighbourhood. Radium, however, does not give rise to any
intermediate product between the element itself and the emanation it
produces, or in other words there is no product in radium corresponding
to Th X in thorium.

Giesel first drew attention to the fact that a radium compound gradually
increased in activity after preparation, and only reached a constant
value after a month’s interval. If a radium compound is dissolved in
water and boiled for some time, or a current of air drawn through the
solution, on evaporation it is found that the activity has been
diminished. The same result is observed if a solid radium compound is
heated in the open air. This loss of activity is due to the removal of
the emanation by the process of solution or heating. Consider the case
of a radium compound which has been kept for some time in solution in a
shallow vessel, exposed to the open air, and then evaporated to dryness.
The emanation which, in the state of solution, was removed as fast as it
was formed, is now occluded, and, together with the active deposit which
it produces, adds its radiations to that of the original radium. The
activity will increase to a maximum value when the rate of production of
fresh emanation balances the rate of change of that already produced.

If now the compound is again dissolved or heated, the emanation escapes.
Since the active deposit is not volatile and is insoluble in water, it
is not removed by the process of solution or heating. Since, however,
the parent matter is removed, the activity due to the active deposit
will immediately begin to decay, and in the course of a few hours will
have almost disappeared. The activity of the radium measured by the α
rays is then found to be about 25 per cent. of its original value. This
residual activity of radium, consisting entirely of α rays, is
non-separable, and has not been further diminished by chemical or
physical means. Rutherford and Soddy[314] examined the effect of
aspiration for long intervals through a radium chloride solution. After
the first few hours the activity was found to be reduced to 25 per
cent., and further aspiration for three weeks did not produce any
further diminution. The radium was then evaporated to dryness, and the
rise of its activity with time determined. The results are shown in the
following table. The final activity in the second column is taken as one
hundred. In column 3 is given the percentage proportion of the activity
recovered.

                     Time in  Activity    Percentage
                        days                Activity
                                           recovered


                           0      25·0             0

                        0·70      33·7          11·7

                        1·77      42·7          23·7

                        4·75      68·5          58·0

                        7·83      83·5          78·0

                       16·0       96·0          95·0

                       21·0      100·0         100·0

The results are shown graphically in Fig. 85.

The decay curve of the radium emanation is shown in the same figure. The
curve of recovery of the lost activity of radium is thus analogous to
the curves of recovery of uranium and thorium which have been freed from
the active products Ur X and Th X respectively. The intensity _I_{t}_ of
the recovered activity at any time is given by

$$ \frac {I_t} {I₀} = 1 − e^{–λt} $$,

where _I₀_ is the final value, and λ is the radio-active constant of the
emanation. The decay and recovery curves are complementary to one
another.

[Illustration: Fig. 85.]

Knowing the rate of decay of activity of the radium emanation, the
recovery curve of the activity of radium can thus at once be deduced,
provided all of the emanation formed is occluded in the radium compound.

When the emanation is removed from a radium compound by solution or
heating, the activity _measured by the_ β _rays_ falls almost to zero,
but increases in the course of a month to its original value. The curve
showing the rise of β and γ rays with time is practically identical with
the curve, Fig. 85, showing the recovery of the lost activity of radium
measured by the α rays. The explanation of this result lies in the fact
that the β and γ rays from radium only arise from the active deposit,
and that the non-separable activity of radium gives out only α rays. On
removal of the emanation, the activity of the active deposit decays
nearly to zero, and in consequence the β and γ rays almost disappear.
When the radium is allowed to stand, the emanation begins to accumulate,
and produces in turn the active deposit, which gives rise to β and γ
rays. The amount of β and γ rays (allowing for a period of retardation
of a few hours) will then increase at the same rate as the activity of
the emanation, which is continuously produced from the radium.


=216. Effect of escape of emanation.= If the radium allows some of the
emanation produced to escape into the air, the curve of recovery will be
different from that shown in Fig. 85. For example, suppose that the
radium compound allows a constant fraction α of the amount of emanation,
present in the compound at any time, to escape per second. If _n_ is the
number of emanation particles present in the compound at the time _t_,
the number of emanation particles changing in the time _dt_ is λ_ndt_,
where λ is the constant of decay of activity of the emanation. If _q_ is
the rate of production of emanation particles per second, the increase
of the number _dn_ in the time _dt_ is given by

                      _dn_ = _qdt_ − λ_ndt_ − α_ndt_,

                  or  _dn_
                      ----- = _q_ − (λ + α)_n_.
                      _dt_

The same equation is obtained when no emanation escapes, with the
difference that the constant λ + α is replaced by λ. When a steady state
is reached, _dn_/_dt_ is zero, and the maximum value of _n_ is equal to
_q_/(λ + α).

If no escape takes place, the maximum value of _n_ is equal to _q_/λ.
The escape of emanation will thus lower the amount of activity recovered
in the proportion λ/(λ + α). If _n₀_ is the final number of emanation
particles stored up in the compound, the integration of the above
equation gives

$$ \frac {n} {n₀} = 1 − e^{-(λ + \alpha) t} $$ .

The curve of recovery of activity is thus of the same general form as
the curve when no emanation escapes, but the constant λ is replaced by λ
+ α.

For example, if α = λ = ¹⁄₄₆₃₀₀₀, the equation of rise of activity is
given by

$$ \frac {n} {n₀} = 1 − e^{−2λ t} $$,

and, in consequence, the increase of activity to the maximum will be far
more rapid than in the case of no escape of emanation.

A very slight escape of emanation will thus produce large alterations
both in the final maximum and in the curve of recovery of activity.

A number of experiments have been described by Mme Curie in her _Thèse
présentée à la Faculté des Sciences de Paris_ on the effect of solution
and of heat in diminishing the activity of radium. The results obtained
are in general agreement with the above view, that 75 per cent. of the
activity of radium is due to the emanation and the excited activity it
produces. If the emanation is wholly or partly removed by solution or
heating, the activity of the radium is correspondingly diminished, but
the activity of the radium compound is spontaneously recovered owing to
the production of fresh emanation. A state of radio-active equilibrium
is reached, when the rate of production of fresh emanation balances the
rate of change in the emanation stored up in the compound. The
differences observed in the rate of recovery of radium under different
conditions were probably due to variations in the rate of escape of the
emanation.


=217.= It has been shown in section 152 that the emanation is produced
at the same rate in the solid as in the solution, and all the results
obtained point to the conclusion that the emanation is produced from
radium at a constant rate, which is independent of physical conditions.
Radium, like thorium, shows a non-separable activity of 25 per cent. of
the maximum activity, and consisting entirely of α rays. The β and γ
rays arise only from the active deposit. The emanation itself (section
156) gives out only α rays. These results thus admit of the explanation
given in the case of thorium (section 136). The radium atoms break up at
a constant rate with the emission of α particles. The residue of the
radium atom becomes the atom of the emanation. This in turn is unstable
and breaks up with the expulsion of an α particle. The emanation is half
transformed in four days. We have seen that this emanation gives rise to
an active deposit. The results obtained up to this stage are shown
diagrammatically below.

                     α _particle_       α _particle_
                    /                       /
                   /                       /
        RADIUM ATOM ——> ATOM OF EMANATION ——> ATOM OF ACTIVE DEPOSIT


=218. Analysis of the active deposit from radium.= We have seen in
chapter VIII that the excited activity produced on bodies, by the action
of the radium emanation, is due to a thin film of active matter
deposited on the surface of bodies. This active deposit is a product of
the decomposition of the radium emanation, and is not due to any action
of the radiations on the surface of the matter.

The curves showing the variation of the excited activity with time are
very complicated, depending not only upon the time of exposure in the
presence of the emanation, but also upon the type of radiation used for
measurement. The greater portion of the activity of this deposit dies
away in the course of 24 hours, but a very small fraction still remains,
which then changes very slowly.

It will be shown in this chapter that at least six successive
transformations occur in the active deposit. The matter initially
produced from the emanation is called radium A, and the succeeding
products B, C, D, E, F. The equations expressing the quantity of A, B,
C,...... present at any time are very complicated, but the comparison of
theory with experiment may be much simplified by temporarily
disregarding some unimportant terms: for example, the products A, B, C
are transformed at a very rapid rate compared with D. The activity due
to D + E + F is, in most cases, negligible compared with that of A or C,
being usually less than ¹⁄₁₀₀₀₀₀ of the initial activity observed for A
or C. The analysis of the active deposit of radium may thus be
conveniently divided into two stages:

    (1) Analysis of the deposit of rapid change, which is mainly
    composed of radium A, B, and C;

    (2) Analysis of the deposit of slow change, which is composed of
    radium D, E, and F.


=219. Analysis of the deposit of rapid change.= In the experiments
described below, a radium solution was placed in a closed glass vessel.
The emanation then collected in the air space above the solution. The
rod, to be made active, was introduced through an opening in the stopper
and exposed in the presence of the emanation for a definite interval. If
the decay was to be measured by the α rays, the rod was made the central
electrode in a cylindrical vessel such as is shown in Fig. 18. A
saturating voltage was applied, and the current between the cylinders
measured by an electrometer. If a very active rod is to be tested, a
sensitive galvanometer can be employed, but, in such a case, a large
voltage is required to produce saturation. A slow current of dust-free
air was continuously circulated through the cylinder, in order to remove
any emanation that may have adhered to the rod. For experiments on the β
and γ rays, it was found advisable to use an electroscope, such as is
shown in Fig. 12, instead of an electrometer. For measurements with the
γ rays, the active rod was placed under the electroscope, and before
entering the vessel the rays passed through a sheet of metal of
sufficient thickness to absorb all the α rays. For measurements with the
γ rays, the electroscope was placed on a lead plate 0·6 cms. thick, and
the active rod placed under the lead plate. The α and β rays were
completely stopped by the lead, and the discharge in the electroscope
was then due to the γ rays alone. The electroscope is very advantageous
for measurements of this character, and accurate observations can be
made simply and readily.

The curve of decay of activity, measured by the α rays, for an exposure
of 1 minute in the presence of the radium emanation is shown in Fig. 86,
curve _BB_.

The curve exhibits three stages:—

    (1) A rapid decay in the course of 15 minutes to less than 10 per
    cent. of the value immediately after removal;

    (2) A period of 30 minutes in which the activity varies very little;

    (3) A gradual decrease almost to zero.

The initial drop decays very approximately according to an exponential
law with the time, falling to half value in about 3 minutes. Three or
four hours after removal the activity again decays according to an
exponential law with the time, falling to half value in about 28
minutes. The family of curves obtained for different times of exposure
have already been shown in Fig. 67. These results thus indicate:—

    (1) An initial change in which half the matter is transformed in 3
    minutes;

    (2) A final change in which half the matter is transformed in 28
    minutes.

[Illustration: Fig. 86.]

Before considering the explanation of the intermediate portion of the
curve further experimental results will be considered.

The curve of decay of the excited activity for a long exposure (24
hours) is shown graphically in Fig. 86, curve _AA_. There is at first a
rapid decrease for the first 15 minutes to about 50 per cent. of the
initial value, then a slower decay, and, after an interval of about 4
hours, a gradual decay nearly to zero, according to an exponential law
with the time, falling to half value in 28 minutes.

The curves of variation with time of the excited activity when measured
by the β _rays_ are shown graphically in Figs. 87 and 88.

Fig. 87 is for a short exposure of 1 minute. Fig. 88 shows the decay for
a long exposure of about 24 hours.

[Illustration: Fig. 87.]

The curves obtained for the β rays are quite different from those
obtained for the α rays. For a short exposure, the activity measured by
the β rays is at first small, then passes through a maximum about 36
minutes after removal. There is then a gradual decrease, and after
several hours the activity decays according to an exponential law,
falling, as in the other cases, to half value in 28 minutes.

The curve shown in Fig. 88 for the β rays is very similar in shape to
the corresponding curve, Fig. 86, curve _AA_, for the α rays, with the
exception that the rapid initial drop observed for the α-ray curve is
quite absent. The later portions of the curve are similar in shape, and,
disregarding the first 15 minutes after removal, the activity decays at
exactly the same rate in both cases.

The curves obtained by means of the γ rays are identical with those
obtained for the β rays. This shows that the β and γ rays always occur
together and in the same proportion.

For increase of the time of exposure from 1 minute to 24 hours the
curves obtained are intermediate in shape between the two representative
limiting curves, Figs. 87 and 88. Some of these curves have already been
shown in Fig. 68.

[Illustration: Fig. 88.]


=220. Explanation of the curves.= It has been pointed out that the rapid
initial drop for curves _A_ and _B_, Fig. 86, is due to a change giving
rise to α rays, in which half of the matter is transformed in about 3
minutes. The absence of the drop in the corresponding curves, when
measured by the β rays, shows that the first 3-minute change does not
give rise to β rays; for if it gave rise to β rays, the activity should
fall off at the same rate as the corresponding α-ray curve.

It has been shown that the activity several hours after removal decays
in all cases according to an exponential law with the time, falling to
half value in about 28 minutes. This is the case whether for a short or
long exposure, or whether the activity is measured by the α, β, or γ
rays. This indicates that the final 28-minute change gives rise to all
three types of rays.

It will be shown that these results can be completely explained on the
supposition that three successive changes occur in the deposited matter
of the following character[315]:—

    (1) A change of the matter A initially deposited in which half is
    transformed in about 3 minutes. This gives rise only to α rays.

    (2) A second “rayless” change in which half the matter B is
    transformed in 21 minutes.

    (3) A third change in which half the matter C is transformed in 28
    minutes. This gives rise to α, β, and γ rays.


=221. Analysis of the β-ray curves=. The analysis of the changes is much
simplified by temporarily disregarding the first 3-minute change. In the
course of 6 minutes after removal, three quarters of the matter A has
been transformed into B and 20 minutes after removal all but 1 per cent.
has been transformed. The variation of the amount of matter B or C
present at any time agrees more closely with the theory, if the first
change is disregarded altogether. A discussion of this important point
is given later (section 228).

The explanation of the β-ray curves (see Figs. 87 and 88), obtained for
different times of exposure, will be first considered. For a very short
exposure, the activity measured by the β rays is small at first, passes
through a maximum about 36 minutes later, and then decays steadily with
the time.

The curve shown in Fig. 87 is very similar in general shape to the
corresponding thorium and actinium curves. It is thus necessary to
suppose that the change of the matter B into C does not give rise to β
rays, while the change of C into D does. In such a case the activity
(measured by the β rays) is proportional to the amount of C present.
Disregarding the first rapid change, the activity _I_{t}_ at any time
_t_ should be given by an equation of the same form (section 207) as for
thorium and actinium, viz.,

$$ \frac {I_t} {I_T} = \frac {e^{–λ_3 t} − e^{–λ_2 t}}
{e^{–λ_3 T} − e^{–λ_2 T}} $$,

where _I_{T}_ is the maximum activity observed, which is reached after
an interval _T_. Since the activity finally decays according to an
exponential law (half value in 28 minutes), one of the values of λ is
equal to 4·13 × 10⁻⁴. As in the case of thorium and actinium, the
experimental curves do not allow us to settle whether this value of λ is
to be given to λ₂ or λ₃. From other data (see section 226) it will be
shown later that it must refer to λ₃. Thus λ₃ = 4·13 × 10⁻⁴ (sec)⁻¹.

The experimental curve agrees very closely with theory if λ₂ = 5·38 ×
10⁻⁴ (sec)⁻¹.

The agreement between theory and experiment is shown by the table given
below. The maximum value _I_{T}_ (which is taken as 100) is reached at a
time _T_ = 36 minutes.

In order to obtain the β-ray curve, the following procedure was adopted.
A layer of thin aluminium was placed inside a glass tube, which was then
exhausted. A large quantity of radium emanation was then suddenly
introduced by opening a stop-cock communicating with the emanation
vessel, which was at atmospheric pressure. The emanation was left in the
tube for 1·5 minutes and then was rapidly swept out by a current of air.
The aluminium was then removed and was placed under an electroscope,
such as is shown in Fig. 12. The α rays from the aluminium were cut off
by an interposed screen of aluminium ·1 mm. thick. The time was reckoned
from a period of 45 seconds after the introduction of the emanation.

                    Time in  Theoretical     Observed
                    minutes     value of     value of
                                activity     activity


                          0            0            0

                         10         58·1           55

                         20         88·6           86

                         30         97·3           97

                         36        100            100

                         40         99·8           99·5

                         50         93·4           92

                         60         83·4           82

                         80         63·7           61·5

                        100         44·8           42·5

                        120         30·8           29

There is thus a good agreement between the calculated and observed
values of the activity measured by the β rays.

The results are satisfactorily explained if it is supposed:—

    (1) That the change B into C (half transformed in 21 minutes) does
    not give rise to β rays;

    (2) That the change C into D (half transformed in 28 minutes) gives
    rise to β rays.


=222.= These conclusions are very strongly supported by observations of
the decay measured by the β rays for a long exposure. The curve of decay
is shown in Fig. 88 and Fig. 89, curve I.

[Illustration: Fig. 89.]

P. Curie and Danne made the important observation that the curve of
decay _C_, corresponding to that shown in Fig. 88, for a long exposure,
could be accurately expressed by an empirical equation of the form

$$ \frac {I_t} {I₀} = ae^{–λ_3 t} − (a − 1) e^{–λ_2 t} $$,

where λ₂ = 5·38 × 10⁻⁴ (sec)⁻¹ and λ₃ = 4·13 × 10⁻⁴ (sec)⁻¹, and α =
4·20 is a numerical constant.

I have found that within the limit of experimental error this equation
represents the decay of excited activity of radium for a long exposure,
measured by the β rays. The equation expressing the decay of activity,
measured by the α rays, differs considerably from this, especially in
the early part of the curve. Several hours after removal the activity
decays according to an exponential law with the time, decreasing to half
value in 28 minutes. This fixes the value of λ₃. The constant α and the
value of λ₂ are deduced from the experimental curve by trial. Now we
have already shown (section 207) that in the case of the active deposit
from thorium, where there are two changes of constants λ₂ and λ₃, in
which only the second change gives rise to a radiation, the intensity of
the radiation is given by

$$ \frac {I_t} {I₀} = \frac {λ_2} {λ_2 − λ_3}
e^{–λ_3 t} − \frac {λ_3} {λ_2 − λ_3}
e^{–λ_2 t} $$

for a long time of exposure (see equation 8, section 198). This is an
equation of the same form as that found experimentally by Curie and
Danne. On substituting the values λ₂, λ₃ found by them,

      $$ \frac {λ_2} {λ_2 − λ_3} = 4\cdot3 $$, and

      $$ \frac {λ_1} {λ_1 − λ_3} = 3\cdot3 $$ .

Thus the theoretical equation agrees in form with that deduced from
observation, and the values of the numerical constants are also closely
concordant. If the first as well as the second change gave rise to a
radiation, the equation would be of the same general form, but the value
of the numerical constants would be different, the values depending upon
the ratio of the ionization in the first and second changes. If, for
example, it is supposed that both changes give out β rays in equal
amounts, it can readily be calculated that the equation of decay would
be

$$ \frac {I_t} {I₀} = \frac {\cdot5λ_2} {λ_2 − λ_3}
e^{–λ_3 t} − \cdot5 (\frac {λ_3} {λ_2 − λ_3} -
1) e^{–λ_2 t} $$ .

Taking the values of λ₂ and λ₃ found by Curie, the numerical factor

$$ e^{–λ_2 t} $$

becomes 2·15 instead of 4·3 and 1·15 instead of 3·3. The theoretical
curve of decay in this case would be readily distinguishable from the
observed curve of decay. The fact that the equation of decay found by
Curie and Danne involves the necessity of an initial rayless change can
be shown as follows:—

Curve I (Fig. 89) shows the experimental curve. At the moment of removal
of the body from the emanation (disregarding the initial rapid change),
the matter must consist of both B and C. Consider the matter which
existed in the form C at the moment of removal. It will be transformed
according to an exponential law, the activity falling by one-half in 28
minutes. This is shown in curve II. Curve III represents the difference
between the ordinates of curves I and II. It will be seen that it is
identical in shape with the curve (Fig. 87) showing the variation of the
activity for a short exposure, measured by the β rays. It passes through
a maximum at the same time (about 36 minutes). The explanation of such a
curve is only possible on the assumption that the first change is a
rayless one. The ordinates of curve III express the activity added in
consequence of the change of the matter B, present after removal, into
the matter C. The matter B present gradually changes into C, and this,
in its change to D, gives rise to the radiation observed. Since the
matter B alone is considered, the variation of activity with time due to
its further changes, shown by curve III, should agree with the curve
obtained for a short exposure (see Fig. 87), and this, as we have seen,
is the case.

The agreement between theory and experiment is shown in the following
table. The first column gives the theoretical curve of decay for a long
exposure deduced from the equation

$$ \frac {I_t} {I₀} = \frac {λ_2} {λ_2 − λ_3}
e^{–λ_3 t} − \frac {λ_3} {λ_2 − λ_3}
e^{–λ_2 t} $$

taking the value of λ₂ = 5·38 × 10⁻⁴ and λ₃ = 4·13 × 10⁻⁴.

                    Time in   Calculated     Observed
                    minutes       values       values


                          0        100          100

                         10         96·8         97·0

                         20         89·4         88·5

                         30         78·6         77·5

                         40         69·2         67·5

                         50         59·9         57·0

                         60         49·2         48·2

                         80         34·2         33·5

                        100         22·7         22·5

                        120         14·9         14·5

The second column gives the observed activity (measured by means of an
electroscope) for a long exposure of 24 hours in the presence of the
emanation.

In cases where a steady current of air is drawn over the active body,
the observed values are slightly lower than the theoretical. This is
probably due to a slight volatility of the product radium B at ordinary
temperatures.

[Illustration: Fig. 90.]


=223. Analysis of the α-ray curves=. The analysis of the decay curves of
the excited activity of radium, measured by the α rays, will now be
discussed. The following table shows the variation of the intensity of
the radiation after a long exposure in the presence of the radium
emanation. A platinum plate was made active by exposure for several days
in a glass tube containing a large quantity of emanation. The active
platinum after removal was placed on the lower of two parallel insulated
lead plates, and a saturating electromotive force of 600 volts was
applied. The ionization current was sufficiently large to be measured by
means of a sensitive high-resistance galvanometer, and readings were
taken as quickly as possible after removal of the platinum from the
emanation vessel. The initial value of the current (taken as 100) was
deduced by continuing the curves backwards to meet the vertical axis
(see Fig. 90), and was found to be 3 × 10⁻⁸ ampere.

                           Time in    Current
                           minutes


                                 0      100

                                 2       80

                                 4       69·5

                                 6       62·4

                                 8       57·6

                                10       52·0

                                15       48·4

                                20       45·4

                                30       40·4

                                40       35·6

                                50       30·4

                                60       25·4

                                80       17·4

                               100       11·6

                               120        7·6

These results are shown graphically in the upper curve of Fig. 90. The
initial rapid decrease is due to the decay of the activity of the matter
A. If the slope of the curve is produced backwards from a time 20
minutes after removal, it cuts the vertical axis at about 50. The
difference between the ordinates of the curves _A_ + _B_ + _C_ and _LL_
at any time is shown in the curve _AA_. The curve _AA_ represents the
activity at any time supplied by the change in radium _A_. The curve
_LL_ starting from the vertical axis is identical with the curve already
considered, representing the decay of activity measured by the β rays
for a long exposure (see Fig. 88).

                    Time in   Calculated     Observed
                    minutes     value of     value of
                                activity     activity


                          0        100          100

                         10         96·8         97·0

                         20         89·4         89·2

                         30         78·6         80·8

                         40         69·2         71·2

                         50         59·9         60·8

                         60         49·2         50·1

                         80         34·2         34·8

                        100         22·7         23·2

                        120         14·9         15·2

This is shown by the agreement of the numbers in the above table. The
first column in the table above gives the theoretical values of the
activity deduced from the equation

$$ \frac {I_t} {I₀} = \frac {λ_2} {λ_2 − λ_3}
e^{–λ_3 t} − \frac {λ_3} {λ_2 − λ_3}
e^{–λ_2 t} $$

for the values of λ₂, λ₃ previously employed. The second column gives
the observed values of the activity deduced from the decay curve _LL_.

The close agreement of the curve _LL_ with the theoretical curve deduced
on the assumption that there are two changes, the first of which does
not emit rays, shows that the change of radium B into C does not emit α
rays. In a similar way, as in the curve I, Fig. 89, the curve _LL_ may
be analysed into its two components represented by the two curves _CC_
and _BB_. The curve _CC_ represents the activity supplied by the matter
C present at the moment of removal. The curve _BB_ represents the
activity resulting from the change of B into C and is identical with the
corresponding curve in Fig. 89. Using the same line of reasoning as
before, we may thus conclude that the change of B into C is not
accompanied by α rays. It has already been shown that it does not give
rise to β rays, and the identity of the β and γ-ray curves shows that it
does not give rise to γ rays. The change of B into C is thus a “rayless”
change, while the change of C into D gives rise to all three kinds of
rays.

An analysis of the decay of the excited activity of radium thus shows
that three distinct rapid changes occur in the matter deposited, viz.:—

    (1) The matter A, derived from the change in the emanation, is half
    transformed in 3 minutes and is accompanied by α rays alone;

    (2) The matter B is half transformed in 21 minutes and gives rise to
    no ionizing rays;

    (3) The matter C is half transformed in 28 minutes and is
    accompanied by α, β, and γ rays;

    (4) A fourth very slow change will be discussed later.


=224. Equations representing the activity curves.= The equations
representing the variation of activity with time are for convenience
collected below, where λ₁ = 3·8 × 10⁻³, λ₂ = 5·38 × 10⁻⁴, λ₃ = 4·13 ×
10⁻⁴:—

(1) Short exposure: activity measured by β rays,

$$ \frac {I_t} {I_T} = 10\cdot3 (e^{–λ_3 t} − e^{–λ_2 t}) $$

where _I_{T}_ is the maximum value of the activity;

(2) Long exposure: activity measured by β rays,

$$ \frac {I_t} {I₀} = 4\cdot3 (e^{–λ_3 t} − 3\cdot3 e^{–λ_2
t}) $$,

where _I₀_ is the initial value;

(3) Any time of exposure _T_: activity measured by the β rays,

$$ \frac {I_t} {I₀} = \frac {ae^{–λ_3 t} − be^{–λ_2 t}} {a −
b} $$,

where

            $$ a = \frac {1 − e^{–λ_3 T}} {λ_3} $$,

            $$ b = \frac {1 − e^{–λ_2 T}} {λ_2} $$;

(4) Activity measured by α rays: long time of exposure,

$$ \frac {I_t} {I₀} = \frac {1}{2} e^{–λ_1 t} + \frac {1}{2}
(4\cdot3 e^{–λ_3 t} − 3\cdot3 e^{–λ_2 t}) $$ .

The equations for the α rays for any time of exposure can be readily
deduced, but the expressions are somewhat complicated.

[Illustration: Fig. 91.]


=225. Equations of rise of excited activity.= The curves expressing the
gradual increase to a maximum of the excited activity produced on a body
exposed in the presence of a constant amount of emanation are
complementary to the curves of decay for a long exposure. The sum of the
ordinates of the rise and decay curves is at any time a constant. This
follows necessarily from the theory and can also be deduced simply from
_à priori_ considerations. (See section 200.)

The curves of rise and decay of the excited activity for both the α and
β rays are shown graphically in Fig. 91. The thick line curves are for
the α rays. The difference between the shapes of the decay curves when
measured by the α or β rays is clearly brought out in the figure. The
equations representing the rise of activity to a maximum are given
below.

For the β and γ rays,

$$ \frac {I_t} {I_{max}} = 1 − (4\cdot3 e^{–λ_3 t} − 3\cdot3
e^{–λ_2 t}) $$ .

For the α rays,

$$ \frac {I_t} {I_{max}} = 1 − \frac {1}{2} e^{–λ_1 t} − \frac
{1}{2} (4\cdot3 e^{–λ_3 t} − 3\cdot3 e^{–λ_2 t}) $$ .


=226. Effect of temperature.= We have so far not considered the evidence
on which the 28-minute rather than the 21-minute change is supposed to
take place in the matter C. This evidence has been supplied by some
recent important experiments of P. Curie and Danne[316] on the
volatilization of the active matter deposited by the emanation. Miss
Gates[317] showed that this active matter was volatilized from a
platinum wire above a red heat and deposited on the surface of a cold
cylinder surrounding the wire. Curie and Danne extended these results by
subjecting an active platinum wire _for a short time_ to the action of
temperatures varying between 15° C. and 1350° C., and then examining at
room temperatures the decay curves not only for the active matter
remaining on the wire, but also for the volatilized part. They found
that the activity of the distilled part always increased after removal,
passed through a maximum, and finally decayed according to an
exponential law to half value in 28 minutes. At a temperature of about
630° C. the active matter left behind on the wire decayed at once
according to an exponential law, falling to half value in 28 minutes. P.
Curie and Danne showed that the matter B is much more volatile than C.
The former is completely volatilized at about 600° C., while the latter
is not completely volatilized even at a temperature of 1300° C. The fact
that the matter C, left behind when B is completely volatilized, decays
at once to half value in 28 minutes shows that the matter C itself and
not B is half transformed in 28 minutes.

Curie and Danne also found that the rate of decay of the active matter
varied with the temperature to which the platinum wire had been
subjected. At 630° C. the rate of decay was normal, at 1100° C. the
activity fell to half value in about 20 minutes, while at 1300° C. it
fell to about half value in about 25 minutes.

I have repeated the experiments of Curie and Danne and obtained very
similar results. It was thought possible that the measured rate of decay
observed after heating might be due to a permanent increase in the rate
of volatilization of C at ordinary temperatures. This explanation,
however, is not tenable, for it was found that the activity decreased at
the same rate whether the activity of the wire was tested in a closed
tube or in the open with a current of air passed over it.

These results are of great importance, for they indicate that the rate
of change of the product C is not a constant, but is affected by
differences of temperature. This is the first case where temperature has
been shown to exert an appreciable influence on the rate of change of
any radio-active product.


=227. Volatility of radium B at ordinary temperature.= Miss Brooks[318]
has observed that a body, made active by exposure to the radium
emanation, possesses the power of exciting secondary activity on the
walls of a vessel in which it is placed. This activity was usually about
¹⁄₁₀₀₀ of the whole, but the amount was increased to about ¹⁄₂₀₀ if the
active wire was washed in water and dried over a gas flame—the method
often adopted to free the wire of any trace of the radium emanation.
This effect of producing activity was most marked immediately after
removal of the wire from the emanation, and was almost inappreciable ten
minutes afterwards.

The effect was particularly noticeable in some experiments with a copper
plate, which was made active by leaving it a short time in a solution of
the active deposit from radium. This active solution was obtained by
placing an active platinum wire in dilute hydrochloric acid. On placing
the copper plate in a testing vessel for a few minutes, and then
removing it, activity was observed on the walls of the vessel amounting
to about one per cent. of the activity of the copper plate.

It was found that this effect was not due to the emission of an
emanation from the active body, but must be ascribed to a slight
volatility of radium B at ordinary temperatures. This was proved by
observations on the variation of the activity of the matter deposited on
the walls of the vessel. The activity was small at first, but rose to a
maximum after about 30 minutes, and then decayed with the time. The
curve of rise was very similar to that shown in Fig. 87, and shows that
the inactive matter radium B was carried to the walls and there changed
into C, which gave rise to the radiation observed.

The product B only escapes from the body for a short time after removal.
This is a strong indication that its apparent volatility is connected
with the presence of the rapidly changing product radium A. Since A
breaks up with an expulsion of an α particle, some of the residual atoms
constituting radium B may acquire sufficient velocity to escape into the
gas, and are then transferred by diffusion to the walls of the vessel.

Miss Brooks observed that the activity was not concentrated on the
negative electrode in an electric field but was diffused uniformly over
the walls of the vessel. This observation is of importance in
considering the explanation of the anomalous effects exhibited by the
active deposit of radium, which will be discussed in the following
section.


=228. Effect of the first rapid change.= We have seen that the law of
decay of activity, measured by the β or γ rays, can be explained very
satisfactorily if the first 3-minute change is disregarded. The full
theoretical examination of the question given in sections 197 and 198
and the curves of Figs. 72 and 73 show, however, that the presence of
the first change should exercise an effect of sufficient magnitude to be
detected in measurements of the activity due to the succeeding changes.
The question is of great interest, for it involves the important
theoretical point whether the substances A and B are produced
independently of one another, or whether A is the parent of B. In the
latter case, the matter A which is present changes into B, and, in
consequence, the amount of B present after A is transformed should be
somewhat greater than if B were produced independently. Since the change
of A is fairly rapid, the effect should be most marked in the early part
of the curve.

In order to examine this point experimentally, the curve of rise of
activity, measured by the β rays, was determined immediately after the
introduction of a large quantity of the radium emanation into a closed
vessel. The curve of decay of activity on a body for a long exposure
after removal of the emanation, and the rise of activity after the
introduction of the emanation, are in all cases complementary to one
another. While, however, it is difficult to measure with certainty
whether the activity has fallen in a given time, for example, from 100
to 99 or 98·5, it is easy to be sure whether the corresponding rise of
activity in the converse experiment is 1 or 1·5 per cent. of the final
amount. Fig. 92, curve I, shows the rise of activity (measured by the β
rays) obtained for an interval of 20 minutes after the introduction of
the emanation. The ordinates represent the percentage amount of the
final activity regained at any time.

Curve III shows the theoretical curve obtained on the assumption that A
is a parent of B. This curve is calculated from equation (9) discussed
in section 198, and λ₁, λ₂, λ₃ are the values previously found.

Curve II gives the theoretical activity at any time on the assumption
that the substances A and B arise independently. This is calculated from
an equation of the same form as (8), section 198.

[Illustration: Fig. 92.]

It is seen that the experimental results agree best with the view that A
and B arise independently. Such a conclusion, however, is of too great
importance to be accepted before examining closely whether the
theoretical conditions are fulfilled in the experiments. In the first
place, it is assumed that the carriers which give rise to excited
activity are deposited on the surface of the body, to be made active
immediately after their formation. There is some evidence, however, that
some of these carriers exist for a considerable interval in the gas
before their deposit on the body. For example, it is found that if a
body is introduced for a short interval, about 1 minute, into a vessel
containing the radium emanation, which has remained undisturbed for
several hours, the activity after the first rapid decay (see Fig. 86,
curve _B_) is in much greater proportion than if an electric field had
been acting for some time previously. This result indicates that the
carriers of B and C both collect in the gas and are swept to the
electrode when an electric field is applied. I have also observed that
if radium emanation, which has stood undisturbed for some time, is swept
into a testing vessel, the rise curve is not complementary to the decay
curve, but indicates that a large amount of radium B and C was present
with the emanation. The experiments of Miss Brooks, previously referred
to, indicate that radium B does not obtain a charge and so will remain
in the gas. Dr  Bronson, working in the laboratory of the writer, has
obtained evidence that a large amount of radium D remains in the gas
even in a strong electric field. If the matter B exists to some extent
in the gas, the difference between the theoretical curves for three
successive changes would be explained; for, in transferring the
emanation to another vessel, the matter B mixed with it would commence
at once to change into C and give rise to a part of the radiation
observed.

The equal division of the activity between the products A and C (see
Fig. 90) supports the view that C is a product of A, for when
radio-active equilibrium is reached, the number of particles of A
changing per second is equal to the number of B or C changing per
second. If each atom of A and C expels an α particle of the same mass
and with the same average velocity, the activity due to the matter A
should be equal to that due to the matter C; and this, as we have seen,
is the case.

While it is a matter of great difficulty to give a definite experimental
proof that radium A and B are consecutive products, I think there is
little doubt of its correctness. Accurate determinations of the curves
of rise and decay may throw further light on the complicated processes
which undoubtedly occur between the breaking up of the atoms of the
emanation and the appearance of the active deposit on the electrodes.


=229. Relative activity supplied by the α-ray products of radium.= There
are four products in radium which give out α rays, viz. radium itself,
the emanation, radium A and C. If these products are in radio-active
equilibrium, the same number of particles of each product are
transformed per second and, if each atom breaks up with the emission of
one α particle, the number of α particles expelled per second should be
the same for each product.

Since, however, the α particles from the different products are not
projected with the same velocity, the activity, measured by the
ionization current in the usual manner, will not be the same for all
products. The activity, when measured by the saturation current between
parallel plates at sufficient distance apart to absorb all the α rays in
the gas, is proportional to the energy of the α particles escaping into
the gas.

It has been shown that the minimum activity of radium after removal of
the emanation, measured by the α rays, is 25 per cent. of the maximum
value. The remaining 75 per cent. is due to the α particles from the
other products. Now the activity supplied by radium A and C is nearly
the same (section 228). If the emanation is introduced into a
cylindrical vessel about 5 cms. in diameter, the activity increases to
about twice its initial value owing to the deposit of radium A and C on
the surface of the vessel. This shows that the activity of the emanation
is of about the same magnitude as that supplied by radium A or C, but an
accurate comparison is beset with difficulty, for the emanation is
distributed throughout the gas, while radium A and C are deposited on
the walls of the vessel. In addition, the relative absorption of the
emanation compared with that of radium A and C is not known.

The writer has made some experiments on the decrease of activity of
radium immediately after heating to a sufficient temperature to drive
off the emanation. The results obtained by this method are complicated
by the alteration of the radiating surface in consequence of the
heating, but indicate that the emanation supplies about 70 per cent. of
the activity of radium A or C.

This points to the conclusion that the α particles from the emanation
are projected with less velocity than those from radium C.

The following table shows approximately the activity supplied by the
different products of radium in radio-active equilibrium.

                         Product    Percentage
                                    proportion
                                    of total
                                    activity

                         Radium     25 per
                                    cent.

                         Emanation  17    „

                         Radium A   29    „

                         Radium B   0    „

                         Radium C   29    „

The products of radium and their radiation are graphically shown later
in Fig. 95.


=230. Active deposit of radium of slow transformation.= It has been
pointed out (section 183) that a body, exposed in the presence of the
radium emanation, does not lose all its activity for a long time after
removal; a small residual activity is always observed. The magnitude of
this residual activity is dependent not only upon the amount of
emanation employed, but also upon the time of exposure of the body in
the presence of the emanation. For an exposure of several hours in the
presence of the emanation, the residual activity is less than
one-millionth of the activity immediately after removal.

An account will now be given of some investigations made by the
writer[319] on the nature of this residual activity and the chemical
properties of the active matter itself. It is first of all necessary to
show that the residual activity arises in consequence of a deposit of
radio-active matter, and is not due to some action of the intense
radiations to which the body made active has been subjected.

The inside of a long glass tube was covered with equal areas of thin
metal, including aluminium, iron, copper, silver, lead, and platinum. A
large amount of radium emanation was introduced into the tube, and the
tube closed. After seven days the metal plates were removed, and, after
allowing two days to elapse for the ordinary excited activity to
disappear, the residual activity of the plates was tested by an
electrometer. The activity of the plates was found to be unequal, being
greatest for copper and silver, and least for aluminium. The activity of
copper was twice as great as that of aluminium. After standing for
another week the activity of the plates was again tested. The activity
of each had diminished in the interval to some extent, but the initial
differences observed had to a large extent disappeared. After reaching a
minimum value the activity of each plate slowly but steadily increased
at the same rate. After a month’s interval the activity of each of the
plates was nearly the same, and more than three times the minimum value.
The initial irregularities in the decay curves of the different metals
are, in all probability, due to slight but different degrees of
absorption of the radium emanation by the metal plates, the absorption
being greatest for copper and silver and least for aluminium. As the
occluded emanation was slowly released or lost its activity, the
activity of the metal fell to a limiting value. The absorption of the
radium emanation by lead, paraffin, and caoutchouc has been noticed by
Curie and Danne (section 182).

The residual activity on the plates comprised both α and β rays, the
latter being present, in all cases, in a very unusual proportion. The
equality of the activity and the identity of the radiation emitted from
each plate show that the residual activity is due to changes of some
form of matter deposited on the plates, and that it cannot be ascribed
to an action of the intense radiations; for if such were the case, it
would be expected that the activity produced on the different plates
would vary not only in quantity, but also in quality. This result is
confirmed by the observation that the active matter can be removed from
a platinum plate by solution in sulphuric acid, and has other
distinctive chemical and physical properties.

The variation with time of the residual activity measured by the α rays
will first be considered. A platinum plate was exposed in the presence
of the radium emanation for seven days. The amount of emanation
initially present was equal to that obtained from about 3 milligrams of
pure radium bromide. The plate immediately after removal gave a
saturation-current, measured between parallel plates by a galvanometer,
of 1·5 × 10⁻⁷ ampere. Some hours after removal, the activity decayed
according to an exponential law with the time, falling to half value in
28 minutes. Three days after removal the active plate gave a
saturation-current, measured by an electrometer, of 5 × 10⁻¹³ ampere;
_i.e._ ¹⁄₃₀0,000 of the initial activity. The activity was observed to
increase steadily with the time. The results are shown in Fig. 93, where
the time is reckoned from the middle of the time of exposure to the
emanation.

The curve is initially nearly a straight line passing through the
origin. The activity increases with the time for the interval of eight
months over which the observations have extended. The latter portions of
the curve, however, fall below the tangent to the curve drawn through
the origin, showing that the activity is not increasing proportionately
with the time.

The active deposit, obtained in a different manner, has been examined
for a still longer period. The emanation from 30 milligrams of radium
bromide was condensed in a glass tube and then sealed. After a month’s
interval, the tube was opened and dilute sulphuric acid introduced. The
acid dissolved off the active deposit in the tube and on driving off the
acid by heat, a radio-active residue was obtained. The activity of this
residue, measured by the α rays, steadily increased for a period of 18
months, but the curve of variation of activity with time plotted as in
Fig. 93 tends to become more flattened, and is obviously approaching a
maximum value.

[Illustration: Fig. 93.]

The explanation of this curve will be considered later in section 236.


=231. Variation of the β ray activity.= The residual activity consists
of both α and β rays, the latter being present initially in an unusually
large proportion. The proportion of α to β rays from the platinum plate,
one month after removal, was at the most one-fiftieth of that from a
thin film of radium bromide in radio-active equilibrium. Unlike the α
ray activity, the activity measured by the β rays remains constant after
the active deposit is about one month old, and, in consequence, the
proportion of α to β rays steadily increases with the time. The
experiments showed that the intensity of the β rays did not vary much,
if at all, over a further period of eighteen months. The want of
proportionality between the α and β rays shows that the two types of
rays arise from different products. This conclusion is confirmed by
experiments, to be described later, which show that the products giving
rise to α and β rays can be temporarily separated from one another by
physical and chemical means.

[Illustration: Fig. 94.]

If observations of the active deposit are begun shortly after its
formation, it is found that the activity, measured by the β rays, is
small at first, but increases with the time, reaching a practical
maximum about 40 days later. Experiments were made on a platinum plate,
which was exposed for 3·75 days in a vessel containing the radium
emanation. The observations of the β ray activity began 24 hours after
removal. The results are shown in Fig. 94, where the time was measured
from the middle of the time of exposure to the emanation. Similar
results were obtained for a negatively charged wire exposed to the
emanation. The curve, if produced back to the origin, is seen to be very
similar to the recovery curves of Ur X, and other active products, and
can be expressed by the equation

$$ \frac {I_t} {I₀} = 1 − e^{–λt} $$,

where _I₀_ is the maximum activity. The activity reaches half its final
value in about six days, and the value of λ is equal to ·115 (day)⁻¹. We
have shown in section 203 that a rising curve of this character
indicates that the β ray activity arises from a product which is
supplied at a constant rate from a primary source. Before discussing in
detail the explanation of these curves, showing the rise with time of
the α and β ray activity, further experimental results will be
considered.


=232. Effect of temperature on the activity.= A platinum plate, made
active in the manner described, was exposed to varying temperatures in
an electric furnace, and the activity tested at atmospheric temperature
after exposure. Four minutes’ exposure in the furnace, at first at 430°
C., and afterwards at 800° C., had little, if any, effect on the
activity. After four minutes at about 1000° C. the activity decreased
about 20 per cent., and a further exposure of eight minutes at a
temperature of about 1050° C. almost completely removed the α ray
activity. On the other hand, the β ray activity, when measured
immediately after removal, was not altered by the heating, but exposure
to a still higher temperature caused it to decrease. These results show
that the active matter consists of two kinds. The part which emits β
rays is not volatile at 1000° C., but the other part, which emits α
rays, is almost completely volatilized at that temperature.

It was found, however, that the β ray activity after heating to about
1000° was not permanent, but decayed according to an exponential law
with the time, the activity decreasing to half value in about 4·5 days.
From the recovery curve of the β ray activity already considered, it was
to be expected that the activity would decay to half value in six days.
This difference in the periods is possibly due to an effect of the high
temperature in altering the rate of decay of radium E. The period of six
days is more probably correct. The results obtained on the rise and
decay of the β rays, taken together, show:—

    (1) That the product giving β rays is supplied at a constant rate
    from some parent matter of very slow rate of change.

    (2) That this parent matter is volatilized at or below 1000° C., and
    the β ray product is left behind. Since the parent matter is
    removed, the product immediately begins to lose its activity at its
    characteristic rate, viz. the activity falls to half value in about
    six days.


=233. Separation of the constituents by means of a bismuth plate.= The
active matter of slow decay was obtained in solution by introducing
dilute sulphuric acid into a glass tube in which the emanation from 30
milligrams of radium bromide had been stored for a month. The solution
showed strong activity and gave out both α and β rays, the latter, as in
other cases, being present in an unusually large proportion.

When a polished bismuth disk was kept for some hours in the solution, it
became strongly active. The active matter deposited on the bismuth gave
out α rays, but no trace of β rays. After several bismuth disks had been
successively left in the solution, the active matter, which emits α
rays, was almost completely removed. This was shown by evaporating down
the solution after treatment. The β ray activity remained unchanged, but
that of the α rays had been reduced to about 10 per cent. of its
original value. Three bismuth disks, made active in this way, were set
aside and their activity measured at regular intervals. The activity
fell off according to an exponential law with the time during the 200
days since their removal, while that of each fell to half value on an
average in about 143 days.

At the same time it was observed that the solution, from which the α ray
activity was removed, gradually regained its activity, showing that the
active substance which gave out α rays was continuously produced from
the matter left behind in the solution.


=234. Explanation of the results.= We have seen that a close examination
of the active deposit of slow change has disclosed,

    (1) the presence of a β ray product which loses half of its activity
    in about six days;

    (2) the presence of an α ray product, which is deposited on bismuth
    and is volatilized at 1000° C. This product loses half of its
    activity in 143 days;

    (3) the presence of a parent substance, which produces the β ray
    product at a constant rate.

This parent product must be transformed very slowly since the β ray
product, which arises from it, soon reaches an equilibrium value, which
does not change appreciably over a period of more than one year. The
experimental evidence points to the conclusion that the parent product
does not give rise to β rays, but that the β rays arise entirely from
the next product. This parent product cannot give rise to α rays, for we
have seen that the initial α ray activity is at first extremely small,
but increases steadily with the time for a period of at least eighteen
months. Thus the parent product does not give rise to either α or β
rays, and must be a “rayless” product.

The first three transition products of the radium emanation, viz. radium
A, B and C, have already been analysed, and shown to be consecutive. It
thus seems probable that the active deposit of slow change must arise
from the successive transformations of the last product radium C. The
results already obtained can be completely explained if it is supposed
that three transition products, viz. radium D, E and F, are present in
the active deposit of slow rate of change. The properties of these
products are summarized below.

    _Radium D_ is a rayless product of very slow rate of change. It will
    be shown later that it is half transformed in about 40 years. It is
    volatile below 1000° C. and is soluble in strong acids.

    _Radium E_ is produced from radium D. In breaking up, it emits β
    (and probably γ) rays but no α rays. It is half transformed in about
    6 days and is not so volatile as radium D and F.

    _Radium F_ is produced from radium E. It emits only α rays and is
    half transformed in 143 days. This substance in solution attaches
    itself to bismuth. It is volatile at about 1000° C.

Apart from their value and interest in showing the stages of
transformation of the radium atom, the results of this analysis have an
important bearing upon the origin of some of the well-known radio-active
substances separated from pitchblende; for it will be shown later that
the product radium F is the radio-active substance present in
radio-tellurium and probably also in polonium. In addition, there is
very strong evidence that the radio-active lead obtained by Hofmann
contains the three products radium D, E and F together.

The changes of radium as far as they are at present known, are shown
diagrammatically in Fig. 95. It is possible that further investigation
will show that the transformation does not end with radium F.

[Illustration: Fig. 95.]

While we have shown that radium D is the parent of E, we have not given
any conclusive evidence that E is the parent of F. This evidence is,
however, supplied by the following experiment. A platinum plate, made
active in the manner already described, was placed in an electric
furnace and heated for four minutes at about 1000° C. Most of the
products D and F were volatilized, but E was left behind. Since the
parent matter D was removed, E at once commenced to lose its β ray
activity. At the same time it was observed that the small α ray
activity, left behind on the platinum plate, increased rapidly at first
and then more slowly, as the activity of E became smaller and smaller.
This experiment shows conclusively that E was the parent of F, the α ray
product.


=235. Rate of transformation of radium D.= It has been observed
experimentally that each of the products of radium, which emit α rays,
supplies about an equal proportion of the activity of radium when in
radio-active equilibrium. Since, when equilibrium is reached, the same
number of particles of each of the successive products must break up per
second, this is an expression of the fact that every atom of each
product breaks up with the expulsion of an equal number (probably one)
of α particles. Now radium D is directly derived from radium C, and,
since the rate of change of D is very slow compared with that of C, the
number of particles of D initially present must be very nearly equal to
the number of particles of radium C which break up during the time that
radium D is being formed. Now D does not itself give out rays, but the
succeeding product E does. The products D and E are practically in
radio-active equilibrium one month after D is set aside, and the
variation of the β ray activity of E then serves as a measure of the
variation of the parent product D. Suppose that a vessel is filled with
a large quantity of radium emanation. After several hours, the product
radium C, which emits β rays, reaches a maximum value, and then
decreases at the same rate as the emanation loses its activity, _i.e._
it falls to half value in 3·8 days. If _N₁_ is the number of β particles
expelled from radium C at its maximum value, the total number _Q₁_ of β
particles expelled during the life of the emanation is given
approximately by

$$ Q_1 = \int₀^{\infty} N_1 e^{–λ_1 t} dt = \frac {N_1}
{λ_1} $$,

where λ₁ is the constant of change of the emanation.

After the emanation has disappeared, and the final products D + E are in
radio-active equilibrium, suppose that the number of β particles _N₂_
expelled per second by radium E is determined. If _Q₂_ is the total
number of particles expelled during the life of D + E, then _Q₂_ as
before is approximately given by _Q₂_ = _N₂_/λ₂ where λ₂ is the constant
of change of radium D. Now we have seen that if each particle of C and
of E gives rise to one β particle, it is to be expected that

                          _Q₁_ = _Q₂_,

                          or

                          λ₂      _N₂_
                         ----  =  ----    .
                          λ₁      _N₁_

The ratio _N₂_/_N₁_ was determined by measuring the activity due to the
β rays from C and E in the same testing-vessel. Then, since _N₂_/_N₁_ is
known, and also the value of λ₁, the value of the constant of change,
λ₂, of radium D is obtained. In this way it was calculated that D is
half transformed in about 40 years.

In the above calculations it is assumed, as a first approximation, that
the β rays from C and E have the same average velocity. This is probably
not accurately the case, but the above number certainly serves to fix
the order of magnitude of the period of the product D. This calculation
is confirmed by observations to be given later on the amount of D and E
in old radium.

It may be of interest to mention that the writer calculated the period
of radium F by a similar method, before its value was experimentally
determined, and found that F should be half transformed in about one
year. This is not very different from the experimental value of 143 days
found later. In addition, it was assumed in the calculation that the α
particles from C and F were projected with the same velocity, and in
consequence produced the same amount of ionization. In practice,
however, it is found that the α particle of F is absorbed in about half
the distance of the α particles of C, and in consequence produces only
about half of the ionization of the latter. If this correction were
made, the calculated period for half transformation would be six months
instead of one year.

A table of the transformation products of radium, together with some of
their physical and chemical properties, is given below.

       Transformation Time to be  Rays            Chemical and
      Products        half                        Physical
                      transformed                 Properties


      Radium          1200 years  α rays          —

      Emanation       3·8 days    α rays          Chemically
                                                  inert gas;
                                                  condenses at
                                                  −150° C.

      Radium A        3 mins.     α rays          Behaves as
      (active deposit                             solid;
      of rapid                                    deposited on
      change)                                     the surface of
                                                  bodies;
                                                  concentrated on
                                                  cathode in
                                                  electric field.
                                                  Soluble in
                                                  strong acids;
                                                  volatile at a
                                                  white heat. B
                                                  is more
                                                  volatile than A
                                                  or C.

      ::   B (same)   21 mins.    no rays         Same

      ::   C (same)   28 mins.    α, β, γ rays    Same

      ::   D (active  about 40    no rays         Soluble in
      deposit of slow years                       strong acids
      change)                                     and volatized
                                                  below 1000° C.

      ::   E (same)   6 days      β (and γ)       Non-volatile at
                                                  1000°C.

      ::   F (same)   143 days    α rays          Volatile at
                                                  1000° C;
                                                  deposited from
                                                  solution on to
                                                  bismuth plate.

      ?               —           —               —


=236. Variation of the activity over long periods of time.= We are now
in a position to calculate the variation of the α and β ray activity of
the active deposit over long periods of time. If it is supposed that the
matter initially deposited consists only of D, the amounts _P_, _Q_ and
_R_ of radium D, E and F existing at any later time are given by the
equations 3, 4, 5, section 197.

Since, however, the intermediate product E has a much more rapid rate of
change than D or F, the equations can be simplified, without much loss
of accuracy, by disregarding the change E, and by supposing that D gives
out β rays and changes directly into the α ray product F.

Let λ₁, λ₂ be the constants of change D and F respectively. Let _n₀_ be
the number of particles of D present initially. Then using the notation
of section 197, the amount _P_ of radium D at any time _t_ is given by

$$ P = n₀ e^{–λ_1 t} $$ .

The amount _Q_ of radium F is given by

$$ Q = \frac {n₀ λ_1} {λ_1 − λ_2} (e^{–λ_2 t} -
e^{–λ_1 t}) $$ .

[Illustration: Fig. 96.]

The number of β particles emitted by D + E per second, some months
afterwards, is

$$ λ_1 n₀ e^{–λ_1 t} $$,

and the number of α particles emitted by radium F is

$$ \frac {λ_1 λ_2 n₀} {λ_1 − λ_2}
(e^{–λ_2 t} − e^{–λ_1 t} ) $$ .

The results are shown graphically in Fig. 96, by the curves _EE_ and
_FF_, in which the ordinates represent the number of β and α particles
expelled per second by the products D and F respectively. The complete
calculation for three changes shows that the number of β particles soon
reaches a practical maximum, and then decays nearly exponentially with
the time, falling to half value in 40 years. The number of α particles
expelled per second increases for several years, but reaches a maximum
after 2·6 years and then diminishes, finally falling off exponentially
with the time to half value in 40 years.

The experimental curve of the rise of α ray activity, shown in Fig. 93,
as far as it has been determined, lies accurately on this curve, if the
maximum is calculated from the above theory. The observed activity after
a period of 250 days is marked by the point _X_ on the curve.


=237. Experiments with old radium.= Since the substance radium D is
produced from radium at a constant rate, the amount present mixed with
the radium will increase with its age. The writer had in his possession
a small quantity of impure radium chloride, kindly presented by
Professors Elster and Geitel four years before. The amount of radium D
present in it was tested in the following way:—The substance was
dissolved in water and kept continuously boiling for a period of about
six hours. Under these conditions the emanation is removed as rapidly as
it is formed, and the β rays from the radium, due to the product radium
C, practically disappear. A newly prepared specimen of radium bromide
under these conditions retains only a fraction of 1 per cent. of its
original β radiation. The old radium, however, showed (immediately after
this treatment) an activity measured by the β rays of about 8 per cent.
of its original amount. The activity could not be reduced any lower by
further boiling or aspiration of air through the solution. This residual
β ray activity was due to the product radium E stored up in the radium.
The β ray activity due to radium E was thus about 9 per cent. of that
due to radium C. Disregarding the differences in the absorption of the β
rays, when the activity of the product E in radium reaches a maximum
value, the β ray activity due to it should be the same as that due to C.
Since the parent product D is half transformed in forty years, the
amount present in the radium after four years should be about 7 per
cent. of the maximum amount; _i.e._ it should show a β ray activity of
about 7 per cent. of that due to radium C. The observed and calculated
values (7 and 9 per cent. respectively) are thus of the same order of
magnitude. The amount of β rays from radium E present in pure radium
bromide about one year old was about 2 per cent. of the total.

The amount of radium F present in old radium was measured by
observations of the activity imparted to a bismuth disk left for several
days in the solution, and was found to be of the same order as the
theoretical value. Radium F is not deposited to an appreciable extent on
the bismuth from a water solution of radium bromide. If, however, a
trace of sulphuric acid is added to the solution, the radium F is
readily deposited on the bismuth. The addition of sulphuric acid to the
radium solution practically effected a separation of radium D, E and F
from the radium proper; for the latter was precipitated as sulphate and
the products D, E and F remained in solution. After filtering, the
solution contained the greater proportion of the products D, E, and F
and very little radium.


=238. Variation of the activity of radium with time.= It has been shown
that the activity of freshly prepared radium increases at first with the
time and practically reaches a maximum value after an interval of about
one month. The results already considered show that there is a still
further slow increase of activity with the time. This is the case
whether the activity is measured by the α or β rays. It will be shown
later that radium is probably half transformed in about 1000 years. From
this it can readily be calculated that after a lapse of about 200 years
the amount of the products radium D, E and F will have reached a maximum
value. The same number of atoms of each of the products C and E will
then break up per second. If each atom of these products in
disintegrating throws off an equal number (probably one) of β particles,
the number of β particles thrown off per second will be twice as great
as from radium a few months old. The number will increase at first at
the rate of about 2 per cent. a year.

Similar considerations apply to the α ray activity. Since, however,
there are four other products of radium besides radium itself which
expel α particles, the number of α particles emitted per second from old
radium will not be more than 25 per cent. greater than the number from
radium a few months old. The activity measured by the α rays will thus
not increase more than 25 per cent., and probably still less, as the α
particles from radium F produce less ionization than the α particles
expelled from the other radium products. The activity of radium will
consequently rise to a maximum after 200 years and then slowly die away
with the time.


=239. Presence of these products in pitchblende.= The products radium D,
E and F must be present in pitchblende in amounts proportional to the
quantity of radium present, and should be capable of separation from the
mineral by suitable chemical methods. The radio-active properties of
these substances, if obtained in the pure state, are summarized below.

_Radium D_ when first separated, should give out very little α or β
radiation. The β ray activity will rapidly increase, reaching half its
maximum value in 6 days. The α ray activity will at first increase
nearly proportionately with the time, and will reach a maximum value
after an interval of about 3 years. The α and β ray activity, after
reaching a maximum, will finally decay, the activity falling to half
value in about 40 years. Since radium D is half transformed in 40 years,
and radium in 1200 years, the maximum β ray activity of radium D, weight
for weight, will be about 300 times that of radium.

The α ray activity, at any time, will be removed by placing a bismuth
disk in the solution.

_Radium F_, after separation, will give out only α rays. Its activity,
after separation, will decrease according to an exponential law, falling
to half value in 143 days. Since radium in radio-active equilibrium
contains four products which emit α rays, the number of α particles
expelled per second from radium F will, weight for weight, be about 800
times as numerous as from new radium in radio-active equilibrium. Since
the α particles from radium F produce only about half as much ionization
as the α particles from the other radium products, the activity of
radium F, measured by the electric method, will be about 400 times that
of radium.


=240. Origin of radio-tellurium and polonium.= It is now necessary to
consider whether these products of radium have been previously separated
from pitchblende, and known by other names.

We shall first consider the α ray product, radium F. The radio-tellurium
of Marckwald and the polonium of Mme Curie both resemble radium F in
giving out only α rays, and in being deposited on a bismuth disk from a
solution. If the active constituent present in radio-tellurium is the
same as radium F, its activity should decay at the same rate as the
latter. The writer[320] has carefully compared the rates of decay of the
activity of radium F and of the radio-tellurium of Marckwald and found
them to be the same within the limits of experimental error. Both lose
half of their activity in about 143 days[321]. A similar value of the
rate of decay of radio-tellurium has been obtained by Meyer and
Schweidler[322].

The experiments on radio-tellurium were made upon the active bismuth
plates supplied by Dr Sthamer of Hamburg, which were prepared under
Marckwald’s directions.

An additional proof[323] of the identity of these two products was
obtained by comparing the absorption of the α rays by aluminium foil.
The α rays from different products are projected with different
velocities, and, in consequence, are unequally absorbed by matter. The
absorption of the rays from the two products by aluminium foil agreed
very closely, indicating the probable identity of the substances from
which they were emitted.

There can thus be no doubt that the active constituent present in the
radio-tellurium of Marckwald is identical with the product radium F.
This is a very interesting result, and shows how the close examination
of the successive transformations of the radio-active bodies may throw
light on the origin of the various substances found in pitchblende.

We have already seen (section 21) that Marckwald, by special chemical
methods, was able to obtain a few milligrams of very active substance by
working over 2 tons of pitchblende. We have already seen (section 239)
that this substance, if obtained in the pure state, should be about 400
times as active as radium. Comparative measurements of the activity of
this substance with radium will thus indicate the amount of impurity
that is present with the former. This method should be of value in
purifying radium F for the purpose of determining its spectrum, which
has not yet been observed.


=241. Polonium.= Since the separation of the active substance by
Marckwald, called by him radio-tellurium, there has been some discussion
as to whether the active constituent is the same as that present in the
polonium of Mme Curie. Both of these substances have similar
radio-active and chemical properties, but the main objection to the view
that the active constituents were identical has rested on an early
statement of Marckwald that the activity of one of his very active
preparations did not decay appreciably in the course of six months. This
objection is now removed, for we have seen that the activity of
radio-tellurium does decay fairly rapidly. It was early recognised that
the activity of the polonium, separated from pitchblende by the methods
of Mme Curie, was not permanent, but decayed with the time. Observations
on the rate of decay have not been very precise, but Mme Curie states
that some of her preparations lost half of their activity in about six
months but in others the rate of decay was somewhat smaller. It is
possible that the initial differences observed in the rates of decay of
different specimens of polonium may be due to the presence of some
radium D with the polonium. The polonium in my possession lost its
activity fairly rapidly, and was reduced to a small portion of its value
in the course of about four years. Rough observations of its activity,
made from time to time, showed that its activity diminished to half
value in about six months. If it is identical with radio-tellurium, the
activity should decay to half value in 143 days, and I think there is
little doubt that more accurate measurement will prove this to be the
case.

While the proof of the identity of the active constituent in polonium is
not so definite as for radio-tellurium, I think there can be no
reasonable doubt that these substances both contain the same active
substance, which is the seventh transformation product of radium.
Marckwald has noticed some chemical differences in the behaviour of
polonium and radio-tellurium, but little weight can be attached to such
observations, for it must be remembered that the active constituent in
both cases is present in minute quantity in the material under
examination, and that the apparent chemical properties of the active
substance are much influenced by the presence of impurities. The most
important and trustworthy test rests upon the identity of the radiations
and the period of decay.


=241 A. Origin of radio-active lead.= Some experiments will now be
discussed which show that the radio-lead first separated from
pitchblende by Hofmann (section 22) contains the products radium D, E
and F. Hofmann has observed that the activity of this substance did not
appreciably decay in the course of several years. In some recent
experiments, Hofmann, Gonder and Wölfl[324] have made a close chemical
examination of the radio-active lead, and have shown the presence of two
radio-active constituents, which are probably identical with the
products radium E and F. The radio-active measurements were
unfortunately not very precise, and the periods of change of the
separated products have not been examined very closely.

Experiments were made on the effect of adding substances to a solution
of radio-lead, and then removing them by precipitation. Small quantities
of iridium, rhodium, palladium, and platinum, in the form of chlorides,
were left in the solution for three weeks, and then precipitated by
formalin or hydroxylamine. All of these substances were found to give
out both α and β rays, the activity being greatest for rhodium and least
for platinum. A large proportion of the β ray activity disappeared in
the course of six weeks, and of the α ray activity in one year. It is
probable that the two products radium E and F were in part removed with
the metals from the radio-lead. We have seen that radium E gives out β
rays and loses half of its activity in about six days, while radium F
gives out only α rays and its activity falls to half value in 143 days.
This conclusion is further confirmed by experiments on the effect of
heat on the activity of these substances. By heating to a full red heat,
the α ray activity was lost in a few seconds. This is in agreement with
the results (section 232) where we have seen that radium F is
volatilized at about 1000° C. and radium E is left behind.

Salts of gold, silver and mercury added to the radio-lead were found to
show only α ray activity on removal. This is in accordance with the view
that radium F alone is removed with these substances. Bismuth salts on
the other hand showed initially α and β ray activity, but the latter
rapidly died away. The presence of β rays in freshly prepared polonium
was early observed by Mme Curie. The α and β ray activity of the
radio-lead is much reduced by the precipitation of bismuth added to the
solution. The α and β ray activity of the radio-lead, however, recovers
itself again. This result is exactly what is to be expected if
radio-lead contains radium D, E and F. Radium E and F are removed with
the bismuth, but the parent substance, radium D, is left behind, and, in
consequence, a fresh supply of radium E and F is produced.

While further experiments are required to settle definitely whether the
products separated from radio-lead are identical with radium E and F,
there can be little doubt that such is the case. This conclusion is
strengthened by some experiments which I have made on a specimen of
radio-lead, which was kindly forwarded to me by Mr Boltwood of New
Haven. This active lead gave out α and β rays, the latter being in
unusually large proportion. The active lead was four months old when
first tested. The β ray activity in the following six months has
remained sensibly constant, but the α ray activity has steadily
increased. These results are to be expected if the radio-lead contains
radium D. Radium E will reach a practical maximum about 40 days after
separation of the product radium D with the lead. The α ray activity due
to radium F should increase to a maximum in about 2·6 years (see section
236).

Further experiments are required to settle whether the lead immediately
after separation from pitchblende contains only radium D, or whether
radium E also appears with it. It seems likely, however, that the
bismuth, which is initially present in solution at the time of
separation of the lead, will retain both radium E and F, and that the
presence of these products in radio-lead is due to their production,
after separation, by the parent substance, radium D.

It would be of scientific value to separate radium D from pitchblende
and obtain it in the pure state, for, a month after removal, the β ray
activity from it would be about 300 times as great as from an equal
weight of radium. By placing a bismuth plate in a solution of this
substance, radium F (polonium) should be separated, and, provided a
sufficient interval is allowed to elapse, a fresh supply of radium F can
at any time be obtained.

The rate of transformation of radium D (half transformed in 40 years) is
sufficiently slow not to interfere seriously with its utility in most
experiments.

The results of the comparison of the products of radium with those
contained in polonium, radio-tellurium and radio-lead are summarized
below.

    Radium D = product in _new radio-lead_, no rays. Half transformed in
    40 years.

    Radium E gives out β rays, separated with bismuth, iridium and
    platinum. Half transformed in 6 days.

    Radium F = product in _polonium_ and _radio-tellurium_. Gives out
    only α rays. Half transformed in 143 days.


=242. Temporary activity of inactive matter separated from radio-active
substances.= We have seen in the last section that the platinum metals
and bismuth acquire temporary activity by their admixture with a
solution of radio-lead, and that these effects are very satisfactorily
explained on the view that some of the products of change of radio-lead
are removed with the inactive substances. Very similar effects have been
observed by Pegram and von Lerch (section 186), when inactive substances
were added to solutions of thorium and of the active deposit of thorium.
These results, too, are almost certainly due to the removal of one or
more of the products of thorium with the inactive matter. Examples of
this character may readily be multiplied, and some of the more
interesting and important of these will be briefly discussed later.

There have been two general points of view regarding the character of
this activity which is temporarily acquired by inactive matter. Some
people have supposed that the inactive molecules of the substance, mixed
with the solution, acquire by “radio-active induction” temporary
activity, the underlying idea being that the close admixture of an
inactive and an active substance has communicated the property of
radiating to some of the molecules of the former. According to the
disintegration theory of radio-activity, on the other hand, the
temporary activity of originally inactive matter is not due to any
alteration of the inactive substance itself, but to an admixture with it
of one or more of the numerous radio-active products. The idea of
“radio-active induction” has no definite experimental evidence in
support of it, while there is much indirect evidence against it.

We shall now consider how these facts are interpreted according to the
disintegration theory. In a specimen of old radium, for example, there
are present, besides radium itself, the seven successive products which
arise from it. Each of these differs in chemical and physical properties
from the others. If now, for example, a bismuth rod is introduced into
the solution, one or more of these products are deposited on the
bismuth. This action is most probably electrolytic in nature, and will
depend upon the electro-chemical behaviour of the bismuth compared with
that of the products in solution. An electro-negative substance will
tend to remove the product or products which are strongly
electro-positive. This point of view serves to explain why different
metals are made active to different degrees, depending upon their
position in the electro-chemical series.

It seems probable that the activity communicated to inactive matter by
precipitation from an active solution occurs only during the
precipitation. The correctness of this view could readily be tested by
observing whether the time that the inactive substance is present in
solution has any effect on the magnitude of the activity imparted to it.

When it is remembered that in pitchblende there are present the
radio-elements uranium, thorium, radium and actinium and their numerous
family of products, it is not surprising that many of the inactive
substances separated from it may show very considerable activity due to
the mixture of products which may be removed with them. In carrying out
experiments on the separation of radium from pitchblende, M. and Mme
Curie observed that the separation of the active substance is fairly
complete if the stage of purification is not far advanced. Copper,
antimony and arsenic can be separated only slightly active, but other
substances like lead and iron always show activity. When the stage of
precipitation is more advanced, every substance separated from the
active solution shows activity.

One of the earliest observations in this direction was made by Debierne,
who found that barium could be made active by solution with actinium.
The active barium removed from the actinium still preserved its activity
after chemical treatment, and, in this way, barium chloride was obtained
whose activity was 6000 times that of uranium. Although the activity of
the barium chloride could be concentrated in the same way as the
activity of radiferous barium chloride, it did not show any of the
spectroscopic lines of radium, and could not have been due to the
admixture of that element with the barium. The activity of the barium
was not permanent, and Debierne states that the activity fell to about
one-third of its value in three months. It seems probable that the
precipitated barium carried down with it the product actinium X, and
also some of the actinium itself, and that the decay observed was due to
the transformation of actinium X. It is interesting to note that barium
is capable of removing a large number of products of the different
radio-elements. This effect is probably connected with its position in
the electro-chemical series, for barium is highly electro-positive.

Giesel showed in 1900 that bismuth could be made active by placing it in
a radium solution, and considered that polonium was in reality bismuth
made active by the process of induction. In later experiments, he found
that the bismuth plate gave out only α rays, and that the activity of
the bismuth could not be ascribed to radium, since no β rays were
present. We have seen that this activity of the bismuth is due to the
product radium F deposited on its surface.

Mme Curie also found that bismuth was made active by solution with a
radium compound, and succeeded in fractionating the above bismuth in the
same way as polonium. In this way bismuth was obtained 2000 times as
active as uranium, but the activity, like that of polonium separated
from pitchblende, decreased with the time. In the light of the
experiments on the transformation products of radium, it is seen that
these early experiments of Mme Curie add additional confirmation to the
view that the product (radium F) separated from radium itself is
identical with the polonium obtained directly from pitchblende.

Footnote 314:

  Rutherford and Soddy, _Phil. Mag._ April, 1903.

Footnote 315:

  Rutherford, _Phil. Trans._ A. p. 169, 1904. Curie and Danne, _C. R._
  p. 748, 1904.

Footnote 316:

  P. Curie and Danne, _Comptes Rendus_, 138, p. 748, 1904.

Footnote 317:

  Miss Gates, _Phys. Rev._ p. 300, 1903.

Footnote 318:

  Miss Brooks, _Nature_, July 21, 1904.

Footnote 319:

  Rutherford, _Phil. Mag._ Nov. 1904. _Nature_, p. 341, Feb. 9, 1905.

Footnote 320:

  Rutherford, _Nature_, p. 341, Feb. 9, 1905.

Footnote 321:

  Marckwald (_Ber. d. D. Chem. Ges._ p. 591, 1905) has recently found
  that the activity of his radio-tellurium falls to half value in 139
  days.

Footnote 322:

  Meyer and Schweidler, _Wien Ber._ Dec. 1, 1904.

Footnote 323:

  Rutherford, _Phil. Trans._ A. p. 169, 1904.

Footnote 324:

  Hofmann, Gonder and Wölfl, _Annal. d. Phys._ 15, p. 615, 1904.




                              CHAPTER XII.
                      RATE OF EMISSION OF ENERGY.


=243.= It was early recognised that a considerable amount of energy is
emitted by the radio-active bodies in the form of their characteristic
radiations. Most of the early estimates of the amount of this energy
were based on the number and energy of the expelled particles, and were
much too small. It has been pointed out (section 114) that the greater
part of the energy emitted from the radio-active bodies in the form of
ionizing radiations is due to the α rays, and that the β rays in
comparison supply only a very small fraction.

Rutherford and McClung[325] made an estimate of the energy of the rays,
emitted by a thin layer of active matter, by determining the total
number of ions produced by the complete absorption of the α rays. The
energy required to produce an ion was determined experimentally by
observations of the heating effect of X rays, and of the total number of
ions produced when the rays were completely absorbed in air. The energy
required to produce an ion in air was found to be 1·90 × 10⁻¹⁰ ergs.
This, as will be shown in Appendix A, is probably an over-estimate, but
was of the right order of magnitude. From this it was calculated that
one gram of uranium oxide spread over a plate in the form of a thin
powdered layer emitted energy into the air at the rate of 0·032 gram
calories per year. This is a very small emission of energy, but in the
case of an intensely radio-active substance like radium, whose activity
is about two million times that of uranium, the corresponding emission
of energy is 69000 gram calories per year. This is obviously an
under-estimate, for it includes only the energy radiated into the air.
The actual amount of energy released in the form of α rays is evidently
much greater than this on account of the absorption of the α rays by the
active matter itself.

It will be shown later that the heating effect of radium and of its
products is a measure of the energy of the expelled α particles.


=244. Heat emission of radium.= P. Curie and Laborde[326] first drew
attention to the striking result that a radium compound kept itself
continuously at a temperature several degrees higher than that of the
surrounding atmosphere. Thus the energy emitted from radium can be
demonstrated by its direct heating effect, as well as by photographic
and electric means. Curie and Laborde determined the rate of the
emission of heat in two different ways. In one method the difference of
temperature was observed by means of an iron-constantine thermo-couple
between a tube containing one gram of radiferous chloride of barium, of
activity about ⅙ of pure radium, and an exactly similar tube
containing one gram of pure barium chloride. The difference of
temperature observed was 1·5° C. In order to measure the rate of
emission of heat, a coil of wire of known resistance was placed in the
pure barium chloride, and the strength of the electric current required
to raise the barium to the same temperature as the radiferous barium was
observed. In the other method, the active barium, enclosed in a glass
tube, was placed inside a Bunsen calorimeter. Before the radium was
introduced, it was observed that the level of the mercury in the stem
remained steady. As soon as the radium, which had previously been cooled
in melting ice, was placed in the calorimeter, the mercury column began
to move at a regular rate. If the radium tube was removed, the movement
of the mercury ceased. It was found from these experiments that the heat
emission from the 1 gram of radiferous barium, containing about ⅙ of
its weight of pure radium chloride, was 14 gram-calories per hour.
Measurements were also made with 0·08 gram of pure radium chloride.
Curie and Laborde deduced from these results that 1 gram of pure radium
emits a quantity of heat equal to about 100 gram-calories per hour. This
result was confirmed by the experiments of Runge and Precht[327] and
others. As far as observation has gone at present, this rate of emission
of heat is continuous and unchanged with lapse of time. Therefore, 1
gram of radium emits in the course of a day 2400, and in the course of a
year 876,000 gram-calories. The amount of heat evolved in the union of
hydrogen and oxygen to form 1 gram of water is 3900 gram-calories. It is
thus seen that 1 gram of radium emits _per day_ nearly as much energy as
is required to dissociate 1 gram of water.

In some later experiments using 0·7 gram of pure radium bromide, P.
Curie[328] found that the temperature of the radium indicated by a
mercury thermometer was 3° C. above that of the surrounding air. This
result was confirmed by Giesel, who obtained a difference of temperature
of 5° C. with 1 gram of radium bromide. The actual rise of temperature
observed will obviously depend upon the size and nature of the vessel
containing the radium.

During their visit to England in 1903 to lecture at the Royal
Institution, M. and Mme Curie performed some experiments with Professor
Dewar, to test by another method the rate of emission of heat from
radium at very low temperatures. This method depended on the measurement
of the amount of gas volatilized when a radium preparation was placed
inside a tube immersed in a liquefied gas at its boiling point. The
arrangement of the calorimeter is shown in Fig. 97.

[Illustration: Fig. 97.]

The small closed Dewar flask _A_ contains the radium in a glass tube
_R_, immersed in the liquid to be employed. The flask _A_ is surrounded
by another Dewar bulb _B_, containing the same liquid, so that no heat
is communicated to _A_ from the outside. The gas liberated in the tube
_A_ is collected in the usual way over water or mercury, and its volume
determined. By this method, the rate of heat emission of the radium was
found to be about the same in boiling carbon dioxide and oxygen, and
also in liquid hydrogen. Especial interest attaches to the result
obtained with liquid hydrogen, for at such a low temperature ordinary
chemical activity is suspended. The fact that the heat emission of
radium is unaltered over such a wide range of temperature indirectly
shows that the rate of expulsion of α particles from radium is
independent of temperature, for it will be shown later that the heating
effect observed is due to the bombardment of the radium by the α
particles.

The use of liquid hydrogen is very convenient for demonstrating the rate
of heat emission from a small amount of radium. From 0·7 gram of radium
bromide (which had been prepared only 10 days previously) 73 c.c. of gas
were given off per minute.

In later experiments P. Curie (_loc. cit._) found that the rate of
emission of heat from a given quantity of radium depended upon the time
which had elapsed since its preparation. The emission of heat was at
first small, but after a month’s interval practically attained a
maximum. If a radium compound is dissolved and placed in a sealed tube,
the rate of heat emission rises to the same maximum as that of an equal
quantity of radium in the solid state.


=245. Connection of the heat emission with the radiations.= The
observation of Curie that the rate of heat emission depended upon the
age of the radium preparation pointed to the conclusion that the
phenomenon of heat emission of radium was connected with the
radio-activity of that element. It had long been known that radium
compounds increased in activity for about a month after their
preparation, when they reached a steady state. It has been shown
(section 215), that this increase of activity is due to the continuous
production by the radium of the radio-active emanation, which is
occluded in the radium compound and adds its radiation to that of the
radium proper. It thus seemed probable that the heating effect was in
some way connected with the presence of the emanation. Some experiments
upon this point were made by Rutherford and Barnes[329]. In order to
measure the small amounts of heat emitted, a form of differential air
calorimeter shown in Fig. 98 was employed. Two equal glass flasks of
about 500 c.c. were filled with dry air at atmospheric pressure. These
flasks were connected through a glass =U=-tube filled with xylene, which
served as a manometer to determine any variation of pressure of the air
in the flasks. A small glass tube, closed at the lower end, was
introduced into the middle of each of the flasks. When a continuous
source of heat was introduced into the glass tube, the air surrounding
it was heated and the pressure was increased. The difference of
pressure, when a steady state was reached, was observed on the manometer
by means of a microscope with a micrometer scale in the eye-piece. On
placing the source of heat in the similar tube in the other flask, the
difference in pressure was reversed. In order to keep the apparatus at a
constant temperature, the two flasks were immersed in a water-bath,
which was kept well stirred.

[Illustration: Fig. 98.]

Observations were first made on the heat emission from 30 milligrams of
radium bromide. The difference in pressure observed on the manometer was
standardized by placing a small coil of wire of known resistance in the
place of the radium. The strength of the current through the wire was
adjusted to give the same difference of pressure on the manometer. In
this way it was found that the heat emission per gram of radium bromide
corresponded to 65 gram-calories per hour. Taking the atomic weight of
radium as 225, this is equivalent to a rate of emission of heat from one
gram of metallic radium of 110 gram-calories per hour.

The emanation from the 30 milligrams of radium bromide was then removed
by heating the radium (section 215). By passing the emanation through a
small glass tube immersed in liquid air, the emanation was condensed.
The tube was sealed off while the emanation was still condensed in the
tube. In this way the emanation was concentrated in a small glass tube
about 4 cms. long. The heating effects of the “de-emanated” radium and
of the emanation tube were then determined at intervals. It was found
that, after removal of the emanation, the heating effect of the radium
decayed in the course of a few hours to a minimum, corresponding to
about 25 per cent. of the original heat emission, and then gradually
increased again, reaching its original value after about a month’s
interval. The heating effect of the emanation tube was found to increase
for the first few hours after separation to a maximum, and then to decay
regularly with the time according to an exponential law, falling to half
its maximum value in about four days. The actual heat emission of the
emanation tube was determined by sending a current through a coil of
wire occupying the same length and position as the emanation tube.

The variation with time of the heating effect from 30 milligrams of
radium and the emanation from it is shown in Fig. 99.

[Illustration: Fig. 99.]

Curve _A_ shows the variation with time of the heat emission of the
radium and curve _B_ of the emanation. The sum total of the rate of heat
emission of the radium and the emanation together, was at any time found
to be equal to that of the original radium. The maximum heating effect
of the tube containing the emanation from 30 milligrams of radium
bromide was 1·26 gram-calories per hour. The emanation together with the
secondary products which arise from it, obtained from one gram of
radium, would thus give out 42 gram-calories per hour. The emanation
stored up in the radium is thus responsible for more than two-thirds of
the total heat emission from radium. It will be seen later that the
decrease to a minimum of the heating effect of radium, after removal of
the emanation, is connected with the decay of the excited activity. In a
similar way, the increase of the heating effect of the emanation to a
maximum some hours after removal is also a result of the excited
activity produced by the emanation on the walls of the containing
vessel. Disregarding for the moment these rapid initial changes in heat
emission, it is seen that the heating effect of the emanation and its
further products, after reaching a maximum, decreases at the same rate
as that at which the emanation loses its activity, that is, it falls to
half value in four days. If _Q__{max.} is the maximum heating effect and
_Q_{t}_ the heating effect at any time _t_ later, then

$$ \frac {Q_t} {Q_{max}} = e^{–λt} $$

where λ is the constant of change of the emanation.

The curve of recovery of the heating effect of radium from its minimum
value is identical with the curve of recovery of its activity measured
by the α rays. Since the minimum heating effect is 25 per cent. of the
total, the heat emission _Q_{t}_ at any time _t_ after reaching a
minimum is given by

$$ \frac {Q_t} {Q_{max}} = \cdot25 + \cdot75 (1 − e^{–λt}) $$,

where _Q__{max.} is the maximum rate of heat emission and λ, as before,
is the constant of change of the emanation.

The identity of the curves of recovery and fall of the heating effect of
radium and its emanation respectively with the corresponding curves for
the rise and fall of radio-activity shows that the heat emission of
radium and its products is directly connected with their radio-activity.
The variation in the heat emission of both radium and its emanation is
approximately proportional to their activity measured by the α rays. It
is not proportional to the activity measured by the β or γ rays, for the
intensity of these rays falls nearly to zero some hours after removal of
the emanation, while the α ray activity, like the heating effect, is 25
per cent. of the maximum value. These results are thus in accordance
with the view that the heat emission of radium accompanies the expulsion
of α particles, and is approximately proportional to the number
expelled. Before such a conclusion can be considered established, it is
necessary to show that the heating effect of the active deposit from the
emanation varies in the same way as its α ray activity. Experiments made
to test this point will now be considered.


=246. Heat emission of the active deposit from the emanation.= New
radium in radio-active equilibrium contains four successive products
which break up with the emission of α particles, viz. radium itself, the
emanation, radium A and C. Radium B does not emit rays at all. The
effect of the later products radium D, E and F may be neglected, if the
radium has not been prepared for more than a year.

It is not easy to settle definitely the relative activity supplied by
each of these products when in radio-active equilibrium, but it has been
shown in section 229 that the activity is not very different for the
four α ray products. The α particles from radium A and C are more
penetrating than those from radium itself and the emanation. The
evidence at present obtained points to the conclusion that the activity
supplied by the emanation is less than that supplied by the other
products. This indicates that the α particles from the emanation are
projected with less velocity than in the other cases.

When the emanation is suddenly released from radium by heat or solution,
the products radium A, B and C are left behind. Since the parent matter
is removed, the amount of the products A, B, C at once commences to
diminish, and at the end of about three hours reaches a very small
value. If the heating effect depends upon the α ray activity, it is thus
to be expected that the heat emission of the radium should rapidly
diminish to a minimum after the removal of the emanation.

When the emanation is introduced into a vessel, the products radium A, B
and C at once appear and increase in quantity, reaching a practical
maximum about 3 hours later. The heating effect of the emanation tube
should thus increase for several hours after the introduction of the
emanation.

In order to follow the rapid changes in the heating effect of radium,
after removal of the emanation, Rutherford and Barnes (_loc. cit._) used
a pair of differential platinum thermometers. Each thermometer consisted
of 35 cms. of fine platinum wire, wound carefully on the inside of a
thin glass tube 5 mms. in diameter, forming a coil 3 cms. long. The
glass tube containing the radium and also the tube containing the
emanation were selected to slide easily into the interior of the coils,
the wire thus being in direct contact with the glass envelope containing
the source of heat. The change in resistance of the platinum
thermometers, when the radium or emanation tube was transferred from one
coil to the other, was readily measured.

[Illustration: Fig. 100.]

The heating effect of the radium in radio-active equilibrium was first
accurately determined. The radium tube was heated to drive off the
emanation, which was rapidly condensed in a small glass tube 3 cms. long
and 3 mms. internal diameter. After allowing a short time for
temperature conditions to become steady, the heating effect of the
radium tube was measured. The results are shown in Fig. 100. An
observation could not be taken until about 12 minutes after the removal
of the emanation, and the heating effect was then found to have fallen
to about 55 per cent. of the maximum value. It steadily diminished with
the time, finally reaching a minimum value of 25 per cent. several hours
later.

It is not possible in experiments of this character to separate the
heating effect of the emanation from that supplied by radium A. Since A
is half transformed in three minutes, its heating effect will have
largely disappeared after 10 minutes, and the decrease is then mainly
due to changes in radium B and C.

The variation with time of the heating effect of the active deposit is
still more clearly brought out by an examination of the rise of the
heating effect when the emanation is introduced into a small tube, and
of the decrease of the heating effect after the emanation is removed.
The curve of rise is shown in the upper curve of Fig. 101. 40 minutes
after the introduction of the emanation, the heating effect had risen to
75 per cent. of the maximum value which was reached after an interval of
about 3 hours.

[Illustration: Fig. 101.]

After the heating effect of the emanation tube had attained a maximum,
the emanation was removed, and the decay with time observed as soon as
possible afterwards. The results are shown in the lower curve of Fig.
101. It is seen that the two curves of rise and decay are complementary
to one another. The first observation was made 10 minutes after removal,
and the heating effect had then dropped to 47 per cent. of the original
value. This sudden drop is due partly to the removal of the emanation,
and partly to the rapid transformation of radium A. The lower curve is
almost identical in shape with the corresponding α ray curve for the
decay of the excited activity after a long exposure (see Fig. 86) and
clearly shows that the heating effect is directly proportional to the
activity measured by the α rays over the whole range examined. The
heating effect decreases according to the same law and at the same rate
as the activity measured by the α rays.

Twenty minutes after the removal of the emanation, radium A has been
almost completely transformed, and the activity is then proportional to
the amount of radium C present, since the intermediate product B does
not give out rays. The close agreement of the activity and heat emission
curves shows that the heating effect is proportional also to the amount
of radium C. We may thus conclude that the rayless product B supplies
little if any of the heat emission observed. If radium B supplied the
same amount as radium C, the curve of decrease of heating effect with
time would differ considerably from the activity curve.

The conclusion that the transformation of radium B is not accompanied by
the release of as much heat as the other changes is to be expected if
the heating effect is mainly due to the energy of motion of the expelled
α particles.

The relative heating effect due to the radium products is shown in the
following table. The initial heating effect of C is deduced by
comparison with the corresponding activity curve.

          Products   Radiation  Initial rate of heat emission

          Radium     α rays     25 per cent. of total

          Emanation  α  „

          Radium A   α  „       44     „        „

          Radium B   no rays     0     „        „

          Radium C   α, β, γ    31     „        „
                     rays

Since radium A and C supply almost an equal proportion of activity, it
is probable that they have equal initial heating effects. If this is the
case, the heating effect of the emanation alone is 13 per cent. of the
total.


=247. Heating effects of the β and= γ =rays=. It has been shown in
section 114 that the kinetic energy of the β particles emitted from
radium is probably not greater than one per cent. of that due to the α
particles. If the heat emission is a result of bombardment by the
particles expelled from its mass, it is to be expected that the heating
effect of the β rays will be very small compared with that due to the α
rays. This anticipation is borne out by experiment. Curie measured the
heating effect of radium (1) when enclosed in a thin envelope, and (2)
when surrounded by one millimetre of lead. In the former case a large
proportion of the β rays escaped, and, in the latter, nearly all were
absorbed. The increase of heating effect in case (2) was not more than
five per cent., and this is probably an over-estimate.

In a similar way, since the total ionization due to the β rays is about
equal to that produced by the γ rays, we should expect that the heating
effect of the γ rays will be very small compared with that arising from
the α rays.

Paschen made some experiments on the heating effect of radium in a
Bunsen ice calorimeter where the radium was surrounded by a thickness of
1·92 cms. of lead—a depth sufficient to absorb a large proportion of the
γ rays. In his first publication[330], results were given which
indicated that the heating effect of the γ rays was even greater than
that of the α rays. This was not confirmed by later observations by the
same method. He concluded that the ice calorimeter could not be relied
on to measure such very small quantities of heat.

After the publication of Paschen’s first paper Rutherford and
Barnes[331] examined the question by a different method. An air
calorimeter of the form shown in Fig. 98 was employed which was found to
give very satisfactory results. The heat emission of radium was measured
(1) when the radium was surrounded by a cylinder of aluminium and (2)
when surrounded by a cylinder of lead of the same dimensions. The
aluminium absorbed only a small fraction of the γ rays while the lead
stopped more than half. No certain difference between the heating effect
in the two cases was observed, although from the earlier experiments of
Paschen a difference of at least 50 per cent. was to be expected.

We must therefore conclude that the β and γ rays together do not supply
more than a small percentage of the total heat emission of radium—a
result which is in accordance with the calculations based on the total
ionization produced by the different types of rays.


=248. Source of the energy.= It has been shown that the heating effect
of radium is closely proportional to the activity measured by the α
rays. Since the activity is generally measured between parallel plates
such a distance apart that most of the α particles are absorbed in the
gas, this result shows that the heating effect is proportional to the
energy of the emitted α particles. The rapid heat emission of radium
follows naturally from the disintegration theory of radio-activity. The
heat is supposed to be derived not from external sources, but from the
internal energy of the radium atom. The atom is supposed to be a complex
system consisting of charged parts in very rapid motion, and in
consequence contains a large store of latent energy, which can only be
manifested when the atom breaks up. For some reason, the atomic system
becomes unstable, and an α particle, of mass about twice that of the
hydrogen atom, escapes, carrying with it its energy of motion. Since the
α particles would be practically absorbed in a thickness of radium of
less than ·001 cm., the greater proportion of the α particles, expelled
from a mass of radium, would be stopped in the radium itself and their
energy of motion would be manifested in the form of heat. The radium
would thus be heated by its own bombardment above the temperature of the
surrounding air. The energy of the expelled α particles probably does
not account for the whole emission of heat by radium. It is evident that
the violent expulsion of a part of the atom must result in intense
electrical disturbances in the atom. At the same time, the residual
parts of the disintegrated atom rearrange themselves to form a
permanently or temporarily stable system. During this process also some
energy is probably emitted, which is manifested in the form of heat in
the radium itself.

The view that the heat emission of radium is due very largely to the
kinetic energy possessed by the expelled α particles is strongly
confirmed by calculations of the magnitude of the heating effect to be
expected on such an hypothesis. It has been shown in section 93 that one
gram of radium bromide emits about 1·44 × 10¹¹ α particles per second.
The corresponding number for 1 gram of radium (Ra = 225) is 2·5 × 10¹¹.
Now it has been calculated from experimental data in section 94, that
the average kinetic energy of the α particles expelled from radium is
5·9 × 10⁻⁶ ergs. Since all of the α particles are absorbed either in the
radium itself or the envelope surrounding it, the total energy of the α
particles emitted per second is 1·5 × 10⁶ ergs. This corresponds to an
emission of energy of about 130 gram calories per hour. Now the observed
heating effect of radium is about 100 gram calories per hour.
Considering the nature of the calculation, the agreement between the
observed and experimental values is as close as would be expected, and
directly supports the view that the heat emission of radium is due very
largely to the bombardment of the radium and containing vessel by the α
particles expelled from its mass.


=249. Heating effect of the radium emanation.= The enormous amount of
heat liberated in radio-active transformations which are accompanied by
the expulsion of α particles is very well illustrated by the case of the
radium emanation.

The heat emission of the emanation released from 1 gram of radium is 75
gram calories per hour at its maximum value. This heat emission is not
due to the emanation alone, but also to its further products which are
included with it. Since the rate of heat emission decays exponentially
with the time to about half value in four days, the total amount of heat
liberated during the life of the emanation from 1 gram of radium is
equal to

$$ \int₀^{\infty} 75 e^{–λt} dt = \frac {75} {λ} $$

= 10,000 gram calories approximately,

since λ = ·0072(hour)⁻¹. Now the volume of the emanation from 1 gram of
radium is about 1 cubic millimetre at standard pressure and temperature
(section 172). Thus 1 cubic centimetre of the emanation would during its
transformation emit 10⁷ gram calories. The heat emitted during the
combination of 1 c.c. of hydrogen and oxygen to form water is about 2
gram calories. The emanation thus gives out during its changes 5 × 10⁶
times as much energy as the combination of an equal volume of hydrogen
and oxygen to form water, although this latter reaction is accompanied
by a larger release of energy than any other known to chemistry.

The production of heat from 1 c.c. of the radium emanation is about 21
gram calories per second. This generation of heat would be sufficient to
heat to redness, if not to melt down, the walls of the glass tube
containing the emanation.

The probable rate of heat emission from 1 gram weight of the emanation
can readily be deduced, assuming that the emanation has about 100 times
the molecular weight of hydrogen. Since 100 c.c. of the emanation would
weigh about 1 gram, the total heat emission from 1 gram of the emanation
is about 10⁹ gram calories.

It can readily be calculated that one pound weight of the emanation
would, at its maximum, radiate energy at the rate of about 10,000
horse-power. This radiation of energy would fall off with the time, but
the total emission of energy during the life of the emanation would
correspond to 60,000 horse-power days.


=250. Heating effects of uranium, thorium, and actinium.= Since the heat
emission of radium is a direct consequence of its bombardment by the α
particles expelled from its mass, it is to be expected that all the
radio-elements which emit α rays should also emit heat at a rate
proportional to their α ray activity.

Since the activity of pure radium is probably about two million times
that of uranium or thorium, the heat emission from 1 gram of thorium or
uranium should be about 5 × 10⁻⁵ gram calories per hour, or about 0·44
gram calories per year. This is a very small rate of generation of heat,
but it should be detectable if a large quantity of uranium or thorium is
employed. Experiments to determine the heating effect of thorium have
been made by Pegram[332]. Three kilograms of thorium oxide, enclosed in
a Dewar bulb, were kept in an ice-bath, and the difference of
temperature between the thorium and ice-bath determined by a set of
iron-constantan thermo-electric couples. The maximum difference of
temperature observed was 0·04° C., and, from the rate of change of
temperature, it was calculated that one gram of thorium oxide liberated
8 × 10⁻⁵ gram calories per hour. A more accurate determination of the
heat emission is in progress, but the results obtained are of the order
of magnitude to be expected.


=251. Energy emitted by a radio-active product.= An important
consequence follows from the fact that the heat emission is a measure of
the energy of the expelled α particles. If each atom of each product
emits α particles, the total emission of energy from 1 gram of the
product can at once be determined. The α particles from the different
products are projected with about the same velocity, and consequently
carry off about the same amount of energy. Now it has been shown that
the energy of each α particle expelled from radium is about 5·9 × 10⁻⁶
ergs. Most of the products probably have an atomic weight in the
neighbourhood of 200. Since there are 3·6 × 10¹⁹ molecules in one cubic
centimetre of hydrogen, it can easily be calculated that there are about
3·6 × 10²¹ atoms in one gram of the product.

If each atom of the product expels one α particle, the total energy
emitted from 1 gram of the matter is about 2 × 10¹⁶ ergs or 8 × 10⁸ gram
calories. The total emission of energy from a product which emits only β
rays is probably about one-hundredth of the above amount.

In this case we have only considered the energy emitted from a single
product independently of the successive products which may arise from
it. Radium, for example, may be considered a radio-active product which
slowly breaks up and gives rise to four subsequent α ray products. The
total heat emission from one gram of radium and products is thus about
five times the above amount, or 4 × 10⁹ gram calories.

The total emission of energy from radium is discussed later in section
266 from a slightly different point of view.


=252. Number of ions produced by an α particle.= In the first edition of
this book it was calculated by several independent methods that 1 gram
of radium emitted about 10¹¹ α particles per second. Since the actual
number has later been determined by measuring the charge carried by the
α rays (section 93) we can, conversely, use this number to determine
with more certainty some of the constants whose values were assumed in
the original calculation.

For example, the total number of ions produced by an α particle in the
gas can readily be determined. The method employed is as follows. 0·484
mgr. of radium bromide was dissolved in water and then spread uniformly
over an aluminium plate. After evaporation, the saturation ionization
current, due to the radium at its minimum activity, was found to be 8·4
× 10⁻⁸ ampere. The plates of the testing vessel were sufficiently far
apart to absorb all the α rays in the gas. The number of α particles
expelled per second into the gas was found experimentally to be 8·7 ×
10⁶. Taking the charge on an ion as 1·13 × 10⁻¹⁹ coulombs (section 36),
the total number of ions produced per second in the gas was 7·5 × 10¹¹.
Thus each α particle on an average produced 86,000 ions in the gas
before it was absorbed.

Now Bragg (section 104) has shown that the α particles from radium at
its minimum activity are stopped in about 3 cms. of air. The results
obtained by him indicate that the ionization of the particles per cm. of
path is less near the radium than some distance away. Assuming, however,
as a first approximation that the ionization is uniform along the path,
the number of ions produced per cm. of path by the α particle is 29,000.
Since the ionization varies directly as the pressure, at a pressure of 1
mm. of mercury the number of ions per unit path would be about 38. Now
Townsend (section 103) found that the maximum number of ions produced
per unit path of air at 1 mm. pressure by an electron in motion was 20,
and in this case a fresh pair of ions was produced at each encounter of
the electron with the molecules in its path. In the present case the α
particle, which has a very large mass compared with the electron,
appears to have a larger sphere of influence than the electron and to
ionize twice as many molecules.

In addition, the α particle produces many more ions per unit path than
an electron moving with the same velocity, for it has been shown
(section 103) that the electron becomes a less efficient ionizer after a
certain velocity is reached. As Bragg (_loc. cit._) has pointed out,
this is to be expected, since the α particle consists of a large number
of electrons and consequently would be a far more efficient ionizer than
an isolated electron. A calculation of the energy required to produce an
ion by an α particle is given in Appendix A.


=253. Number of β particles expelled from one gram of radium.= It is of
importance to compare the total number of β particles expelled from one
gram of radium in radio-active equilibrium, as, theoretically, this
number should bear a definite relation to the total number of α
particles emitted. We have seen that new radium in radio-active
equilibrium contains four products which emit α rays, viz. radium
itself, the emanation, radium A and radium C. On the other hand, β rays
are expelled from only one product, radium C. The same number of atoms
of each of these successive products in equilibrium break up per second.
If the disintegration of each atom is accompanied by the expulsion of
one α particle and, in the case of radium C, also of one β particle, the
number of α particles emitted from radium in radio-active equilibrium
will be four times the number of β particles.

The method employed by Wien to determine the number of β particles
emitted from a known quantity of radium has already been discussed in
section 80. On account of the absorption of some of the β particles in
the radium envelope and in the radium itself, the number found by him is
far too small. It has been shown in section 85 that a number of easily
absorbed β rays are projected from radium, many of which would be
stopped in the radium itself or in the envelope containing it.

In order to eliminate as far as possible the error due to this
absorption, in some experiments made by the writer, the active deposit
obtained from the radium emanation rather than radium itself was used as
a source of β rays. A lead rod, 4 cms. long and 4 mms. in diameter, was
exposed as the negative electrode in a large quantity of the radium
emanation for three hours. The rod was then removed and the γ ray effect
from it immediately measured by an electroscope and compared with the
corresponding γ ray effect from a known weight of radium bromide in
radio-active equilibrium. Since the active deposit contains the product
radium C which alone emits β rays, and, since the intensities of the β
and γ rays are always proportional to each other, the number of β
particles expelled from the lead rod per second is equal to the
corresponding number from the weight of radium bromide which gives the
same γ ray effect as the lead rod.

The rod was then enveloped in a thickness of aluminium foil of ·0053
cms.—a thickness just sufficient to absorb the α rays—and made the
insulated electrode in a cylindrical metal vessel which was rapidly
exhausted to a low pressure. The current in the two directions was
measured at intervals by an electrometer, and, as we have seen in
section 93, the algebraic sum of these currents is proportional to _ne_,
where _n_ is the number of β particles expelled per second from the lead
rod, and _e_ the charge on each particle. The activity of the radium C
decayed with the time, but, from the known curve of decay, the results
could be corrected in terms of the initial value immediately after the
rod was removed from the emanation.

Taking into account that half of the β particles emitted by the active
deposit were absorbed in the radium itself, and reckoning the charge on
the β particle as 1·13 × 10⁻¹⁹ coulombs, two separate experiments gave
7·6 × 10¹⁰ and 7·0 × 10¹⁰ as the total number of β particles expelled
per second from one gram of radium. Taking the mean value, we may
conclude that the total number of β particles expelled per second from
one gram of radium in radio-active equilibrium is about 7·3 × 10¹⁰.

The total number of α particles expelled from one gram of radium at its
minimum activity has been shown to be 6·2 × 10¹⁰ (section 93). The
approximate agreement between these numbers is a strong indication of
the correctness of the theoretical views previously discussed. It is to
be expected that the number of β particles, deduced in this way, will be
somewhat greater than the true value, since the β particles give rise to
a secondary radiation consisting also of negatively charged particles
moving at a high speed. These secondary β particles, arising from the
impact of the β particles on the lead, will pass through the aluminium
screen and add their effect to the primary β rays.

The results, however, indicate that four α particles are expelled from
radium in radio-active equilibrium for each β particle and thus confirm
the theory of successive changes.

Footnote 325:

  _Phil. Trans._ A. p. 25, 1901.

Footnote 326:

  P. Curie and Laborde, _C. R._ 136, p. 673, 1903.

Footnote 327:

  Runge and Precht, _Sitz. Ak. Wiss. Berlin_, No. 38, 1903.

Footnote 328:

  P. Curie, Société de Physique, 1903.

Footnote 329:

  Rutherford and Barnes, _Nature_, Oct. 29, 1903. _Phil. Mag._ Feb.
  1904.

Footnote 330:

  Paschen, _Phys. Zeit._ Sept. 15, 1904.

Footnote 331:

  Rutherford and Barnes, _Nature_, Dec. 18, 1904; _Phil. Mag._ May,
  1905.

Footnote 332:

  Pegram, _Science_, May 27, 1904.




                             CHAPTER XIII.
                        RADIO-ACTIVE PROCESSES.


=254. Theories of radio-activity.= In previous chapters, a detailed
account has been given of the nature and properties of the radiations,
and of the complex processes taking place in the radio-active
substances. The numerous products arising from the radio-elements have
been closely examined, and have been shown to result from a
transformation of the parent element through a number of well-marked
stages. In this chapter, the application of the disintegration theory to
the explanation of radio-active phenomena will be considered still
further, and the logical deductions to be drawn from the theory will be
discussed briefly.

A review will first be given of the working hypotheses which have served
as a guide to the investigators in the field of radio-activity. These
working theories have in many cases been modified or extended with the
growth of experimental knowledge.

The early experiments of Mme Curie had indicated that radio-activity was
an atomic and not a molecular phenomenon. This was still further
substantiated by later work, and the detection and isolation of radium
from pitchblende was a brilliant verification of the truth of this
hypothesis.

The discovery that the β rays of the radio-elements were similar to the
cathode rays produced in a vacuum tube was an important advance, and has
formed the basis of several subsequent theories. J. Perrin[333], in
1901, following the views of J. J. Thomson and others, suggested that
the atoms of bodies consisted of parts and might be likened to a
miniature planetary system. In the atoms of the radio-elements, the
parts composing the atoms more distant from the centre might be able to
escape from the central attraction and thus give rise to the radiation
of energy observed. In December 1901, Becquerel[334] put forward the
following hypothesis, which, he stated, had served him as a guide in his
investigations. According to the view of J. J. Thomson, radio-active
matter consists of negatively and positively charged particles. The
former have a mass about ¹⁄₁₀₀₀ of the mass of the hydrogen atom, while
the latter have a mass about one thousand times greater than that of the
negative particle. The negatively charged particles (the β rays) would
be projected with great velocity, but the larger positive particles with
a much lower velocity forming a sort of gas (the emanation) which
deposits itself on the surface of bodies. This in turn would subdivide,
giving rise to rays (excited activity).

In a paper communicated to the Royal Society in June 1900, Rutherford
and McClung[335] estimated that the energy, radiated in the form of
ionizing rays into the gas, was 3000 gram-calories per year for radium
of activity 100,000 times that of uranium. Taking the latest estimate of
the activity of a pure radium compound as 2,000,000, this would
correspond to an emission of energy into the gas in the form of α rays
of about 66,000 gram-calories per gram per year. The suggestion was made
that this energy might be derived from a re-grouping of the constituents
of the atom of the radio-elements, and it was pointed out that the
possible energy to be derived from a greater concentration of the
components of the atom was large compared with that given out in
molecular reactions.

In the original papers[336] giving an account of the discovery of the
emanation of thorium and the excited radio-activity produced by it, the
view was taken that both of these manifestations were due to
radio-active material. The emanation behaved like a gas, while the
matter which caused excited activity attached itself to solids and could
be dissolved in some acids but not in others. Rutherford and Miss Brooks
showed that the radium emanation diffused through air like a gas of
heavy molecular weight. At a later date Rutherford and Soddy showed that
the radium and thorium emanations behaved like chemically inert gases,
since they were unaffected by the most drastic physical and chemical
treatment.

On the other hand, P. Curie, who, in conjunction with Debierne, had made
a series of researches on the radium emanation, expressed dissent from
this view. P. Curie[337] did not consider that there was sufficient
evidence that the emanation was material in nature, and pointed out that
no spectroscopic evidence of its presence had yet been obtained, and
also that the emanation disappeared when contained in a sealed vessel.
It was pointed out by the writer[338] that the failure to detect
spectroscopic lines was probably a consequence of the minute quantity of
the emanation present, under ordinary conditions, although the
electrical and phosphorescent actions produced by this small quantity
are very marked. This contention is borne out by later work. P. Curie at
first took the view that the emanation was not material, but consisted
of centres of condensation of energy attached to the gas molecules and
moving with them.

M. and Mme Curie have throughout taken a very general view of the
phenomena of radio-activity, and have not put forward any definite
theory. In Jan. 1902, they gave an account of the general working
theory[339] which had guided them in their researches. Radio-activity is
an atomic property, and the recognition of this fact had created their
methods of research. Each atom acts as a constant source of emission of
energy. This energy may either be derived from the potential energy of
the atom itself, or each atom may act as a mechanism which instantly
regains the energy which is lost. They suggested that this energy may be
borrowed from the surrounding air in some way not accounted for by the
principle of Carnot.

In the course of a detailed study of the radio-activity of thorium,
Rutherford and Soddy[340] found that it was necessary to suppose that
thorium was continuously producing from itself new kinds of active
matter, which possess temporary activity and differ in chemical
properties from the thorium itself. The constant radio-activity of
thorium was shown to be the result of equilibrium between the processes
of production of active matter and the change of that already produced.
At the same time, the theory was advanced that the production of active
matter was a consequence of the disintegration of the atom. The work of
the following year was devoted to an examination of the radio-activity
of uranium and radium on similar lines, and it was found that the
conclusions already advanced for thorium held equally for uranium and
radium[341]. The discovery of a condensation of the radio-active
emanations[342] gave additional support to the view that the emanations
were gaseous in character. In the meantime, the writer[343] had found
that the rays consisted of positively charged bodies atomic in size,
projected with great velocity. The discovery of the material nature of
these rays served to strengthen the theory of atomic disintegration, and
at the same time to offer an explanation of the connection between the α
rays and the changes occurring in the radio-elements. In a paper
entitled “Radio-active Change,” Rutherford and Soddy[344] put forward in
some detail the theory of atomic disintegration as an explanation of the
phenomena of radio-activity, and at the same time some of the more
important consequences which follow from the theory were discussed.

In a paper announcing the discovery of the heat emission of radium, P.
Curie and Laborde[345] state that the heat energy may be equally well
supposed to be derived from a breaking up of the radium atom or from
energy absorbed by the radium from some external source.

J. J. Thomson in an article on “Radium,” communicated to _Nature_[346],
put forward the view that the emission of energy from radium is probably
due to some change within the atom, and pointed out that a large store
of energy would be released by a contraction of the atom.

Sir William Crookes[347], in 1899, proposed the theory that the
radio-active elements possess the property of abstracting energy from
the gas. If the moving molecules, impinging more swiftly on the
substance, were released from the active substance at a much lower
velocity, the energy released from the radio-elements might be derived
from the atmosphere. This theory was advanced again later on to account
for the large heat emission of radium, discovered by P. Curie and
Laborde.

F. Re[348] recently advanced a very general theory of matter with a
special application to radio-active bodies. He supposes that the parts
of the atom were originally free, constituting a nebula of extreme
tenuity. These parts have gradually become united round centres of
condensation, and have thus formed the atoms of the elements. On this
view an atom may be likened to an extinct sun. The radio-active atoms
occupy a transitional stage between the original nebula and the more
stable chemical atoms, and in the course of their contraction give rise
to the heat emission observed.

Lord Kelvin in a paper to the British Association meeting, 1903, has
suggested that radium may obtain its energy from external sources. If a
piece of white paper is put into one vessel and a piece of black paper
into an exactly similar vessel, on exposure of both vessels to the light
the vessel containing the black paper is found to be at a higher
temperature. He suggests that radium in a similar manner may keep its
temperature above the surrounding air by its power of absorption of
unknown radiations.

Richarz and Schenck[349] have suggested that radio-activity may be due
to the production and breaking up of ozone which is known to be produced
by radium salts.


=255. Discussion of Theories.= From the survey of the general hypotheses
advanced as possible explanations of radio-activity, it is seen that
they may be divided broadly into two classes, one of which assumes that
the energy emitted from the radio-elements is obtained at the expense of
the internal energy of the atom, and the other that the energy is
derived from external sources, but that the radio-elements act as
mechanisms capable of transforming this borrowed energy into the special
forms manifested in the phenomena of radio-activity. Of these two sets
of hypotheses the first appears to be the more probable, and to be best
supported by the experimental evidence. Up to the present not the
slightest experimental evidence has been adduced to show that the energy
of radium is derived from external sources.

J. J. Thomson (_loc. cit._) has discussed the question in the following
way:—

“It has been suggested that the radium derives its energy from the air
surrounding it, that the atoms of radium possess the faculty of
abstracting the kinetic energy from the more rapidly moving air
molecules while they are able to retain their own energy when in
collision with the slowly moving molecules of air. I cannot see,
however, that even the possession of this property would explain the
behaviour of radium; for imagine a portion of radium placed in a cavity
in a block of ice; the ice around the radium gets melted; where does the
energy for this come from? By the hypothesis there is no change in the
air-radium system in the cavity, for the energy gained by the radium is
lost by the air, while heat cannot flow into the cavity from the
outside, for the melted ice round the cavity is hotter than the ice
surrounding it.”

The writer has recently found that the activity of radium is not altered
by surrounding it with a large mass of lead. A cylinder of lead was cast
10 cms. in diameter and 10 cms. high. A hole was bored in one end of the
cylinder to the centre, and the radium, enclosed in a small glass tube,
was placed in the cavity. The opening was then hermetically closed. The
activity was measured by the rate of discharge of an electroscope by the
γ rays transmitted through the lead, but no appreciable change was
observed during a period of one month.

M. and Mme Curie early made the suggestion that the radiation of energy
from the radio-active bodies might be accounted for by supposing that
space is traversed by a type of Röntgen rays, and that the
radio-elements possess the property of absorbing them. Recent
experiments (section 279) have shown that there is present at the
surface of the earth a very penetrating type of rays, similar to the γ
rays of radium. Even if it were supposed that the radio-elements
possessed the power of absorbing this radiation, the energy of the rays
is far too minute to account even for the energy radiated from an
element of small activity like uranium. In addition, all the evidence so
far obtained points to the conclusion that the radio-active bodies do
not absorb the type of rays they emit to any greater extent than would
be expected from their density. It has been shown (section 86) that this
is true in the case of uranium. Even if it were supposed that the
radio-elements possess the property of absorbing the energy of some
unknown type of radiation, which is able to pass through ordinary matter
with little absorption, there still remains the fundamental difficulty
of accounting for the peculiar radiations from the radio-elements, and
the series of changes that occur in them. It is not sufficient for us to
account for the heat emission only, for it has been shown (chapter XII)
that the emission of heat is directly connected with the radio-activity.

In addition, the distribution of the heat emission of radium amongst the
radio-active products which arise from it is extremely difficult to
explain on the hypothesis that the energy emitted is borrowed from
external sources. It has been shown that more than two-thirds of the
heat emitted by radium is due to the emanation together with the active
deposit which is produced by the emanation. When the emanation is
separated from the radium, its power of emitting heat, after reaching a
maximum, decreases with the time according to an exponential law. It
would thus be necessary on the absorption hypothesis to postulate that
most of the heat emission of radium, observed under ordinary conditions,
is not due to the radium itself but to something produced by the radium,
whose power of absorbing energy from external sources diminishes with
time.

A similar argument also applies to the variation with time of the
heating effect of the active deposit produced from the emanation. It has
been shown in the last chapter that most of the heating effect observed
in radium and its products must be ascribed to the bombardment of the α
particles expelled from these substances. It has already been pointed
out (section 136) that it is difficult to imagine any mechanism, either
internal or external, whereby such enormous velocity can suddenly be
impressed upon the α particles. We are forced to the conclusion that the
α particle did not suddenly acquire this energy of motion, but was
initially in rapid motion in the atom, and for some reason, was suddenly
released with the velocity which it previously possessed in its orbit.

The strongest evidence against the hypothesis of absorption of external
energy is that such a theory ignores the fact, that, whenever
radio-activity is observed, it is always accompanied by some change
which can be detected by the appearance of new products having chemical
properties distinct from those of the original substances. This leads to
some form of “chemical” theory, and other results show that the change
is atomic and not molecular.


=256. Theory of radio-active change.= The processes occurring in the
radio-elements are of a character quite distinct from any previously
observed in chemistry. Although it has been shown that the
radio-activity is due to the spontaneous and continuous production of
new types of active matter, the laws which control this production are
different from the laws of ordinary chemical reactions. It has not been
found possible in any way to alter either the rate at which the matter
is produced or its rate of change when produced. Temperature, which is
such an important factor in altering the rate of chemical reactions, is,
in these cases, almost entirely without influence. In addition, no
ordinary chemical change is known which is accompanied by the expulsion
of charged atoms with great velocity. It has been suggested by Armstrong
and Lowry[350] that radio-activity may be an exaggerated form of
fluorescence or phosphorescence with a very slow rate of decay. But no
form of phosphorescence has yet been shown to be accompanied by
radiations of the character of those emitted by the radio-elements.
Whatever hypothesis is put forward to explain radio-activity must
account not only for the production of a series of active products,
which differ in chemical and physical properties from each other and
from the parent element, but also for the emission of rays of a special
character. Besides this, it is necessary to account for the large amount
of energy continuously radiated from the radio-elements.

The radio-elements, besides their high atomic weights, do not possess in
common any special chemical characteristics which differentiate them
from the other elements, which do not possess the property of
radio-activity to an appreciable degree. Of all the known elements,
uranium, thorium, and radium possess the greatest atomic weights, viz.:
radium 225, thorium 232·5, and uranium 240.

If a high atomic weight is taken as evidence of a complicated structure
of the atom, it might be expected that disintegration would occur more
readily in heavy than in light atoms. At the same time, there is no
reason to suppose that the elements of the highest atomic weight must be
the most radio-active; in fact, radium is far more active than uranium,
although its atomic weight is less. This is seen to be the case also in
the radio-active products; for example, the radium emanation is
enormously more active weight for weight than the radium itself, and
there is every reason to believe that the emanation has an atom lighter
than that of radium.

In order to explain the phenomena of radio-activity, Rutherford and
Soddy have advanced the theory that the atoms of the radio-elements
suffer spontaneous disintegration, and that each disintegrated atom
passes through a succession of well-marked changes, accompanied in most
cases by the emission of α rays.

A preliminary account of this hypothesis has already been given in
section 136, while the mathematical theory of successive changes, which
is based upon it, has been discussed in chapter IX. The general theory
has been utilized in chapters X and XI to account for the numerous
active substances found in uranium, thorium, actinium and radium.

The theory supposes that, on an average, a definite small proportion of
the atoms of each radio-active substance becomes unstable at a given
time. As a result of this instability, the atoms break up. In most
cases, the disintegration is explosive in violence and is accompanied by
the ejection of an α particle with great velocity; in a few cases, α and
β particles are expelled together, while in others a β particle alone
escapes. In a few cases, the change in the atom appears to be less
violent in character, and is not accompanied by the expulsion of either
an α or β particle. The explanation of these rayless changes is
considered in section 259. The expulsion of an α particle, of mass about
twice that of the hydrogen atom, leaves behind it a new system lighter
than the original one, and possessing chemical and physical properties
quite different from those of the original element. This new system
again becomes unstable, and expels another α particle. The process of
disintegration, once started, proceeds from stage to stage at a definite
measurable rate in each case.

At any time after the disintegration has commenced, there exists a
proportion of the original matter, which is unchanged, mixed with the
part which has undergone change. This is in accordance with the observed
fact that the spectrum of radium, for example, does not change
progressively with time. The radium breaks up so slowly that only a
small fraction has been transformed in the course of a few years. The
unchanged part still shows its characteristic spectrum, and will
continue to do so as long as any radium exists. At the same time it is
to be expected that, in old radium, the spectrum of those products which
exist in any quantity should also appear.

The term metabolon has been suggested as a convenient expression for
each of these changing atoms, derived from the successive disintegration
of the atoms of the radio-elements. Each metabolon, on an average,
exists only for a limited time. In a collection of metabolons of the
same kind the number _N_, which are unchanged at a time _t_ after
production, is given by

$$ N = N₀ e^{–λt} $$,

where _N₀_ is the original number. Now _dN_/_dt_ = -λ_N_, or the
fraction of the metabolons present, which change in unit time, is equal
to λ. The value 1/λ may be taken as the _average life_ of each
metabolon.

This may be simply shown as follows:—At any time _t_ after _N₀_
metabolons have been set aside, the number which change in the time _dt_
is equal to λ_Ndt_ or

$$ λ N₀ e^{–λt} dt $$ .

Each metabolon has a life _t_, so that the average life of the whole
number is given by

$$ \int₀^{\infty} λ te^{–λt} dt = \frac {1} {λ} $$

The various metabolons from the radio-elements are distinguished from
ordinary matter by their great instability and consequent rapid rate of
change. Since a body which is radio-active must _ipso facto_ be
undergoing change, it follows that none of the active products, for
example, the emanations and Th X, can consist of any known kind of
matter; for there is no evidence to show that inactive matter can be
made radio-active, or that two forms of the same element can exist, one
radio-active and the other not. For example, half of the matter
constituting the radium emanation has undergone change after an interval
of four days. After the lapse of about one month the emanation as such
has nearly disappeared, having been transformed through several stages
into other and more stable types of matter, which are in consequence
difficult to detect by their radio-activity.

The striking difference in chemical and physical properties which exists
in many cases between the various products themselves, and also between
the primary active substance and its products, has already been drawn
attention to in chapter IX. Some of the products show distinctive
electro-chemical behaviour and can be removed from a solution by
electrolysis. Others show differences in volatility which have been
utilized to effect a partial separation. There can be no doubt that each
of these products is a definite new chemical substance, and if it could
be collected in sufficient quantity to be examined by ordinary chemical
means, would be found to behave like a distinct chemical element. It
would differ, however, from the ordinary chemical element in the
shortness of its life, and the fact that it is continuously changing
into another substance. We shall see later (section 261) that there is
every reason to believe that radium itself is a metabolon in the true
sense of the term, since it is continuously changing, and is itself
produced from another substance. The main point of difference between it
and the other products lies in the comparative slowness of its rate of
change.

It is for this reason that radium exists in pitchblende in greater
quantity than the other more rapidly changing products. By working up a
large amount of the mineral, we have seen that a sufficient quantity of
the pure product has been obtained for chemical examination.

On account of the short life of the emanation, it exists in pitchblende
in much less quantity than radium, but it, too, has been isolated
chemically and its volume measured. The extraordinary properties of this
emanation, or gas, have already been discussed, and there can be no
doubt that, while it exists, it must be considered a new element allied
in chemical properties to the argon-helium group of gases.

There can be no doubt that in the radio-elements we are witnessing the
spontaneous transformation of matter, and that the different products
which arise mark the stages or halting-places in the process of
transformation, where the atoms are able to exist for a short time
before again breaking up into new systems.


=257. Radio-active products.= The following table gives the list of the
active products or metabolons known to result from the disintegration of
the three radio-elements. In the second column is given the value of the
radio-active constant λ for each active product, _i.e._ the proportion
of the active matter undergoing change per second; in the third column
the time _T_ required for the activity to fall to one-half, _i.e._ the
time taken for half the active product to undergo change; in the fourth
column, the nature of the rays from each active product, not including
the rays from the products which result from it; in the fifth column, a
few of the more marked physical and chemical properties of each
metabolon.

    Products     λ(sec)⁻¹     T          Nature of  Chemical and
                                         the rays   Physical
                                                    properties of
                                                    the product

    ────────────────────────────────────────────────────────────────
    Uranium      —            —          α          Soluble in
                                                    excess of
                                                    ammonium
                                                    carbonate,
                                                    soluble in
                                                    ether.
    Uranium X    3·6 × 10⁻⁷   22 days    β and γ    Insoluble in
                                                    excess of
                                                    ammonium
                                                    carbonate,
                                                    soluble in ether
                                                    and water.
    ────────────────────────────────────────────────────────────────
    Thorium      —            —          α          Insoluble in
                                                    ammonia.
    Thorium X    2·0 × 10⁻⁶   4 days     α          Soluble in
                                                    ammonia and
                                                    water.
    Emanation    1·3 × 10⁻²   53 secs.   α          Chemically inert
                                                    gas of heavy
                                                    molecular
                                                    weight.
                                                    Condenses at
                                                    −120° C.
    Thorium A    1·74 × 10⁻⁵  11 hours   no rays    Deposited on
                                                    bodies;
                                                    concentrated on
                                                    the cathode in
                                                    an electric
                                                    field. Soluble
                                                    in some acids;
                                                    Th A more
                                                    volatile than Th
                                                    B; shows
                                                    definite
                                                    electro-chemical
                                                    behaviour.
    Thorium B    2·2 × 10⁻⁴   55 mins.   α, β, γ    Same
    ?            —            —          —
    ────────────────────────────────────────────────────────────────
    Actinium     —            —          no rays    Insoluble in
                                                    ammonia.
    Actinium X   7·8 × 10⁻⁷   10·2 days  α (and β?) Soluble in
                                                    ammonia.
    Emanation    ·17          3·9 secs.  α          Behaves like a
                                                    gas.
    Actinium A   3·2 × 10⁻⁴   36 mins.   no rays    Deposited on
                                                    bodies;
                                                    concentrated on
                                                    the cathode in
                                                    an electric
                                                    field, soluble
                                                    in ammonia and
                                                    strong acids;
                                                    volatilized at a
                                                    temperature of
                                                    100° C., A and B
                                                    can be separated
                                                    by electrolysis.
    Actinium B   5·4 × 10⁻³   2·15 mins. α, β, γ    Same
    ?            —            —          —
    ────────────────────────────────────────────────────────────────
    Radium       —            1300 years α          Allied
                                                    chemically to
                                                    barium.
    Emanation    2·1 × 10⁻⁶   3·8 days   α          Chemically inert
                                                    gas of heavy
                                                    molecular
                                                    weight;
                                                    condenses at
                                                    −150° C.
    Radium A     3·85 × 10⁻³  3 mins.    α          Deposited on
    (active                                         surface of
    deposit of                                      bodies;
    rapid                                           concentrated on
    change)                                         cathode in
                                                    electric field;
                                                    soluble in
                                                    strong acids; B
                                                    volatized at
                                                    about 700° C., A
                                                    and C at about
                                                    1000° C.
    Radium B     5·38 × 10⁻⁴  21 mins.   no rays    Same
    (same)
    Radium C     4·13 × 10⁻⁴  28 mins.   α, β, γ    Same
    (same)
    Radium D     —            about 40   no rays    Soluble in
    (active                                         acids; volatile
    deposit of                                      below 1000° C.
    slow change)
    Radium E     1·3 × 10⁻⁶   6 days     β and γ    Non-volatile at
    (same)                                          1000° C.
    Radium F     5·6 × 10⁻⁸   143 days   α          Deposited on
    (same)                                          bismuth from
                                                    solution;
                                                    volatile at
                                                    about 1000° C.,
                                                    same properties
                                                    as
                                                    radio-tellurium
                                                    and polonium.

The products and their radiations are indicated graphically in Fig. 102
on page 450.

[Illustration: Fig. 102.]

One product has been observed in uranium, four in thorium, four in
actinium and seven in radium. It is not improbable that a closer
examination of the radio-elements may reveal still further changes. If
any very rapid transformations exist, they would be very difficult to
detect. The change of thorium X into the emanation, for example, would
probably not have been discovered if the product of the change had not
been gaseous in character. The electrolysis of solutions is, in many
cases, a very powerful method of separating active products from one
another, and its possibilities have not yet been exhausted. The main
family of changes of the radio-elements, as far as they are known, have
been investigated closely, and it is not likely that any product of
comparatively slow rate of change has been overlooked. There is a
possibility, however, that two radio-active products may in some cases
arise from the disintegration of a single substance. This point is
discussed further in section 260.

The remarkable way in which the disintegration theory can be applied to
unravel the intricacies of the succession of radio-active changes is
very well illustrated in the case of radium. Without its aid, it would
not have been possible to disentangle the complicated processes which
occur. We have already seen that this analysis has been instrumental in
showing that the substances polonium, radio-tellurium and radio-lead are
in reality products of radium.

After the radio-active substances have undergone the succession of
changes traced above, a final stage is reached where the atoms are
either permanently stable, or change so slowly that it is difficult to
detect their presence by means of their radio-activity. It is probable,
however, that the process of transformation still continues through
further slow stages.

There is now considerable evidence that the elements uranium, radium and
actinium are intimately connected together. The two latter probably
result from the breaking up of uranium. The evidence in support of this
idea is given in section 262, but there still remains much work to be
done to bridge over the gaps which at present appear to separate these
elements from one another.

After the series of transformations have come to an end, there will
probably remain a product or products which will be inactive, or active
only to a minute extent. In addition, since the α particles, expelled
during the transformation, are material in nature, and are
non-radio-active, they must collect in some quantity in radio-active
matter. The probability that the α particles consist of helium is
considered later in section 268.

The value of _T_, the time for a product to be half-transformed, may be
taken as a comparative measure of the stability of the different
metabolons. The stability of the products varies over a very wide range.
For example, the value of _T_ for radium D is 40 years, and for the
actinium emanation 3·9 secs. This corresponds to a range of stability
measured by 3·8 × 10⁸. The range of stability is still further extended,
when it is remembered that the atoms of the radio-elements themselves
are very slowly changing.

The only two metabolons of about the same stability are thorium X and
the radium emanation. In each case, the transformation is half completed
in about four days. I consider that the approximate agreement of the
numbers is a mere coincidence, and that the two types of matter are
quite distinct from one another; for, if the metabolons were identical,
it would be expected that the changes which follow would take place in
the same way and at the same rate, but such is not the case. Moreover,
Th X and the radium emanation have chemical and physical properties
quite distinct from one another.

It is very remarkable that the three radio-active substances, radium,
thorium and actinium, should exhibit such a close similarity in the
succession of changes which occur in them. Each of them at one stage of
its disintegration emits a radio-active gas, and in each case this gas
is transformed into a solid which is deposited upon the surface of
bodies. It would appear that, after disintegration of an atom of any of
these has once begun, there is a similar succession of changes, in which
the resulting systems have allied chemical and physical properties. Such
a connection is of interest as indicating a possible origin of the
recurrence of properties in the atoms of the elements, as exemplified by
the periodic law. The connection between thorium and actinium is
especially close both as regards the number and nature of the products.
The period of transformation of the successive products, though
differing in magnitude, rises and falls in a very analogous manner. This
indicates that the atoms of these two elements are very similarly
constituted.


=258. Amount of the products.= By application of the theory of
successive changes, the probable amount of each of the products present
in radium and the other radio-elements can readily be estimated.

Since each radio-atom expels one α particle of atomic weight about that
of hydrogen or helium, the atoms of the intermediate products will not
differ much in weight from the parent atom.

The approximate weight of each product present in a gram of radium can
be readily deduced. Let _N_{A}_, _N_{B}_, _N_{C}_ be the number of atoms
of the products A, B, C present per gram in radio-active equilibrium.
Let λ_{_A_}, λ_{_B_}, λ_{_C_} be the corresponding constants of change.
Then if _q_ is the number of the parent atoms breaking up per second,
per gram,

          _q_ = λ_{_A_}_N_{A}_ = λ_{_B_}_N_{B}_ = λ_{_C_}_N_{C}_.

Consider the case of the radium products, where the value of _q_ is 6·2
× 10¹⁰ (section 93). Knowing the value of λ and _q_, the value of _N_
can at once be calculated. The corresponding weight can be deduced,
since in one gram of matter of atomic weight about 200, there are about
4 × 10²¹ atoms (section 39). The results are shown in the following
table:—

          Product      Value of λ   Number of    Weight of
                       (sec)⁻¹      atoms, _N_,  product gram
                                    present per  of radium
                                    gram


          Radium       2·0 × 10⁻⁶   3·2 × 10¹⁶    8 × 10⁻³
          emanation

          Radium A     3·8 × 10⁻³   1·7 × 10¹³   4 × 10⁻⁶

          Radium B     5·4 × 10⁻⁴   1·3 × 10¹⁴    3 × 10⁻⁵

          Radium C     4·1 × 10⁻⁴   1·6 × 10¹⁴    4 × 10⁻⁵

With the small quantities of radium available, the amounts of the
products radium A, B and C are too small to weigh. It may be possible,
however, to detect their presence by means of the spectroscope.

In the case of thorium, the weight of the product Th X, which is present
in greatest quantity, is far too small to be detected. Since the value
of λ for Th X is about the same as for the radium emanation, the maximum
weight present per gram is about 4 × 10⁻¹² of a gram, remembering that
_q_ for radium is about 2 × 10⁶ times the value for thorium. Even with a
kilogram of thorium, the amount of Th X is far too small to be detected
by its weight.

This method can be used generally to calculate the relative amounts of
any successive products in radio-active equilibrium, provided the value
of λ for each product is known. For example, it will be shown later that
uranium is the parent of radium and is half transformed in about 6 × 10⁸
years, while radium and radium D are half transformed in 1300 and 40
years respectively. The weight of radium present in one gram of uranium,
when equilibrium is established, is thus 2 × 10⁻⁶ grams, and the weight
of radium D is 7 × 10⁻⁸ grams. In a mineral containing a ton of uranium
there should be about 1·8 grams of radium and ·063 grams of radium D.
Some recent experiments described in section 262 show that these
theoretical estimates are about twice too great.


=259. Rayless Changes.= The existence of well-marked changes in radium,
thorium, and actinium, which are not accompanied by the expulsion of α
or β particles, is of great interest and importance.

Since the rayless changes are not accompanied by any appreciable
ionization of the gas, their presence cannot be detected by direct
means. The rate of change of the substance can, however, be determined
indirectly, as we have seen, by measurement of the variation with time
of the activity of the succeeding product. The law of change has been
found to be the same as for the changes which give rise to α rays. The
rayless changes are thus analogous, in some respects, to the
monomolecular changes observed in chemistry, with the difference that
the changes are in the atom itself, and are not due to the decomposition
of a molecule into simpler molecules or into its constituent atoms.

It must be supposed that a rayless change is not of so violent a
character as one which gives rise to the expulsion of α or β particles.
The change may be accounted for either by supposing that there is a
rearrangement of the components of the atom, or that the atom breaks up
without the expulsion of its parts with sufficient velocity to produce
ionization by collision with the gas. The latter point of view, if
correct, at once indicates the possibility that undetected changes of a
similar character may be taking place slowly in the non-radio-active
elements; or, in other words, that all matter may be undergoing a slow
process of change. The changes taking place in the radio-elements have
been observed only in consequence of the expulsion with great velocity
of the parts of the disintegrated atom. Some recent experiments
described in Appendix A show that the α particle from radium ceases to
ionize the gas when its velocity falls below about 10⁹ cms. per second.
It is thus seen that α particles may be projected with a great velocity,
and yet fail to produce ionization in the gas. In such a case, it would
be difficult to follow the changes by the electrical method, as the
electrical effects would be very small in comparison with those produced
by the known radio-active bodies.


=260. Radiations from the products.= We have seen that the great
majority of the radio-active products break up with the expulsion of α
particles, and that the β particle with its accompaniment of the γ ray
appears in most cases only in the last rapid change. In the case of
radium, for example, which has been most closely investigated on account
of its great activity, radium itself, the emanation and radium A emit
only α particles; radium B emits no rays at all; while radium C emits
all three kinds of rays. It is difficult to settle with certainty
whether the products thorium X and actinium X emit β particles or not,
but the β and γ rays certainly appear in each case in the last rapid
change in the active deposit, and, in this respect, behave in a similar
manner to radium.

The very slow moving electrons which accompany the particles emitted
from radium (section 93) are not taken into account, for they appear to
be liberated as a result of the impact of α particles on matter, and are
expelled with a speed insignificant compared with that of the β
particles emitted from radium C.

The appearance of β and γ rays only in the last rapid changes of the
radio-elements is very remarkable, and cannot be regarded as a mere
coincidence. The final expulsion of a β particle results in the
appearance of a product of great stability, or, in the case of radium,
of a product (radium D) which has far more stability than the preceding
one. It would appear that the initial changes are accompanied by the
expulsion of an α particle, and that once the β particle is expelled,
the components of the residual atom fall into an arrangement of fairly
stable equilibrium, where the rate of transformation is very slow. It
thus appears probable that the β particle, which is finally expelled,
may be regarded as the active agent in promoting the disintegration of
the radio-atom through the successive stages. A discussion of this
question will be given with more advantage later (section 270), when the
general question of the stability of the atom is under consideration.

It is significant that the change in which the three types of rays
appear is far more violent in character than the preceding changes. Not
only are the α particles expelled with greater velocity than in any
other change, but the β particles are projected with a velocity very
closely approaching that of light.

There is always a possibility that, in such a violent explosion in the
atom, not only may the α and β particles be expelled, but the atom
itself may be disrupted into several fragments. If the greater
proportion of the matter resulting from the disintegration is of one
kind, it would be difficult to detect the presence of a small quantity
of rapidly changing matter from observations of the rate of decay; but,
if the products have distinctive electro-chemical behaviour, a partial
separation should, in some cases, be effected by electrolysis. It has
already been pointed out that the results of Pegram and von Lerch
(section 207) on the electrolysis of thorium solutions may be explained
on the supposition that thorium A and B have distinctive
electro-chemical behaviour. Pegram, however, in addition observed the
presence of a product which decayed to half value in six minutes. This
active product was obtained by electrolysing a solution of pure thorium
salt, to which a small quantity of copper nitrate had been added. The
copper deposit was slightly active and lost half of its activity in
about six minutes.

The presence of such radio-active products, which do not come under the
main scheme of changes, indicates that, at some stage of the
disintegration, more than one substance results. In the violent
disintegration which occurs in radium C and thorium B, such a result is
to be expected, for it is not improbable that there are several
arrangements whereby the constituents of the atom form a system of some
slight stability. The two products resulting from the disintegration
would probably be present in unequal proportion, and, unless they gave
out different kinds of rays, would be difficult to separate from each
other.


=261. Life of radium.= Since the atoms of the radio-elements are
continuously breaking up, they must also be considered to be metabolons,
the only difference between them and metabolons such as the emanations
Th X and others being their comparatively great stability and consequent
very slow rate of change. There is no evidence that the process of
change, traced above, is reversible under present conditions, and in the
course of time a quantity of radium, uranium, or thorium left to itself
must gradually be transformed into other types of matter.

There seems to be no escape from this conclusion. Let us consider, for
example, the case of radium. The radium is continuously producing from
itself the radium emanation, the rate of production being always
proportional to the amount of radium present. All the radium must
ultimately be changed into emanation, which in turn is transformed
through a succession of stages into other kinds of matter. There is no
doubt that the emanation is chemically quite different from radium
itself. The quantity of radium must diminish, to compensate for the
emanation which is formed; otherwise it is necessary to assume that
matter in the form of emanation is created from some unknown source.

An approximate estimate of the rate of change of radium can easily be
made by two different methods depending upon (1) the number of atoms of
radium breaking up per second, and (2) the amount of emanation produced
per second.

It has been shown experimentally (section 93) that 1 gram of radium at
its minimum activity expels 6·2 × 10¹⁰ α particles per second. The
heating effect of radium and also its volume agree closely with
calculation, if it is supposed that each atom of each product in
breaking up emits one α particle. On this supposition it is seen that
6·2 × 10¹⁰ atoms of radium break up per second.

Now it has been shown experimentally (section 39) that one cubic
centimetre of hydrogen at standard pressure and temperature contains 3·6
× 10¹⁹ molecules. Taking the atomic weight of radium as 225, the number
of atoms in 1 gram of radium is equal to 3·6 × 10²¹. The fraction λ of
radium which breaks up is thus 1·95 × 10⁻¹¹ per second, or 5·4 × 10⁻⁴
per year. It follows that in each gram of radium about half a milligram
breaks up per year. The average life of radium is about 1800 years, and
half of the radium is transformed in about 1300 years.

We shall now consider the calculation, based on the observed result of
Ramsay and Soddy, that the volume of emanation to be obtained from one
gram of radium is about 1 cubic millimetre. The experimental evidence
based on diffusion results indicates that the molecular weight of the
emanation is about 100. If the disintegration theory is correct, the
emanation is an atom of radium minus one particle, and therefore must
have a molecular weight of at least 200. This high value is more likely
to be correct than the experimental number, which is based on evidence
that must necessarily be somewhat uncertain. Now the rate of production
of emanation per second is equal to λ_N₀_, where _N₀_ is the equilibrium
amount. Taking the molecular weight as 200, the weight of emanation
produced per second from 1 gram of radium = 8·96 × 10⁻⁶ λ = 1·9 × 10⁻¹¹
gram.

Now the weight of emanation produced per second is very nearly equal to
the weight of radium breaking up per second. Thus the fraction of radium
breaking up per second is about 1·9 × 10⁻¹¹, which is in agreement with
the number previously calculated by the first method.

We may thus conclude that _radium is half transformed in about 1300
years_.

Taking the activity of pure radium as about two million times that of
uranium, and remembering that only one change, which gives rise to α
rays, occurs in uranium and four in radium, it can readily be calculated
that the fraction of uranium changing per year is about 10⁻⁹. From this
it follows that uranium should be half transformed in about 6 × 10⁸
years.

If thorium is a true radio-active element, the time for half
transformation is about 2·4 × 10⁹ years, since thorium has about the
same activity as uranium but contains four products which emit α rays.
If the activity of thorium is due to some radio-active impurity, no
estimate of the length of its life can be made until the primary active
substance has been isolated and its activity measured.


=262. Origin of radium.= The changes in radium are thus fairly rapid,
and a mass of radium if left to itself should in the course of a few
thousand years have lost a large proportion of its radio-activity.
Taking the above estimate of the life of radium, the value of λ is 5·4 ×
10⁻⁴, with a year as the unit of time. A mass of radium left to itself
should be half transformed in 1300 years and only one-millionth part
would remain after 26,000 years. Thus supposing, for illustration, that
the earth was originally composed of pure radium, its activity per gram
26,000 years later would not be greater than the activity observed
to-day in a good specimen of pitchblende. Even supposing this estimate
of the life of radium is too small, the time required for the radium
practically to disappear is short compared with the probable age of the
earth. We are thus forced to the conclusion that radium is being
continuously produced in the earth, unless the very improbable
assumption is made, that radium was in some way suddenly formed at a
date recent in comparison with the age of the earth. It was early
suggested by Rutherford and Soddy[351] that radium might be a
disintegration product of one of the radio-elements found in
pitchblende. Both uranium and thorium fulfil the conditions required in
a possible source of production of radium. Both are present in
pitchblende, have atomic weights greater than that of radium, and have
rates of change which are slow compared with that of radium. In some
respects, uranium fulfils the conditions required better than thorium;
for it has not been observed that minerals rich in thorium contain much
radium, while on the other hand, the pitchblendes containing the most
radium contain a large proportion of uranium.

If radium is not produced from uranium, it is certainly a remarkable
coincidence that the greatest activity of pitchblende yet observed is
about five or six times that of uranium. Since radium has a life short
compared with that of uranium, the amount of radium produced should
reach a maximum value after a few thousand years, when the rate of
production of fresh radium—which is also a measure of the rate of change
of uranium—balances the rate of change of that product. In this respect
the process would be exactly analogous to the production of the
emanation by radium, with the difference that the radium changes much
more slowly than the emanation. But since radium itself in its
disintegration gives rise to at least five changes with the
corresponding production of α rays, the activity due to the radium
(measured by the α rays), when in a state of radio-active equilibrium
with uranium, should be about five times that of the uranium that
produces it; for it has been shown that only one change has so far been
observed in uranium in which α rays are expelled. Taking into account
the presence of actinium in pitchblende, the activity observed in the
best pitchblende is about the same as would be expected if the radium
were a disintegration product of uranium. If this hypothesis is correct,
the amount of radium in any pitchblende should be proportional to the
amount of uranium present, provided the radium is not removed from the
mineral by percolating water.

This question has been experimentally attacked by Boltwood[352],
McCoy[353] and Strutt[354]. McCoy measured the relative activities of
different minerals in the form of powder by means of an electroscope,
and determined the amount of uranium present by chemical analysis. His
results indicated that the activity observed in the minerals was very
approximately proportional to their content of uranium. Since actinium
is present as well as uranium and its products, this would indicate that
the amount of radium and actinium taken together is proportional to the
amount of uranium. This problem has been attacked more directly by
Boltwood and Strutt by measuring the relative amount of the radium
emanation evolved by different minerals. By dissolving the mineral and
then setting it aside in a closed vessel, the amount of emanation
present reaches a maximum value after about a month’s interval. The
emanation is then introduced into a closed vessel containing a gold-leaf
electroscope similar to that shown in Fig. 12. The rate of movement of
the gold-leaf is proportional to the amount of emanation from the
solution, and this in turn is proportional to the amount of radium.
Boltwood has made in this way a very complete and accurate comparison of
the radium content of different varieties of pitchblende and other ores
containing radium. It was found that many of the minerals in the solid
state allowed a considerable fraction of the emanation to escape into
the air. The percentage fraction of the total amount of emanation lost
in this way is shown in Column II of the following table. Column I gives
the maximum amount of emanation present in 1 gram of the mineral in
arbitrary units when none of the emanation escapes; Column III the
weight in grams of uranium contained in 1 gram; and Column IV the ratio
obtained by dividing the quantity of emanation by the quantity of
uranium. The numbers in Column IV should be constant, if the amount of
radium is proportional to the amount of uranium.

                         Locality         I   II    III   IV
            Substance


            Uraninite    North        170·0 11·3 0·7465  228
                         Carolina

            Uraninite    Colorado     155·1  5·2 0·6961  223

            Gummite      North        147·0 13·7 0·6538  225
                         Carolina

            Uraninite    Joachimsthal 139·6  5·6 0·6174  226

            Uranophane   North        117·7  8·2 0·5168  228
                         Carolina

            Uraninite    Saxony       115·6  2·7 0·5064  228

            Uranophane   North        113·5 22·8 0·4984  228
                         Carolina

            Thorogummite North         72·9 16·2 0·3317  220
                         Carolina

            Carnotite    Colorado      49·7 16·3 0·2261  220

            Uranothorite Norway        25·2  1·3 0·1138  221

            Samarskite   North         23·4  0·7 0·1044  224
                         Carolina

            Orangite     Norway        23·1  1·1 0·1034  223

            Euxenite     Norway        19·9  0·5 0·0871  228

            Thorite      Norway        16·6  6·2 0·0754  220

            Fergusonite  Norway        12·0  0·5 0·0557  215

            Aeschynite   Norway        10·0  0·2 0·0452  221

            Xenotine     Norway        1·54 26·0 0·0070  220

            Monazite     North         0·88      0·0043  205
            (sand)       Carolina

            Monazite     Norway        0·84  1·2 0·0041  207
            (crys.)

            Monazite     Brazil        0·76      0·0031  245
            (sand)

            Monazite     Conn.         0·63      0·0030  210
            (massive)

With the exception of some of the monazites, the numbers show a
surprisingly good agreement, and, taking into consideration the great
variation of the content of uranium in the different minerals, and the
wide range of locality from which they were obtained, the results afford
a direct and satisfactory proof that the amount of radium in the
minerals is directly proportional to the amount of uranium.

In this connection, it is of interest to note that Boltwood found that a
considerable quantity of radium existed in various varieties of
monazite, although most of the previous analyses agreed in stating that
no uranium was present. A careful examination was in consequence made to
test this point, and it was found by special methods that uranium was
present, and in about the amount to be expected from the theory. The
ordinary methods of analysis failed to give correct results on account
of the presence of phosphates. Results of a similar character have
recently been given by Strutt[355].

The weight of radium in a mineral per gram of uranium is thus a definite
constant of considerable practical importance. Its value was recently
determined by Boltwood by comparison of the emanation, liberated from a
known weight of uraninite, with that liberated from a known quantity of
pure radium bromide, supplied for the purpose by the writer. A measured
weight of radium bromide was taken from a stock which gave out heat at a
rate of slightly over 100 gram calories per hour per gram, and was thus
probably pure. This was dissolved in water, and, by successive
dilutions, a standard solution was made up containing 10⁻⁷ gram of
radium bromide per c.c. Taking the constitution of radium bromide as
RaBr₂, it was deduced that the weight of radium per gram of uranium in
any mineral was 8·0 × 10⁻⁷ gram. The amount of radium in a mineral per
ton of uranium is thus 0·72 gram.

Strutt (_loc. cit._) obtained a value nearly twice as great, but he had
no means of ascertaining the purity of his radium bromide.

This amount of radium per gram of uranium is of the right order of
magnitude to be expected on the disintegration theory, if uranium is the
parent of radium. The activity of pure radium, compared with uranium, is
not known with sufficient accuracy to determine with accuracy the
theoretical proportion of radium to uranium.

The production of radium from uranium, while very strongly supported by
these experiments, cannot be considered definitely established until
direct experimental evidence is obtained of the growth of radium in
uranium. The rate of production of radium to be expected on the
disintegration theory can readily be estimated. The fraction of uranium
breaking up per year has been calculated (section 261) and shown to be
about 10⁻⁹ per year. This number represents the weight of radium
produced per year from 1 gram of uranium. The emanation, released from
the amount of radium produced in one year from 1 gram of uranium, would
cause an ordinary gold-leaf electroscope to be discharged in about
half-an-hour. If a kilogram of uranium is used, the amount of radium
produced in a single day should be easily detectable.

Experiments to detect the growth of radium in uranium have been made by
several observers. Soddy[356] examined the amount of emanation given off
at different times from one kilogram of uranium nitrate in solution,
which was originally freed from the small trace of radium present by a
suitable chemical process. The solution was kept stored in a closed
vessel, and the amount of emanation which collected in the solution was
measured at regular intervals.

Preliminary experiments showed that the actual rate of production of
radium was far less than the amount to be expected theoretically, and at
first very little indication was obtained that radium was produced at
all. After allowing the uranium to stand for eighteen months, Soddy
states that the amount of emanation was distinctly greater than at
first. The solution after this interval contained about 1·5 × 10⁻⁹ gram
of radium. This gives the value of about 2 × 10⁻¹² for the fraction of
uranium changing per year, while the theoretical value is about 10⁻⁹.

Whetham[357] also found that a quantity of uranium nitrate which had
been set aside for a year showed an appreciable increase in the content
of radium, and considers that the rate of production is faster than that
found by Soddy. In his case, the uranium was not originally completely
freed from radium.

Observations extending over years will be required before the question
can be considered settled, for the accurate estimation of small
quantities of radium by the amount of emanation is beset with
difficulties. This is especially the case where observations are made
over wide intervals of time.

The writer has made an examination to see if radium is produced from
actinium or thorium. It was thought possible that actinium might prove
to be an intermediate product between uranium and radium. The solutions,
freed from radium, have been set aside for a year, but no certain
increase in the content of radium has been observed.

There is little doubt that the production of radium by uranium first
proceeds at only a small fraction of the rate to be expected from
theory. This is not surprising when we consider that probably several
changes intervene between the product Ur X and the radium. In the case
of radium, for example, it has been shown that a number of slow changes
follow the rapid changes ordinarily observed. On account of the feeble
activity of uranium, it would not be easy to detect directly the
occurrence of such changes. If, for example, one or more rayless
products occurred between Ur X and radium, which were removed from the
uranium by the same chemical process used to free it from radium, the
rate of production of radium would be very small at first, but would be
expected to increase with time as more of the intermediary products were
stored up in the uranium. The fact that the contents of uranium and
radium in radio-active minerals are always proportional to each other,
coupled with definite experimental evidence that radium is produced from
uranium, affords an almost conclusive proof that uranium is in some way
the parent of radium.

The general evidence which has been advanced to show that radium must be
continuously produced from some other substance applies also to
actinium, which has an activity of the same order of magnitude as that
of radium. The presence of actinium with radium in pitchblende would
indicate that this substance also is in some way derived from uranium.
It is possible that actinium may prove to be produced either from radium
or to be the intermediary substance between uranium and radium. If it
could be shown that the amount of actinium in radio-active minerals is,
like radium, proportional to the amount of uranium, this would afford
indirect proof of such a connection. It is not so simple to settle this
point for actinium as for radium, since actinium gives out a very
short-lived emanation, and the methods adopted to determine the content
of radium in minerals cannot be applied without considerable
modifications to determine the amount of actinium present.

The experimental data, so far obtained, do not throw much light upon the
origin of the primary active matter in thorium. Hofmann and others
(section 23) have shown that thorium separated from minerals containing
uranium is always more active the greater the quantity of uranium
present. This would indicate that the active substance in thorium also
may be derived from uranium.

While much work still remains to be done, a promising beginning has
already been made in determining the origin and relation of the
radio-elements. We have seen that the connection between polonium,
radio-tellurium, and radio-lead with radium has already been
established. Radium itself is now added to the list, and it is probable
that actinium will soon follow.

While the experiments undoubtedly show that there is a definite relation
between the amount of uranium and radium present in the ordinary
radio-active minerals, Danne[358] has recently called attention to a
very interesting apparent exception. Considerable quantities of radium
were found in certain deposits in the neighbourhood of Issy-l’Evêque in
the Saône-Loire district, although no trace of uranium was present. The
active matter is found in pyromorphite (phosphate of lead), in clays
containing lead, and in pegmatite, but the radium is usually present in
greater quantities in the former. The pyromorphite is found in veins of
the quartz and feldspar rocks. The veins are always wet owing to the
presence of a number of springs in the neighbourhood. The content of
uranium in the pyromorphite varies considerably, but Danne considers
that about a centigram of radium is present per ton. It seems probable
that the radium found in this locality has been deposited from water
flowing through it, possibly in past times. The presence of radium is
not surprising, since crystals of autunite have been found about 40
miles distant, and probably there are deposits containing uranium in
that region. This result is of interest, as suggesting that radium may
be removed with water and deposited by physical or chemical action some
distance away.

It will be shown in the next chapter that radium has been found very
widely distributed over the surface of the earth, but generally in very
small quantities.


=263. Does the radio-activity of radium depend upon its concentration?=
We have seen that the radio-active constant λ of any product is
independent of the concentration of the product. This result has been
established over a very wide range for some substances, and especially
for the radium emanation. No certain difference in the rate of decay of
the emanation has been observed, although the amount present in unit
volume of the air has been varied a millionfold.

It has been suggested by J. J. Thomson[359] that the rate of
disintegration of radium may be influenced by its own radiations. This,
at first sight, appears very probable, for a small mass of a pure radium
compound is subjected to an intense bombardment by the radiations
arising from it, and the radiations are of such a character that they
might be expected to produce a breaking up of the atoms of matter which
they traverse. If this be the case, the radio-activity of a given
quantity of radium should be a function of its concentration, and should
be greater in the solid state than when disseminated through a large
mass of matter.

The writer has made an experiment to examine this question. Two glass
tubes were taken, in one of which was placed a few milligrams of pure
radium bromide in a state of radio-active equilibrium, and in the other
a solution of barium chloride. The two tubes were connected near the top
by a short cross tube, and the open ends sealed off. The activity of the
radium in the solid state was tested immediately after its introduction
by placing it in a definite position near an electroscope made of thin
metal of the type shown in Fig. 12. The increased rate of discharge of
the electroscope due to the β and γ rays from the radium was observed.
When a lead plate 6 mms. in thickness was placed between the radium and
the electroscope, the rate of discharge observed was due to the γ rays
alone. By slightly tilting the apparatus, the barium solution flowed
into the radium tube and dissolved the radium. The tube was well shaken,
so as to distribute the radium uniformly throughout the solution. No
appreciable change of the activity measured by the γ rays was observed
over the period of one month. The activity measured by the β and γ rays
was somewhat reduced, but this was not due to a decrease of the
radio-activity, but to an increased absorption of the β rays in their
passage through the solution. The volume of the solution was at least
1000 times greater than that of the solid radium bromide, and, in
consequence, the radium was subjected to the action of a much weaker
radiation. I think we may conclude from this experiment that the
radiations emitted by radium have little if any influence in causing the
disintegration of the radium atoms.

Voller[360] recently published some experiments which appeared to show
that the life of radium varied enormously with its concentration. In his
experiments, solutions of radium bromide of known strengths were
evaporated down in a platinum vessel 1·2 sq. cms. in area, and their
activity tested from time to time. The activity of the radium, so
deposited, at first showed the normal rise to be expected on account of
the production of the emanation, but after reaching a maximum, it
rapidly decayed. For a weight of 10⁻⁶ mgrs. of radium bromide, the
activity for example, practically disappeared in 26 days after reaching
its maximum. The time taken for the activity to disappear increased
rapidly with the amount of radium present. In another set of
experiments, he states that the activity observed on the vessel was not
proportional to the amount of radium present. For example, the activity
only increased 24 times for a millionfold increase of the radium
present, from 10⁻⁹ mgrs. to 10⁻³ mgrs.

These results, however, have not been confirmed by later experiments
made by Eve. He found that, over the range examined, the activity was
directly proportional to the amount of radium present, within the limits
of experimental error. The following table illustrates the results
obtained. The radium was evaporated down in platinum vessels 4·9 sq.
cms. in area.

                    Weight of       Activity in
                    radium in       arbitrary units
                    milligrams

                    10⁻⁴            1000

                    10⁻⁵             106

                    10⁻⁶              11·8

                    10⁻⁷               1·25

For an increase of one-thousandfold of the quantity of radium, the
activity increased 800 times, while Voller states that the activity, in
his experiments, only increased 3 to 4 times.

In the experiments of Eve, the activity was measured by observing the
increased rate of discharge of a gold-leaf electroscope when the
platinum vessel containing the active deposit was placed inside the
electroscope. The activity of 10⁻⁸ mgrs. was too small to be measured
with accuracy in the electroscope employed, while 10⁻³ mgrs. gave too
rapid a rate of discharge. On the other hand, the method of measurement
employed by Voller was unsuitable for the measurement of very weak
radio-activity.

Eve also found that a small quantity of radium _kept in a closed vessel_
did not lose its activity with time. A silvered glass vessel contained a
gold-leaf system, such as is shown in Fig. 12. A solution containing
10⁻⁶ mgrs. of radium bromide was evaporated over the bottom of the
vessel of area 76 sq. cms. The activity, after reaching a maximum, has
remained constant over the 100 days during which observations have so
far been made.

These experiments of Eve, as far as they go, show that the activity of
radium is proportional to the amount of radium present, and that radium,
kept in a closed vessel, shows no signs of decreasing in activity. On
the other hand, I think there is no doubt that a very small quantity of
radium deposited on a plate and _left in the open air_ does lose its
activity fairly rapidly. This loss of activity has nothing whatever to
do with the shortness of life of the radium itself, but is due to the
escape of the radium from the plate into the surrounding gas. Suppose,
for example, that a solution containing 10⁻⁹ mgrs. of radium bromide is
evaporated in a vessel of one sq. cm. in area. This amount of radium is
far too small to form even a layer of molecular thickness. It seems
likely that, during the process of evaporation, the radium would tend to
collect in small masses and be deposited on the surface of the vessel.
These would very readily be removed by slow currents of air and so
escape from the plate. The disappearance of such minute amounts of
radium is to be expected, and would probably occur with all kinds of
matter present in such minute amount. Such an effect has nothing to do
with an alteration of the life of radium and must not be confused with
it.

The result that the total radiation from a given quantity of radium
depends only on the quantity of radium and not on the degree of its
concentration is of great importance, for it allows us to determine with
accuracy the content of radium in minerals and in soils in which the
radium exists in a very diffused state.


=264. Constancy of the radiations.= The early observations on uranium
and thorium had shown that their radio-activity remained constant over
the period of several years during which they were examined. The
possibility of separating from uranium and thorium the active products
Ur X and Th X respectively, the activity of which decayed with the time,
seemed at first sight to contradict this. Further observation, however,
showed that the total radio-activity of these bodies was not altered by
the chemical processes, for it was found that the uranium and thorium
from which the active products were removed, spontaneously regained
their radio-activity. At any time after removal of the active product,
the sum total of the radio-activity of the separated product together
with that of the substance from which it has been separated is always
equal to that of the original compound before separation. In cases where
active products, like Ur X and the radium emanation, decay with time
according to an exponential law, this follows at once from the
experimental results. If _I₁_ is the activity of the product at any time
t after separation, and _I₀_ the initial value, we know that

$$ \frac {I_1} {I₀} = e^{–λt} $$ .

At the same time the activity _I₂_ recovered during the same interval
_t_ is given by

$$ \frac {I_2} {I₀} = 1 − e^{–λt} $$,

where λ is the same constant as before. It thus follows that _I₁_ + _I₂_
= _I₀_, which is an expression of the above result. The same is also
true whatever the law of decay of activity of the separated product (see
section 200). For example, the activity of Th X after separation from
thorium at first increases with the time. At the same time, the activity
of the residual thorium compound at first decreases, and at such a rate
that the sum of the activities of the thorium and its separated product
is always equal to that of the original thorium.

This apparent constancy of the total radiation follows from the general
result that the radio-active processes cannot in any way be changed by
the action of known forces. It may be recalled that the constant of
decay of the activity of a radio-active product has a definite fixed
value under all conditions. It is independent of the concentration of
the active matter, of the pressure, and of the nature of the gas in
which the substance is placed, and it is not affected by wide ranges of
temperature. The only observed exception is the product radium C. Its
value of λ increases with temperature to some extent at about 1000° C.,
but at 1200° C. returns nearly to the normal value. In the same way, it
has not been found possible to alter the rate of production of active
matter from the radio-elements. In addition, there is not a single
well-authenticated case where radio-activity has been altered or
destroyed in any active body or created in an inactive element.

Certain cases have been observed, which at first sight seem to indicate
a destruction of radio-activity. For example, the excited radio-activity
is removed from a platinum wire when heated above a red heat. It has
been shown, however, by Miss Gates (section 187) that the radio-activity
is not destroyed, but is deposited in unaltered amount on the colder
bodies surrounding it. Thorium oxide has been shown to lose to a large
extent its power of emanating by ignition to a white heat. But a close
examination shows that the emanation is still being produced at the same
rate, but is occluded in the compound.

The total radio-activity of a given mass of a radio-element, measured by
the peculiar radiations emitted, is a quantity which can neither be
increased nor diminished, although it may be manifested in a series of
products which are capable of separation from the radio-element. The
term “conservation of radio-activity” is thus a convenient expression of
the facts known at the present time. It is quite possible, however, that
further experiments at very high or very low temperatures may show that
the radio-activity does vary.

Although no difference has been observed in the radio-activity of
uranium over an interval of five years, it has been shown (section
261) that on theoretical grounds the radio-activity of a _given
quantity_ of a radio-element should decrease with the time. The change
will, however, be so slow in uranium, that probably millions of
years must elapse before a measurable change can take place, while the
total radio-activity of a given quantity of matter left to itself should
thus decrease, but it ought to be constant for a _constant mass_ of the
radio-element. It has already been pointed out (section 238) that the
activity of radium, measured by the α and β rays, will probably increase
for several hundred years after its separation. This is due to the
appearance of fresh products in the radium. Ultimately, however, the
activity must decrease according to an exponential law with the time,
falling to half value in about 1300 years.

The conservation of radio-activity applies not only to the radiations
taken as a whole, but also to each specific type of radiation. If the
emanation is removed from a radium compound, the amount of β radiation
of the radium at once commences to decrease, but this is compensated by
the appearance of β rays in the radiations from the vessel in which the
separated emanation is stored. At any time the sum total of the β
radiations from the radium and the emanation vessel is always the same
as that from the radium compound before the emanation was removed.

Similar results have also been found to hold for the γ rays. This was
tested by the writer in the following way. The emanation from some solid
radium bromide was released by heat, and condensed in a small glass tube
which was then sealed off. The radium so treated, and the emanation
tube, were placed together under an electroscope, with a screen of lead
1 cm. thick interposed in order to let through only the γ rays. The
experiments were continued over three weeks, but the sum total of the γ
rays from the radium and the emanation tube was, over the whole
interval, equal to that of the original radium. During this period the
amount of γ rays from the radium at first decreased to only a few per
cent. of the original value, and then slowly increased again, until at
the end of the three weeks it had nearly regained its original value,
before the emanation was removed. At the same time the amount of γ rays
from the emanation tube rose from zero to a maximum and then slowly
decreased again at the same rate as the decay of the activity of the
emanation in the tube. This result shows that the amount of γ rays from
radium was a constant quantity over the interval of observation,
although the amount of γ rays from the radium and emanation tube had
passed through a cycle of changes.

There is one interesting possibility in this connection that should be
borne in mind. The rays from the active substances carry off energy in a
very concentrated form, and this energy is dissipated by the absorption
of the rays in matter. The rays might be expected to cause a
disintegration of the atoms of inactive matter on which they fall and
thus give rise to a kind of radio-activity. This effect has been looked
for by several observers. Ramsay and W. T. Cooke[361] state that they
have noticed such an action, using about a decigram of radium as a
source of radiation. The radium, sealed in a glass vessel, was
surrounded by an external glass tube and exposed to the action of the β
and γ rays of radium for several weeks. The inside and outside of the
glass tube were found to be active, and the active matter was removed by
solution in water. The radio-activity observed was very minute,
corresponding to only about 1 milligram of uranium. The writer has, at
various times, tried experiments of this character but with negative
results. The greatest care is necessary in such experiments to ensure
that the radio-activity is not due to other causes besides the rays from
the radium. This care is especially necessary in laboratories where
considerable quantities of the radium emanation have been allowed to
escape into the air. The surface of every substance becomes coated with
the slow transformation products of radium, viz. radium D, E, and F. The
activity communicated in this way to originally inactive matter is often
considerable. This infection by the radium emanation extends throughout
the whole laboratory, on account of the distribution of the emanation by
convection and diffusion. For example, Eve[362] found that every
substance which he examined in the laboratory of the writer showed much
greater activity than the normal. In this case the radium had been in
use in the building for about two years.


=265. Loss of weight of the radio-elements.= Since the radio-elements
are continually throwing off α particles atomic in size, an active
substance, enclosed in a vessel sufficiently thin to allow the α
particles to escape, must gradually lose in weight. This loss of weight
will be small under ordinary conditions, since the greater proportion of
the α rays produced are absorbed in the mass of the substance. If a very
thin layer of a radium compound were spread on a very thin sheet of
substance, which did not appreciably absorb the α particles, a loss of
weight due to the expulsion of α particles might be detectable. Since
_e_/_m_ = 6 × 10³ for the α particle and _e_ = 1·1 × 10⁻²⁰
electromagnetic units and 2·5 × 10¹¹ α particles are expelled per
second per gram of radium, the proportion of the mass expelled is 4·8 ×
10⁻¹³ per second and 10⁻⁵ per year. There is one condition, however,
under which the radium should lose in weight fairly rapidly. If a
current of air is slowly passed over a radium solution, the emanation
produced would be removed as fast as it was formed. Since the atom of
the emanation has a mass probably not much smaller than the radium atom,
the fraction of the mass removed per year should be nearly equal to the
fraction of the radium which changes per year, _i.e._ one gram of radium
should diminish in weight about half a milligram (section 261) per year.

If it is supposed that the β particles have weight, the loss of weight
due to their expulsion is very small compared with that due to the
emission of α particles. The writer has shown (section 253) that about
7 × 10¹⁰ β particles are projected per second from 1 gram of radium.
The consequent loss of weight would only be about 10⁻⁹ grams per year.

Except under very special experimental conditions, it would thus be
difficult to detect the loss of weight of radium due to the expulsion of
β particles from its mass. There is, however, a possibility that radium
might change in weight even though none of the radio-active products
were allowed to escape. For example, if the view is taken that
gravitation is the result of forces having their origin in the atom, it
is possible that, if the atom were disintegrated, the weight of the
parts might not be equal to that of the original atom.

A large number of experiments have been made to see if radium
preparations, kept in a sealed tube, alter in weight. With the small
quantities of radium available to the experimenter, no difference of
weight of radium preparations with time has yet been established with
certainty. Heydweiller stated that he had observed a loss of weight of
radium and Dorn also obtained a slight indication of change in weight.
These results have not, however, been confirmed. Forch, later, was
unable to observe any appreciable change.

J. J. Thomson[363] has made experiments to see if the ratio of weight to
mass for radium is the same as for inactive matter. We have seen in
section 48 that a charge in motion possesses an apparent mass which is
constant for slow speeds but increases as the speed of light is
approached. Now radium emits some electrons at a velocity comparable
with the velocity of light, and presumably these electrons were in rapid
motion in the atom before their expulsion. It might thus be possible
that the ratio for radium would differ from that for ordinary matter.
The pendulum method was used, and the radium was enclosed in a small
light tube suspended by a silk fibre. Within the limit of experimental
error the ratio of weight to mass was found to be the same as for
ordinary matter, so that we may conclude that the number of electrons
moving with a velocity approaching that of light is small compared with
the total number present.


=266. Total emission of energy from the radio-element.= It has been
shown that 1 gram of radium emits energy at the rate of 100
gram-calories per hour or 876,000 gram-calories per year. If 1 gram of
radium in radio-active equilibrium be set apart, its radio-activity and
consequent heat emission is given at a time _t_ by

$$ qe^{–λt} $$,

where λ is the constant of decay of activity of radium and of the
initial heating effect; the total heat emission from 1 gram of radium is
given by

$$ \int₀^{\infty} qe^{–λt} dt = \frac {q} {λ} $$ .

Now on the estimate of the life of radium given in section 261 the value
of λ is ¹⁄₁₈₅₀ when 1 year is taken as the unit of time. The total heat
emission from 1 gram of radium during its life is thus 1·6 × 10⁹
gram-calories. The heat emitted in the union of hydrogen and oxygen to
form 1 gram of water is about 4 × 10³ gram-calories, and in this
reaction more heat is given out for equal weights than in any other
chemical reaction known. It is thus seen that the total energy emitted
from 1 gram of radium during its changes is about one million times
greater than in any known molecular change. That matter is able, under
special conditions, to emit an enormous amount of energy, is well
exemplified by the case of the radium emanation. Calculations of the
amount of this energy have already been given in section 249.

Since the other radio-elements only differ from radium in the slowness
of their change, the total heat emission from uranium and thorium must
be of a similar high order of magnitude. There is thus reason to believe
that there is an enormous store of latent energy resident in the atoms
of the radio-elements. This store of energy could not have been
recognized if the atoms had not been undergoing a slow process of
disintegration. The energy emitted in radio-active changes is derived
from the internal energy of the atoms. The emission of this energy does
not disobey the law of the conservation of energy, for it is only
necessary to suppose that, when the radio-active changes have ceased,
the energy stored up in the atoms of the final products is less than
that of the original atoms of the radio-elements. The difference between
the energy originally possessed by the matter which has undergone the
change, and the final inactive products which arise, is a measure of the
total amount of energy released.

There seems to be every reason to suppose that the atomic energy of all
the elements is of a similar high order of magnitude. With the exception
of their high atomic weights, the radio-elements do not possess any
special chemical characteristics which differentiate them from the
inactive elements. The existence of a latent store of energy in the
atoms is a necessary consequence of the modern view developed by J. J.
Thomson, Larmor, and Lorentz, of regarding the atom as a complicated
structure consisting of charged parts in rapid oscillatory or orbital
motion in regard to one another. The energy may be partly kinetic and
partly potential, but the mere concentration of the charged particles,
which probably constitute the atom, in itself implies a large store of
energy in the atom, in comparison with which the energy emitted during
the changes of radium is insignificant.

The existence of this store of latent energy does not ordinarily
manifest itself, since the atoms cannot be broken up into simpler forms
by the physical or chemical agencies at our disposal. Its existence at
once explains the failure of chemistry to transform the atoms, and also
accounts for the rate of change of the radio-active processes being
independent of all external agencies. It has not so far been found
possible to alter in any way the rate of emission of energy from the
radio-elements. If it should ever be found possible to control at will
the rate of disintegration of the radio-elements, an enormous amount of
energy could be obtained from a small quantity of matter.


=267. Production of helium from radium and the radium emanation.= Since
the final products, resulting from a disintegration of the
radio-elements, are not radio-active, they should in the course of
geologic ages collect in some quantity, and should always be found
associated with the radio-elements. Now the inactive products resulting
from the radio-active changes are the α particles expelled at each
stage, and the final inactive product or products which remain, when the
process of disintegration can no longer be traced by the property of
radio-activity.

Pitchblende, in which the radio-elements are mostly found, contains in
small quantity a large proportion of all the known elements. In
searching for a possible disintegration product common to all the
radio-elements, the presence of helium in the radio-active minerals is
noteworthy; for helium is only found in the radio-active minerals, and
is an invariable companion of the radio-elements. Moreover, the presence
in minerals of a light, inert gas like helium had always been a matter
of surprise. The production by radium and thorium of the radio-active
emanations, which behave like chemically inert gases of the helium-argon
family, suggested the possibility that one of the final inactive
products of the disintegration of the radio-elements might prove to be a
chemically inert gas. The later discovery of the material nature of the
α rays added weight to the suggestion; for the measurement of the ratio
_e_/_m_ of the α particle indicated that if the α particle consisted of
any known kind of matter, it must either be hydrogen or helium. For
these reasons, it was suggested in 1902 by Rutherford and Soddy[364]
that helium might be a product of the disintegration of the
radio-elements.

Sir William Ramsay and Mr Soddy in 1903 undertook an investigation of
the radium emanation, with the purpose of seeing if it were possible to
obtain any spectroscopic evidence of the presence of a new substance.
First of all, they exposed the emanation to very drastic treatment
(section 158), and confirmed and extended the results previously noted
by Rutherford and Soddy that the emanation behaved like a chemically
inert gas, and in this respect possessed properties analogous to the
gases of the helium-argon group.

On obtaining 30 milligrams of pure radium bromide (prepared about three
months previously) Ramsay and Soddy[365] examined the gases, liberated
by solution of the radium bromide in water, for the presence of helium.
A considerable quantity of hydrogen and oxygen was released by the
solution (see section 124). The hydrogen and oxygen were removed by
passing the liberated gases over a red-hot spiral of partially
oxidized copper-wire and the resulting water vapour was absorbed in
a phosphorus pentoxide tube.

The gas was then passed into a small vacuum tube which was in connection
with a small =U= tube. By placing the =U= tube in liquid air, most of
the emanation present was condensed, and also most of the CO₂ present in
the gas. On examining the spectrum of the gas in the vacuum tube, the
characteristic line _D₃_ of helium was observed.

This experiment was repeated with 30 milligrams of radium bromide about
four months old, lent for the purpose by the writer. The emanation and
CO₂ were removed by passing them through a =U= tube immersed in liquid
air. A practically complete spectrum of helium was observed, including
the lines of wave-lengths 6677, 5876, 5016, 4972, 4713 and 4472. There
were also present three other lines of wave-lengths about 6180, 5695,
5455 which have not yet been identified.

In later experiments, the emanation from 50 milligrams of the radium
bromide was conveyed with oxygen into a small =U= tube, cooled in liquid
air, in which the emanation was condensed. Fresh oxygen was added, and
the =U= tube again pumped out. The small vacuum tube, connected with the
=U= tube, showed at first no helium lines when the liquid air was
removed. The spectrum obtained was a new one, and Ramsay and Soddy
considered it to be probably that of the emanation itself. After
allowing the emanation tube to stand for four days, the helium spectrum
appeared with all the characteristic lines, and in addition, three new
lines present in the helium obtained by solution of the radium. These
results have since been confirmed. The experiments, which have led to
such striking and important results, were by no means easy of
performance, for the quantity of helium and of emanation released from
50 mgrs. of radium bromide is extremely small. It was necessary, in all
cases, to remove almost completely the other gases, which were present
in sufficient quantity to mask the spectrum of the substance under
examination. The success of the experiments has been largely due to the
application, to this investigation, of the refined methods of gas
analysis, previously employed by Sir William Ramsay with so much skill
in the separation of the rare gases xenon and krypton, which exist in
minute proportions in the atmosphere. The fact that the helium spectrum
was not present at first, but appeared _after_ the emanation had
remained in the tube for some days, shows that the helium must have been
produced from the emanation. The emanation cannot be helium itself, for,
in the first place, helium is not radio-active, and in the second place,
the helium spectrum was not present at first, when the quantity of
emanation in the tube was at its maximum. Moreover, the diffusion
experiments, already discussed, point to the conclusion that the
emanation is of high molecular weight. There can thus be no doubt that
the helium is derived from the emanation of radium in consequence of
changes of some kind occurring in it.

These results were confirmed later by other observers. Curie and
Dewar[366] performed the following experiment: A weight of about ·42 gr.
of radium bromide was placed in a quartz tube, and the tube exhausted
until no further gas came off. The radium was then heated to fusion,
about 2·6 c.c. of gas being liberated in the process. The tube was then
sealed, and some weeks afterwards the spectrum of the gas liberated in
the tube by the radium was examined by Deslandres and found to give the
entire spectrum of helium. The gas, liberated during the initial heating
of the radium, was collected and found to contain a large amount of
emanation, although the gas had been passed through two tubes immersed
in liquid air. The tube containing these gases was very luminous and
rapidly turned violet, while more than half of the gases was absorbed.
The spectrum of the phosphorescent light was found to be discontinuous,
consisting of three nitrogen bands. No sign of the helium spectrum was
observed, although helium must have been present.

Himstedt and Meyer[367] placed 50 mgrs. of radium bromide in a =U= tube
connected with a small vacuum tube. The tube was carefully exhausted and
then sealed off. The spectrum of hydrogen and carbon dioxide alone was
observed for three months, but after four months the red, yellow, green
and blue lines of the helium spectrum were visible. The slow appearance
of the helium spectrum was probably due to the presence in the tube of a
considerable quantity of hydrogen. In another experiment, some radium
sulphate which had been heated to a bright red heat in a quartz tube was
connected with a small vacuum tube. After three weeks, some of the lines
of helium were clearly seen, and increased in brightness with time.


=268. Connection between helium and the α particles=. The appearance of
helium in a tube containing the radium emanation may indicate either
that the helium is one of the final products, which appear at the end of
the series of radio-active changes, or that the helium is in reality the
expelled α particle. The evidence at present points to the latter as
being the more probable explanation. In the first place, the emanation
diffuses like a gas of heavy molecular weight, and it appears probable
that after the expulsion of a few α particles, the atomic weight of the
final product is comparable with that of the emanation. On the other
hand, the value of _e_/_m_ determined for the projected α particle
points to the conclusion that, if it consists of any known kind of
matter, it is either hydrogen or helium.

There has been a tendency to assume that the helium produced from the
radium emanation is the last transformation product of that substance.
The evidence, however, does not support this view. We have seen that the
emanation, after the initial rapid changes, is transformed very slowly.
If the helium were the final product, the amount present in the
emanation tube after a few days or weeks would be insignificant, since
the product radium D intervenes, which takes 40 years to be half
transformed. Since the helium cannot be the final product of the series
of changes, and since all the other products are radio-active, and
almost certainly of high atomic weight, it is difficult to see what
position the helium atom occupies in the scheme of transformation,
unless it be the α particle expelled during the successive changes.

It is a matter of great difficulty to settle definitely whether the α
particle is a projected helium atom or not. On account of the very small
deflection of the α rays in an electric field, and the complex nature of
the α radiation from radium, an accurate determination of the value
_e_/_m_ for the α particle is beset with difficulties.

It may be possible to settle the question by accurate measurements of
the volume of gas in a tube, filled originally with the radium
emanation. Since the emanation itself, and two of the rapidly changing
products which result from it, emit α particles, the final volume of the
α particles, if they can exist in the gaseous state, would be three
times the volume of the emanation. Ramsay and Soddy (section 172) have
made experiments of this kind, but the results obtained were very
contradictory, depending upon the kind of glass employed. In one case,
the volume of the residual gases shrank almost to zero, in another the
initial volume increased to about ten times its initial value. In the
latter experiment a brilliant spectrum of helium was observed in the
residual gas. This difference of behaviour is probably due to different
degrees of absorption of helium by the glass tubes.

If the α particles are helium atoms, we may expect that a large
proportion of the helium, which is produced in a tube containing the
radium emanation, will be buried in the wall of the glass tube; for the
α particles are projected with sufficient velocity to penetrate some
distance into the glass. This helium may either remain in the glass, or
in some cases rapidly diffuse out again. In any case, a fraction of the
helium would be liberated when an intense electric discharge is passed
through the tube. Ramsay and Soddy have in some instances observed that
a slight amount of helium is liberated on heating the walls of the tube
in which the emanation had been stored for some time.

The volume of helium produced per year by 1 gram of radium can easily be
calculated on the assumption that the α particle is in reality a helium
atom.

It has been shown that 2·5 × 10¹¹ α particles are projected per second
from 1 gram of radium. Since there are 3·6 × 10¹⁹ molecules in one cubic
centimetre of any gas at standard pressure and temperature, the volume
of the α particles released per second is 7 × 10⁻⁹ c.c. and per year
0·24 c.c. It has already been pointed out that, on this hypothesis, the
volume of helium released by the emanation is three times the volume of
the latter. The amount of helium to be obtained from the emanation
released from 1 gram of radium in radio-active equilibrium is thus about
3 cubic mms.

Ramsay and Soddy have tried to estimate experimentally the probable
volume of helium produced per second by one gram of radium. The helium,
obtained from 50 mgrs. of radium bromide, which had been kept in
solution in a closed vessel for 60 days, was introduced into a vacuum
tube. Another similar tube was placed in series with it, and the amount
of the helium in the latter adjusted until on passing a discharge
through the two tubes in series the helium lines in each tube were of
about the same brightness. In this way they calculated that the amount
of helium present was 0·1 cubic mm. On this estimate, the amount of
helium produced per year per gram of radium is about 20 cubic mms. We
have seen that the calculated amount is about 240 cubic mms., on the
assumption that the α particle is a helium atom. Ramsay and Soddy
consider that the presence of argon in one of the tubes may have
seriously interfered with the correctness of the estimation. On account
of the great uncertainty attaching to estimates of the above character,
the value deduced by Ramsay and Soddy does not exclude the probability
that the calculated volume may be of the right order of magnitude.

In order to explain the presence of helium in radium on ordinary
chemical lines, it has been suggested that radium is not a true element,
but a molecular compound of helium with some substance known or unknown.
The helium compound gradually breaks down, giving rise to the helium
observed. It is at once obvious that this postulated helium compound is
of a character entirely different from that of any other compound
previously observed in chemistry. Weight for weight, it emits during its
change an amount of energy at least one million times greater than any
molecular compound known (see section 249). In addition, it must be
supposed that the rate of breaking up of the helium compound is
independent of great ranges of temperature—a result never before
observed in any molecular change. The helium compound in its breaking up
must give rise to the peculiar radiations and also pass through the
successive radio-active changes observed in radium.

Thus in order to explain the production of helium and radio-activity on
this view, a unique kind of molecule must be postulated—a molecule, in
fact, which is endowed with every single property which on the
disintegration theory is ascribed to the atom of the radio-elements. On
the other hand, radium as far as it has been examined, has fulfilled
every test required for an element. It has a well-marked and
characteristic spectrum, and there is no reason to suppose that it is
not an element in the ordinarily accepted sense of the term.

On the theory that the radio-elements are undergoing atomic
disintegration, the helium must be considered to be a constituent of the
radium atom, or, in other words, the radium atom is built up of parts,
one of which, at least, is the atom of helium. The theory that the heavy
atoms are all built up of some simple fundamental unit of matter or
protyle has been advanced at various times by many prominent chemists
and physicists. Prout’s hypothesis that all elements are built up out of
hydrogen is an example of this point of view of regarding the subject.

On the disintegration theory, the changes occurring in the radio-atoms
involve an actual transformation of the atoms through successive
changes. This change is so slow in uranium and thorium that at least a
million years would be required before the amount of change could be
measured by the balance. In radium it is a million times faster, but
even in this case it is doubtful whether any appreciable change would
have been observed by ordinary chemical methods for many years had not
the possibility of such a change been suggested from other lines of
evidence.

The similarity of the α particles from the different radio-elements
indicates that they consist of expelled particles of the same kind. On
this view, helium should be produced by each of the radio-elements. Its
presence in minerals containing thorium, for example in monazite sand
and the Ceylon mineral described by Ramsay, indicates that helium may be
a product of thorium as well as of radium. Strutt[368] has recently
suggested that most of the helium observed in radio-active minerals may
be a decomposition product of thorium rather than of uranium and radium;
for he finds that minerals rich in helium always contain thorium, while
many uranium minerals nearly free from thorium contain little helium.
The evidence in support of this view is, however, not altogether
satisfactory, for some of the uranium minerals in question are secondary
uranium minerals (see Appendix B), deposited by the action of water or
other agencies at a comparatively late date, and are also, in many
cases, highly emanating, and consequently could not be expected to
retain more than a fraction of the helium produced in them.

Taking the view that the α particles are projected helium atoms, we must
regard the atoms of the radio-elements as compounds of some known or
unknown substance with helium. These compounds break up spontaneously,
and at a very slow rate even in the case of radium. The disintegration
takes place in successive stages, and at most of the stages a helium
atom is projected with great velocity. This disintegration is
accompanied by an enormous emission of energy. The liberation of such a
large amount of energy in the radio-active changes at once explains the
constancy of the rate of change under the action of any of the physical
and chemical agencies at our command. On this view, uranium, thorium and
radium are in reality compounds of helium. The helium, however, is held
in such strong combination that the compound cannot be broken up by
chemical or physical forces, and, in consequence, these bodies behave as
chemical elements in the ordinary accepted chemical sense.

It appears not unlikely that many of the so-called chemical elements may
prove to be compounds of helium, or, in other words, that the helium
atom is one of the secondary units with which the heavier atoms are
built up. In this connection it is of interest to note that many of the
elements differ in their atomic weight by four—the atomic weight of
helium.

If the α particle is a helium atom, at least three α particles must be
expelled from uranium (238·5) to reduce its atomic weight to that of
radium (225). It is known that five α particles are expelled from radium
during its successive transformations. This would make the atomic weight
of the final residue 225 − 20 = 205. This is very nearly the atomic
weight of lead, 206·5. I have, for some time, considered it probable
that lead is the end or final product of radium. The same suggestion has
recently been made by Boltwood[369]. This point of view is supported by
the fact that lead is always found in small quantity in all uranium
minerals, and that the relative proportions of lead and helium in the
radio-active minerals are about the same as would be expected if lead
and helium were both decomposition products of radium. Dr Boltwood has
drawn my attention to the fact that the proportion of lead in many
radio-active minerals varies with the content of helium. A mineral rich
in helium in nearly all cases contains more lead than a mineral poor in
helium. This cannot be considered, at present, more than a speculation,
but the facts as they stand are very suggestive.


=269. Age of radio-active minerals.= Helium is only found in the
radio-active minerals, and this fact, taken in conjunction with the
liberation of helium by radium, indicates that the helium must have been
produced as a result of the transformation of radium and the other
radio-active substances contained in the minerals. Now in a mineral
about half the helium is, in many cases, released by heat and the
residue by solution. It seems probable that the helium produced
throughout the mass of the mineral is mechanically imprisoned in it.
Moss[370] found that, by grinding pitchblende in vacuo, helium is
evolved, apparently showing that the helium exists in cavities of the
mineral. Travers[371] has suggested that, since helium is liberated on
heating, the effect may be due to the heat generated by grinding. The
escape of the helium from the heated mineral is probably connected with
the fact observed by Jaquerod[372] that helium passes through the walls
of a quartz tube, heated above 500° C. The substance of the mineral
probably possesses a similar property. Travers considers that helium is
present in the mineral in a state of supersaturated solid solution, but
the facts are equally well explained by assuming that the helium is
mechanically imprisoned in the mass of the mineral.

The sudden rise of temperature observed in the mineral fergusonite, at
the time the helium is released, has been found to have nothing to do
with the presence of helium, for it also takes place in minerals not
containing helium. The old view that helium was in a state of chemical
combination with the mineral must be abandoned in the light of these
more recent experiments.

Since the helium is only released from some minerals by the action of
high temperatures and solution, it appears probable that a large
proportion of the helium found in the minerals is unable to escape under
normal conditions. Thus if the rate of production of helium by the
radio-active substance were definitely known, it should be possible to
calculate the age of the mineral by observing the volume of helium
liberated from it by solution.

In the absence of such definite information, an approximate calculation
will be made to indicate the order of magnitude of the time that has
elapsed since the mineral was formed or was at a temperature low enough
to prevent the escape of the helium.

Let us take, for example, the mineral fergusonite, which was found by
Ramsay and Travers[373] to evolve 1·81 c.c. of helium. The fergusonite
contained about 7 per cent. of uranium. Now uranium in old minerals
probably contains about 8 × 10⁻⁷ of its weight of radium (see section
262). One gram of the mineral thus contained about 5·6 × 10⁻⁸ grams of
radium. Now if the α particle is helium, it has been shown that 1 gram
of radium produces 0·24 c.c. of helium per year. The volume of helium
produced per year in 1 gram of fergusonite is thus 1·3 × 10⁻⁸ c.c.
Assuming that the rate of production of helium has been uniform, the
time required to produce 1·81 c.c. per gram is about 140 million years.
If the calculated rate of production of helium by radium is an
over-estimate, the time is correspondingly lengthened.

I think that, when the constants required for these calculations are
more definitely fixed, this method will probably give fairly trustworthy
information as to the probable age of some of the radio-active minerals
of the earth’s crust, and indirectly as to the age of the strata in
which they are found.

In this connection it is of interest to note that Ramsay[374] found that
a Ceylon mineral, thorianite, contained as much as 9·5 c.c. of helium
per gram. According to the analysis by Dunstan, this mineral contains
about 76 per cent. of thorium and 12 per cent. of uranium. The unusually
large amount of helium evolved from this mineral would indicate that it
was formed at an earlier date than the fergusonite previously
considered.


=270. Possible causes of disintegration.= In order to explain the
phenomena of radio-activity, it has been supposed that a certain small
fraction of the radio-atoms undergoes disintegration per second, but no
assumptions have been made as to the cause which produces the
instability and consequent disintegration. The instability of the atoms
may be supposed to be brought about either by the action of external
forces or by that of forces inherent in the atoms themselves. It is
conceivable, for example, that the application of some slight external
force might cause instability and consequent disintegration, accompanied
by the liberation of a large amount of energy, on the same principle
that a detonator is necessary to start some explosives. It has been
shown that the number of atoms of any radio-active product which break
up per second is always proportional to the number present. This law of
change does not throw any light on the question, for it would be
expected equally on either hypothesis. It has not been found possible to
alter the rate of change of any product by the application of any known
physical or chemical forces, unless possibly it is assumed that the
force of gravitation which is not under our control may influence in
some way the stability of the radio-atoms.

It seems likely therefore that the cause of the disruption of the atoms
of the radio-elements and their products resides in the atoms
themselves. According to the modern views of the constitution of the
atom, it is not so much a matter of surprise that some atoms
disintegrate as that the atoms of the elements are so permanent as they
appear to be. In accordance with the hypothesis of J. J. Thomson, it may
be supposed that the atoms consist of a number of small positively and
negatively charged particles in rapid internal movement, and held in
equilibrium by their mutual forces. In a complex atom, where the
possible variations in the relative motion of the parts are very great,
the atom may arrive at such a phase that one part acquires sufficient
kinetic energy to escape from the system, or that the constraining
forces are momentarily neutralised, so that the part escapes from the
system with the velocity possessed by it at the instant of its release.

Sir Oliver Lodge[375] has advanced the view that the instability of the
atom may be a result of radiation of energy by the atom. Larmor has
shown that an electron, subject to acceleration, radiates energy at a
rate proportional to the square of its acceleration. An electron moving
uniformly in a straight line does not radiate energy, but an electron,
constrained to move in a circular orbit with constant velocity, is a
powerful radiator, for in such a case the electron is continuously
accelerated towards the centre. Lodge considered the simple case of a
negatively charged electron revolving round an atom of mass relatively
large but having an equal positive charge and held in equilibrium by
electrical forces. This system will radiate energy, and, since the
radiation of energy is equivalent to motion in a resisting medium, the
particle tends to move towards the centre, and its speed consequently
increases. The rate of radiation of energy will increase rapidly with
the speed of the electron. When the speed of the electron becomes very
nearly equal to the velocity of light, according to Lodge, another
effect supervenes. It has been shown (section 82) that the apparent mass
of an electron increases very rapidly as the speed of light is
approached, and is theoretically infinite at the speed of light. There
will be at this stage a sudden increase of the mass of the revolving
atom, and, on the supposition that this stage can be reached, a
consequent disturbance of the balance of forces holding the system
together. Lodge considers it probable that, under these conditions, the
parts of the system will break asunder and escape from the sphere of one
another’s influence.

It seems probable that the primary cause of the disintegration of the
atom must be looked for in the loss of energy of the atomic system due
to electromagnetic radiation (section 52). Larmor[376] has shown that
the condition to be fulfilled in order that a system of rapidly moving
electrons may persist without loss of energy is that the vector sum of
the accelerations towards the centre should be permanently zero. While a
single electron moving in a circular orbit is a powerful radiator of
energy, it is remarkable how rapidly the radiation of energy diminishes
if several electrons are revolving in a ring. This has recently been
shown by J. J. Thomson[377], who examined mathematically the case of a
system of negatively electrified corpuscles, situated at equal intervals
round the circumference of a circle, and rotating in one plane with
uniform velocity round its centre. For example, he found that the
radiation from a group of six particles moving with a velocity of ⅒
of the velocity of light is less than one-millionth part of the
radiation from a single particle describing the same orbit with the same
velocity. When the velocity is ¹⁄₁₀₀ of that of light the amount of
radiation is only 10⁻¹⁶ that of a single particle moving with the same
velocity in the same orbit.

Results of this kind indicate that an atom consisting of a large number
of revolving electrons may radiate energy extremely slowly, and yet,
finally, this minute but continuous drain of energy from the atom must
result either in a rearrangement of its component parts into a new
system, or of an expulsion of electrons or groups of electrons from the
atom.

Simple models of atoms to imitate the behaviour of polonium in shooting
out α particles, and of radium in shooting out β particles have been
discussed by Lord Kelvin[378]. It is possible to devise certain stable
arrangements of the positively and negatively electrified particles,
supposed to constitute an atom, which, on the application of some
disturbing force, break up with the expulsion of a part of the system
with great velocity.

J. J. Thomson[379] has mathematically investigated the possible stable
arrangements of a number of electrons moving about in a sphere of
uniform positive electrification. The properties of such a model atom
are very striking, and indirectly suggest a possible explanation of the
periodic law in chemistry. He has shown that the electrons, if in one
plane, arrange themselves in a number of concentric rings; and
generally, if they are not constrained to move in one plane, in a number
of concentric shells like the coats of an onion.

The mathematical problem is much simplified if the electrons are
supposed to rotate in rings in one plane, the electrons in each ring
being arranged at equal angular intervals. The ways in which the number
of electrons group themselves, for numbers ranging from 60 to 5 at
intervals of 5, are shown in the following table:—

                   Number      60  55  50  45  40  35
                   of
                   electrons


                   Number in   20  19  18  17  16  16
                   successive
                   rings

                               16  16  15  14  13  12

                               13  12  11  10   8   6

                                8   7   5   4   3   1

                                3   1   1

                   Number      30  25  20  15  10   5
                   of
                   electrons


                   Number in   15  13  12  10   8   5
                   successive
                   rings

                               10   9   7   5   2

                                5   3   1

In the next table is given the possible series of arrangements of
electrons which can have an outer ring of 20:—

             Number      59  60  61  62  63  64  65  66  67
             of
             electrons


             Number in   20  20  20  20  20  20  20  20  20
             successive
             rings

                         16  16  16  17  17  17  17  17  17

                         13  13  13  13  13  13  14  14  15

                          8   8   9   9  10  10  10  10  10

                          2   3   3   3   3   4   4   5   5

The smallest number of electrons which can have an outer ring of 20 is
59, while 67 is the greatest.

The various arrangements of electrons can be classified into families,
in which the groupings of the electrons have certain features in common.
Thus the group of 60 electrons consists of the same arrangement of
electrons as the group of 40 with the addition of an outer ring of 20
electrons; the group of 40 is the same as the group of 24 with an
additional ring outside; and the group of 24 in turn is the same as the
group of 11 with an extra ring. A series of model atoms may be formed in
this way, in which each atom is derived from the preceding member by an
additional ring of electrons. Such atoms would be expected to possess
many properties in common, and would correspond to the elements in the
same vertical column of the periodic table of Mendeléef.

Different arrangements of electrons vary widely in stability. Some may
acquire an extra electron or two and yet remain stable, others readily
lose an electron without disturbing their stability. The former would
correspond to an electro-negative atom, the latter to an
electro-positive.

Certain arrangements of electrons are stable if the electrons move with
an angular velocity greater than a certain value, but become unstable
when the velocity falls below this value. Four electrons in motion, for
example, are stable in one plane, but when the velocity falls below a
certain critical value, the system is unstable, and the electrons tend
to arrange themselves at the corners of a regular tetrahedron. J. J.
Thomson (_loc. cit._) applies this property to explain why an atom of
radio-active matter breaks up, as follows:—

“Consider now the properties of an atom containing a system of
corpuscles (electrons) of this kind. Suppose the corpuscles were
originally moving with velocities far exceeding the critical velocity;
in consequence of the radiation from the moving corpuscles, their
velocity will slowly—very slowly—diminish; when, after a long interval,
the velocity reaches the critical velocity, there will be what is
equivalent to an explosion of the corpuscles, the corpuscles will move
far away from their original position, their potential energy will
decrease, while their kinetic energy will increase. The kinetic energy
gained in this way might be sufficient to carry the system out of the
atom, and we should have, as in the case of radium, a part of the atom
shot off. In consequence of the very slow dissipation of energy by
radiation the life of the atom would be very long. We have taken the
case of the four corpuscles as the type of a system which, like a top,
requires for its stability a certain amount of rotation. Any system
possessing this property would, in consequence of the gradual
dissipation of energy by radiation, give to the atom containing it
radio-active properties similar to those conferred by the four
corpuscles.”


=271. Heat of the sun and earth.= It was pointed out by Rutherford and
Soddy[380] that the maintenance of the sun’s heat for long intervals of
time did not present any fundamental difficulty if a process of
disintegration, such as occurs in the radio-elements, were supposed to
be taking place in the sun. In a letter to _Nature_ (July 9, 1903) W. E.
Wilson showed that the presence of 3·6 grams of radium in each cubic
metre of the sun’s mass was sufficient to account for the present rate
of emission of energy by the sun. This calculation was based on the
estimate of Curie and Laborde that 1 gram of radium emits 100
gram-calories per hour, and on the observation of Langley that each
square centimetre of the sun’s surface emits 8·28 × 10⁶ gram-calories
per hour. Since the average density of the sun is 1·44, the presence of
radium in the sun, to the extent of 2·5 parts by weight in a million,
would account for its present rate of emission of energy.

An examination of the spectrum of the sun has not so far revealed any of
the radium lines. It is known, however, from spectroscopic evidence that
helium is present, and this indirectly suggests the existence of
radio-active matter also. It can readily be shown[381] that the absence
of penetrating rays from the sun at the surface of the earth does not
imply that the radio-elements are not present in the sun. Even if the
sun were composed of pure radium, it would hardly be expected that the γ
rays emitted would be appreciable at the surface of the earth, since the
rays would be almost completely absorbed in passing through the
atmosphere, which corresponds to a thickness of 76 centimetres of
mercury.

In the Appendix E of Thomson and Tait’s _Natural Philosophy_, Lord
Kelvin has calculated the energy lost in the concentration of the sun
from a condition of infinite dispersion, and concludes that it seems “on
the whole probable that the sun has not illuminated the earth for
100,000,000 years and almost certain that he has not done so for
500,000,000 years. As for the future we may say, with equal certainty,
that inhabitants of the earth cannot continue to enjoy the light and
heat essential to their life for many million years longer, unless
sources now unknown to us are prepared in the great storehouses of
creation.”

The discovery that a small mass of a substance like radium can emit
spontaneously an enormous quantity of heat renders it possible that this
estimate of the age of the sun’s heat may be much increased. In a letter
to _Nature_ (Sept. 24, 1903) G. H. Darwin drew attention to this
probability, and at the same time pointed out that, on Kelvin’s
hypotheses, his estimate of the duration of the sun’s heat was probably
much too high, and stated that, “The lost energy of the sun, supposed to
be a homogeneous sphere of mass _M_ and radius _a_, is (⅗)μ_M²_/_a_
where μ is the constant of gravitation. On introducing numerical values
for the symbols in this formula, I find the lost energy to be 2·7 × 10⁷
_M_ calories where _M_ is expressed in grams. If we adopt Langley’s
value of the solar constant, this heat suffices to give a supply for 12
million years. Lord Kelvin used Pouillet’s value for that constant, but
if he had been able to use Langley’s, his 100 million would have been
reduced to 60 million. The discrepancy between my results of 12 million
and his of 60 million is explained by a conjectural augmentation of the
lost energy to allow for the concentration of the solar mass towards its
central parts.” Now it has been shown (section 266) that one gram of
radium emits during its life an amount of heat corresponding to 1·6 ×
10⁹ gram-calories. It has also been pointed out that there is every
reason to suppose that a similar amount of energy is resident in the
chemical atoms of the inactive elements. It is not improbable that, at
the enormous temperature of the sun, the breaking up of the elements
into simpler forms may be taking place at a more rapid rate than on the
earth. If the energy resident in the atoms of the elements is thus
available, the time during which the sun may continue to emit heat at
the present rate may be at least 50 times longer than the value computed
from dynamical data.

Similar considerations apply to the question of the age of the earth. A
full discussion of the probable age of the earth, computed from its
secular cooling from a molten mass, is given by Lord Kelvin in Appendix
D of Thomson and Tait’s _Natural Philosophy_. He has there shown that
about 100 million years after the earth was a molten mass, the gradual
cooling due to radiation from its surface would account for the average
temperature gradient of ¹⁄₅₀° F. per foot, observed to-day near the
earth’s surface.

Some considerations will now be discussed which point to the probability
that the present temperature gradient observed in the earth cannot be
used as a guide to estimate the length of time that has elapsed since
the earth has been at a temperature capable of supporting animal and
vegetable life; for it will be shown that probably there is sufficient
radio-active matter on the earth to supply as much heat to the earth as
is lost by radiation from its surface. Taking the average conductivity
_K_ of the materials of the earth as ·004 (C.G.S. units) and the
temperature gradient _T_ near the surface as ·00037° C. per cm., the
heat _Q_ in gram-calories conducted to the surface of the earth per
second is given by

                             _Q_ = 4π_R²KT_,

where _R_ is the radius of the earth.

Let _X_ be the average amount of heat liberated per second per cubic
centimetre of the earth’s volume owing to the presence of radio-active
matter. If the heat _Q_ radiated from the earth is equal to the heat
supplied by the radio-active matter in the earth,

                          _X_ . (⁴⁄₃)π_R³_ = 4π_R²KT_,

                   or
                         3_KT_
                   _X_ = ------   .
                          _R_

Substituting the values of these constants,

                  _X_ = 7 × 10⁻¹⁵ gram-calories per second
                      = 2·2 × 10⁻⁷ gram-calories per year.

Since 1 gram of radium emits 876,000 gram-calories per year, the
presence of 2·6 × 10⁻¹³ grams of radium per unit volume, or 4·6 × 10⁻¹⁴
grams per unit mass, would compensate for the heat lost from the earth
by conduction.

Now it will be shown in the following chapter that radio-active matter
seems to be distributed fairly uniformly through the earth and
atmosphere. In addition, it has been found that all substances are
radio-active to a feeble degree, although it is not yet settled whether
this radio-activity may not be due mainly to the presence of a
radio-element as an impurity. For example, Strutt[382] observed that a
platinum plate was about ¹⁄₃₀₀₀ as active as a crystal of uranium
nitrate, or about 2 × 10⁻¹⁰ as active as radium. This corresponds to a
far greater activity than is necessary to compensate for the loss of
heat of the earth. A more accurate deduction, however, can be made from
data of the radio-activity exhibited by matter dug out of the earth.
Elster and Geitel[383] filled a dish of volume 3·3 × 10³ c.c. with clay
dug up from the garden, and placed it in a vessel of 30 litres capacity
in which was placed an electroscope to determine the conductivity of the
enclosed gas. After standing for several days, they found that the
conductivity of the air reached a constant maximum value, corresponding
to three times the normal. It will be shown later (section 284) that the
normal conductivity observed in sealed vessels corresponds to the
production of about 30 ions per c.c. per second. The number of ions
produced per second in the vessel by the radio-active earth was thus
about 2 × 10⁶. This would give a saturation current through the gas of
2·2 × 10⁻¹⁴ electromagnetic units. Now the emanation from 1 gram of
radium stored in a metal cylinder gives a saturation current of about
3·2 × 10⁻⁵ electromagnetic units. Elster and Geitel considered that
most of the conductivity observed in the gas was due to a radio-active
emanation, which gradually diffused from the clay into the air in the
vessel. The increased conductivity in the gas observed by Elster and
Geitel would thus be produced by the emanation from 7 × 10⁻¹⁰ gram of
radium. Taking the density of clay as 2, this corresponds to about 10⁻¹³
gram of radium per gram of clay. But it has been shown that if 4·6 ×
10⁻¹⁴ gram of radium were present in each gram of earth, the heat
emitted would compensate for the loss of heat of the earth by conduction
and radiation. The amount of activity observed in the earth is thus
about the right order of magnitude to account for the heat emission
required. In the above estimate, the presence of uranium and thorium
minerals in the earth has not been considered. Moreover, it is probable
that the total amount of radio-activity in the clay was considerably
greater than that calculated, for it is likely that other radio-active
matter was present which did not give off an emanation.

If the earth is supposed to be in a state of thermal equilibrium in
which the heat lost by radiation is supplied from radio-active matter,
there must be an amount of radio-active matter in the earth
corresponding to about 270 million tons of radium. If there were more
radium than this in the earth, the temperature gradient would be greater
than that observed to-day. This may appear to be a very large quantity
of radium, but recent determinations (section 281) of the amount of
radium emanation in the atmosphere strongly support the view that a
large quantity of radium must exist in the surface soil of the earth.
Eve found, on a minimum estimate, that the amount of emanation always
present in the atmosphere is equivalent to the equilibrium amount
derived from 100 tons of radium. There is every reason to believe that
the emanation found in the atmosphere is supplied both by the diffusion
of the emanation from the soil and by the action of springs. Since the
emanation loses half its activity in four days, it cannot diffuse from
any great depth. Assuming that the radium is uniformly distributed
throughout the earth, the quantity of the radium emanation produced in a
thin shell of earth about thirteen metres in depth, is sufficient to
account for the amount ordinarily observed in the atmosphere.

I think we may conclude that the present rate of loss of heat of the
earth might have continued unchanged for long periods of time in
consequence of the supply of heat from radio-active matter in the earth.
It thus seems probable that the earth may have remained for very long
intervals of time at a temperature not very different from that observed
to-day, and that, in consequence, the time during which the earth has
been at a temperature capable of supporting the presence of animal and
vegetable life may be very much longer than the estimate made by Lord
Kelvin from other data.


=272. Evolution of matter.= Although the hypothesis that all matter is
composed of some elementary unit of matter or protyle has been advanced
as a speculation at various times by many prominent physicists and
chemists, the first definite experimental evidence showing that the
chemical atom was not the smallest unit of matter was obtained in 1897
by J. J. Thomson in his classic research on the nature of the cathode
rays produced by an electric discharge in a vacuum tube. We have seen
that Sir William Crookes, who was the first to demonstrate the
remarkable properties of these rays, had suggested that they consisted
of streams of projected charged matter and represented—as he termed it—a
new or “fourth state of matter.”

J. J. Thomson showed by two distinct methods (section 50), that the
cathode rays consisted of a stream of negatively charged particles
projected with great velocity. The particles behaved as if their mass
was only about ¹⁄₁₀₀₀ of the mass of the atom of hydrogen, which is the
lightest atom known. These corpuscles, as they were termed by Thomson,
were found at a later date to be produced from a glowing carbon filament
and from a zinc plate exposed to the action of ultra-violet light. They
acted as isolated units of negative electricity, and, as we have seen,
may be identified with the electrons studied mathematically by Larmor
and Lorentz. Not only were these electrons produced by the action of
light, heat, and the electric discharge, but similar bodies were also
found to be emitted spontaneously from the radio-elements with a
velocity far greater than that observed for the electrons in a vacuum
tube.

The electrons produced in these various ways were all found to carry a
negative charge, and to be apparently identical; for the ratio _e_/_m_
of the charge of the electron to its mass was in all cases the same
within the limits of experimental error. Since electrons, produced from
different kinds of matter and under different conditions, were in all
cases identical, it seemed probable that they were a constituent part of
all matter. J. J. Thomson suggested that the atom is built up of a
number of these negatively charged electrons combined in some way with
corresponding positively charged bodies.

On this view the atoms of the chemical elements differ from one another
only in the number and arrangement of the component electrons.

The removal of an electron from the atom in the case of ionization does
not appear to affect permanently the stability of the system, for no
evidence has so far been obtained to show that the passage of an intense
electric discharge through a gas results in a permanent alteration of
the structure of the atom. On the other hand, in the case of the
radio-active bodies, a positively charged particle of mass about twice
that of the hydrogen atom escapes from the heavy radio-atom. This loss
appears to result at once in a permanent alteration of the atom, and
causes a marked change in its physical and chemical properties. In
addition there is no evidence that the process is reversible.

The expulsion of a β particle with great velocity from an atom of
radio-active matter also results in a transformation of the atom. For
example radium E emits a β particle, and, in consequence, gives rise to
a distinct substance radium F (polonium). A case of this kind, where the
expulsion of a β particle with great velocity causes a complete
rearrangement of the parts of an atom, is probably quite distinct from
the process which occurs during ionization, where a slow speed electron
escapes from the atom without apparently affecting the stability of the
atom left behind.

The only direct experimental evidence of the transformation of matter
has been derived from a study of the radio-active bodies. If the
disintegration theory, advanced to account for the phenomena of
radio-activity, is correct in the main essentials, then the
radio-elements are undergoing a spontaneous and continuous process of
transformation into other and different kinds of matter. The rate of
transformation is slow in uranium and thorium, but is fairly rapid in
radium. It has been shown that the fraction of a mass of radium which is
transformed per year is about ¹⁄₂₀₀₀ of the total amount present. In the
case of uranium and thorium probably a million years would be required
to produce a similar amount of change. Thus the process of
transformation in uranium and thorium is far too slow to be detected
within a reasonable time by the use of the balance or spectroscope, but
the radiations which accompany the transformation can easily be
detected. Although the process of change is slow it is continuous, and
in the course of ages the uranium and thorium present in the earth must
be transformed into other types of matter.

Those who have considered the possibility of atoms undergoing a process
of transformation have generally thought that the matter as a whole
would undergo a progressive change, with a gradual alteration of
physical and chemical properties of the whole mass of substance. On the
theory of disintegration this is not the case. Only a minute fraction of
the matter present breaks up in unit time, and in each of the successive
stages through which the disintegrated atoms pass, there is in most
cases a marked alteration in the chemical and physical properties of the
matter. The transformation of the radio-elements is thus a
transformation of a part _per saltum_, and not a progressive change of
the whole. At any time after the process of transformation has been in
progress there will thus remain a part of the matter which is unchanged,
and, mixed with it, the products which have resulted from the
transformation of the remainder.

The question naturally arises whether the process of degradation of
matter is confined to the radio-elements or is a universal property of
matter. It will be shown in chapter XIV that all matter, so far
examined, exhibits the property of radio-activity to a slight degree. It
is very difficult, however, to make certain that the observed
radio-activity is not due to the presence in the matter of a slight
trace of a radio-element. If ordinary matter is radio-active, it is
certain that its activity is much less than that of uranium, and
consequently that its rate of transformation must be excessively slow.
There is, however, another possibility to be considered. The changes
occurring in the radio-elements would probably never have been detected
if the change had not been accompanied by the expulsion of charged
particles with great velocity. It does not seem unlikely that an atom
may undergo disintegration without projecting a part of its system with
sufficient velocity to ionize the gas. In fact, we have seen that, even
in the radio-elements, several of the series of changes in both thorium,
radium, and actinium are unaccompanied by ionizing rays. The
experimental results given in Appendix A strongly support this point of
view. It may thus be possible that all matter is undergoing a slow
process of transformation, which has so far only been detected in the
radio-elements on account of the expulsion of charged particles with
great velocity during the change. This process of degradation of matter
continuing for ages must reduce the constituents of the earth to the
simpler and more stable forms of matter.

The idea that helium is a transformation product of radium suggests the
probability that helium is one of the more elementary substances of
which the heavier atoms are composed. Sir Norman Lockyer, in his
interesting book on “Inorganic Evolution,” has pointed out that the
spectra of helium and of hydrogen predominate in the hottest stars. In
the cooler stars the more complex types of matter appear. Sir Norman
Lockyer has based his theory of evolution of matter on evidence of a
spectroscopic examination of the stars, and considers that temperature
is the main factor in breaking up matter into its simpler forms. The
transformation of matter occurring in the radio-elements is on the other
hand spontaneous, and independent of temperature over the range
examined.

Footnote 333:

  Perrin, _Revue Scientifique_, April 13, 1901.

Footnote 334:

  Becquerel, _C. R._ 133, p. 979, 1901.

Footnote 335:

  Rutherford and McClung, _Phil. Trans._ A, p. 25, 1901.

Footnote 336:

  Rutherford, _Phil. Mag._ Jan. and Feb. 1900.

Footnote 337:

  P. Curie, _C. R._ 136, p. 223, 1903.

Footnote 338:

  Rutherford, _Phil. Mag._ April, 1903.

Footnote 339:

  M. and Mme Curie, _C. R._ 134, p. 85, 1902.

Footnote 340:

  Rutherford and Soddy, _Trans. Chem. Soc._ 81, pp. 321, 837, 1902.
  _Phil. Mag._ Sept. and Nov. 1902.

Footnote 341:

  Rutherford and Soddy, _Phil. Mag._ April, 1903.

Footnote 342:

  Rutherford and Soddy, _Phil. Mag._ May, 1903.

Footnote 343:

  Rutherford, _Phys. Zeit._ 4, p. 235, 1902. _Phil. Mag._ Feb. 1903.

Footnote 344:

  Rutherford, _Phil. Mag._ May, 1903.

Footnote 345:

  Curie and Laborde, _C. R._ 136, p. 673, 1903.

Footnote 346:

  J. J. Thomson, _Nature_, p. 601, 1903.

Footnote 347:

  Crookes, _C. R._ 128, p. 176, 1899.

Footnote 348:

  F. Re, _C. R._ p. 136, p. 1393, 1903.

Footnote 349:

  Richarz and Schenck, _Berl. Ber._ p. 1102, 1903. Schenck, _Berl. Ber._
  p. 37, 1904.

Footnote 350:

  Armstrong and Lowry, _Proc. Roy. Soc._ 1903. _Chem. News_, 88, p. 89,
  1903.

Footnote 351:

  Rutherford and Soddy, _Phil. Mag._ May, 1903.

Footnote 352:

  Boltwood, _Nature_, May 25, p. 80, 1904. _Phil. Mag._ April, 1905.

Footnote 353:

  McCoy, _Ber. d. D. Chem. Ges._ No. 11, p. 2641, 1904.

Footnote 354:

  Strutt, _Nature_, March 17 and July 7, 1904. _Proc. Roy. Soc._ March
  2, 1905.

Footnote 355:

  Strutt, _Proc. Roy. Soc._ March 2, 1905.

Footnote 356:

  Soddy, _Nature_, May 12, 1904; Jan. 19, 1905.

Footnote 357:

  Whetham, _Nature_, May 5, 1904; Jan. 26, 1905.

Footnote 358:

  Danne, _C. R._ Jan. 23, 1905.

Footnote 359:

  J. J. Thomson, _Nature_, April 30, p. 601, 1903.

Footnote 360:

  Voller, _Phys. Zeit._ 5, No. 24, p. 781, 1904.

Footnote 361:

  Ramsay and Cooke, _Nature_, Aug. 11, 1904.

Footnote 362:

  Eve, _Nature_, March 16, 1905.

Footnote 363:

  J. J. Thomson, International Electrical Congress, St Louis, Sept.
  1904.

Footnote 364:

  Rutherford and Soddy, _Phil. Mag._ p. 582, 1902; pp. 453 and 579,
  1903.

Footnote 365:

  Ramsay and Soddy, _Nature_, July 16, p. 246, 1903. _Proc. Roy. Soc._
  72, p. 204, 1903; 73, p. 346, 1904.

Footnote 366:

  Curie and Dewar, _C. R._ 138, p. 190, 1904. _Chem. News_, 89, p. 85,
  1904.

Footnote 367:

  Himstedt and Meyer, _Ann. d. Phys._ 15, p. 184, 1904.

Footnote 368:

  Strutt, _Proc. Roy. Soc._ March 2, 1905.

Footnote 369:

  Boltwood, _Phil. Mag._ April, 1905.

Footnote 370:

  Moss, _Trans. Roy. Soc. Dublin_, 1904.

Footnote 371:

  Travers, _Nature_, p. 248, Jan. 12, 1905.

Footnote 372:

  Jaquerod, _C. R._ p. 789, 1904.

Footnote 373:

  Ramsay and Travers, _Zeitsch. Physik. Chem._ 25, p. 568, 1898.

Footnote 374:

  Ramsay, _Nature_, April 7, 1904.

Footnote 375:

  Lodge, _Nature_, June 11, p. 129, 1903.

Footnote 376:

  Larmor, _Aether and Matter_, p. 233.

Footnote 377:

  J. J. Thomson, _Phil. Mag._ p. 681, Dec. 1903.

Footnote 378:

  Lord Kelvin, _Phil. Mag._ Oct. 1904.

Footnote 379:

  Thomson, _Phil. Mag._ March, 1904.

Footnote 380:

  Rutherford and Soddy, _Phil. Mag._ May, 1903.

Footnote 381:

  See Strutt and Joly, _Nature_, Oct. 15, 1903.

Footnote 382:

  Strutt, _Phil. Mag._ June, 1903.

Footnote 383:

  Elster and Geitel, _Phys. Zeit._ 4, No. 19, p. 522, 1903. _Chem.
  News_, July 17, p. 30, 1903.




                              CHAPTER XIV.
      RADIO-ACTIVITY OF THE ATMOSPHERE AND OF ORDINARY MATERIALS.


=273. Radio-activity of the atmosphere.= The experiments of Geitel[384]
and C. T. R. Wilson[385] in 1900 showed that a positively or negatively
charged conductor placed inside a closed vessel gradually lost its
charge. This loss of charge was shown to be due to a small ionization of
the air inside the vessel. Elster and Geitel also found that a charged
body exposed in the open air lost its charge rapidly, and that the rate
of discharge was dependent on the locality and on atmospheric
conditions. A more detailed description and discussion of these results
will be given later in section 284.

In the course of these experiments, Geitel observed that the rate of
discharge increased slightly for some time after the vessel had been
closed. He considered that this might possibly be due to the existence
of some radio-active substances in the air, which produced excited
activity on the walls of the vessel and so increased the rate of
dissipation of the charge. In 1901 Elster and Geitel[386] tried the bold
experiment of seeing whether it were possible to extract a radio-active
substance from the air. The experiments of the writer had shown that the
excited radio-activity from the thorium emanation could be concentrated
on the negative electrode in a strong electric field. This result
indicated that the carriers of the radio-activity had a positive charge
of electricity. Elster and Geitel therefore tried an experiment to see
whether positively charged carriers, possessing a similar property, were
present in the atmosphere. For this purpose a cylinder of wire-netting,
charged negatively to 600 volts, was exposed for several hours in the
open air. The cylinder was then removed, and quickly placed in a large
bell-jar, inside which was placed an electroscope to detect the rate of
discharge. It was found that the rate of discharge was increased to a
slight extent. In order to multiply the effect a wire about 20 metres in
length was exposed at some height from the ground, and was kept charged
to a high potential by connecting it to the negative terminal of an
influence machine. After exposure for some hours, this wire was removed
and placed inside the dissipation vessel. The rate of discharge was
found to be increased many times by the presence of the wire. No
increase was observed when the wire was charged positively instead of
negatively. The results also showed that the radio-active matter could
be removed from the wire in the same way as from a wire made active by
exposure in the presence of the thorium emanation. A piece of leather
moistened with ammonia was rubbed over the active wire. On testing the
leather, it was found to be strongly radio-active. When a long wire was
used, the amount of activity obtained on the leather was comparable with
that possessed by a gram of uranium oxide.

The activity produced on the wire was not permanent, but disappeared to
a large extent in the course of a few hours. The amount of activity
produced on a wire of given size, exposed under similar conditions, was
independent of the material of the wire. Lead, iron and copper wires
gave about equal effects.

The amount of activity obtained was greatly increased by exposing a
negatively charged wire in a mass of air which had been undisturbed for
a long time. Experiments were made in the great cave of Wolfenbüttel,
and a very large amount of activity was observed. By transferring the
activity to a piece of leather it was found that the rays could
appreciably light up a screen of barium platinocyanide in the dark[387].
The rays also darkened a photographic plate through a piece of aluminium
0·1 mm. in thickness.

These remarkable experiments show that the excited radio-activity
obtained from the atmosphere is very similar in character to the excited
activity produced by the emanations of radium and thorium. No
investigators have contributed more to our knowledge of the
radio-activity and ionization of the atmosphere than Elster and Geitel.
The experiments here described have been the starting-point of a series
of researches by them and others on the radio-active properties of the
atmosphere, which have led to a great extension of our knowledge of that
important subject.

Rutherford and Allan[388] determined the rate of decay of the excited
activity produced on a negatively charged wire exposed in the open air.
A wire about 15 metres long was exposed in the open air, and kept
charged by an influence machine to a potential of about −10,000 volts.
An hour’s exposure was sufficient to obtain a large amount of excited
activity on the wire. The wire was then rapidly removed and wound on a
framework which formed the central electrode in a large cylindrical
metal vessel. The ionization current for a saturation voltage was
measured by means of a sensitive Dolezalek electrometer. The current,
which is a measure of the activity of the wire, was found to diminish
according to an exponential law with the time, falling to half value in
about 45 minutes. The rate of decay was independent of the material of
the wire, of the time of exposure, and of the potential of the wire.

An examination was also made of the nature of the rays emitted by the
radio-active wire. For this purpose a lead wire was made radio-active in
the manner described, and then rapidly wound into the form of a flat
spiral. The penetrating power of the rays was tested in a vessel similar
to that shown in Fig. 17. Most of the ionization was found to be due to
some very easily absorbed rays, which were of a slightly more
penetrating character than the α rays emitted from a wire made active by
the radium or thorium emanations. The intensity of the rays was cut down
to half value by about 0·001 cm. of aluminium. The photographic action
observed by Elster and Geitel through 0·1 mm. of aluminium showed that
some penetrating rays were also present. This was afterwards confirmed
by Allan, who used the electric method. These penetrating rays are
probably similar in character to the β rays from the radio-elements.


=274.= The excited activity produced on the negatively charged wire
cannot be due to an action of the strong electric field on the surface
of the wire; for very little excited activity is produced if the wire is
charged to the same potential inside a closed cylinder.

We have seen that the excited activity produced on the wire can be
partially removed by rubbing and by solution in acids, and, in this
respect, it is similar to the excited activity produced in bodies by the
emanations of radium and thorium. The very close similarity of the
excited activity obtained from the atmosphere to that obtained from the
radium and thorium emanations suggests the probability that a
radio-active emanation exists in the atmosphere. This view is confirmed
by a large amount of indirect evidence discussed in sections 276, 277
and 280.

Assuming the presence of a radio-active emanation in the atmosphere, the
radio-active effects observed receive a simple explanation. The
emanation in the air gradually breaks up, giving rise in some way to
positively charged radio-active carriers. These are driven to the
negative electrode in the electric field, and there undergo a further
change, giving rise to the radiations observed at the surface of the
wire. The matter which causes excited activity will thus be analogous to
the active deposit of radium and thorium.

Since the earth is negatively electrified with regard to the upper
atmosphere, these positive radio-active carriers produced in the air are
continuously deposited on the surface of the earth. Everything on the
surface of the earth, including the external surface of buildings, the
grass, and leaves of trees, must be covered with an invisible deposit of
radio-active material. A hill, or mountain peak, or any high mass of
rock or land, concentrates the earth’s electric field at that point and
consequently will receive more excited radio-activity per unit area than
the plain. Elster and Geitel have pointed out that the greater
ionization of the air observed in the neighbourhood of projecting peaks
receives a satisfactory explanation on this view.

If the radio-active carriers are produced at a uniform rate in the
atmosphere, the amount of excited activity _I_{t}_, produced on a wire
exposed under given conditions, will, after exposure for a time _t_, be
given by

$$ I_t = I₀ (1 − e^{–λt} ) $$,

where _I₀_ is the maximum activity on the wire and λ is the constant of
decay of the excited activity. Since the activity of a wire after
removal falls to half value in about 45 minutes, the value of λ is 0·92
(hour)⁻¹. Some experiments made by Allan[389] are in rough agreement
with the above equation. Accurate comparative results are difficult to
obtain on account of the inconstancy of the radio-activity of the open
air. After an exposure of a wire for several hours, the activity reached
a practical maximum, and was not much increased by continued exposure.

We have seen (section 191) that the carriers of the active deposit of
radium and thorium move in an electric field with about the same
velocity as the ions. We should expect therefore that a long wire
charged to a high negative potential would abstract the active carriers
from the atmosphere for a considerable distance. This does not appear to
be the case, for Eve (see section 281) has found that the carriers are
only abstracted from the air for a radius of less than one metre, for a
potential of the wire of −10,000 volts. It seems probable that the
carriers of the active matter are deposited on the numerous fine dust
particles present in the air and thus move very slowly even in a strong
electric field.

The amount of excited activity produced on a wire, supported some
distance from the surface of the earth, should increase steadily with
the voltage, for the greater the potential, the greater the volume of
air from which the radio-active carriers are abstracted.

The presence of radio-active matter in the atmosphere will account for a
considerable portion of the ionization of the air observed near the
earth. This important question is discussed in more detail in section
281.


=275. Radio-activity of freshly fallen rain and snow.= C. T. R.
Wilson[390] tried experiments to see if any of the radio-active material
from the air was carried down by rain. For this purpose a quantity of
freshly fallen rain was collected, rapidly evaporated to dryness in a
platinum vessel, and the activity of the residue tested by placing the
vessel in an electroscope. In all cases, the rate of discharge of the
electroscope was considerably increased. From about 50 c.c. of rain
water, an amount of activity was obtained sufficient to increase the
rate of discharge of the electroscope four or five times, after the rays
had traversed a thin layer of aluminium or gold-leaf. The activity
disappeared in the course of a few hours, falling to half value in about
30 minutes. Rain water, which had stood for some hours, showed no trace
of activity. Tap water, when evaporated, left no active residue.

The amounts of activity obtained from a given quantity of rain water
were all of the same order of magnitude, whether the rain was
precipitated in fine or in large drops, by night or by day, or whether
the rain was tested at the beginning or at the end of a heavy rainfall
lasting several hours.

The activity obtained from rain is not destroyed by heating the platinum
vessel to a red heat. In this and other respects it resembles the
excited activity obtained on negatively charged wires exposed in the
open air.

C. T. R. Wilson[391] obtained a radio-active precipitate from rain water
by adding a little barium chloride and precipitating the barium with
sulphuric acid. An active precipitate was also obtained when alum was
added to the water, and the aluminium precipitated by ammonia. The
precipitates obtained in this way showed a large activity. The filtrate
when boiled down was quite inactive, showing that the active matter had
been completely removed by precipitation. This effect is quite analogous
to the production of active precipitates from a solution containing the
active deposit of thorium (see section 185).

The radio-activity of freshly fallen snow was independently observed by
C. T. R. Wilson[392] in England, and Allan[393] and McLennan[394] in
Canada. In order to obtain a large amount of activity, the surface layer
of snow was removed, and evaporated to dryness in a metal vessel. An
active residue was obtained with radio-active properties similar to
those observed for freshly fallen rain. Both Wilson and Allan found that
the activity of rain and snow decayed at about the same rate, the
activity falling to half value in about 30 minutes. McLennan states that
he found a smaller amount of radio-activity in the air after a prolonged
fall of snow.

Schmauss[395] has observed that drops of water falling through air
ionized by Röntgen rays acquire a negative charge. This effect is
ascribed to the fact that the negative ions in air diffuse faster than
the positive. On this view the drops of rain and flakes of snow would
acquire a negative charge in falling through the air. They would in
consequence act as collectors of the positive radio-active carriers from
the air. On evaporation of the water the radio-active matter would be
left behind.


=276. Radio-active emanations from the earth.= Elster and Geitel
observed that the air in caves and cellars was, in most cases,
abnormally radio-active, and showed very strong ionization. This action
might possibly be due to an effect of stagnant air, by which it produced
a radio-active emanation from itself, or to a diffusion of a
radio-active emanation from the soil. To test whether this emanation was
produced by the air itself, Elster and Geitel shut up the air for
several weeks in a large boiler, but no appreciable increase of the
activity or ionization was observed. To see whether the air imprisoned
in the capillaries of the soil was radio-active, Elster and Geitel[396]
put a pipe into the earth and sucked up the air into a testing vessel by
means of a water pump.

The apparatus employed to test the ionization of the air is shown in
Fig. 103. _C_ is an electroscope connected with a wire net, _Z_. The
active air was introduced into a large bell-jar of 27 litres capacity,
the inside of which was covered with wire netting, _MM´_. The bell-jar
rested on an iron plate _AB_. The electroscope could be charged by the
rod _S_. The rate of discharge of the electroscope, before the active
air was introduced, was noted. On allowing the active air to enter, the
rate of discharge increased rapidly, rising in the course of a few hours
in one experiment to 30 times the original value. They found that the
emanation produced excited activity on the walls of the containing
vessel. The air sucked up from the earth was even more active than that
observed in caves and cellars. There can thus be little doubt that the
abnormal activity observed in caves and cellars is due to a radio-active
emanation, present in the earth, which gradually diffuses to the surface
and collects in places where the air is not disturbed.

Results similar to those obtained by Elster and Geitel for the air
removed from the earth at Wolfenbüttel were also obtained later by Ebert
and Ewers[397] at Munich. They found a strongly active emanation in the
soil, and, in addition, examined the variation with time of the activity
due to the emanation in a sealed vessel. After the introduction of the
active air into the testing vessel, the activity was observed to
increase for several hours, and then to decay, according to an
exponential law, with the time, falling to half value in about 3·2 days.
This rate of decay is more rapid than that observed for the radium
emanation, which decays to half value in a little less than four days.
The increase of activity with time is probably due to the production of
excited activity on the walls of the vessel by the emanation. In this
respect it is analogous to the increase of activity observed when the
radium emanation is introduced into a closed vessel. No definite
experiments were made by Ebert and Ewers on the rate of decay of this
excited activity. In one experiment the active emanation, after standing
in the vessel for 140 hours, was removed by sucking ordinary air of
small activity through the apparatus. The activity rapidly fell to about
half value, and this was followed by a very slow decrease of the
activity with time. This result indicates that about half the rate of
discharge observed was due to the radiation from the emanation and the
other half to the excited activity produced by it.

The apparatus employed by Ebert and Ewers in these experiments was very
similar to that employed by Elster and Geitel, shown in Fig. 103. Ebert
and Ewers observed that, when the wire net attached to the electroscope
was charged negatively, the rate of discharge observed was always
greater than when it was charged positively. The differences observed
between the two rates of discharge varied between 10 and 20 per cent. A
similar effect has been observed by Sarasin, Tommasina and Micheli[398]
for a wire made active by exposure to the open air. This difference in
the rates of discharge for positive and negative electricity is probably
connected with the presence of particles of dust or small water globules
suspended in the gas. The experiments of Miss Brooks (section 181) have
shown that the particles of dust present in the air containing the
thorium emanation become radio-active. A large proportion of these dust
particles acquire a positive charge and are carried to the negative
electrode in an electric field. This effect would increase the rate of
discharge of the electroscope when charged negatively. In later
experiments, Ebert and Ewers noticed that, in some cases, when the air
had been kept in the vessel for several days, the effect was reversed,
and the electroscope showed a great rate of discharge when charged
positively.

[Illustration: Fig. 103.]

J. J. Thomson[399] has observed that the magnitude of the ionization
current depends on the direction of the electric field, if fine water
globules are suspended in the ionized gas.

In later experiments, Ebert[400] found that the radio-active emanation
could be removed from the air by condensation in liquid air. This
property of the emanation was independently discovered by Ebert before
he was aware of the results of Rutherford and Soddy on the condensation
of the emanations of radium and thorium. To increase the amount of
radio-active emanation in a given volume of air, a quantity of the
active air, obtained by sucking the air from the soil, was condensed by
a liquid air machine. The air was then allowed partially to evaporate,
but the process was stopped before the point of volatilization of the
emanation was reached. This process was repeated with another quantity
of air and the residues added together. Proceeding in this way, he was
able to concentrate the emanation in a small volume of air. On allowing
the air to evaporate, the ionization of the air in the testing vessel
increased rapidly for a time and then slowly diminished. Ebert states
that the maximum for the emanation which had been liquefied for some
time was reached earlier than for fresh air. The rate of decay of
activity of the emanation was not altered by keeping it at the
temperature of liquid air for some time. In this respect it behaves like
the emanations of radium and thorium.

J. J. Thomson[401] found that air bubbled through Cambridge tap water
showed much greater conductivity than ordinary air. The air was drawn
through the water by means of a water pump into a large gasometer, when
the ionization current was tested with a sensitive electrometer. When a
rod charged negatively was introduced into this conducting air it became
active. After an exposure for a period of 15 to 30 minutes in the
conducting gas, the rod, when introduced into a second testing vessel,
increased the saturation current in the vessel to about five times the
normal amount. Very little effect was produced when the rod was
uncharged or charged positively for the same time. The activity of the
rod decayed with the time, falling to half value in about 40 minutes.
The amount of activity produced on a wire under constant conditions was
independent of the material of the wire. The rays from the rod were
readily absorbed in a few centimetres of air.

These effects were, at first, thought to be due to the action of the
small water drops suspended in the gas, for it was well known that air
rapidly drawn through water causes a temporary increase in its
conductivity. Later results, however, showed that there was a
radio-active emanation present in Cambridge tap water. This led to an
examination of the waters from deep wells in various parts of England,
and J. J. Thomson found that, in some cases, a large amount of emanation
could be obtained from the well water. The emanation was released either
by bubbling air through the water or by boiling the water. The gases
obtained by boiling the water were found to be strongly active. A sample
of air mixed with the radio-active emanation was condensed. The
liquefied gas was allowed to evaporate, and the earlier and later
portions of the gas were collected in separate vessels. The final
portion was found to be about 30 times as active as the first portion.

An examination of the radio-active properties of the active gases so
obtained has been made by Adams[402]. He found that the activity of the
emanation decayed, according to an exponential law, with the time,
falling to half value in about 3·4 days. This is not very different from
the rate of decay of the activity of the radium emanation, which falls
to half value in a little less than four days. The excited activity
produced by the emanation decayed to half value in about 35 minutes. The
decay of the excited activity from radium is at first irregular, but
after some time falls off, according to an exponential law, diminishing
to half value in 28 minutes. Taking into account the uncertainty
attaching to measurements of the very small ionization observed in these
experiments, the results indicate that the emanation obtained from well
water in England is similar to, if not identical with, the radium
emanation. Adams observed that the emanation was slightly soluble in
water. After well water had been boiled for a while and then put aside,
it was found to recover its power of giving off an emanation. The amount
obtained after standing for some time was never more than 10 per cent.
of the amount first obtained. Thus it is probable that the well water,
in addition to the emanations mixed with it, has also a slight amount of
a permanent radio-active substance dissolved in it. Ordinary rain water
or distilled water does not give off an emanation.

Bumstead and Wheeler[403] have made a very careful examination of the
radio-activity of the emanation obtained from the surface water and soil
at New Haven, Connecticut. The emanation, obtained from the water by
boiling, was passed into a large testing cylinder, and measurements of
the current were made by means of a sensitive electrometer. The current
gradually rose to a maximum, after the introduction of the emanation, in
exactly the same way as the current increases in a vessel after the
introduction of the radium emanation. The decay of activity of the
emanations obtained from the water and soil was carefully measured, and,
within the limits of experimental error, agreed with the rate of decay
of activity observed for the radium emanation. The identity of the
emanations from the water and soil with the radium emanation was still
further established by experiments on the rate of diffusion of the
emanation through a porous plate. By comparative tests it was found that
the coefficient of diffusion of the emanations from the water and soil
was the same as for the radium emanation. Also, by comparison of the
rate of diffusion of carbonic acid, it was found that the density of the
emanation was about four times that of carbonic acid, a result in good
agreement with that found for the radium emanation (sections 161 and
162).

Bumstead[404] has found that a considerable amount of thorium as well as
radium emanation exists in the air of New Haven. For a three hour
exposure in the open air, 3 to 5 per cent. of the excited activity on
the wire is due to thorium. For a twelve hour exposure, the thorium
activity was sometimes 15 per cent. of the whole. On account of the
comparatively slow decay of the excited activity of thorium, the
activity on the wire after removal for three or four hours was due
almost entirely to thorium. The rate of decay could then be measured
accurately, and was found to be the same as for a wire exposed in the
presence of the thorium emanation.

Dadourian[405] has made an examination of the underground air in New
Haven, and has found that this too contains a large quantity of the
thorium emanation. A circular hole about 50 cms. in diameter and 2
metres deep was dug in the ground. A number of wires were wound on an
insulated frame and suspended in the hole, the top of the hole then
being covered over. The wire was charged negatively by a Wimshurst
machine. After a long exposure the excited activity on the wire
diminished at a rate that showed it to be a mixture of the excited
activities of thorium and radium.

A very large amount of work has been done in examining various hot and
mineral springs for the presence of the radium emanation, and it is not
possible here to refer more than briefly to a few of the very numerous
papers that have been published on this subject both in Europe and
America. H. S. Allen and Lord Blythswood[406] have observed that the hot
springs at Bath and Buxton gave off a radio-active emanation. This was
confirmed by Strutt[407], who found that the escaping gases contained
the radium emanation, and also that the mud deposited from the springs
contained a trace of radium salts. These results are of considerable
interest, for Lord Rayleigh has observed that helium is contained among
the gases evolved by the springs. It appears probable that the helium
observed is produced from the radium or radio-active deposits through
which the water flows. Many mineral and hot springs which are famous for
their curative properties have been found to contain traces of radium
and also considerable amounts of radium emanation. It has been suggested
that the curative properties may be due to some extent to the presence
of these minute quantities of radium.

Himstedt[408] found that the thermal springs at Baden Baden contained
the radium emanation, while Elster and Geitel[409] examined the deposits
formed by these springs and found them to contain small quantities of
radium salts. Results of a similar character were obtained for a number
of waters in Germany by Dorn[410], Schenck[411], and H. Mache[412].

Curie and Laborde[413] have tested the waters of a large number of
mineral springs and found that the great majority contain the radium
emanation. In this connection, it is of interest to note that Curie and
Laborde found very little emanation in the waters of Salins-Moutiers,
while Blanc[414] observed, on the other hand, that the sediment from the
spring was very active. A closer examination of this deposit by Blanc
revealed the fact that it contained a considerable quantity of thorium.
This was proved by finding that it gave out an emanation, which lost
half of its activity in one minute, and produced excited activity, which
fell to half value in about 11 hours. Boltwood[415] has tested a number
of samples of spring water from different sources in America and has
found that many of them contain the radium emanation.

Most of the results upon the amount of radium emanation from different
sources have been expressed in arbitrary units without, in many cases,
any comparative standard being given. Boltwood (_loc. cit._) has
described a satisfactory method for collecting and testing the emanation
from different waters, and has suggested that the rate of discharge
observed by the electroscope or the electrometer should be expressed in
terms of the effect due to the emanation liberated on solution of a
definite weight of the mineral uraninite. Since in every mineral so far
examined, the amount of radium present is proportional to the amount of
uranium, such a standard would be sufficiently definite for practical
purposes. The emanation liberated from a few centigrams of the mineral
is sufficient to give a convenient rate of discharge of an electroscope.
Such a method is preferable to using a known quantity of a radium
compound as a standard, since it is difficult to know with certainty the
activity of the preparations of radium which may be in the possession of
the different experimenters.


=277. Radio-activity of constituents of the earth.= Elster and
Geitel[416] observed that, although in many cases the conductivity of
the air was abnormally high in underground enclosures, the conductivity
varied greatly in different places. In the Baumann Cave, for example,
the conductivity of the air was nine times the normal, but in the Iberg
Cave only three times the normal. In a cellar at Clausthal the
conductivity was only slightly greater than the normal, but the excited
radio-activity obtained on a negatively charged wire exposed in it was
only ¹⁄₁₁ of the excited radio-activity obtained when the wire was
exposed in the free air. They concluded from these experiments that the
amount of radio-activity in the different places probably varied with
the nature of the soil. Observations were then made on the conductivity
of the air sucked up from the earth at different parts of the country.
The clayey and limestone soils at Wolfenbüttel were found to be strongly
active, the conductivity varying from four to sixteen times the normal
amount. A sample of air from the shell limestone of Würzburg and from
the basalt of Wilhelmshöhe showed very little activity.

Experiments were made to see whether any radio-active substance could be
detected in the soil itself. For this purpose some earth was placed on a
dish and introduced under a bell-jar, similar to that shown in Fig. 103.
The conductivity of the air in the bell-jar increased with the time,
rising to three times the normal value after several days. Little
difference was observed whether the earth was dry or moist. The activity
of the soil seemed to be permanent, for no change in the activity was
observed after the earth had been laid aside for eight months.

Attempts were then made to separate the radio-active constituent from
the soil by chemical treatment. For this purpose a sample of clay was
tested. By extraction with hydrochloric acid all the calcium carbonate
was removed. On drying the clay the activity was found to be reduced,
but it spontaneously regained its original activity in the course of a
few days. It seems probable, therefore, that an active product had been
separated from the soil by the acid. Elster and Geitel consider that an
active substance was present in the clay, which formed a product more
readily soluble in hydrochloric acid than the active material itself.
There seemed to be a process of separation analogous to that of Th X
from thorium by precipitation with ammonia.

Experiments were also made to see whether substances placed in the earth
acquired any radio-activity. For this purpose samples of potter’s clay,
whitening, and heavy spar, wrapped in linen, were placed in the earth 50
cms. below the surface. After an interval of a month, these were dug up
and their activity examined. The clay was the only substance which
showed any activity. The activity of the clay diminished with the time,
showing that activity had been excited in it by the emanations present
in the soil.

Elster and Geitel[417] have found that a large quantity of the
radio-active emanation can be obtained by sucking air through clay. In
some cases, the conductivity of the air in the testing vessel was
increased over 100 times. They have also found that the so-called
“fango”—a fine mud obtained from hot springs in Battaglia, Northern
Italy—gives off three or four times as much emanation as clay. By
treating the fango with acid, the active substance present was
dissolved. On adding some barium chloride to the solution, and
precipitating the barium as sulphate, the active substance was removed,
and in this way a precipitate was obtained over 100 times as active,
weight for weight, as the original fango. Comparisons were made of the
rate of decay of the excited activity, due to the emanation from fango,
with that due to the radium emanation, and within the limits of error,
the decay curves obtained were found to be identical. There can thus be
no doubt that the activity observed in fango is due to the presence of a
small quantity of radium. Elster and Geitel calculate that the amount of
radium, contained in it, is only about one-thousandth of the amount to
be obtained from an equal weight of pitchblende from Joachimsthal.

Vincenti and Levi Da Zara[418] have found that the waters and sediments
of a number of hot springs in Northern Italy contain the radium
emanation. Elster and Geitel observed that natural carbonic acid
obtained from great depths of old volcanic soil was radio-active, while
Burton[419] found that the petroleum from a deep well in Ontario,
Canada, contained a large quantity of emanation, probably of radium,
since its activity fell to half value in 3·1 days, while the excited
activity produced by the emanation fell to half value in about 35
minutes. A permanently active deposit was left behind after
volatilization of the oil, indicating that probably one or more of the
radio-elements were present in minute quantity.

Elster and Geitel[420] have found that the active sediments obtained
from springs at Nauheim and Baden Baden showed abnormal rates of decay
of the excited activity. This was finally traced to the presence in the
deposit of both thorium and radium. By suitable chemical methods, the
two active substances were separated from each other and were then
tested separately.


=278. Effect of meteorological conditions upon the radio-activity of the
atmosphere.= The original experiments of Elster and Geitel on the
excited radio-activity derived from the atmosphere were repeated by
Rutherford and Allan[421] in Canada. It was found that a large amount of
excited radio-activity could be derived from the air, and that the
effects were similar to those observed by Elster and Geitel in Germany.
This was the case even on the coldest day in winter, when the ground was
covered deeply with snow and wind was blowing from the north over
snow-covered lands. The results showed that the radio-activity present
in the air was not much affected by the presence of moisture, for the
air during a Canadian winter is extremely dry. The greatest amount of
excited activity on a negatively charged wire was obtained in a strong
wind. In some cases the amount produced for a given time of exposure was
ten to twenty times the normal amount. A cold bright day of winter
usually gave more effect than a warm dull day in summer.

Elster and Geitel[422] have made a detailed examination of the effect of
meteorological conditions on the amount of excited radio-activity to be
derived from the atmosphere. For this purpose a simple portable
apparatus was devised by them and used for the whole series of
experiments. A large number of observations were taken, extending over a
period of twelve months. They found that the amount of excited activity
obtained was subject to great variations. The extreme values obtained
varied in the ratio of 16 to 1. No direct connection could be traced
between the amount of ionization in the atmosphere and the amount of
excited activity produced. They found that the greatest amount of
excited activity was obtained during a fog, when the amount of
ionization in the air was small. This result, however, is not
necessarily contradictory to the view that the ionization and activity
of the air are to a certain extent connected. From the experiments of
Miss Brooks on the effect of dust in acting as carriers of excited
activity, more excited activity should be obtained during a fog than in
clear air. The particles of water become centres for the deposit of
radio-active matter. The positive carriers are thus anchored and are not
removed from the air by the earth’s field. In a strong electric field,
these small drops will be carried to the negative electrode and manifest
their activity on the surface of the wire. On the other hand, the
distribution of water globules throughout the air causes the ions in the
air to disappear rapidly in consequence of their diffusion to the
surface of the drops (see section 31). For this reason the denser the
fog, the smaller will be the conductivity observed in the air.

Lowering the temperature of the air had a decided influence. The average
activity observed below 0° C. was 1·44 times the activity observed above
0° C. The height of the barometer was found to exert a marked influence
on the amount of excited activity to be derived from the air. The lower
the barometer the greater was the amount of excited activity in the air.
The effect of variation of the height of the barometer is intelligible,
when it is considered that probably a large proportion of the
radio-activity observed in the air is due to the radio-active emanations
which are continuously diffusing from the earth into the atmosphere.
Elster and Geitel have suggested that a lowering of the pressure of the
air would cause the air from the ground to be drawn up from the
capillaries of the earth into the atmosphere. This, however, need not
necessarily be the case if the conditions of the escape of the emanation
into the atmosphere are altered by the variation of the position of
underground water or by a heavy fall of rain.

The amount of excited activity to be derived from the air on the Baltic
Coast was only one-third of that observed inland at Wolfenbüttel.
Experiments on the radio-activity of the air in mid-ocean would be of
great importance in order to settle whether the radio-activity observed
in the air is due to the emanations from the soil alone. It is probable
that the radio-activity of the air at different points of the earth may
vary widely, and may largely depend on the nature of the soil.

Saake[423] has found that the amount of emanation present in the air at
high altitudes in the valley of Arosa in Switzerland is much greater
than the normal amount at lower levels. Elster and Geitel have observed
that there is also a larger number of ions in the air at high altitudes,
and suggest that the curative effect of thermal springs and the
physiological actions of the air at high levels may be connected with
the presence of an unusual amount of radio-active matter in the
atmosphere. Simpson[424] made experiments on the amount of excited
activity at Karasjoh, Norway, at a height of about 150 feet above sea
level. The sun did not rise above the level of the horizon during the
time the observations were taken. The average amount of excited activity
obtained from the air was considerably greater than the normal amount
observed by Elster and Geitel in Germany. This was the more surprising
as the ground was frozen hard and covered with deep snow. Allan, working
in Montreal, Canada, early observed that the amount of activity to be
obtained from the air was about the same in summer as in winter,
although, in the latter case, the whole earth was deeply frozen and
covered with snow, and the winds blew from the north over snow-covered
lands. Under such conditions, a diminution of the amount of activity is
to be expected since the diffusion of the emanation must be retarded, if
not altogether stopped, by the freezing of the soil. On the other hand,
it appears difficult to escape from the conclusion of Elster and Geitel
that the emanation present in the atmosphere is evolved from the earth
itself.

Some interesting experiments have been made by McLennan[425] on the
amount of excited radio-activity to be derived from the air when filled
with fine spray. The experiments were made at the foot of the American
Fall at Niagara. An insulated wire was suspended near the foot of the
Fall, and the amount of excited activity on the wire compared with the
amount to be obtained on the same wire for the same exposure in Toronto.
The amount of activity obtained from the air at Toronto was generally
five or six times that obtained from the air at the Falls. In these
experiments it was not necessary to use an electric machine to charge
the wire negatively, for the falling spray kept the insulated wire
permanently charged to a potential of about −7500 volts. These results
indicate that the falling spray had a negative charge and electrified
the wire. The small amount of the excited radio-activity at the Falls
was probably due to the fact that the negatively charged drops
abstracted the positively charged radio-active carriers from the
atmosphere, and in falling carried them to the river below. On
collecting the spray and evaporating it, no active residue was obtained.
Such a result is, however, to be expected on account of the minute
proportion of the spray tested compared with that present in the air.


=279. A very penetrating radiation from the earth’s surface.=
McLennan[426], and Rutherford and Cooke[427] independently, observed the
presence of a very penetrating radiation inside buildings. McLennan
measured the natural conductivity of the air in a large closed metal
cylinder by means of a sensitive electrometer. The cylinder was then
placed inside another and the space between filled with water. For a
thickness of water between the cylinders of 25 cms. the conductivity of
the air in the inner cylinder fell to about 63 per cent. of its initial
value. This result shows that part of the ionization in the inner
cylinder was due to a penetrating radiation from an external source,
which radiation was partially or wholly absorbed in water.

Rutherford and Cooke observed that the rate of discharge of a sealed
brass electroscope was diminished by placing a lead screen around the
electroscope. A detailed investigation of the decrease of the rate of
discharge in the electroscope, when surrounded by metal screens, was
made later by Cooke[428]. A thickness of 5 cms. of lead round the
electroscope decreased the rate of discharge about 30 per cent. Further
increase of the thickness of the screen had no effect. When the
apparatus was surrounded by 5 tons of pig-lead the rate of discharge was
about the same as when it was surrounded by a plate about 3 cms. thick.
An iron screen also diminished the rate of discharge to about the same
extent as the lead. By suitably arranging lead screens it was found that
the radiation came equally from all directions. It was of the same
intensity by night as by day. In order to be sure that this penetrating
radiation did not arise from the presence of radio-active substances in
the laboratory, the experiments were repeated in buildings in which
radio-active substances had never been introduced, and also on the open
ground far removed from any building. In all cases a diminution of the
rate of discharge of the electroscope, when surrounded by lead screens,
was observed. These results show that a penetrating radiation is present
at the surface of the earth, arising partly from the earth itself and
partly from the atmosphere.

The result is not surprising when the radio-activity of the earth and
atmosphere is taken into account. The writer has found that bodies made
active by exposure to the emanations from thorium and radium give out γ
rays. We may expect then that the very similar excited radio-activity
which is present in the earth and atmosphere should also give rise to γ
rays of a similar character. More recent work, however (section 286),
indicates that this explanation is not sufficient to explain all the
facts observed.


=280. Comparison of the radio-activity of the atmosphere with that
produced by the radio-elements.= The radio-active phenomena observed in
the earth and atmosphere are very similar in character to those produced
by thorium and radium. Radio-active emanations are present in the air of
caves and cellars, in natural carbonic acid, and in deep well water, and
these emanations produce excited radio-activity on all bodies in contact
with them. The question now arises whether these effects are due
entirely to known radio-elements present in the earth or to unknown
kinds of radio-active matter. The simplest method of testing this point
is to compare the rate of decay of the radio-active product in the
atmosphere with those of the known radio-active products of thorium and
radium. A cursory examination of the facts at once shows that the
radio-activity of the atmosphere is much more closely allied to effects
produced by radium than to those due to thorium. The activity of the
emanation released from well water, and also that sucked up from the
earth, decays to half value in about 3·3 days, while the activity of the
radium emanation decays to half value in an interval of 3·7 to 4 days.
Considering the difficulty of making accurate determinations of these
quantities, the rates of decay of the activity of the emanations from
the earth and from radium agree within the limits of experimental error.
A large number of observers have found that the radium emanation is
present in the water of thermal springs and in the sediment deposited by
them. Bumstead and Wheeler have shown that the emanation from the soil
and surface water of New Haven is identical with that from radium. If
the emanations from the earth and from radium are the same, the excited
activities produced should have the same rate of decay. The emanation
from well water in England approximately fulfils this condition (section
276), but an observation recorded by Ebert and Ewers (section 276) seems
to show that the excited activity due to the emanation sucked up from
the earth decays at a very slow rate compared with that due to radium.

Bumstead has given undoubted evidence that the thorium as well as the
radium emanation is also present in the atmosphere at New Haven, while
Dadourian has shown that it is emitted by New Haven soil. Blanc, and
Elster and Geitel, have also found that thorium is present in the
sediment from some thermal springs.

If the active matter in the atmosphere consists mainly of the radium
emanation, the active deposit on a negatively charged wire, exposed in
the open air, should initially consist of radium A, B and C. The curve
of decay should be identical with the decay curve of the excited
activity of radium, measured by the α rays, that is, there should be a
rapid initial drop corresponding to the initial 3 minute change, then a
slow rate of variation, the activity after several hours decaying to
half value in about 28 minutes (see section 222). The rapid initial drop
has been observed by Bumstead for the air at New Haven. Allan[429] did
not observe this initial drop in Montreal, but found the activity fell
to half value in about 45 minutes, reckoning from a time about 10
minutes after the removal of the active wire. This is about the rate of
decay to be expected for the active deposit of radium over the same
interval. Allan obtained evidence that there were several kinds of
active matter deposited on the wire. For example, the activity
transferred from the active wire to a piece of leather, moistened with
ammonia, fell to half value in 38 minutes; for a piece of absorbent felt
treated similarly, the activity fell to half value in 60 minutes, the
normal time for the untreated wire being 45 minutes.

It is probable that this variation of the rate of decay is due to the
fact that unequal proportions of radium B and C were transferred from
the wire to the rubber. If a greater proportion of B than of C were
removed, the decay would be slower and _vice versa_.

The fact that the activity of rain and snow falls to half value in about
30 minutes is a strong indication that the radium emanation is present
in the atmosphere. The active matter with the rain and snow after
standing some time would consist mainly of radium C and this should
decay exponentially with the time, falling to half value in 28 minutes.

On account of the rapid decay of the thorium emanation—half value in one
minute—it is not likely that much of the activity of the atmosphere can
be ascribed to it. Its effect would be most marked near the surface of
the soil.

There can be little doubt, that a large part of the radio-activity of
the atmosphere is due to the radium emanation, which is continually
diffusing into the atmosphere from the pores of the earth. Since
radio-activity has been observed in the atmosphere at all points at
which observations have, so far, been made, radio-active matter must be
distributed in minute quantities throughout the soil of the earth. The
volatile emanations escape into the atmosphere by diffusion, or are
carried to the surface in spring water or by the escape of underground
gases, and cause the radio-active phenomena observed in the atmosphere.
The observation of Elster and Geitel that the radio-activity of the air
is much less near the sea than inland is explained at once, if the
radio-activity of the atmosphere is due mainly to the diffusion of
emanations from the soil into the air above it.

The rare gases helium and xenon which exist in the atmosphere have been
tested and found to be non-radio-active. The radio-activity of the air
cannot be ascribed to a slight radio-activity possessed by either of
these gases.


=281. Amount of the radium emanation in the atmosphere.= It is a matter
of great interest to form an estimate of the amount of radium emanation
present in the atmosphere, for since it comes from the earth, it
indirectly serves as a means of estimating the amount of radium which is
distributed over a thin crust of the earth.

Some experiments in this direction have been made by Eve in the
laboratory of the writer. The experiments are not yet completed but the
results so far obtained allow us to calculate the probable amount of
emanation per cubic kilometre of the atmosphere near the earth.

Experiments were first made with a large iron tank 154 cms. square and
730 cms. deep, in a building in which no radium or other radio-active
material had ever been introduced. The saturation ionization current for
the air in the tank was first measured by means of an electroscope,
connected with an insulated electrode passing up the centre of the
closed tank. Assuming that the ionization in the tank was uniform, the
number of ions produced per c.c. of the air in the tank was found to be
10. This is a considerably lower value than has usually been observed in
a small closed vessel (see section 284). Cooke obtained the value 10 for
a well cleaned brass electroscope, surrounded by lead, while Schuster
obtained a value about 12 for the air in the laboratory of Owens
College, Manchester.

In order to measure the amount of the excited activity from the tank, a
central insulated wire was charged negatively to about 10,000 volts by a
Wimshurst machine. After two hours, the wire was removed and wound on an
insulated frame connected with a gold-leaf electroscope. The rate of
decay of the activity on the wire was found to be about the same as for
the excited activity produced by the radium emanation. In order to
estimate the amount of radium emanation present in the large tank,
special experiments were made with a smaller tank in which a known
quantity of the radium emanation was introduced by employing a solution
of pure radium bromide of known concentration. A central wire was made
the negative electrode as before, and, after removal, it was wound on
the frame and its activity tested. In this way it was found that the
amount of radium emanation present in the large tank, in order to
produce the excited activity observed, must have been equal to the
equilibrium or maximum amount to be obtained from 9·5 × 10⁻⁹ grams of
pure radium bromide. The volume of the large tank was 17 cubic metres,
so that the amount of emanation present per cubic metre was equivalent
to that liberated from 5·6 × 10⁻¹⁰ grams of radium bromide in
radio-active equilibrium.

If the amount of the emanation in the tank is taken as the average
amount existing in the outside air, _the amount of radium emanation
present per cubic kilometre of the air is equivalent to that supplied by
0·56 grams of radium bromide_.

For the purpose of calculation, suppose the emanation is uniformly
distributed over the land portion of the earth (¼ of the total
surface), and to extend to an average height of 5 kilometres. The air
over the sea is not taken into account as its radio-activity has not
been examined. The total amount of emanation present in the atmosphere
under these conditions corresponds to that supplied by about 400 tons of
radium bromide. In order to maintain this amount of emanation in the
atmosphere, it must be supplied at a constant rate from the earth’s
surface. Since the greater amount of the emanation probably escapes into
the air by transpiration and diffusion through the soil, the emanation
cannot reach the surface except from a very thin layer of the earth. The
probable thickness of this layer can be estimated if it is assumed that
the present loss of heat from the earth is supplied from the
radio-active matter contained in it. We have seen (section 271) that, on
this hypothesis, there must be an amount of active matter in the earth
corresponding to about 300 million tons of radium. If this is supposed
to be uniformly distributed, a thickness of layer of about 13 metres
will suffice to maintain the calculated amount of emanation in the
atmosphere. This thickness of layer is about the order of magnitude to
be expected from general considerations.

These results lead indirectly to the conclusion that a large amount of
emanation does undoubtedly exist in the surface crust of the earth.

Experiments were also made by Eve with a large zinc cylinder exposed in
the open air. Volume for volume, the average amount of excited activity
derived from it was only about one-third of that obtained from the large
iron tank. This would reduce the amount of emanation, previously
deduced, to about one-third.

Before such calculations can be considered at all definite, it will be
necessary to make comparative measurements of the amount of emanation in
the atmosphere at various parts of the earth. The air at Montreal is not
abnormally active, so that the calculations probably give the right
order of magnitude of the quantities.

Eve also observed that the amount of activity to be obtained per unit
length of the wire in the zinc cylinder of about 70 cms. in diameter was
about the same as for a wire ·5 mms. in diameter charged to 10,000 volts
in the open air, supported 20 feet from the ground. This shows that such
a potential does not draw in the carriers of excited activity which are
more than half a metre away, and probably the range is even less.

It is of great importance to find how large a proportion of the number
of ions produced in the atmosphere is due to the radio-active matter
distributed throughout it. The results of Eve with the large iron tank,
already referred to, indicate that a large proportion of the ionization
in the tank was due to the radio-active matter contained in it, for the
ratio of the excited activity on the central electrode to the total
ionization current in the tank was about ⁷⁄₁₀ of the corresponding ratio
for a smaller tank into which a supply of the radium emanation had been
introduced.

This result requires confirmation by experiments at other parts of the
earth, but the results point to the conclusion that a large part, if not
all, of the ionization at the earth’s surface is due to radio-active
matter distributed in the atmosphere. A constant rate of production of
30 ions per second per c.c. of air, which has been observed in the open
air at the surface of the earth in various localities, would be produced
by the presence in each c.c. of the air of the amount of emanation
liberated from 2·4 × 10⁻¹⁵ grams of radium bromide in radio-active
equilibrium. It is not likely, however, that the ionization of the upper
part of the atmosphere is due to this cause alone. In order to explain
the maintenance of the large positive charge, which generally exists in
the upper atmosphere, there must be a strong ionization of the upper
air, which may possibly be due to ionizing radiations emitted by the
sun.


=282. Ionization of atmospheric air.= A large number of measurements
have been made during the last few years to determine the relative
amount of ionization in the atmosphere in different localities and at
different altitudes. Measurements of this character were first
undertaken by Elster and Geitel with a special type of electroscope. A
charged body exposed to the air was attached to a portable electroscope,
and the rate of loss of charge was observed by the movement of the gold
or aluminium leaf. The rates of discharge of the electroscope for
positive and negative electricity were generally different, the ratio
depending on the locality and the altitude, and on the meteorological
conditions. This apparatus is not suitable for quantitative measurements
and the deductions to be drawn from the observations are of necessity
somewhat indefinite.

Ebert[430] has designed a portable apparatus in which the number of ions
per c.c. of the air can be determined easily. A constant current of air
is drawn between two concentric cylinders by means of a fan actuated by
a falling weight. The inner cylinder is insulated and connected with an
electroscope. Knowing the capacity of the apparatus, and the velocity of
the current of air, the rate of movement of the gold-leaf affords a
measure of the number of ions present in unit volume of the air drawn
between the cylinders.

In this way Ebert found that the number of ions in the air was somewhat
variable, but on an average corresponded to about 2600 per c.c. in the
particular locality where the measurements were made.

This is the equilibrium number of ions present per c.c. when the rate of
production balances the rate of recombination. If _q_ is the number of
ions produced per second per unit volume of the air and _n_ is the
equilibrium number, then _q_ = α_n²_ where α is the constant of
recombination (section 30).

By a slight addition to the apparatus of Ebert, Schuster[431] has shown
that the constant of recombination for the particular sample of air
under investigation can be determined. The value so obtained for air in
the neighbourhood of Manchester was variable, and two or three times as
great as for dust-free air. The results of some preliminary measurements
showed that the number of ions present per c.c. of the air in different
localities varied from 2370 to 3660, while the value of _q_, the number
of ions produced per c.c. per second, varied between 12 and 38·5.

Rutherford and Allan and Eberts showed that the ions in the air had
about the same mobility as the ions produced in air by Röntgen rays and
radio-active substances. In some recent determinations by Mache and Von
Schweidler[432], the velocity of the positive ion was found to be about
1·02 cms. per second, and that of the negative 1·25 cms., for a
potential gradient of one volt per cm.

Langevin[433] has recently shown that in addition to these swift moving
ions, there are also present in the atmosphere some ions which travel
extremely slowly in an electric field. The number of these slowly moving
ions in the air in Paris is about 40 times as great as the number of the
swifter ions. This result is of great importance, for in the apparatus
of Ebert these ions escape detection, since the electric field is not
strong enough to carry them to the electrodes during the time of their
passage between the cylinders.


=283. Radio-activity of ordinary materials.= It has been shown that
radio-active matter seems to be distributed fairly uniformly over the
surface of the earth and in the atmosphere. The very important question
arises whether the small radio-activity observed is due to known or
unknown radio-elements present in the earth and atmosphere, or to a
feeble radio-activity of matter in general, which is only readily
detectable when large quantities of matter are present. The experimental
evidence is not yet sufficient to answer this question, but undoubted
proof has been obtained that many of the metals show a very feeble
radio-activity. Whether this radio-activity is due to the presence of a
slight trace of the radio-elements or is an actual property of the
metals themselves will be discussed in more detail in section 286.

Schuster[434] has pointed out that every physical property hitherto
discovered for one element has been found to be shared by all the others
in varying degrees. For example, the property of magnetism is most
strongly marked in iron, nickel, and cobalt, but all other substances
are found to be either feebly magnetic or diamagnetic. It might thus be
expected on general principles that all matter should exhibit the
property of radio-activity in varying degrees. On the view developed in
chapter X., the presence of this property is an indication that the
matter is undergoing change accompanied by the expulsion of charged
particles. It does not, however, by any means follow that because the
atom of one element in the course of time becomes unstable and breaks
up, that, therefore, the atoms of all the other elements pass through
similar phases of instability.

It has already been mentioned (section 8), that Mme Curie made a very
extensive examination of most of the elements and their compounds for
radio-activity. The electric method was used, and any substance
possessing an activity of ¹⁄₁₀₀ of that of uranium would certainly have
been detected. With the exception of the known radio-elements and the
minerals containing uranium and thorium, no other substances were found
to be radio-active even to that degree.

Certain substances like phosphorus[435] possess the property of ionizing
a gas under special conditions. The air which is drawn over the
phosphorus is conducting, but it has not yet been settled whether this
conductivity is due merely to ions formed at the surface of the
phosphorus or to ions produced by the phosphorus nuclei or emanations,
as they have been termed, which are carried along with the current of
air. It does not however appear that the ionization of the gas is in any
way due to the presence of a penetrating type of radiation such as is
emitted by the radio-active bodies. Le Bon (section 8) observed that
quinine sulphate, after being heated to a temperature below the melting
point and then allowed to cool, showed for a time strong phosphorescence
and was able rapidly to discharge an electroscope. The discharging
action of quinine sulphate under varying conditions has been very
carefully examined by Miss Gates[436]. The ionization could not be
observed through thin aluminium foil or gold-leaf, but appeared to be
confined to the surface of the sulphate. The current observed by an
electrometer was found to vary with the direction of the electric field,
indicating that the positive and negative ions had very different
mobilities. The discharging action appears to be due either to an
ionization of the gas very close to the surface by some short
ultra-violet light waves, accompanying the phosphorescence, or to a
chemical action taking place at the surface.

Thus, neither phosphorus nor quinine sulphate can be considered to be
radio-active, even under the special conditions when they are able to
discharge an electrified body. No evidence in either case has been found
that the ionization is due to the emission of a penetrating radiation.

No certain evidence has yet been obtained that any body can be made
radio-active by exposure to Röntgen rays or cathode rays. A metal
exposed to the action of Röntgen rays gives rise to a secondary
radiation which is very readily absorbed in a few centimetres of air. It
is possible that this secondary radiation may prove to be analogous in
some respects to the α rays from the radio-elements. The secondary
radiation, however, ceases immediately the Röntgen rays are cut off.
Villard[437] stated that a piece of bismuth produced a feeble
photographic action after it had been exposed for some time to the
action of the cathode rays in a vacuum. It has not however been shown
that the bismuth gives out rays of a character similar to those of the
radio-active bodies. The experiments of Ramsay and Cooke on the
production of apparent activity in inactive matter by the radiations
from radium have already been discussed in section 264.

The existence of a very feeble radio-activity of ordinary matter has
been deduced from the study of the conductivity of gases in closed
vessels. The conductivity is extremely minute, and special methods are
required to determine it with accuracy. A brief account will now be
given of the gradual growth of our knowledge on this important question.


=284. Conductivity of air in closed vessels.= Since the time of Coulomb
onwards several investigators have believed that a charged conductor
placed inside a closed vessel lost its charge more rapidly than could be
explained by the conduction leak across the insulating support.
Matteucci, as early as 1850, observed that the rate of loss of charge
was independent of the potential. Boys, by using quartz insulators of
different lengths and diameters, arrived at the conclusion that the
leakage must in part take place through the air. This loss of charge in
a closed vessel was believed to be due in some way to the presence of
dust particles in the air.

On the discovery that gases become temporary conductors of electricity
under the influence of Röntgen rays and the rays from radio-active
substances, attention was again drawn to this question. Geitel[438] and
C. T. R. Wilson[439] independently attacked the problem, and both came
to the conclusion that the loss of charge was due to a constant
ionization of the air in the closed vessel. Geitel employed in his
experiments an apparatus similar to that shown in Fig. 103. The loss of
charge of an Exner electroscope, with the cylinder of wire netting _Z_
attached, was observed in a closed vessel containing about 30 litres of
air. The electroscope system was found to diminish in potential at the
rate of about 40 volts per hour, and this leakage was shown not to be
due to a want of insulation of the supports.

Wilson, on the other hand, used a vessel of very small volume, in order
to work with air which could be completely freed from dust. In the first
experiments a silvered glass vessel with a volume of only 163 c.c. was
employed. The experimental arrangement is shown in Fig. 104.

[Illustration: Fig. 104.]

The conductor, of which the loss of charge was to be measured, was
placed near the centre of the vessel _A_. It consisted of a narrow strip
of metal with a gold-leaf attached. The strip of metal was fixed to the
upper rod by means of a small sulphur bead. The upper rod was connected
with a sulphur condenser with an Exner electroscope _B_ attached to
indicate its potential. The gold-leaf system was initially charged to
the same potential as the upper rod and condenser by means of a fine
steel wire which was caused to touch the gold-leaf system by the
attraction of a magnet brought near it. The rate of movement of the
gold-leaf was measured by means of a microscope provided with a
micrometer eye-piece. By keeping the upper rod at a slightly higher
potential than the gold-leaf system, it was ensured that the loss of
charge of the gold-leaf system should not be due in any way to a
conduction leakage across the sulphur bead.

The method employed by Wilson in these experiments is very certain and
convenient when an extremely small rate of discharge is to be observed.
In this respect the electroscope measures with certainty a rate of loss
of charge much smaller than can be measured by a sensitive electrometer.

Both Geitel and Wilson found that the leakage of the insulated system in
dust-free air was the same for a positive as for a negative charge, and
was independent of the potential over a considerable range. The leakage
was the same in the dark as in diffuse daylight. The independence of
leakage of the potential is strong evidence that the loss of charge is
due to a constant ionization of the air. When the electric field acting
on the gas exceeds a certain value, all the ions are carried to the
electrodes before recombination occurs. A saturation current is reached,
and it will be independent of further increase of the electric field,
provided, of course, a potential sufficiently high to cause a spark to
pass is not applied.

C. T. R. Wilson has recently devised a striking experiment to show the
presence of ions in dust-free air which is not exposed to any external
ionizing agency. Two large metal plates are placed in a glass vessel
connected with an expansion apparatus similar to that described in
section 34. On expanding the air, the presence of the ions is shown by
the appearance of a slight cloud between the plates. These condensation
nuclei carry an electric charge, and are apparently similar in all
respects to the ions produced in gases by X rays, or by the rays from
active substances.

Wilson found that the loss of charge of the insulated system was
independent of the locality. The rate of discharge was unaltered when
the apparatus was placed in a deep tunnel, so that it did not appear
that the loss of charge was due to an external radiation. From
experiments already described, however (section 279), it is probable
that about 30 per cent. of the rate of discharge observed was due to
a very penetrating radiation. This experiment of Wilson’s indicates
that the intensity of the penetrating radiation was the same in the
tunnel as at the earth’s surface. Wilson found that the ionization
of the air was about the same in a brass vessel as in one of glass,
and came to the conclusion that the air was spontaneously ionized.

Using a brass vessel of volume about 471 c.c., Wilson determined the
number of ions that must be produced in air per unit volume per second,
in order to account for the loss of charge of the insulated system. The
leakage system was found to have a capacity of about 1·1 electrostatic
units, and lost its charge at the rate of 4·1 volts per hour for a
potential of 210 volts, and 4·0 volts per hour for a potential of 120
volts. Taking the charge on an ion as 3·4 × 10⁻¹⁰ electrostatic units,
this corresponds to a production of 26 ions per second.

Rutherford and Allan[440] repeated the results of Geitel and Wilson,
using an electrometer method. The saturation current was observed
between two concentric zinc cylinders of diameter 25·5 and 7·5 cms.
respectively and length 154 cms. It was found that the saturation
current could practically be obtained with a potential of a few volts.
Saturation was however obtained with a lower voltage after the air had
remained undisturbed in the cylinders for several days. This was
probably due to the gradual settling of the dust originally present in
the air.

Later observations of the number of ions produced in air in sealed
vessels have been made by Patterson[441], Harms[442], and Cooke[443].
The results obtained by different observers are shown in the following
table. The value of the charge on an ion is taken as 3·4 × 10⁻¹⁰
electrostatic units:

              Material of   Number of ions Observer
              vessel          produced per
                           c.c. per second


              Silvered             36      C. T. R. Wilson
              glass

              Brass                26      „       „

              Zinc                 27      Rutherford and
                                           Allan

              Glass          53 to 63      Harms

              Iron                 61      Patterson

              Cleaned              10      Cooke
              brass

It will be shown later that the differences in these results are
probably due to differences in the radio-activity of the containing
vessel.


=285. Effect of pressure and nature of gas.= C. T. R. Wilson (_loc.
cit._) found that the rate of leakage of a charged conductor varied
approximately as the pressure of the air between the pressures examined,
viz. 43 mms. and 743 mms. of mercury. These results point to the
conclusion that, in a good vacuum, a charged body would lose its charge
extremely slowly. This is in agreement with an observation of Crookes,
who found that a pair of gold-leaves retained their charge for several
months in a high vacuum.

Wilson[444] at a later date investigated the leakage for different
gases. The results are included in the following table, where the
ionization produced in air is taken as unity:

                      Gas      Relative   (Relative
                               ionization ionization) /
                                          (density)


                Air            1·00       1·00

                Hydrogen       0·184      2·7

                Carbon dioxide 1·69       1·10

                Sulphur        2·64       1·21
                dioxide

                Chloroform     4·7        1·09

With the exception of hydrogen, the ionization produced in different
gases is approximately proportional to their density. The relative
ionization is very similar to that observed by Strutt (section 45) for
gases exposed to the influence of the α and β rays from radio-active
substances, and points to the conclusion that the ionization observed
may be due either to a radiation from the walls of the vessel or from
external sources.

Jaffé[445] has made a careful examination of the natural ionization in
the very heavy gas nickel-carbonyl, Ni(CO)₄, in a small silvered glass
vessel. The ionization of this gas was 5·1 times that of air at normal
pressure while its density is 5·9 times that of air. The leak of the
electroscope was nearly proportional to the pressures except at low
pressure, when the leak was somewhat greater than would be expected if
the pressure law held. The fact that a gas of such high density and
complicated structure behaves like the simpler and lighter gases is a
strong indication that the ionization itself is due to a radiation from
the walls of the vessel and not to a spontaneous ionization of the gas.

Patterson[446] examined the variation of the ionization of air with
pressure in a large iron vessel of diameter 30 cms. and length 20 cms.
The current between a central electrode and the cylinder was measured by
means of a sensitive Dolezalek electrometer. He found that the
saturation current was practically independent of the pressure for
pressures greater than 300 mms. of mercury. Below a pressure of 80 mms.
the current varied directly as the pressure. For air at atmospheric
pressure, the current was independent of the temperature up to 450° C.
With further increase of temperature, the current began to increase, and
the increase was more rapid when the central electrode was charged
negatively than when it was charged positively. This difference was
ascribed to the production of positive ions at the surface of the iron
vessel. The results obtained by Patterson render it very improbable that
the ionization observed in air is due to a spontaneous ionization of the
enclosed air: for we should expect the amount of this ionization to
depend on the temperature of the gas. On the other hand, these results
are to be expected if the ionization of the enclosed air is mainly due
to an easily absorbed radiation from the walls of the vessel. If this
radiation had a penetrating power about equal to that observed for the α
rays of the radio-elements, the radiation would be absorbed in a few
centimetres of air. With diminution of pressure, the radiations would
traverse a greater distance of air before complete absorption, but the
total ionization produced by the rays would still remain about the same,
until the pressure was reduced sufficiently to allow the radiation to
traverse the air space in the vessel without complete absorption. With
still further diminution of pressure, the total ionization produced by
the radiation, and in consequence the current observed, would vary
directly as the pressure.


=286. Examination of ordinary matter for radio-activity.= Strutt[447],
McLennan and Burton[448], and Cooke[449], independently observed about
the same time that ordinary matter is radio-active to a slight degree.
Strutt, by means of an electroscope, observed that the ionization
produced in a closed vessel varied with the material of the vessel. A
glass vessel with a removable base was employed and the vessel was lined
with the material to be examined. The following table shows the relative
results obtained. The amount of leakage observed is expressed in terms
of the number of scale divisions of the eye-piece passed over per hour
by the gold-leaf:

                    Material of     Leakage in
                    lining of       scale divisions
                    vessel          per hour


                    Tinfoil         3·3

                    „    another    2·3
                    sample

                    Glass coated    1·3
                    with phosphoric
                    acid

                    Silver          1·6
                    chemically
                    deposited on
                    glass

                    Zinc            1·2

                    Lead            2·2

                    Copper (clean)  2·3

                    „    (oxidized) 1·7

                    Platinum        2·0, 2·9, 3·9
                    (various
                    samples)

                    Aluminium       1·4

There are thus marked differences in the leakage observed for different
materials and also considerable differences in different samples of the
same metal. For example, one specimen of platinum caused nearly twice
the leakage of another sample from a different stock.

McLennan and Burton, on the other hand, measured by means of a sensitive
electrometer the ionization current produced in the air in a closed iron
cylinder 25 cms. in diameter and 130 cms. in length, in which an
insulated central electrode was placed. The open cylinder was first
exposed for some time at the open window of the laboratory. It was then
removed, the top and bottom closed, and the saturation current through
the gas determined as soon as possible. In all cases it was observed
that the current diminished for two or three hours to a minimum and then
very slowly increased again. In one experiment, for example, the initial
current observed corresponded to 30 on an arbitrary scale. In the course
of four hours the current fell to a minimum of 6·6, and 44 hours later
had risen to a practical maximum of 24. The initial decrease observed is
probably due to a radio-activity of the enclosed air or walls of the
vessel, which decayed rapidly with the time. The decay of the excited
activity produced on the interior surface of the cylinder when exposed
to the air was probably responsible for a part of the decrease observed.
McLennan ascribes the increase of current with time to a radio-active
_emanation_ which is given off from the cylinder, and ionizes the
enclosed air. On placing linings of lead, tin, and zinc in the iron
cylinder, considerable differences were observed both for the minimum
current and also for the final maximum. Lead gave about twice the
current due to zinc, while tin gave an intermediate value. These results
are similar in character to those obtained by Strutt.

McLennan and Burton also investigated the effect of diminution of
pressure on the current. The cylinder was filled with air to a pressure
of 7 atmospheres, and allowed to stand until the current reached a
constant value. The air was then allowed to escape and the pressure
reduced to 44 mms. of mercury. The current was found to vary
approximately as the pressure over the whole range. These results are
not in agreement with the results of Patterson already described, nor
with some later experiments of Strutt. McLennan’s results however point
to the conclusion that the ionization was mainly due to an emanation
emitted from the metal. Since the air was rapidly removed, a
proportionate amount of the emanation would be removed also, and it
might thus be expected that the current would vary directly as the
pressure. If this is the case the current through the gas at low
pressures should increase again to a maximum if time is allowed for a
fresh emanation to form.

H. L. Cooke, using an electroscopic method, obtained results very
similar to those given by Strutt. Cooke observed that a penetrating
radiation was given out from brick. When a brass vessel containing the
gold-leaf system was surrounded by brick, the discharge of the
electroscope was increased by 40 to 50 per cent. This radiation was of
about the same penetrating power as the rays from radio-active
substances. The rays were completely absorbed by surrounding the
electroscope with a sheet of lead 2 mms. in thickness. This result is in
agreement with the observation of Elster and Geitel, already mentioned,
that radio-active matter was present in clay freshly dug up from the
earth.

Cooke also observed that the ionization of the air in a brass
electroscope could be reduced to about one-third of its usual value if
the interior surface of the brass was carefully cleaned. By removing the
surface of the brass he was able to reduce the ionization of the
enclosed air from 30 to 10 ions per c.c. per second. This is an
important observation, and indicates that a large proportion of the
radio-activity observed in ordinary matter is due to a deposit of
radio-active matter on its surface. It has already been shown that
bodies which have been exposed in the presence of the radium emanation
retain a residual activity which decays extremely slowly. There can be
no doubt that the radium emanation is present in the atmosphere, and the
exposed surface of matter, in consequence, will become coated with an
invisible film of radio-active matter, deposited from the atmosphere. On
account of the slow decay of this activity it is probable that the
activity of matter exposed in the open air would steadily increase for a
long interval. Metals, even if they are originally inactive, would thus
acquire a fairly permanent activity, but it should be possible to get
rid of this by removing the surface of the metal or by chemical
treatment. The rapid increase of activity of all matter left in a
laboratory in which a large quantity of emanation has been released has
been drawn attention to by Eve[450]. This superficial activity, due to
the products radium D, E, and F, was mainly removed by placing the metal
in strong acid.

A number of experiments have been made by J. J. Thomson, N. R. Campbell,
and A. Wood in the Cavendish laboratory to examine whether the
radio-activity observed in ordinary matter is a specific property of
such matter or is due to the presence of some radio-active impurity. An
account of these experiments was given by Professor J. J. Thomson in a
discussion on the Radio-activity of Ordinary Matter at the British
Association meeting at Cambridge, 1904. The results[451], as a whole,
support the view that each substance gives out a characteristic type or
types of radiation and that the radiation is a specific property of the
substance. J. J. Thomson[452] has made experiments to observe the action
of different substances in cutting off the external very penetrating
radiation (section 279) observed by Cooke and McLennan. He found that
some substances cut off this external radiation, while others had little
if any effect. For example, the ionization in a closed vessel was
reduced 17 per cent. by surrounding it with a thick lead envelope; but,
on surrounding it with an equivalent absorbing thickness of water, or
water mixed with sand, no sensible diminution was observed. In other
experiments Wood[453] found that the diminution of the ionization by a
given screen depended upon the material of the vessel. For example, the
ionization in a lead vessel, surrounded by a lead screen, was reduced 10
per cent., while in an iron vessel it was reduced 24 per cent. He
concludes from his experiments that the ionization observed in a closed
vessel has a threefold origin. Part of it is due to an external
penetrating radiation, part to a secondary radiation set up by it, while
the remainder is due to an intrinsic radiation from the walls,
altogether independent of the external radiation.

In some experiments of Campbell[454], the variation of the ionization
current between two parallel plates was observed for a progressive
increase of the distance between them. The effects observed are shown in
Fig. 105. The curves at first rise rapidly, then bend over and finally
become a straight line. The knee of the curve is at a different distance
for the different substances. The shape of these curves indicates that
two types of radiation are present, one of which is readily absorbed in
the gas while the other, a more penetrating type of radiation, extends
over the whole distance between the plates. In another series of
experiments, one side of the testing vessel was of thin aluminium, and
the ionization current was observed when an exterior screen was brought
up to it. Lead gave a considerable increase, but the radiation from it
was readily absorbed by an interposed screen. The radiation emitted by
carbon and zinc was more than twice as penetrating as from lead.

[Illustration: Fig. 105.]

Attempts were made to see whether a radio-active emanation was given off
by dissolving solid substances and then keeping the solutions in a
closed vessel and afterwards testing the activity of the air drawn from
them. In some cases an emanation was observed, but the amount varied
with different specimens of the same material; in others no effect was
detected.

When linings of different substances were placed in a closed testing
vessel, the ionization current in most cases fell at first, passed
through a minimum, and then slowly increased to a maximum. For lead the
maximum was reached in 9 hours, for tin in 14 and for zinc in 18 hours.
These results indicate that an emanation is given off from the metal,
and that the amount reaches a maximum value at different intervals in
the various cases. This was confirmed by an examination of a piece of
lead which was left in radium-free nitric acid. Twenty times the normal
effect was observed after this treatment. This is probably due to the
increase of porosity of the lead which allows a greater fraction of the
emanation produced in the metal to diffuse out with the gas.

The activity observed in ordinary matter is extremely small. The lowest
rate of production of ions yet observed is 10 per cubic centimetre per
second in a brass vessel. Suppose a spherical brass vessel is taken of
capacity 1 litre. The area of the interior surface would be about 480
sq. cms. and the total number of ions produced per second would be about
10⁴. Now it has been shown, in section 252, that an α particle projected
from radium itself gives rise to 8·6 × 10⁴ ions before it is absorbed in
the gas. An expulsion of one α particle every 8 seconds from the whole
vessel, or of one α particle from each square centimetre of surface _per
hour_ would thus account for the minute conductivity observed. Even if
it were supposed that this activity is the result of a breaking up of
the matter composing the vessel, the disintegration of one atom per
second per gram, provided it was accompanied by the expulsion of an α
particle, would fully account for the conductivity observed.

While the experiments, already referred to, afford strong evidence that
ordinary matter does possess the property of radio-activity to a feeble
degree, it must not be forgotten that the activity observed is
excessively minute, compared even with a weak radio-active substance
like uranium or thorium. The interpretation of the results is
complicated, too, by the presence of the radium emanation in the
atmosphere, for we have seen that the surface of every body exposed to
the open air must become coated with the slowly changing transformation
products of the radium emanation. The distribution of radio-active
matter throughout the constituents of the earth renders it difficult to
be certain that any substance, however carefully prepared, is freed from
radio-active impurities. If matter in general is radio-active, it must
be undergoing transformation at an excessively slow rate, unless it be
supposed (see Appendix A) that changes of a similar character to those
observed in the radio-elements may occur without the appearance of their
characteristic radiations.

Footnote 384:

  Geitel, _Phys. Zeit._ 2, p. 116, 1900.

Footnote 385:

  C. T. R. Wilson, _Proc. Camb. Phil. Soc._ 11, p. 32, 1900. _Proc. Roy.
  Soc._ 68, p. 151, 1901.

Footnote 386:

  Elster and Geitel, _Phys. Zeit._ 2, p. 590, 1901.

Footnote 387:

  Elster and Geitel, _Phys. Zeit._ 3, p. 76, 1901.

Footnote 388:

  Rutherford and Allan, _Phil. Mag._ Dec. 1902.

Footnote 389:

  Allan, _Phil. Mag._ Feb. 1904.

Footnote 390:

  C. T. R. Wilson, _Proc. Camb. Phil. Soc._ 11, p. 428, 1902.

Footnote 391:

  C. T. R. Wilson, _Proc. Camb. Phil. Soc._ 11, p. 428, 1902; 12, p. 17,
  1903.

Footnote 392:

  C. T. R. Wilson, _Proc. Camb. Phil. Soc._ 12, p. 85, 1903.

Footnote 393:

  Allan, _Phys. Rev._ 16, p. 106, 1903.

Footnote 394:

  McLennan, _Phys. Rev._ 16, p. 184, 1903.

Footnote 395:

  Schmauss, _Annal. d. Phys._ 9, p. 224, 1902.

Footnote 396:

  Elster and Geitel, _Phys. Zeit_. 3, p. 574, 1902.

Footnote 397:

  Ebert and Ewers, _Phys. Zeit._ 4, p. 162, 1902.

Footnote 398:

  Sarasin, Tommasina and Micheli, _C. R._ 139, p. 917, 1905.

Footnote 399:

  J. J. Thomson, _Phil. Mag._ Sept. 1902.

Footnote 400:

  Ebert, _Sitz. Akad. d. Wiss. Munich_, 33, p. 133, 1903.

Footnote 401:

  J. J. Thomson, _Phil. Mag._ Sept. 1902.

Footnote 402:

  Adams, _Phil. Mag._ Nov. 1903.

Footnote 403:

  Bumstead and Wheeler, _Amer. Journ. Science_, 17, p. 97, Feb. 1904.

Footnote 404:

  Bumstead, _Amer. Journ. Science_, 18, July, 1904.

Footnote 405:

  Dadourian, _Amer. Journ. Science_, 19, Jan. 1905.

Footnote 406:

  H. S. Allen and Lord Blythswood, _Nature_, 68, p. 343, 1903; 69, p.
  247, 1904.

Footnote 407:

  Strutt, _Proc. Roy. Soc._ 73, p. 191, 1904.

Footnote 408:

  Himstedt, _Ann. d. Phys._ 13, p. 573, 1904.

Footnote 409:

  Elster and Geitel, _Phys. Zeit._ 5, No. 12, p. 321, 1904.

Footnote 410:

  Dorn, _Abhandl. d. Natur. Ges. Halle_, 25, p. 107, 1904.

Footnote 411:

  Schenck, Thesis Univ. Halle, 1904.

Footnote 412:

  Mache, _Wien. Ber._ 113, p. 1329, 1904.

Footnote 413:

  Curie and Laborde, _C. R._ 138, p. 1150, 1904.

Footnote 414:

  Blanc, _Phil. Mag._ Jan. 1905.

Footnote 415:

  Boltwood, _Amer. Journ. Science_, 18, Nov. 1904.

Footnote 416:

  Elster and Geitel, _Phys. Zeit._ 4, p. 522, 1903.

Footnote 417:

  Elster and Geitel, _Phys. Zeit._ 5, No. 1, p. 11, 1903.

Footnote 418:

  Vincenti and Levi Da Zara, _Atti d. R. Instit. Veneto d. Scienze_, 54,
  p. 95, 1905.

Footnote 419:

  Burton, _Phil. Mag._ Oct. 1904.

Footnote 420:

  Elster and Geitel, _Phys. Zeit._ 6, No. 3, p. 67, 1905.

Footnote 421:

  Rutherford and Allan, _Phil. Mag._ Dec. 1902.

Footnote 422:

  Elster and Geitel, _Phys. Zeit._ 4, p. 138, 1902; 4, p. 522, 1903.

Footnote 423:

  Saake, _Phys. Zeit._ 4, p. 626, 1903.

Footnote 424:

  Simpson, _Proc. Roy. Soc._ 73, p. 209, 1904.

Footnote 425:

  McLennan, _Phys. Rev._ 16, p. 184, 1903, and _Phil. Mag._ 5, p. 419,
  1903.

Footnote 426:

  McLennan, _Phys. Rev._ No. 4, 1903.

Footnote 427:

  Rutherford and Cooke, _Americ. Phys. Soc._ Dec. 1902.

Footnote 428:

  Cooke, _Phil. Mag._ Oct. 1903.

Footnote 429:

  Allan, _Phil. Mag._ Feb. 1904.

Footnote 430:

  Ebert, _Phys. Zeit._ 2, p. 622, 1901. _Zeitschr. f. Luftschiffahrt_,
  4, Oct. 1902.

Footnote 431:

  Schuster, _Proc. Manchester Phil. Soc._ p. 488, No. 12, 1904.

Footnote 432:

  Mache and Von Schweidler, _Phys. Zeit._ 6, No. 3, p. 71, 1905.

Footnote 433:

  Langevin, _C. R._ 140, p. 232, 1905.

Footnote 434:

  Schuster, British Assoc. 1903.

Footnote 435:

  J. J. Thomson, _Conduction of Electricity through Gases_, p. 324,
  1903.

Footnote 436:

  Miss Gates, _Phys. Rev._ 17, p. 499, 1903.

Footnote 437:

  Villard, _Société de Physique_, July, 1900.

Footnote 438:

  Geitel, _Phys. Zeit._ 2, p. 116, 1900.

Footnote 439:

  C. T. R. Wilson, _Proc. Camb. Phil. Soc._ 11, p. 52, 1900. _Proc. Roy.
  Soc._ 68, p. 152, 1901.

Footnote 440:

  Rutherford and Allan, _Phil. Mag._ Dec. 1902.

Footnote 441:

  Patterson, _Phil. Mag._ August, 1903.

Footnote 442:

  Harms, _Phys. Zeit._ 4, No. 1, p. 11, 1902.

Footnote 443:

  Cooke, _Phil. Mag._ Oct. 1903.

Footnote 444:

  Wilson, _Proc. Roy. Soc._ 69, p. 277, 1901.

Footnote 445:

  Jaffé, _Phil. Mag._ Oct. 1904.

Footnote 446:

  Patterson, _Phil. Mag._ Aug. 1903.

Footnote 447:

  Strutt, _Phil. Mag._ June, 1903. _Nature_, Feb. 19, 1903.

Footnote 448:

  McLennan and Burton, _Phys. Rev._ No. 4, 1903. J. J. Thomson,
  _Nature_, Feb. 26, 1903.

Footnote 449:

  Cooke, _Phil. Mag._ Aug. 6, 1903. Rutherford, _Nature_, April 2, 1903.

Footnote 450:

  Eve, _Nature_, March 16, 1905.

Footnote 451:

  See article in _Le Radium_, No. 3, p. 81, Sept. 15, 1904.

Footnote 452:

  J. J. Thomson, _Proc. Camb. Phil. Soc._ 12, p. 391, 1904.

Footnote 453:

  Wood, _Phil. Mag._ April, 1905.

Footnote 454:

  Campbell, _Nature_, p. 511, March 31, 1904. _Phil. Mag._ April, 1905.




                              APPENDIX A.
                       PROPERTIES OF THE α RAYS.


A brief account is given here of some investigations made by the writer
on the properties of the α rays from radium—investigations which were
not completed in time for the results to be incorporated in the text.

The experiments were undertaken primarily with a view of determining
accurately the value of _e_/_m_ of the α particle from radium, in order
to settle definitely whether or not it is an atom of helium. In the
previous experiments of the writer, Becquerel, and Des Coudres, on this
subject (sections 89, 90, and 91), a thick layer of radium in
radio-active equilibrium has been used as a source of α rays. Bragg
(section 103) has shown that the rays emitted from radium under such
conditions are complex, and consist of particles projected over a
considerable range of velocity. In order to obtain a homogeneous pencil
of rays it is necessary to use a very thin layer of a simple
radio-active substance as a source of rays. In the experiments that
follow, this condition was fulfilled by using a fine wire which was made
active by exposure for several hours in the presence of a large quantity
of radium emanation. By charging the wire negatively the active deposit
was concentrated upon the wire, which was made intensely active. The
active deposit initially contains radium A, B, and C. The activity of
radium A practically disappears in about fifteen minutes, and the α
radiation is then due entirely to the single product radium C, since
radium B is a rayless product. The activity of radium C decreases to
about 15 per cent. of its initial value after two hours.


=Magnetic deflection of the α rays.= The photographic method was
employed to determine the deviation of the pencil of rays in a magnetic
field. The experimental arrangement is shown in Fig. 106. The rays from
the active wire, which was placed in a slot, passed through a narrow
slit and fell normally on a photographic plate, placed at a known
distance above the slit. The apparatus was enclosed in a brass tube
which could be exhausted rapidly to a low pressure by means of a Fleuss
pump. The apparatus was placed in a strong uniform magnetic field
parallel to the plane of the slit. The magnetic field was reversed every
ten minutes, so that on developing the plate two narrow bands were
observed, the distance between which represented twice the deviation
from the normal of the pencil of rays by the magnetic field. The width
of the band was found to be the same whether the magnetic field was
applied or not, showing that the pencil of rays was homogeneous and
consisted of α particles projected with the same velocity.

[Illustration: Fig. 106.]

By placing the photographic plate at different distances from the slit
it was found that the rays, after entering the magnetic field, described
the arc of a circle of radius ρ equal to 42·0 cms. The strength of field
_H_ was 9470 C.G.S. units, so that the value of _H_ρ for the α particles
expelled from radium C is 398,000. This is in good agreement with the
maximum values of _H_ρ, previously found for radium rays (see section
92).

The electric deviation of the rays from radium C has not yet been
accurately measured, but an approximate determination of _e_/_m_ for the
α particles can be obtained by assuming that the heating effect of
radium C is a measure of the kinetic energy of the α particles expelled
from it. We have seen in section 246 that the heating effect of the
radium C present in one gram of radium in radio-active equilibrium is 31
gram calories per hour, which corresponds to an emission of energy of
3·6 × 10⁵ ergs per second. Now when radio-active equilibrium is reached,
the number of α particles expelled from radium C per second is equal to
the number of α particles expelled per second from radium at its minimum
activity. This number, _n_, is 6·2 × 10¹⁰ (section 93).

                   Then      ½ _mnv²_ = 3·6 × 10⁵,

                   or        (_m_/_e_)_v²_ = 1·03 × 10¹⁶,

substituting the value of _n_, and the value of the ionic charge _e_.
The value of _e_ in this case has not been assumed, since _n_ = _i_/_e_,
where _i_ was the measured current due to the charge carried by the α
rays.

From the magnetic deflection, it is known that

                         (_m_/_e_)_v_ = 3·98 × 10⁵.

From these two equations we obtain

                _v_ = 2·6 × 10⁹ cms. per second.
                _e_/_m_ = 6·5 × 10³ electromagnetic units.

These values are in surprisingly good agreement with the previous values
of the writer and Des Coudres (section 91). On account of the
uncertainty attaching to the value of _n_, not much weight can be
attached to the determination by this method of the constants of the α
particles.


=Decrease of velocity of the α particles in passing through matter.=
Some experiments were made to determine the velocity of the α particles
from radium C after passing through known thicknesses of aluminium. The
previous apparatus was employed, and the distance between the
photographic bands was observed for successive layers of aluminium foil,
each ·00031 cms. thick, placed over the active wire. The photographic
plate was placed 2 cms. above the slit, and the magnetic field extended
1 cm. below the slit. The amount of deviation of the rays is inversely
proportional to their velocity after traversing the aluminium screens.
The impressions on the plate were clear and distinct, and about the same
in all cases, showing that the rays were still homogeneous after passing
through the aluminium.

A clear photographic impression was obtained for 12 layers of foil, but
it was not found possible to obtain any effect through 13 layers. This
result shows that the photographic action of the rays, like the ionizing
action, ceases very abruptly.

The results obtained are shown in the following table. Assuming that the
value of _e_/_m_ is constant, the third column gives the velocity of the
α particles after traversing the aluminium. This is expressed in terms
of _V₀_, the velocity of the α particle when the screens are removed.

                    Number of     Distance   Velocity
                    layers of      between       of α
                     aluminum bands on the  particles
                         foil        plate

                            0    1·46 mms.       1·00
                                                  _V₀_

                            5      1·71  „   ·85    „

                            8      1·91  „   ·76    „

                           10      2·01  „   ·73    „

                           12      2·29  „   ·64    „

                           13           No
                              photographic
                                    effect

The velocity of the α particle is thus reduced only about 36 per cent.
of its initial value when it fails to produce any action on the
photographic plate.

Now Bragg has shown (section 104) that the α particle produces
approximately the same number of ions per cm. of path in air over its
whole range. Consequently, the simplest assumption to make is that the
energy of the α particle is diminished by a constant amount in
traversing each layer of foil. After passing through 12 layers the
kinetic energy is reduced to 41 per cent. of the maximum. Each layer of
foil thus absorbs 4·9 per cent. of the maximum energy. The observed
kinetic energy of the α particle after passing through successive layers
of foil, and the value calculated on the above assumptions, are shown in
the following table.

                     Number of   Observed Calculated
                     layers of     energy     energy
                      aluminum
                          foil

                             0        100        100

                             5         73         75

                             8         58         61

                            10         53         51

                            12         41         41

The experimental and theoretical values agree within the limits of
experimental error. We may thus conclude, as a first approximation, that
the same proportion of the total energy is abstracted from the α
particles in passing through equal distances of the absorbing screen.


=Range of ionization and photographic action in air.= The abrupt falling
off of the photographic impression after the rays had passed through 12
layers of foil suggested that it might be directly connected with the
corresponding abrupt falling off of the ionization in air, so clearly
brought out by Bragg. This was found to be the case. It was found
experimentally that the absorption in each layer of aluminium foil was
equivalent to that produced by a distance of ·54 cms. of air. Twelve
layers of foil thus corresponded to 6·5 cms. of air. Now Bragg found
that the α rays from radium C ionize the air for a distance 6·7 cms.,
and that the ionization then falls off very rapidly. We may thus
conclude that the α rays cease to affect the photographic plate at the
same velocity as that at which they cease to ionize the gas. This is a
very important result, and, as we shall see later, suggests that the
action on the photographic plate is due to an ionization of the
photographic salts.

The velocity of the α particles from the different radio-active products
can at once be calculated, knowing the maximum range in air of the α
rays from each product. The latter have been experimentally determined
by Bragg. The velocity is expressed in terms of _V₀_, the initial
velocity of the α particles from radium C. The rays from radium C are
projected with a greater velocity than the rays from the other products
of radium.

                    Product    Maximum    Velocity
                               range of α of α
                               particles  particles
                               in air


                    Radium     3 cms.     ·82 _V₀_

                    Emanation  3·8 or 4·4 ·87 or ·90
                               cms.       _V₀_

                    Rad. A     4·4 or 3·8 ·90 or ·87
                                „         _V₀_

                    Rad. C     6·7        1·00 _V₀_
                                „

It is difficult to determine from the experiments whether the range 3·8
cms. belongs to the rays from the emanation or from radium A. The mean
velocity of the α particles is thus ·90 _V₀_, and the maximum variation
for the individual products does not vary more than 10 per cent. from
the mean value.

The results of Becquerel, discussed in section 92, at once receive an
explanation on the above results. The α particles, expelled from radium
in radio-active equilibrium, have all ranges lying between 0 and 6·7
cms. of air. The velocity of the α particles which are able to produce a
photographic impression varies between ·64 _V₀_ and _V₀_. The particles
which have only a short range in air are projected with a smaller
velocity than those which have a greater range. The former are in
consequence more bent by a magnetic field. It is thus to be expected
that the apparent curvature of the path of rays in a uniform magnetic
field will be greater close to the radium than at some distance away.


=Range of phosphorescent action in air.= Some experiments were also made
to see whether the action of the α rays in producing luminosity in
substances like zinc sulphide, barium platinocyanide, and willemite,
ceased at the same distance as the ionizing action.

A very active wire was placed on a movable plate, the distance of which
from a fixed screen of phosphorescent substance could be varied. The
distance at which the phosphorescent action ceased could be determined
fairly accurately. Different thicknesses of aluminium foil were then
placed over the active wire, and the corresponding distance at which the
luminosity disappeared was measured. The results are shown graphically
in Fig. 107, where the ordinates represent the distance of the
phosphorescent screen from the active wire, and the abscissae the number
of layers of aluminium foil, each ·00031 cms. thick.

[Illustration: Fig. 107.]

It is seen that the curve joining the points is a straight line. 12·5
thicknesses of foil absorbed the rays to the same extent as 6·8 cms. of
air, so that each thickness of aluminium corresponded in absorbing power
to ·54 cms. of air. For a screen of zinc sulphide, the phosphorescent
action ceased at a distance of air of 6·8 cms., showing that the
photographic and phosphorescent ranges of the α rays in air were
practically identical.

The experiments with barium platinocyanide and willemite were more
difficult, as the β and γ rays from the active wire produced a
luminosity comparable with that produced by the α rays. Fairly
concordant results, however, were obtained by introducing a thin sheet
of black paper between the active wire and the screen. If the luminosity
was sensibly changed, it was concluded that the α rays still produced an
effect, and in this way the point of cessation of phosphorescent action
could be approximately determined. For example, with eight thicknesses
of foil over the active wire the additional thickness of air required to
cut off the phosphorescent effect of the a rays was 2·5 cms. for
willemite, and 2·1 cms. for barium platinocyanide.

The corresponding distance for zinc sulphide was 2·40 cms., a value
intermediate between the other two.

Since eight layers of foil are equivalent to 4·3 cms. of air, the ranges
in air of phosphorescent action for zinc sulphide, barium
platinocyanide, and willemite correspond to 6·7, 6·8, and 6·4 cms.
respectively. The differences observed are quite likely to be due to
experimental error.


=Discussion of results.= We have seen that the ionizing, phosphorescent,
and photographic actions of the α rays emitted from radium C cease after
traversing very nearly the same distance of air. This is a surprising
result when it is remembered that the α particle, after passing through
this depth of air, still possesses a velocity of at least 60 per cent.
of its initial value. Taking the probable value of the initial velocity
of the α particle from radium C as 2·5 × 10⁹ cms. per sec., the
ionizing, phosphorescent, and photographic actions cease when the
velocity of the α particle falls below 1·5 × 10⁹ cms. per second, that
is, a velocity of about ¹⁄₂₀ of that of light. The particle still
possesses nearly 40 per cent. of its initial energy of projection at
this stage.

These results show that the property of the α rays of producing
ionization in gases, of producing luminosity in some substances, and of
affecting a photographic plate, ceases when the velocity of the α
particle falls below a certain fixed value which is the same in each
case. It seems reasonable, therefore, to suppose that these three
properties of the α rays must be ascribed to a common cause. Now the
absorption of the α rays in gases is mainly a consequence of the energy
absorbed in the production of ions in the gas. When the α particles are
completely absorbed in the gas, the same total amount of ionization is
produced, showing that the energy required to produce an ion is the same
for all gases. On the other hand, for a constant source of radiation,
the ionization per unit volume of the gas is approximately proportional
to its density. Since the absorption of the α rays in solid matter is
approximately proportional to the density of the absorbing medium
compared with air, it is probable that this absorption is also a result
of the energy used up in producing ions in the solid matter traversed,
and that about the same amount of energy is required to produce an ion
in matter whether solid, liquid, or gaseous.

It is probable, therefore, that the production of ions in the
phosphorescent material and in the photographic film would cease at
about the same velocity for which the α particle is unable to ionize the
gas. On this view, then, the experimental results receive a simple
explanation. The action of the α rays in producing photographic and
phosphorescent actions is primarily a result of ionization. This
ionization may possibly give rise to secondary actions which influence
the effects observed.

This point of view is of interest in connection with the origin of the
“scintillations” observed in zinc sulphide and other substances when
exposed to the action of the α rays. This effect is ascribed by
Becquerel to the cleavage of the crystals under the bombardment of the α
particles. These results, however, show that we must look deeper for the
explanation of this phenomenon. The effect is primarily due to the
production of ions in the phosphorescent material and not to direct
bombardment, for we have seen that the α particle produces no
scintillations when it still possesses a large amount of kinetic energy.
It seems not unlikely that the scintillations produced by the α rays
must be ascribed to the recombination of the ions which are produced by
the α particle in the crystalline mass. It is difficult to see how this
ionization could result in a cleavage of the crystals.

This close connection of the photographic and phosphorescent actions of
the α rays with their property of producing ions, raises the question
whether photographic and phosphorescent actions in general may not, in
the first place, be due to a production of ions in the substance.


=Ionization curve for the α rays from radium C.= Mr McClung, working in
the laboratory of the writer, has recently determined the relative
ionization per unit path of the α particles projected from radium C,
using the method first employed by Bragg and discussed in section 104.
An active wire, exposed for several hours to the emanation from radium,
was used as a source of rays. The α particles were homogeneous, since
the film of radio-active matter was extremely thin.

The relation between the ionization observed over the cross section of
the narrow cone of rays and the distance from the source of rays is
shown in Fig. 108.

[Illustration: Fig. 108.]

The curve exhibits the same peculiarities as those given by Bragg for a
thin film of matter of one kind. The ionization of the α particle per
unit path increases slowly for about 4 cms. There is then a more rapid
increase just before the α particle ceases to ionize the gas, and then a
rapid falling off. The ionization does not appear to end so abruptly as
is really the case, since there is a correction to be applied for the
angle subtended by the cone of rays. The maximum range of the α rays in
air was 6·7 cms., a number in agreement with that obtained by Bragg by
measurements on the range of the rays from radium.

These results show that the ionization per unit path of the α particle
increases at first slowly and then rapidly with decrease of velocity
until the rays cease to ionize the gas.


=Energy required to produce an ion.= From the above results the energy
required to produce an ion by collision of the α particle with the gas
molecules can readily be deduced. The α particles, emitted from radium
itself, are initially projected with a velocity ·88_V₀_ where _V₀_ is
the initial velocity of projection of the α particles from radium C. The
α particles cease to ionize the gas at a velocity ·64_V₀_. From this it
can at once be deduced that ·48 of the total energy of the α particle,
shot out by radium itself, is absorbed when it ceases to ionize the gas.
Assuming that the heating effect of radium at its minimum activity—25
gram calories per hour per gram—is a measure of the kinetic energy of
the expelled α particles, it can be calculated that the kinetic energy
of each α particle is 4·7 × 10⁻⁶ ergs. The amount of energy absorbed
when the α particle just ceases to ionize the gas is 2·3 × 10⁻⁶ ergs.
Assuming that this energy is used up in ionization, and remembering that
the α particle from radium itself produces 86000 ions in its path
(section 252), the average energy required to produce an ion is 2·7 ×
10⁻¹¹ ergs. This is equivalent to the energy acquired by an ion moving
freely between two points differing in potential by 24 volts.

Townsend found that fresh ions were produced by an electron for a
corresponding difference of potential of 10 volts. Stark, from other
data, obtained a value 45 volts, while Langevin considers that 60 volts
is an average value. The value obtained by Rutherford and McClung for
ionization by X-rays was 175 volts, and is probably too high.


=Rayless changes.= We have seen that the α particles from the
radio-active substances are projected with an average velocity not more
than 30 per cent. greater than the minimum velocity, below which the α
particles are unable to produce any ionizing, photographic, or
phosphorescent action. Such a conclusion suggests that the property of
the radio-active substances of emitting α particles has been detected
because the α particles were projected slightly above this minimum
velocity. A similar disintegration of matter may be taking place in
other substances at a rate much greater than in uranium without
producing much electrical effect, provided the α particles are projected
below the critical velocity.

The α particle, on an average, produces about 100,000 ions in the gas
before it is absorbed, so that the electrical effect observed is about
100,000 times as great as that due to the charge carried by the α
particles alone.

It is not unlikely that the numerous rayless products which have been
observed may undergo disintegration of a similar character to the
products which obviously emit α rays. In the rayless product the α
particle may be expelled with a velocity less than 1·5 × 10⁹ cms. per
second and so fail to produce much electrical effect.

These considerations have an important bearing on the question whether
matter in general is radio-active. The property of emitting α particles
above the critical velocity may well be a property only of a special
class of substances, and need not be exhibited by matter in general. At
the same time the results suggest that ordinary matter may be undergoing
transformation accompanied by the expulsion of α particles at a rate
much greater than that shown by uranium, without producing appreciable
electrical or photographic action.




                              APPENDIX B.
                         RADIO-ACTIVE MINERALS.


Those natural mineral substances which possess marked radio-active
properties have been found to contain either uranium or thorium, one of
these elements being always present in sufficient proportion readily to
permit its chemical separation and identification by the ordinary
analytical methods[455].

A large number of uranium and thorium minerals are known at the present
time, but they are for the most part found very sparingly, and some of
them have been observed to occur only in a single locality. The chief
commercial sources of uranium are uraninite, gummite, and carnotite,
while thorium is obtained almost exclusively from monazite.

Rutherford and Soddy (_Phil. Mag._ 65, 561 (1903)), were the first to
call attention to the important fact that the relations between the
various radio-active substances and the other elements could best be
determined from the study of the natural minerals in which these bodies
occur, since these minerals represent mixtures of extreme antiquity,
which have remained more or less undisturbed for almost countless ages.
In dealing with these matters, however, it is highly important that we
bring to our aid the data furnished by geology and mineralogy, from
which it is often possible to determine the relative ages of the
different substances with at least a rough degree of approximation.
Thus, for example, if a certain mineral occurs as a primary constituent
of a rock of remote geological period, it can safely be assumed that its
age is greater than that of a similar or different mineral occurring in
a later formation. It is, moreover, quite evident that those minerals
which are obviously produced by the decomposition and alteration of the
primary minerals, through the action of percolating water and other
agencies acting from the surface downward, are of less antiquity than
the primary minerals from which they originated. Through the application
of these considerations it should, in general, be possible to arrange
the various minerals roughly in the order of their probable ages.

The most familiar and widely known uranium mineral is uraninite,
commonly called pitchblende, which consists essentially of uranium
dioxide (UO₂), uranium trioxide (UO₃), and lead oxide (PbO), present in
varying proportions. The uraninites can be distinguished as primary,
namely, those which occur as a primary constituent of pegmatitic dikes
and coarse granites, and secondary, when they occur in metalliferous
veins associated with the sulphides of silver, lead, copper, nickel,
iron, and zinc. The former varieties are quite frequently crystalline in
character, contain a larger proportion of the rare earths and helium,
and have a higher specific gravity than the latter, which are always
massive and botryoidal.

The following are the most prominent localities in which primary
uraninites occur:

1. North Carolina, U.S.A. (especially in Mitchell and Yancey counties).
The uraninite is found in a coarse pegmatitic dike which is mined for
the mica constituent. The associated feldspar of the dike is
considerably decomposed through the action of meteoric waters and gases,
and the uraninite itself is largely altered into the secondary minerals
gummite and uranophane through the same agencies. Among the associated
primary minerals are allanite, zircon, columbite, samarskite,
fergusonite and monazite, while the secondary minerals include gummite,
thorogummite, uranophane, autunite, phosphuranylite, hatchettolite, and
cyrtolite. The geological period of this formation is difficult to
establish with certainty, but is stated to be perhaps Archean, or
possibly to correspond with the close of the Ordovician or with the
Permian.

2. Connecticut, U.S.A. The best known localities are Glastonbury, where
the uraninite is found in the feldspar quarries, and Branchville, where
it occurs in an albitic granite. Both of these localities have furnished
fine crystals. The geological period probably corresponds with the close
of the Ordovician or Carboniferous eras, and is stated to be certainly
Post-Cambrian and Pre-Triassic. Among the associated minerals are
(primary) columbite, (secondary) torbernite and autunite.

3. Southern Norway, particularly in the neighbourhood of Moss. Here
uraninite occurs in the augite-syenite and pegmatite. The varieties
found are known as cleveite and bröggerite, and among the primary
associated minerals are orthite, fergusonite, monazite, and thorite. The
period is stated to be Post-Devonian.

4. Llano County, Texas. The variety of uraninite known as nivenite is
found here in a quartzose pegmatite, associated with the primary
minerals gadolinite, allanite and fergusonite, and the secondary
minerals cyrtolite, yttrialite, gummite, and thorogummite.

Secondary uraninite is found at Johanngeorgenstadt, Marienberg and
Schneeberg in Saxony, at Joachimsthal and Pribam in Bohemia, at
Cornwall in England, and at Black Hawk, Colorado, and in the Black
Hills, South Dakota, in the United States. The exact geological period
of most of these secondary occurrences is somewhat uncertain, but they
are undoubtedly very much later than the primary occurrences mentioned
above.

As a matter of general interest the analysis of a typical primary
uraninite (No. 1) and of a typical secondary uraninite (No. 2) is given
below[456]:

                                 No. 1               No. 2
                          Glastonbury, Johanngeorgenstadt,
                                 Conn.              Saxony

             Sp. Gr.              9·59               6·89

             UO₃                 26·48              60·05

             UO₂                 57·43              22·33

             ThO₂                 9·79                ...

             CeO₂                 0·25                ...

             La₂O                 0·13                ...

             Y₂O₃                 0·20                ...

             PbO                  3·26               6·39

             CaO                  0·08               1·00

             He                   und.               und.

             H₂O                  0·61               3·17

             Fe₂O₃                0·40               0·21

             SiO₂                 0·25               0·50

             Al₂O₃                ...                0·20

             Bi₂O₃                ...                0·75

             CuO                  ...                0·17

             MnO                  ...                0·09

             MgO                  ...                0·17

             Na₂O                 ...                0·31

             P₂O₅                 ...                0·06

             SO₃                  ...                0·19

             As₂O₃                ...                2·34

             Insoluble            0·70                ...

The following list comprises the more important radio-active minerals,
with their approximate chemical composition and some notes on their
occurrence and probable origin.

                 Name         Composition   Remarks


              Uraninite,      Oxides of     Occurs primary
              Cleveite,       uranium and   as a
              Bröggerite,     lead. Usually constituent of
              Nivenite,       contains      rocks and
              Pitchblende     thorium,      secondary in
                              other rare    veins with
                              earths and    metalliferous
                              helium.       sulphides
                              Uranium
                              50-80%.
                              Thorium 0-10%

              Gummite         (Pb, Ca)      An alteration
                              U₃SiO₁₂        product of
                              . 6H₂O?       uraninite.
                              Uranium       Formed by the
                              50-65%        action of
                                            percolating
                                            waters

              Uranophane,     CaO . 2UO₃     An alteration
              Uranotil        . 2SiO₂ .      product of
                              6H₂O          uraninite
                              Uranium       through
                              44-56%        gummite

              Carnotite       A vanadate of Occurs as a
                              uranium and   secondary
                              potassium.    mineral
                              Uranium       impregnating a
                              42-51%        porous,
                                            sedimentary
                                            sandstone.
                                            Found in
                                            Colorado and
                                            Utah

              Uranosphaerite  Bi₂O₃ .       Alteration
                              2UO₃ .        product of
                              3H₂O.         other uranium
                              Uranium 41%   minerals

              Torbernite,     CuO . 2UO₃    „      „
              Cuprouranite    . P₂O₅
                              . 8H₂O.
                              Uranium
                              44-51%

              Autunite,       CaO . 2UO₃    „      „
              Calciouranite   . P₂O₅
                              . 8H₂O.
                              Uranium
                              45-51%

              Uranocircite    BaO . 2UO₃    „      „
                              . P₂O₅
                              . 8H₂O.
                              Uranium 46%

              Phosphuranylite 3UO₃ .        „      „
                              P₂O₅ .
                              6H₂O.
                              Uranium
                              58-64%

              Zunerite        CuO . 2UO₃    „      „
                              . As₂O₅
                              . 8H₂O.
                              Uranium 46%

              Uranospinite    CaO . 2UO₃    „      „
                              . As₂O₅
                              . 8H₂O.
                              Uranium 49%

              Walpurgite      5Bi₂O₃        „      „
                              . 3UO₃ .
                              As₂O₅ .
                              12H₂O.
                              Uranium 16%

              Thorogummite    UO₃ .         A variety of
                              3ThO₂ .       gummite
                              3SiO₂ .
                              6H₂O?
                              Uranium 41%

              Thorite,        ThSiO₄.       A primary
              Orangite,       Uranium       constituent of
              Uranothorite    1-10%.        pegmatite
                              Thorium oxide dikes
                              48-71%

              Thorianite      Oxide of      Occurs as a
                              thorium,      primary
                              uranium, the  constituent of
                              rare earths   a pegmatite
                              and lead.     dike in
                              Contains a    Ceylon.
                              relatively    Geological age
                              large         probably
                              proportion of Archean
                              helium.
                              Uranium
                              9-10%.
                              Thorium oxide
                              73-77%

              Samarskite      Niobate and   Primary
                              tantalate of  constituent of
                              rare earths.  pegmatite
                              Uranium 8-10% dikes

              Fergusonite     Metaniobate   „      „
                              and tantalate
                              of rare
                              earths.
                              Uranium 1-6%

              Euxenite        Niobate and   „      „
                              titanate of
                              rare earths.
                              Uranium 3-10%

              Monazite        Phosphate of  „      „
                              the rare
                              earths,
                              chiefly
                              cerium.
                              Uranium
                              0·3-0·4%

Footnote 455:

  An apparent exception has been observed by Danne in the case of
  certain lead minerals which occur under peculiar conditions at
  d’Issy-l’Évêque, France. See p. 465.

Footnote 456:

  Hillebrand, _Am. J. Sci._ 40, 384 (1890); _ibid._ 42, 390 (1891).




                                 INDEX.


_The numbers refer to the pages._

 α rays
   discovery of, 141
   nature of, 141
   magnetic deviation of, 142 _et seq._
   electrostatic deviation of, 146
   velocity of, 148
   value of _e_/_m_ for, 148
   charge carried by, 151 _et seq._
   number of α particles expelled from one gram of radium, 155
   mass and energy of, 156
   origin of, in atomic disintegration, 157
   scintillations produced by, 158 _et seq._
   absorption of, by matter, 161 _et seq._
   increase of absorption with thickness of matter traversed, 163
   relative absorption of α rays from radio-elements, 164
   absorption of, by gases, 165 _et seq._
   connection between absorption and density, 169
   relation between ionization and absorption, 170
   theory of absorption of, 170 _et seq._
   range of ionization of, 172 _et seq._
   complexity of α rays from radium, 174 _et seq._
   effect of thickness of layer of radiating matter on emission of, 195
   relative ionization produced by α and β rays, 196 _et seq._
   phosphorescence by α rays, 202 _et seq._
   connection of, with radio-active changes, 235, 444 _et seq._, 455
   from the emanations, 263
   emission of energy from radio-elements in form of α rays, 419 _et
      seq._
   connection of heat emission of radium with α rays, 421 _et seq._
   number of ions produced by an α particle, 433
   absence of, in rayless changes, 454
   emission from active products, 454 _et seq._
   loss of weight due to expulsion of, 473
   α particles consist of helium, 479 _et seq._
   magnetic deflection of, from radium C, 543
   velocity and _e_/_m_ for, from radium C, 543 _et seq._
   diminution of velocity of, in passing through matter, 545
   diminution in velocity of, in passing through aluminium, 545
   velocity of, when ionization ceases, 545 _et seq._
   connection of phosphorescent, photographic, and ionization effects
      produced by, 546 _et seq._
   energy required to produce an ion by α rays, 551

 Abraham
   apparent mass of moving charged body, 71, 127

 Absorption
   law of, in gases, 64 _et seq._
   relative absorption of α, β and γ rays by matter, 111
   connection between absorption and ionization, 134 _et seq._, 170 _et
      seq._
   of β rays by solids, 134 _et seq._
   connection between absorption and density for β rays, 137
   of β rays in radio-active matter, 140
   of α rays by solids, 161 _et seq._
   of α rays in gases, 167, 170 _et seq._
   connection between absorption and density for α rays, 169
   theory of, 170 _et seq._
   of γ rays by solids, 179 _et seq._
   connection between absorption and density for γ rays, 181
   of rays from the emanations, 263
   of penetrating rays from the earth, 520, 540


 Actinium
   methods of separation of, 20 _et seq._
   properties of, 21
   similarity to “emanating substance” of Giesel, 21
   possible connection with radio-activity of thorium, 28
   emanation from, 249
   excited activity produced by, 311
   effect of magnetic field on excited activity from, 324
   separation of actinium X, 365
   decay of actinium X, 365
   source of actinium emanation, 365
   analysis of active deposit of, 366
   radiations from products of, 368
   penetrating power of β and γ rays from, 368
   products of, 369
   table of products of, 449
   possible origin of, 464

 Actinium A
   separation and period of, 367 _et seq._

 Actinium B
   period of, 368
   properties of, 368

 Actinium X
   separation and decay of, 364 _et seq._
   production of emanation by, 365

 Adams
   decay of activity of emanation from well water, 511
   decay of excited activity from the emanation, 511

 Age
   of radium, 457
   of sun and earth, 492 _et seq._

 Allan, S. J.
   increase with time of excited activity from atmosphere, 505
   radio-activity of snow, 506
   effect of conditions on decay of activity from air, 519, 523

 Allan and Rutherford
   decay of excited activity from the atmosphere, 503
   ionization of air in closed vessels, 534

 Allen, H. S. and Lord Blythswood
   radium emanation in Bath springs, 513

 Anderson and Hardy
   action of radium rays on the eye, 217

 Armstrong and Lowry
   radio-activity and phosphorescence, 444

 Arnold
   rays from phosphorescent substances, 4

 Aschkinass and Caspari
   action of radium rays on microbes, 216

 Atmosphere
   excited activity from, 501 _et seq._
   radio-activity of, due to emanations, 504
   diffusion of emanations into, from the earth, 507
   effect of temperature, pressure, &c. on radio-activity of, 517 _et
      seq._
   presence of very penetrating radiation in, 520
   comparison of radio-activity of, with radio-elements, 521 _et seq._
   amount of radium emanation in, 524 _et seq._
   ionization of, due to radium emanation, 526

 Atom
   number of per c.c., 54
   disintegration of, 234 _et seq._
   complex nature of, 235
   changing atoms, 444 _et seq._
   possible causes of disintegration of, 486
   evolution of, 496

 Atomic weight
   of radium by chemical methods, 17
   from spectroscopic evidence, 18
   emanations, 273
   of radio-elements and connection with radio-activity, 445



 β rays
   discovery of, 113
   magnetic deflection of, 114
   complexity of, 116
   examination by the electrical method, 118
   effect of, on a fluorescent screen, 119
   charge carried by the, 120 _et seq._
   electrostatic deviation of, 124
   velocity of, and value of _e_/_m_ for, 126
   variation of _e_/_m_ with velocity of, 127 _et seq._
   distribution of velocity amongst β particles, 131
   absorption of, 134 _et seq._
   variation of absorption with density, 136 _et seq._
   number of β particles stopped by matter, 137 _et seq._
   variation of intensity of, with thickness of layer, 140
   secondary β rays, 189 _et seq._
   relative ionization produced by α and β rays, 196
   relative energy emitted in form of α and β rays, 196 _et seq._
   phosphorescent action of, 201 _et seq._
   physical action produced by, 207 _et seq._
   chemical action of, 213
   physiological action of, 216
   from Ur X, 347
   from active deposit of radium, 377 _et seq._
   significance of appearance of, only in last radio-active changes, 455
   change of weight due to expulsion of, 473

 Barium platinocyanide
   phosphorescence of, under radium rays, 203
   change of colour due to radium rays, 205

 Barkla
   polarization of X rays, 80

 Barnes and Rutherford
   heating effect of radium emanation, 421, 429
   connection of heating effect with radio-activity, 421
   heating effect of active deposit, 425
   heating effect of γ rays, 429
   heating effect of emanation, 431
   division of heating effect among active products, 433

 Bary
   phosphorescence under radium rays, 202

 Baskerville
   activity of thorium, 29
   phosphorescence of kunzite under radium rays, 203

 Baskerville and Kunz
   phosphorescence of substances under radium rays, 204

 Beattie, Smolan and Kelvin
   discharging power of uranium rays, 7

 Becquerel
   rays from calcium sulphide, 4
   rays from uranium, 5 _et seq._
   permanence of uranium rays, 6
   discharging power of uranium rays, 6
   magnetic deflection of radium rays by photographic method, 114 _et
      seq._
   curvature of radium rays in a magnetic field, 115 _et seq._
   complexity of radium rays, 116 _et seq._
   electrostatic deflection of β rays of radium, 124 _et seq._
   value of _e_/_m_ for β rays of radium, 126 _et seq._
   magnetic deviation of α rays of radium and polonium, 145
   trajectory of rays of radium in magnetic field, 148
   scintillations due to cleavage of crystals, 160
   γ rays from radium, 179
   secondary rays produced by active substances, 187
   phosphorescence produced by radium rays, 201
   conductivity of paraffin under radium radiation, 210
   effect of temperature on uranium rays, 210
   chemical action of radium rays, 214
   removal of activity from uranium by precipitation with barium, 219
   theory of radio-activity, 438

 Bemont et M. et Mme Curie
   discovery of radium, 13

 Berndt
   spectrum of polonium, 23

 Blanc
   thorium in sediments from hot springs, 514

 Blythswood, Lord and Allen, H. S.
   radium emanation in Bath springs, 513

 Bödlander and Runge
   evolution of gases from radium, 215

 Boltwood
   origin of radium, 460
   amount of radium in minerals, 460
   proportionality of uranium and radium in minerals, 461
   production of lead by uranium, 484
   radium emanation in spring water, 514
   method of standardization of amount of emanation in waters, 514

 Boys
   rate of dissipation of charge, 531

 Bragg and Kleeman
   theory of absorption of α rays, 172 _et seq._
   relation between ionization and absorption, 174 _et seq._
   range of α rays in air, 174
   four sets of α rays from radium, 174 _et seq._

 Bronson
   use of steady deflection method with an electrometer, 104
   decay of thorium emanation, 242
   decay of excited activity from actinium, 312

 Brooks, Miss
   variation of excited activity from thorium for short exposures, 304
   effect of dust on distribution of excited activity, 305
   decay curves of excited activity of radium measured by α and β rays,
      307 _et seq._
   decay curves of excited activity from actinium, 312

 Brooks and Rutherford
   absorption of α rays by matter, 161
   comparison of absorption of α rays from radio-elements, 164
   diffusion of radium emanation, 270
   decay of excited activity from radium, 306

 Bumstead
   presence of thorium emanation in atmosphere, 512

 Bumstead and Wheeler
   diffusion of radium emanation, 273
   emanation from surface water and the soil, 512, 522
   identity of emanation from soil with radium emanation, 512, 522

 Burton
   radium emanation in petroleum, 516

 Burton and McLennan
   penetrating radiation from the earth, 520
   radio-activity of ordinary materials, 537
   emanation from ordinary matter, 538


 Campbell
   radio-activity of ordinary materials, 540

 Canal rays
   discovery of, 78
   magnetic and electric deflection of, 78
   value of _e_/_m_ for, 78
   similarity of, to α rays, 110

 Capacity
   of electroscopes, 87
   of electrometers, 94, 102
   standards of, 102

 Carbonic acid
   radio-activity of natural, 516

 Caspari and Aschkinass
   action of radium rays on microbes, 216

 Cathode rays
   discovery of, 73
   magnetic and electric deflection of, 74
   value of _e_/_m_ for, 75
   radiation of energy from, 79
   comparison of, with β rays, 120
   absorption of, by matter, 136, 137
   _see also_ β rays

 Caves
   radio-active matter present in air of, 514 _et seq._
   radio-activity of air of, due to emanation from the soil, 515

 Changes
   (_see_ Transformations)

 Charge
   carried by the ions, 50 _et seq._
   negative charge carried by β rays, 120
   measurement of charge carried by β rays, 121 _et seq._
   positive charge carried by α rays, 145
   measurement of charge carried by α rays, 151 _et seq._

 Chemical nature
   of emanation, 267
   of active deposit, 312

 Chemical actions of radium rays
   production of ozone, 213
   coloration of glass and rock-salt, 213
   on phosphorus, 214
   on iodoform, 214
   on globulin, 214
   evolution of hydrogen and oxygen, 215

 Child
   potential gradient between electrodes, 65
   variation of current with voltage for surface ionization, 66

 Clouds
   formation of, by condensation of water round ions, 46 _et seq._
   difference between positive and negative ions in formation of, 49

 Collie and Ramsay
   spectrum of emanation, 292

 Collision
   ionization by, 39, 57
   number of ions produced by β rays per cm. of path, 434
   total number of ions produced by collisions of α particles, 434

 Coloration
   of crystals of radiferous barium, 15
   of bunsen flame by radium, 15
   of glass by radium rays, 213
   of rock-salt, fluor-spar and potassium sulphate by radium rays, 213

 Concentration
   of excited activity on negative electrode, 297
   activity of radium independent of, 466

 Condensation
   of water round the ions, 46 _et seq._
   of emanations, 277
   experimental illustration of, 279
   temperature of, 280
   difference between point of, for emanations of thorium and radium,
      283
   from air sucked up from the earth, 510

 Conductivity
   of gases exposed to radiations, 31 _et seq._
   variation of, with pressure, 61 _et seq._
   variation of, with nature of gas, 64
   comparison of, for gases exposed to α, β, and γ rays, 64
   comparison of, when exposed to γ rays and to hard X rays, 184
   of insulators, 209
   of air in caves and cellars, 507 _et seq._
   of air in closed vessels, 531 _et seq._
   variation of, in closed vessels with pressure and nature of gas, 534
   variation of, with temperature for air in closed vessels, 536
   increase of, with time, in a closed vessel, 537

 Conservation of radio-activity
   examples of, 469 _et seq._

 Cooke, H. L.
   penetrating rays from the earth, 520
   number of ions per c.c. in closed vessels, 534
   radio-activity from ordinary matter, 536

 Cooke, W. T. and Ramsay
   radio-activity produced by radiations of radium, 472

 Corpuscle
   (_see_ Electron)

 Crookes, Sir W.
   spectrum of radium, 17
   spectrum of polonium, 23
   nature of cathode rays, 73
   nature of α rays, 142
   scintillations produced by radium, 158
   spinthariscope, 158
   number of scintillations independent of pressure and temperature, 159
   phosphorescence of diamond, 204
   separation of Ur X, 219
   theory of radio-activity, 441

 Crookes and Dewar
   absence of nitrogen spectrum in phosphorescent light of radium at low
      pressures, 206

 Crystallization
   effect of, on activity of uranium, 349

 Curie, Mme
   permanence of uranium rays, 6
   discovery of radio-activity of thorium, 10
   radio-activity of uranium and thorium minerals, 11
   relative activity of compounds of uranium, 12
   coloration of radium crystals, 15
   spectrum of radium, 16
   discovery of polonium, 22
   nature of a rays, 142
   absorption of α rays from polonium, 163
   secondary radiation tested by electric method, 188
   slowly decaying excited activity from radium, 311
   recovery of activity of radium, 375
   bismuth made active by solution of barium, 417

 Curie, P.
   magnetic deviation of radium rays by electric method, 114
   conductivity of dielectrics under radium rays, 209
   radio-activity of radium unaffected by temperature, 210
   decay of activity of radium emanation, 247
   discovery of excited radio-activity from radium, 295
   heat emission of radium at low temperature and variation of heat
      emission with age of radium, 419
   nature of the emanation, 439

 Curie, M. et Mme
   discovery of radium, 13
   charge carried by β rays, 121
   luminosity of radium compounds, 205
   production of ozone by radium rays, 213
   coloration of glass by radium rays, 213
   theory of radio-activity, 439
   possible absorption by radio-elements of unknown radiations, 442

 Curie, J. et P.
   quartz piezo-électrique, 105 _et seq._

 Curie, P. et Danne
   diffusion of radio-active emanation, 272
   decay of excited activity from radium, 309
   decay curves of radium and equation, 309
   occlusion of radium emanation in solids, 310
   changes in radium, 381
   effect of temperature on active deposit, 390

 Curie, P. and Debierne
   evolution of gas from radium, 215
   active gases evolved from radium, 251
   phosphorescence produced by radium emanation, 252
   distribution of luminosity, 252
   rate of production of emanation independent of pressure, 266
   effect of pressure on amount of excited activity, 266, 317

 Curie and Dewar
   production of helium by radium, 479

 Curie, P. and Laborde
   heat emission of radium, 419
   origin of heat from radium, 440
   radium emanation in waters of hot springs, 514

 Current
   through gases, 31 _et seq._
   variation of, with distance between the plates, 59 _et seq._
   variation of, with pressure of gas, 61 _et seq._
   variation of, with nature of gas, 64
   measurement of, by galvanometer, 84
   measurement of, by electroscope, 85 _et seq._
   measurement of, by electrometer, 90 _et seq._
   measurement of, by quartz piezo-électrique, 105


 Dadourian
   presence of thorium emanation in the earth, 512

 Danne
   on deposit of radium not containing uranium, 465

 Danne et Curie
   diffusion of radio-active emanation, 272
   decay of excited activity from radium, 309
   decay curves of radium and equation, 309
   occlusion of radium emanation in solids, 310
   changes in radium, 381
   effect of temperature on active deposit, 390

 Danysz
   action of radium rays on skin, 216

 Darwin, G. H.
   age of sun, 492

 Debierne
   actinium, 21
   emanation from actinium, 249
   decay of excited activity from actinium, 311
   effect of magnetic field on activity excited from actinium, 324
   barium made active by actinium, 417

 Debierne and Curie
   evolution of gas from radium, 215
   active gases evolved from radium, 251
   phosphorescence produced by radium emanation, 252
   distribution of luminosity, 252
   rate of production of emanation independent of pressure, 266
   effect of pressure on amount of excited activity, 266, 317

 Decay
   of activity of Th X, 221
   of activity of Ur X, 223
   significance of law of, 229
   effect of conditions on the rate of, 232
   of activity of thorium emanation, 241
   of activity of radium emanation, 247
   of activity of actinium emanation, 249
   of excited activity due to thorium for long exposure, 302
   of excited activity due to thorium for short exposure, 304
   of excited activity due to radium, 306 _et seq._
   excited activity of slow decay due to radium, 311
   of excited activity from actinium, 311
   of radium A, B and C, 377 _et seq._
   of radium D, E and F, 397 _et seq._
   of heating effect of emanation, 423
   of excited activity from atmosphere, 502
   of activity of rain and snow, 506
   of emanation from the earth, 508
   differences in, of excited activity from atmosphere, 521 _et seq._

 Demarçay
   spectrum of radium, 16

 Deposit, active
   connection of, with excited activity, 301
   physical and chemical properties of, 312
   electrolysis of, 313
   effect of temperature on, 315
   effect of pressure on distribution of, 317
   transmission of, by positive carriers, 318 _et seq._
   nomenclature of, 328
   theory of changes in, 331 _et seq._
   theory of activity due to, 337 _et seq._
   theory of rayless change in, 341 _et seq._
   of thorium, 302 _et seq._, 351 _et seq._
     analysis of, 351
     rayless change in, 352
     effect of temperature on, 354
     period of products of, 355
   of actinium, 311 _et seq._
     decay curves of, 311
     analysis of, 367
     rayless change in, 367
     period of products of, 368
     radiations from, 368
   of radium, 376 _et seq._
     connection of excited activity with, 306
     general analysis of, 376 _et seq._
     analysis of, of rapid change, 377 _et seq._
     analysis of α ray curves, 377
     α ray curves of, 378
     β ray curves of, 379
     analysis of β ray curves, 381
     equations of activity curves, 389
     effect of temperature on, 390
     volatility of, 391
     of slow transformation, 311, 397
     variation of α ray activity of, 398
     variation of β ray activity of, 399
     separation of constituents of, 401 _et seq._
     successive products in, 402
     variation of activity of, for long periods, 407
     presence in old radium, 408
     effect of, on variation of activity of radium with time, 409
     presence in pitchblende, 410
     connection with radio-tellurium, 411
     connection with polonium, 411, 412
     connection with radio-lead, 413
     connection of, with radio-active induction, 415 _et seq._
     heat emission of, 425 _et seq._
     use of, to determine number of β particles from radium, 435
     use of, as source of α rays, 543

 Des Coudres
   magnetic and electric deviation of α rays, 148
   determination of _e_/_m_ for α rays, 148

 Dewar
   emission of heat from radium in liquid hydrogen, 420

 Dewar and Crookes
   absence of nitrogen spectrum in phosphorescent light of radium at low
      pressures, 206

 Dewar and Curie
   production of helium by radium, 479

 Dielectrics
   condition of, under radium rays, 209

 Diffusion
   of ions, 51 _et seq._
   of radium emanation into gases, 270
   of thorium emanation into gases, 275
   of radium emanation into liquids, 276

 Discharge
   action of rays on spark and electrodeless, 208

 Disintegration
   account of theory of, 234, 325, 445
   list of products of, 449
   rate of, in radio-elements, 457
   emission of energy in consequence of, 474 _et seq._
   helium a product of, 476 _et seq._
   possible causes of, 486 _et seq._
   of matter in general, 496 _et seq._

 Dissipation of charge
   in caves and cellars, 514 _et seq._
   in closed vessels, 531
   effect of pressure and nature of gas on, 534 _et seq._
   effect of material of vessel on, 536 _et seq._

 Dolezalek
   electrometer, construction of, 94 _et seq._

 Dorn
   charge carried by β rays, 122
   electrostatic deflection of β rays from radium, 124
   discovery of radium emanation, 246
   effect of moisture on emanating power of thorium, 255
   electrolysis of radium solution, 313
   loss of weight of radium, 474
   radium emanation in springs, 513

 Dreyer and Salomonsen
   coloration of quartz by radium rays, 213

 Dunston
   analysis of thorianite, 486

 Durack
   ionization by collision of electrons of great velocity, 171

 Dust
   effect of, on recombination of ions, 42
   effect of, on distribution of excited activity, 305


 Earth
   amount of radium in, 493 _et seq._
   age of, 496
   excited activity deposited on, 504
   activity concentrated on peaks of, 504
   emanation from, 507
   very penetrating radiation from, 520

 Ebert
   condensation of emanation from the earth, 510
   apparatus for determining number of ions per c.c. in air, 527
   velocity of ions in air, 528

 Ebert and Ewers
   emanation from the earth, 508

 Electrolysis
   separation of radio-tellurium by, 25
   of solutions of active deposit, 313
   of radium solutions, 313
   of thorium solutions, 314

 Electrometer
   description of, 90 _et seq._
   use of, in measurements, 90
   construction of, 91 _et seq._
   Dolezalek, 94
   adjustment and screening of, 95
   special key for, 97
   application of, to measurements of radio-activity, 97 _et seq._
   measurement of current by, 100
   capacity of, 101
   use with steady deflection, 103
   use with quartz piezo-électrique, 105


 Electron
   definition of, 56
   production of, under different conditions, 76 _et seq._
   identity of β rays with electrons, 120 _et seq._
   variation of apparent mass of electron with velocity, 127 _et seq._
   evidence that mass of electron is electromagnetic, 129 _et seq._
   diameter of, 131

 Electroscope
   description of, used by Curie, 85
   construction of, for accurate measurements, 86
   use of, in measurements of minute currents, 86
   of C. T. R. Wilson, 89
   use of, in measuring conductivity of air in closed vessels, 531 _et
      seq._
   use of, for determining radio-activity of ordinary matter, 537

 Elster and Geitel
   radio-active lead, 27
   effect of magnetic field on conductivity produced in air by β rays,
      113
   scintillations produced by active substances, 158
   action of radium rays on spark, 208
   photo-electric action of body, coloured by radium rays, 214
   radio-active matter in earth, 494
   discovery of excited activity in atmosphere, 501
   emanations from the earth, 507
   radio-activity of air in caves, 507
   radio-activity of the soil, 515
   radio-activity of fango, 516
   variation of radio-activity in atmosphere with meteorological
      conditions, 517
   effect of temperature and pressure on atmospheric radio-activity, 518

 Emanation
   of thorium, discovery and properties of, 238
   methods of measurement of, 240
   decay of activity of, 241
   effect of thickness of layer on amount of, 243
   increase of, with time to a maximum, 245
   of radium, 246
   decay of activity of, 247
   of actinium, properties of, 249
   of radium, phosphorescence produced by, 251
   rate of emission of, 254
   effect of conditions on rate of emission of, 255
   regeneration of emanating power, 256
   continuous rate of production of, 257
   source of thorium emanation, 261
   source of radium and actinium emanation, 263
   radiations from, 263
   effect of pressure on production of, 265
   chemical nature of, 267
   experiments to illustrate gaseous nature of, 268
   rate of diffusion of radium emanation, 269
   rate of diffusion of thorium emanation, 275
   diffusion of, into liquids, 276
   condensation of, 277
   temperature of condensation of, 280
   volume of, from one gram of radium and thorium, 288
   measurement of volume of, from radium, 289
   diminution of volume of, 290
   spectrum of emanation, 292
   connection between emanation and excited activity, 298
   effect of removal of, on activity of radium, 371 _et seq._
   fraction of activity of radium due to, 374
   effect of rate of escape of, on activity of radium, 374
   heat emission of, 420, 431
   variation of heat emission with time, 421 _et seq._
   enormous emission of energy from emanation, 431
   radio-activity of atmosphere due to emanations, 504
   sucked up from the earth, 507
   in caves, 507 _et seq._
   rate of decay of activity of, from the earth, 508
   condensation of, from the atmosphere, 510
   in well water and springs, 510 _et seq._
   from “fango,” 516
   effect of meteorological conditions on amount of, in atmosphere, 517
      _et seq._
   from metals, 538

 Emanating power
   measurement of, 254
   effect of conditions on, 255
   regeneration of, 256

 Emanium or “emanating substance” of Giesel (_see_ Actinium)
   discovery of, 21
   separation and properties of, 21
   similarity of, to actinium, 21
   emanation from, 249
   excited activity produced by, 311
   action of an electric field on, 323

 Energy
   of α particle, 156
   of β particle, 196
   comparison of, for α and β particles, 196
   emitted from radium in form of heat, 419 _et seq._
   emission of, from the emanation, 431
   emission of, from radio-active products of radium, 433
   total emission of, from 1 gram of radio-elements, 474 _et seq._
   latent store of, in matter, 475

 Eve
   conductivity of gases exposed to X rays, 64
   conductivity of gases exposed to X rays and γ rays, 183, 184
   secondary rays produced by β and γ rays, 189 _et seq._
   magnetic deflection of secondary rays from γ rays, 193
   variation of activity of radium with concentration, 467
   amount of radium emanation in the atmosphere, 524 _et seq._
   ionization due to emanation in atmosphere, 526 _et seq._

 Evolution of matter
   evidence of, 497

 Ewers and Ebert
   emanation from the earth, 508


 Excited radio-activity
   discovery and properties of, 295 _et seq._
   concentration of, on negative electrode, 297
   connection of, with the emanations, 298
   removal of, by acids, 300
   decay of, due to thorium, 302
   decay of, for short exposure to thorium, 304
   effect of dust on distribution of, 305
   decay curves for different times of exposure, 306 _et seq._
   decay of, from radium, 306
   decay curves of, measured by α rays, 308
   decay curves of, measured by β rays, 309
   decay curves of, from actinium, 311
   of radium, of very slow decay, 311
   effect of solution on, 312
   electrolysis of active solutions, 313
   effect of temperature on, 315
   variation with electric field, of amount of, 316
   effect of pressure on distribution of, 317
   transmission of, 318
   from actinium and emanium, 323
   heat emission due to, 425 _et seq._
   from the atmosphere, 501 _et seq._
   decay of, 502
   due to emanation in atmosphere, 504
   distribution of, on surface of the earth, 504
   concentration of, on prominences of the earth, 504
   of rain and snow, 506
   produced by emanation from tap water, 510
   effect of meteorological conditions on amount of, 517 _et seq._
   amount of, at Niagara Falls, 520
   rate of decay of, dependent on conditions, 522 _et seq._

 Exner and Haschek
   spectrum of radium, 17

 Eye
   action of radium rays on, 217


 Fehrle
   distribution of excited activity on a plate in electric field, 318

 Fluorescence
   produced in substances by radium rays, 18
   produced in substances by radium and polonium rays, 201 _et seq._

 Fog
   large amount of excited activity during, 518

 Forch
   loss of weight of radium, 474


 γ rays
   relative conductivity of gas exposed to γ and hard X rays, 64, 184
   discovery of, 179
   absorption of, by matter, 179 _et seq._
   connection between absorption of, and density, 182
   discussion of nature of rays, 182 _et seq._
   secondary rays produced by γ rays, 189
   measurement of radio-activity by means of, 442, 467
   conservation of radio-activity measured by, 471

 Gases
   evolved by radium, 215
   presence of helium in gases from radium, 216

 Gates, Miss F.
   effect of temperature on excited activity, 315
   discharge of quinine sulphate, 530

 Geitel
   natural conductivity of air in closed vessels, 501, 531

 Geitel and Elster
   radio-active lead, 27
   effect of magnetic field on conductivity produced by radium rays, 113
   scintillations produced by active substances, 158
   action of radium rays on spark, 208
   photo-electric action of bodies coloured by radium rays, 214
   radio-active matter in earth, 494
   discovery of radio-active matter in atmosphere, 501
   emanations from the earth, 507
   radio-activity of air in caves, 507
   radio-activity of the soil, 515
   radio-activity of fango, 516
   variation of radio-activity of air with meteorological conditions,
      517
   effect of temperature and pressure on radio-activity in atmosphere,
      518

 Giesel
   coloration of bunsen flame by radium, 15
   separation of radium by crystallization of bromide, 15
   emanating substance, 21
   radio-active lead, 27
   magnetic deviation of β rays, 113
   decrease with time of luminosity of radio-active screen, 205
   spectrum of phosphorescent light of emanium due to didymium, 206, 207
   coloration of bodies by radium rays, 213
   evolution of gases from radium, 215
   action of radium rays on the eye, 217
   emanation from the emanating substance, 250
   luminosity produced by radium emanation, 251
   decay of excited activity of emanium, 312
   activity of radium dependent on age, 371
   bismuth made active by radio-active solution, 417
   temperature of radium bromide above air, 420

 Gimingham and Rossignol
   decay of thorium emanation, 242

 Glass
   coloration produced in, by radium rays, 213
   phosphorescence produced in, by emanation, 252

 Glew
   simple form of spinthariscope, and scintillations, 159

 Globulin
   action of radium rays on, 214

 Godlewski
   effect of crystallization on activity of uranium, 349
   diffusion of uranium X, 350
   separation of actinium X, 365
   source of actinium emanation, 365
   recovery and decay curves of actinium, 366
   penetrating power of β and γ rays from actinium, 368
   radiations from active products, 368

 Goldstein
   canal rays, 78
   coloration of bodies by radium rays, 213

 Gonder, Hofmann, and Wölfl
   properties of radio-active lead, 27, 413

 Grier and Rutherford
   magnetic deviation of β rays of thorium, 114
   relative current due to α and β rays, 195
   nature of rays from Ur X, 347


 Hardy
   coagulation of globulin by radium rays, 214

 Hardy and Miss Willcock
   coloration of iodoform solutions by radium rays, 214

 Hardy and Anderson
   action of radium rays on the eye, 217

 Harms
   number of ions per c.c. in closed vessel, 534

 Hartmann
   spectrum of phosphorescent light of emanium, 206

 Haschek and Exner
   spectrum of radium, 17

 Heat
   rate of emission of, from radium, 419 _et seq._
   emission of, from radium at low temperatures, 420
   connection of heat emission with the radio-activity, 421 _et seq._
   source of heat energy, 421 _et seq._
   rate of emission of, after removal of the emanation, 422 _et seq._
   rate of emission of, by emanation, 423, 431
   variation with time of heat emission of radium, and of its emanation,
      423
   heating effect of the emanation, 423, 431
   heating effect of active deposit, 425
   proportion of heating effect due to radio-active products, 433
   origin of, in radium, 442 _et seq._
   total heat emission during life of radio-elements, 474 _et seq._
   heating of earth by radio-active matter, 493

 Heaviside
   apparent mass of moving charged body, 71, 127

 Helium
   produced by radium and its emanation, 476 _et seq._
   amount of, from radium, 480
   origin of, 480

 Helmholtz and Richarz
   action of ions on steam jet, 47

 Hemptinne
   action of rays on spark and electrodeless discharge, 208

 Henning
   resistance of radium solutions, 208
   effect of voltage on amount of excited activity, 316

 Henning and Kohlrausch
   conductivity of solutions of radium bromide, 208

 Hertz
   electric deviation of cathode rays, 73

 Heydweiler
   loss of weight of radium, 474

 Himstedt
   action of radium rays on selenium, 208
   radium emanation in springs of Baden, 513

 Himstedt and Meyer
   production of helium by radium, 479

 Himstedt and Nagel
   action of radium rays on the eye, 217

 Hofmann, Gonder, and Wölfl
   properties of radio-active lead, 27, 413

 Hofmann and Strauss
   radio-active lead, 27

 Hofmann and Zerban
   connection of activity of thorium with uranium, 29

 Huggins, Sir W. and Lady
   spectrum of phosphorescent light of radium bromide, 205

 Hydrogen
   production of, by radium rays, 215

 Induced radio-activity (_see_ Excited radio-activity)

 Induction
   radio-active, 24
   meaning and examples of, 415 _et seq._

 Insulators
   conduction of, under radium rays, 209

 Iodoform
   coloration produced in, by radium rays, 214

 Ionization
   theory of, to explain conductivity of gases, 31 _et seq._
   by collision, 39, 57
   variation of, with pressure of gas, 61 _et seq._
   variation of, with nature of gas, 64
   comparison of, produced by rays, 111, 194
   production of, in insulators, 209
   total, produced by 1 gram of radium, 433 _et seq._
   natural ionization of gases, 531 _et seq._
   connection of, with phosphorescent and photographic actions, 549

 Ions
   in explanation of conductivity of gases, 31 _et seq._
   production of, by collision, 39, 57
   rate of recombination of, 40 _et seq._
   mobility of, 42 _et seq._
   difference between mobility of positive and negative, 43 _et seq._
   condensation of water around, 46 _et seq._
   difference between positive and negative, 49
   charge carried by, 50
   diffusion of, 51 _et seq._
   charge on an ion same as on hydrogen atom, 54
   number of, produced per c.c., 54
   size and nature of, 55 _et seq._
   definition of, 56 _et seq._
   velocity acquired by, between collisions, 58
   energy required to produce, 58, 551
   comparative number of, produced in gases, 65
   disturbance of potential gradient by movement of, 65
   production of, in insulators, 209
   number of, produced by α particle, 433
   number produced per c.c. in closed vessels, 533 _et seq._


 Joly
   motion of radium in an electric field, 211
   absorption of radium rays by atmosphere, 492 (see foot-note)


 Kaufmann
   velocity of cathode rays, 75
   variation of _e_/_m_ with velocity of electron, 127 _et seq._

 Kelvin
   theory of radio-activity, 441
   age of sun and earth, 492, 493

 Kelvin, Smolan and Beattie
   discharging power of uranium rays, 7

 Kleeman and Bragg
   theory of absorption of α rays, 172 _et seq._
   relation between ionization and absorption, 174 _et seq._
   range of α rays in air, 174
   four sets of α rays from radium, 174 _et seq._

 Kohlrausch
   conductivity of water altered by radium rays, 208

 Kohlrausch and Henning
   conductivity of solutions of radium bromide, 208

 Kunz
   phosphorescence of willemite and kunzite, 203

 Kunz and Baskerville
   phosphorescence of substance under radium rays, 204

 Kunzite
   phosphorescence of, under radium rays, 203


 Laborde and Curie
   heat emission of radium, 419
   origin of heat from radium, 440
   radium emanation in waters of hot springs, 514

 Langevin
   coefficient of recombination of ions, 41
   velocity of ions, 45 _et seq._
   energy required to produce an ion, 58
   secondary radiation produced by X rays, 187
   slow moving ions in air, 528

 Larmor
   radiation theory, 77
   radiation of energy from moving electron, 79
   structure of the atom, 157

 Lead, radio-active
   preparation of, 26
   radiations from, 26

 Le Bon
   rays from bodies exposed to sunlight, 5
   discharging power of quinine sulphate, 9, 530

 Lenard
   ionization of gases by ultra-violet light, 9
   action of ions on a steam jet, 47
   penetrating power of cathode rays, 73
   negative charge carried by Lenard rays, 120
   absorption of cathode rays proportional to density, 136, 137

 Lerch, von
   chemical properties of active deposit of thorium, 313
   electrolysis of solution of active deposit, 313
   effect of temperature on excited activity, 315
   temporary activity of active deposit from thorium, 415

 Lockyer
   inorganic evolution, 499

 Lodge, Sir Oliver
   electronic theory, 69
   instability of atoms, 487

 Lorentz
   structure of atoms, 157

 Lowry and Armstrong
   radio-activity and phosphorescence, 444

 Luminosity
   of radium compounds, 205
   change of, in radium compounds with time, 205
   spectrum of phosphorescent light from radium bromide, 206
   of radium compounds unaffected by temperature, 210


 Mache
   radium emanation in hot springs, 513

 Mache and von Schweidler
   velocity of ions in air, 528

 Makower
   diffusion of radium emanation, 274
   diffusion of thorium emanation, 276

 Marckwald
   preparation of radio-tellurium, 25
   rate of decay of radio-tellurium, 411

 Mass
   apparent mass of electron, 71, 127
   variation of mass of electron with speed, 127 _et seq._
   of α particle, 147 _et seq._

 Materials
   radio-activity of ordinary, 528, 536 _et seq._

 Matteucci
   rate of dissipation of charge in closed vessels, 531

 McClelland
   absorption of γ rays, 181
   secondary rays from β and γ rays from radium, 192

 McClung
   coefficient of recombination of ions, 41
   conductivity of gases exposed to X rays, 64
   ionization by α rays from radium C, 550

 McClung and Rutherford
   energy required to produce an ion, 58
   variation of current with thickness of layer of uranium, 195
   estimate of energy radiated from radio-elements, 418
   radiation of energy from radium, 438

 McLennan
   absorption of cathode rays, 65
   radio-activity of snow, 506
   excited radio-activity at Niagara Falls, 519

 McLennan and Burton
   penetrating radiation from the earth, 520
   radio-activity of ordinary materials, 537
   emanation from ordinary matter, 538

 Metabolon
   definition of, 446
   table of metabolons, 449
   radio-elements as metabolons, 457

 Meteorological conditions
   effect of, on radio-activity of atmosphere, 517

 Methods of measurement
   in radio-activity, 82 _et seq._
   comparison of photographic and electrical, 83 _et seq._
   description of electrical, 84 _et seq._

 Meyer and Himstedt
   production of helium by radium, 479

 Meyer and Schweidler
   magnetic deviation of β rays by electrical method, 113
   absorption of β rays of radium by matter, 136
   activity proportional to amount of uranium, 195
   emanation from uranium, 348
   effect of crystallization on activity of uranium, 349
   rate of decay of radio-tellurium, 411

 Minerals, radio-active
   constant ratio of radium to uranium, 459 _et seq._
   list of minerals, 461
   age of, 485
   composition of, 554 _et seq._

 Mobility
   of ions, 43 _et seq._

 Moisture
   effect of, on velocity of ions, 43, 45
   effect of, on emanating power, 255

 Molecule
   number of, in 1 c.c. of hydrogen, 54

 Molecular weight
   of radium emanation, 273
   of thorium emanation, 275


 Nagel and Himstedt
   action of radium rays on the eye, 217

 Niewenglowski
   rays from sulphide of calcium, 4

 Nomenclature
   of successive products, 328 _et seq._

 Number
   of molecules per c.c. of hydrogen, 54
   of ions produced in gas by active substances, 55
   of β particles expelled from 1 gram of radium, 124
   of α particles emitted per gram of radium, 155
   of ions produced per c.c. in closed vessels, 534


 Occlusion
   of emanation in thorium and radium, 258
   of radium emanation by solids, 310

 Owens
   saturation current affected by dust, 42
   penetrating power of rays independent of compound, 164
   absorption of α rays varies directly as the pressure of gas, 169
   effect of air currents on conductivity produced by thorium, 238

 Oxygen
   change into ozone, by radium rays, 213
   production of, from radium solutions, 215

 Ozone
   production of, by radium rays, 213


 Paraffin
   objection to, as an insulator, 96
   conductivity of, under radium rays, 210

 Paschen
   distribution of velocity amongst β particles, 131 _et seq._
   absence of magnetic deflection of γ rays, 183
   γ rays and electrons, 185
   heating effect of γ rays, 186, 429

 Patterson
   number of ions per c.c. in closed vessel, 534
   natural conductivity of air due to an easily absorbed radiation, 536
   effect of temperature on natural conductivity of air, 536

 Peck and Willows
   action of radium rays on spark, 208

 Pegram
   electrolysis of thorium solutions, 314
   temporary activity of substances separated from thorium, 415

 Penetrating power
   comparison of, for α, β and γ rays, 111
   variation in, of β rays, 134 _et seq._
   variation of, with density for β rays, 137
   comparison of, for α rays from radio-elements, 164
   variation of, with density for α rays, 169
   variation of, with density for γ rays, 182

 Penetrating radiation
   from the earth and atmosphere, 520

 Perrin
   charge carried by cathode rays, 73
   theory of radio-activity, 437

 Phosphorescence
   production of, by radium, 19
   production of, by radium and polonium rays, 201 _et seq._
   comparison of, produced by α and β rays, 202
   of zinc sulphide, 202
   of barium platinocyanide, 203
   of willemite and kunzite, 203
   produced by radium emanation in substances, 203, 252
   diminution of, with time, 205
   of radium compounds, 205
   spectrum of phosphorescent light of radium bromide, 205
   spectrum of phosphorescent light of “emanium,” 206
   production of by heat (thermo-luminescence), 207
   use of, to illustrate condensation of emanations, 279
   connection of with ionization, 547 _et seq._

 Phosphorus
   action of radium rays on, 214
   ionization produced by, 529

 Photo-electric action
   produced by radium rays in certain substances, 214

 Photographic
   method, advantages and disadvantages of, 83
   relative photographic action of rays, 83
   connection of photographic action with ionization, 546

 Physical action of radium rays
   on sparks, 208
   on electrodeless discharge, 208
   on selenium, 208
   on conductivity of insulators, 209

 Physiological action of radium rays
   production of burns, 216
   effect on bacteria, 216
   effect on eye, 217

 Piezo-électrique of quartz
   description of, 105

 Pitchblendes
   comparison of radio-activity of, 11
   radio-elements separated from, 13 _et seq._
   radium continually produced from, 459
   constitution of, 557

 Polarization of uranium rays
   absence of, 7

 Polonium
   methods of separation of, 22
   rays from, 23
   decay of activity of, 23
   discussion of nature of, 24
   similarity to radio-tellurium, 26
   magnetic deviation of α rays from, 146, 150
   slow moving electrons, 153
   increase of absorption with thickness of matter traversed, 163
   connection of, with radium F, 411

 Potential
   required to produce saturation, 32 _et seq._
   fall of potential needed to produce ions at each collision, 58
   gradient due to movement of ions, 65

 Precht and Runge
   spectrum of radium, 17
   atomic weight of radium, 18
   heating effect of radium, 420

 Pressure
   effect of, on velocity of ions, 46
   effect of, on current through gases, 61 _et seq._
   production of emanation independent of, 265
   effect of, on distribution of excited activity, 317
   effect of, on natural conductivity of air in closed vessels, 534

 Products, radio-active
   list of, from radio-elements, 449
   properties of, 449
   amount of in radium, 452 _et seq._
   radiations from, 455


 Quartz piezo-électrique
   use of, in measurement of current, 105

 Quinine sulphate
   discharging power of, 530
   phosphorescence of, 530


 Radiations
   emitted by uranium, 8
   emitted by thorium, 10
   emitted by radium, 18
   emitted by actinium, 21
   emitted by polonium, 23
   method of measurement of, 82 _et seq._
   methods of comparison of, 108
   three kinds of, 109
   analogy to rays from a Crookes tube, 110
   relative ionizing and penetrating power of, 111
   difficulties of comparative measurement of, 112
   β rays, 113
   α rays, 141
   γ rays, 179
   secondary rays, 187
   comparison of ionization of α and β rays, 194
   phosphorescent effect of, 201 _et seq._
   physical actions of, 207 _et seq._
   chemical actions of, 213 _et seq._
   physiological actions of, 216
   from the emanation, 263
   from Ur X, 347
   connection of, with heat emission, 421 _et seq._
   from different active products, 455
   conservation of energy of each specific type of, 469 _et seq._

 Radio-lead
   connection of, with polonium, 411 _et seq._
   connection of, with radium D, 413

 Radio-tellurium
   rate of decay of, 411
   connection of with radium F, 411

 Radium
   discovery of, 13
   separation of, 13
   spectrum of, 16
   atomic weight of, 17
   radiations from, 18
   compounds of, 19
   nature of radiations from, 109
   β rays from, 113
   α rays from, 141
   γ rays from, 179
   secondary rays from, 187
   production of phosphorescence by, 201 _et seq._
   spectrum of phosphorescent light of, 206
   physical actions of, 207 _et seq._
   chemical actions of, 213 _et seq._
   physiological actions of, 216
   emanation from, 246
   properties of emanation from, 247 _et seq._
   chemical nature of emanation from, 267
   diffusion of emanation from, 269
   condensation of emanation from, 277
   amount of emanation from, 288
   volume of emanation from, 289
   spectrum of emanation from, 292
   excited radio-activity from, 295 _et seq._
   decay of excited activity from, 306 _et seq._
   difference in properties of radium and the emanation, 327
   nomenclature of products, 328
   theory of successive changes in, 330
   alteration of activity of, by removal of emanation, 371 _et seq._
   recovery of activity of, after removal of emanation, 372
   effect of escape of emanation on recovery of activity of, 374
   non-separable activity of, 375
   period and properties of radium A, B and C, 376 _et seq._
   analysis of active deposit of rapid changes of radium, 377
   analysis of β ray curves, 381 _et seq._
   analysis of α ray curves, 386 _et seq._
   equations of activity curves, 389
   effect of temperature on active deposit of, 390
   relative activity due to products of, 395
   active deposit of slow transformation, 397
   physical and chemical properties of radium D, E and F, 398 _et seq._
   effect of temperature on active deposit of slow change, 401
   separation of radium F by bismuth, 402
   products of, 402 _et seq._
   rate of transformation of radium D, 404 _et seq._
   variation of the activity of the active deposit over long periods of
      time, 407
   amounts of radium D, E and F in old radium, 408
   variation of activity of, with time, 409
   products of in pitchblende, 410
   origin of radio-tellurium, 411
   origin of polonium, 411, 412
   origin of radio-lead, 413
   temporary activity of inactive matter separated from pitchblende, 415
      _et seq._
   heat emission of, 419 _et seq._
   heat emission of emanation from, 420, 431
   heating effects due to products of, 433
   theories of radio-activity of, 437 _et seq._
   discussion of theories of radio-activity of, 441 _et seq._
   energy of radiations, not derived from external source, 442 _et seq._
   theory of radio-active change, 444 _et seq._
   list of active products of, 449
   amount of products of, 452
   rate of change of, 457
   life of radium, 457
   origin of, 459 _et seq._
   production of, by uranium, 459 _et seq._
   amount of in 1 gram of uranium, 461
   amount of, in minerals, 461
   radio-activity of, independent of concentration, 466 _et seq._
   disappearance of, 467
   life of, independent of concentration, 468
   conservation of radio-activity of, 469 _et seq._
   loss of weight of, 473
   experiments to determine loss of weight of, 474
   total emission of energy from 1 gram of, 474 _et seq._
   production of helium from, 476
   helium, disintegration product of, 479 _et seq._
   amount of helium from, 480
   possible causes of disintegration of, 486 _et seq._
   amount of, to account for heat of sun, 491
   possible connection of with heat of sun, 491
   possible connection of with heat of earth, 493
   probable amount of, in earth, 495
   amount of, in atmosphere, 495, 524
   presence of, in atmosphere, 521 _et seq._

 Radium A
   decay curve of, 378
   radiation from, 381
   effect of, on activity curves, 386 _et seq._
   connection with later changes, 392
   activity supplied by, 393

 Radium B
   absence of rays in, 381
   effect of, on activity curves, 381 _et seq._
   effect of temperature on, 390
   volatility of, 390
   absence of heating effect of, 433
   nature of rayless change in, 454, 552

 Radium C
   radiations from, 381
   analysis of β ray curves of, 381 _et seq._
   analysis of α ray curves of, 386 _et seq._
   effect of temperature on, 390
   activity supplied by, 394 _et seq._
   heating effect of, 425
   use of, as a source of β rays, 435
   explosive nature of change in, 456
   magnetic deflection of rays from, 543
   velocity and value of _e_/_m_ for rays from, 544

 Radium D
   origin of name of, 376
   connection of, with active deposit, 403
   period of transformation of, 406
   effect of, on variation of activity, 407
   presence in old radium, 408
   effect of, on activity of old radium, 409
   presence in pitchblende, 410
   connection with radio-lead, 413
   amount of, in 1 ton of uranium, 454

 Radium E
   effect of temperature on, 401
   connection of, with β ray activity active deposit, 403, 400
   connection with radio-lead, 413

 Radium F
   variation of activity due to, 398
   effect of temperature on, 401
   separation of, on bismuth plate, 402
   connection with active deposit, 403
   variation of activity of, over long periods of time, 407
   presence in old radium, 409
   effect of, on activity of old radium, 409
   presence in pitchblende, 410
   connection with radio-tellurium, 411
   connection with polonium, 411, 412
   connection with radio-lead, 413

 Rain
   radio-activity of, 505
   decay of activity of, 506

 Ramsay, Sir W.
   amount of helium in thorianite, 486

 Ramsay and Collie
   spectrum of emanation, 292

 Ramsay and Cooke
   radio-activity produced by radiation from radium, 472

 Ramsay and Soddy
   evolution of gas from radium, 215
   production of hydrogen and oxygen from radium, 215
   chemical nature of the emanation, 268
   gaseous nature of the emanation, 268
   volume of emanation, and change with time, 289
   helium from radium emanation, 291
   amount of helium produced by radium, 480

 Ramsay and Travers
   amount of helium in fergusonite, 486

 Rayless changes
   discussion of, 454, 552

 Re, F.
   theory of radio-activity, 441

 Recombination
   of ions, 40 _et seq._
   constant of, 42

 Recovery
   of activity of thorium after removal of Th X, 221
   of activity of uranium after removal of Ur X, 223
   significance of law of, 224
   effect of conditions on rate of, 232
   of activity of radium after removal of emanation, 372
   of heating effect of radium, 423

 Reflection
   no evidence of direct reflection for uranium rays, 7
   diffuse reflection of rays, 7

 Refraction
   no evidence of, for uranium rays, 7

 Regeneration
   of emanating power, 256

 Richarz and von Helmholtz
   action of ions on steam jet, 47

 Richarz and Schenck
   theory of radio-activity, 441

 Rossignol and Gimingham
   decay of thorium emanation, 242

 Runge
   spectrum of radium, 17

 Runge and Bödlander
   evolution of gas from radium, 215

 Runge and Precht
   spectrum of radium, 17
   atomic weight of radium, 18
   heating effect of radium, 420

 Russel
   photographic action of substances, 83


 Saake
   amount of emanation in air at high altitudes, 519

 Salomonsen and Dreyer
   coloration of quartz by radium rays, 213

 Saturation current
   meaning of, 33 _et seq._
   application of, to measurements of radio-activity, 84
   measurement of, 100 _et seq._

 Schenck
   radium emanation in springs, 513

 Schenck and Richarz
   theory of radio-activity, 441

 Schmidt
   discovery of radio-activity of thorium, 10

 Schmidt and Wiedemann
   thermo-luminescence, 207

 Schuster
   number of ions per c.c. in air of Manchester, 528
   radio-activity of matter, 529

 Schweidler and Mache
   velocity of ions in air, 528

 Schweidler and Meyer
   magnetic deviation of β rays by electrical method, 113
   absorption of β rays of radium by matter, 136
   activity proportional to amount of uranium, 195
   emanation from uranium, 348
   effect of crystallization on activity of uranium, 349
   rate of decay of radio-tellurium, 411

 Scintillations
   discovery of, in zinc sulphide screen, 158
   connection of, with α rays, 158
   illustration of, by spinthariscope, 158
   cause of, 160
   production of, by action of electric field, 160

 Searle
   apparent mass of moving charged body, 71, 127

 Secondary rays
   examination of, by photographic method, 187
   examination of, by electrical method, 188
   production of, by β and γ rays, 189 _et seq._
   from different materials, 191
   amount of, depends upon atomic weight, 192
   magnetic deflection of, 193

 Seitz
   absorption of electrons by matter, 137 _et seq._

 Selenium
   action of radium rays on, 208

 Simon
   value of _e_/_m_ for cathode rays, 75, 129

 Simpson
   amount of excited activity in north of Norway, 519

 Slater, Miss
   effect of temperature on active deposit of thorium, 354

 Smolan, Beattie and Kelvin
   discharging power of uranium rays, 7

 Snow
   radio-activity of, 506
   decay of activity of, 507

 Soddy
   comparison of photographic and electrical action of uranium rays, 83
   nature of rays from Ur X, 347
   production of radium from uranium, 463

 Soddy and Ramsay
   evolution of gas from radium, 215
   production of hydrogen and oxygen from radium, 215
   chemical nature of the emanation, 268
   gaseous nature of the emanation, 268
   volume of the emanation, and change with time, 289
   helium from radium emanation, 291
   amount of helium produced by radium, 480

 Soddy and Rutherford
   separation of Th X, 220
   decay of activity of Th X, 221
   recovery of activity of thorium freed from Th X, 221
   decay of activity of Ur X, 223
   recovery of activity of uranium freed from Ur X, 223
   explanation of decay and recovery curves, 224
   rate of production of Th X, 227
   theory of decay of activity, 229
   influence of conditions on rate of decay and recovery of activity,
      233
   disintegration hypothesis, 234
   decay of activity of radium emanation, 247
   measurements of emanating power, 254
   effect of temperature, moisture, and solution, on emanating power,
      255
   regeneration of emanating power, 256
   constant rate of production of emanation of radium and thorium, 257
   source of thorium emanation, 261
   radiations from the emanation, 264
   chemical nature of emanation, 267
   condensation of emanations of radium and thorium, 277
   temperature of condensation of emanation, 278
   effect of successive precipitations on activity of thorium, 358
   recovery of activity of radium, 372
   theory of radio-activity, 439
   theory of radio-active change, 445
   conservation of radio-activity, 469

 Soil
   radio-activity of, 507 _et seq._
   difference in activity of, 508 _et seq._

 Solution
   coloration of, by radium, 15
   of active deposit in acids, 312
   electrolysis of active, 313

 Source
   of thorium emanation, 261
   of radium and actinium emanations, 263

 Spark
   action of radium rays on, 208

 Spectrum
   spark spectrum of radium, 15, 16
   flame spectrum of radium, 17
   effect of a magnetic field on spectrum of radium, 17
   of polonium, 23
   of phosphorescent light of radium bromide, 206
   of emanation, 292
   of helium in radium gases and emanation, 477

 Spinthariscope
   description of, 158

 Springs
   emanation from water of, 513

 Stark
   energy to produce an ion, 58

 Stoney, Johnstone
   use of term electron, 76

 Strauss and Hofmann
   radio-active lead, 27

 Strutt
   conductivity of gases for radiation, 63, 64
   conductivity of gases produced by γ rays, 64, 183
   negative charge carried by β rays, 122 _et seq._
   absorption of β rays proportional to density, 136
   nature of α rays, 142
   attempt to measure charge of α rays, 153
   constant ratio of uranium to radium in minerals, 462
   connection of thorium with helium, 483
   absorption of radium rays from sun by atmosphere, 492
   presence of radium in Bath waters, 513
   radio-activity of ordinary matter, 536

 Sun
   effect of radium in, 491
   age of, 492


 Temperature
   effect of, on intensity of radiations from uranium and radium, 210
   effect of, on luminosity, 210
   rate of decay of radium emanation unaffected by, 249
   of condensation of emanations, 283
   rate of decay of thorium emanation unaffected by, 287
   effect of, on excited activity, 315
   effect of, on active deposit of thorium, 354
   effect of, on active deposit of actinium, 368
   effect of, on active deposit of rapid change of radium, 390
   effect of, on active deposit of slow change, 401
   of radium above surrounding space, 419
   effect of, on amount of excited activity in atmosphere, 518
   effect of, on natural ionization of air, 536

 Theories
   of radio-activity, review of, 437 _et seq._
   discussion of, 441 _et seq._
   disintegration theory, 445 _et seq._

 Thermo-luminescence, 207

 Thomson, J. J.
   relation between current and voltage for ionized gases, 34
   difference between ions as condensation nuclei, 49
   charge on ion, 50
   magnetic field produced by an ion in motion, 69
   apparent mass of electron, 71
   action of magnetic field on moving ion, 72
   determination of _e_/_m_ for cathode stream, 73
   origin of X rays, 80
   slow velocity electrons from radio-tellurium, 153
   charge carried by α rays, 154
   theory of radio-activity, 440
   cause of heat emission from radium, 442
   structure of atom, 487
   possible causes of disintegration of radium, 487
   nature of electrons, 496
   emanation from tap-water and deep wells, 510
   radio-activity of ordinary materials, 539

 Thomson, J. J. and Rutherford
   ionization theory of gases, 31 _et seq._

 Thorium
   discovery of radio-activity of, 10
   emanation from, 11
   preparation of non-radio-active thorium, 29
   nature of radiations from, 109
   β rays from, 114
   α rays from, 141
   γ rays from, 180
   separation of Th X from, 220
   recovery of activity of, 221
   disintegration of, 234
   emanation from, 238
   properties of emanation from, 239
   diffusion of emanation from, 275
   condensation of emanation from, 277
   excited radio-activity from, 295 _et seq._
   analysis of active deposit of, 351 _et seq._
   rayless change in, 352
   explanation of initial portion of decay curve, 358
   explanation of initial portion of recovery curve, 358
   effect of successive precipitations on, 358
   recovery curve after large number of precipitations, 359
   products of, 363
   non-separable activity of, 363
   radiations from active products of, 363
   division of activity amongst active products of, 363
   rate of emission of energy by, 432
   theories of radio-activity of, 438
   discussion of theories of radio-activity, 441 _et seq._
   source of energy of radiations, 442 _et seq._
   theory of radio-active change, 444 _et seq._
   table of radio-active products of, 449
   rate of change of, 458
   life of, 458
   conservation of radio-activity of, 469
   total emission of energy from 1 gram of, 475
   possible causes of disintegration of, 486 _et seq._

 Thorium A
   period and properties of, 352 _et seq._
   absence of rays in, 352
   effect of temperature on, 354

 Thorium B
   period and properties of, 352 _et seq._
   effect of temperature on, 354
   radiations from, 363

 Thorium X
   methods of separation of, 220
   law of decay of activity of, 221
   law of recovery of activity of, 221
   theory to explain production of, 224
   material nature of, 226
   continuous production of, 227
   explanation of decay of activity of, 229
   effect of conditions on the rate of change of, 233
   disintegration hypothesis to explain production of, 234
   minute amount of, produced, 237
   effect of successive separations of, on activity of thorium, 358 _et
      seq._
   analysis of decay and recovery curves of, 358
   radiations from, 363

 Tommasina
   scintillations produced by electrification, 160

 Townsend
   ions by collision, 39, 57
   coefficient of recombination, 41
   diffusion of ions, 51 _et seq._
   comparison of charge on ion with that on hydrogen atom in
      electrolysis, 53
   number of molecules per c.c. of gas, 54
   ionization by collision for different speeds, 171


 Transformations, successive
   theory of, 325 _et seq._
   nomenclature of, 328
   activity due to, 337
   detection of a rayless change in, 341
   in uranium, 346 _et seq._
   in thorium, 351 _et seq._
   in actinium, 364 _et seq._
   in radium, 371 _et seq._
   list of, 449
   origin of radium in, 459
   helium, a result of, 476 _et seq._
   possible cause of, 486 _et seq._
   application of, to evolution of matter, 497 _et seq._

 Transmission
   of excited radio-activity of radium and thorium, 318 _et seq._
   of excited radio-activity of actinium, 323

 Travers and Ramsay
   amount of helium in fergusonite, 486

 Troost
   rays from hexagonal blende, 4


 Uranium
   discovery of radio-activity of, 5
   persistence of radiations of, 6
   discharging power of rays, 7
   absence of reflection, refraction and polarization, 7
   examination of uranium minerals, 11 _et seq._
   relative activity of compounds of uranium, 12
   nature of radiations from, 109
   β rays from, 114
   α rays from, 141
   γ rays from, 180
   separation of Ur X from, 219
   recovery of activity of, 219
   changes in, 346 _et seq._
   non-separable activity of, 347
   radiations from Ur X, 347 _et seq._
   method of measurement of activity of Ur X, 347
   emission of energy by, 418
   theories of radio-activity of, 437 _et seq._
   discussion of theories of radio-activity, 441 _et seq._
   source of energy of radiation, 442 _et seq._
   theory of radio-active change, 444 _et seq._
   table of active products, 449
   rate of change of, 458
   life of, 458
   radium probable product of, 459 _et seq._
   amount of radium in, 460 _et seq._
   amount of, in radio-active minerals, 461
   growth of radium in, 463
   conservation of radio-activity of, 469
   total emission of energy from 1 gram of, 475
   possible causes of disintegration of, 486 _et seq._

 Uranium X
   separation of, by Crookes, 219
   separation of, by Becquerel, 219
   decay of activity of, 223
   recovery of activity of, 223
   theory to explain production of, 224 _et seq._
   material nature of, 226
   explanation of decay of activity of, 229
   changes in, 346 _et seq._
   radiations from, 347 _et seq._
   method of measurement of radiations from, 347
   effect of crystallization on activity of, 349
   diffusion of, 350


 Velocity
   of ions in electric field, 42 _et seq._
   difference between, of positive and negative ions, 43 _et seq._
   of β particle or electron, 126 _et seq._
   variation of mass of electron with, 127
   of α particle, 148
   of transmission of carriers of excited activity, 320 _et seq._
   of ions in atmosphere, 528

 Villard
   discovery of γ rays from radium, 179
   alteration of X ray screen with time, 205
   activity produced by cathode rays, 530

 Vincenti and Levi Da Zara
   radium emanation in spring waters, 516

 Voller
   variation of activity of radium with concentration, 467

 Volume
   of radium emanation, calculation of, 289
   decrease of, of radium emanation, 290


 Walker, G. W.
   theory of electrometer, 90

 Walkhoff
   action of radium rays on skin, 216

 Wallstabe
   diffusion of radium emanation into liquids, 276

 Water
   emanation from, 510 _et seq._
   decay of activity of emanation from, 511 _et seq._

 Water-falls
   amount of excited activity produced at Niagara, 520
   electrification produced near, 520

 Watts, Marshall
   atomic weight of radium, 18

 Weichert
   velocity of cathode rays, 76

 Weight
   loss of by radio-elements, 473
   attempts to measure loss of in radium, 474

 Wheeler and Bumstead
   diffusion of radium emanation, 273
   emanation from surface water and the soil, 512, 522
   identity of emanation from soil with radium emanation, 512, 522

 Whetham
   effect of valency of ion on colloidal solutions, 215
   production of radium from uranium, 463

 Wiedemann
   thermo-luminescence, 207

 Wiedemann and Schmidt
   thermo-luminescence, 207

 Wien
   value of _e_/_m_ for canal rays, 78
   positive charge of canal rays, 78
   amount of charge carried by β rays, 124

 Willcock, Miss and Hardy
   coloration of iodoform solution by radium rays, 214

 Willemite
   phosphorescence of, under radium rays, 203
   use of, to show condensation of emanation, 279

 Willows and Peck
   action of radium rays on spark, 208

 Wilson, C. T. R.
   ions as nuclei of condensation, 47 _et seq._
   difference between positive and negative ions as condensation nuclei,
      49
   equality of charges carried by positive and negative ions, 50
   construction of electroscope, 86, 88
   natural ionization of air in vessels, 501
   radio-activity of rain and snow, 505, 506
   loss of charge in closed vessels, 531 _et seq._, 534
   presence of ions in dust-free air shown by condensation, 533
   number of ions produced per c.c., 533
   effect of pressure and nature of gas on ionization in sealed vessels,
      534

 Wilson, H. A.
   charge on ion, 51

 Wilson, W. E.
   radium in sun, 491

 Wölfl, Hofmann and Gonder
   properties of radio-active lead, 27, 413

 Wood, A.
   radio-activity of ordinary materials, 540


 Zara, Levi Da and Vincenti
   radium emanation in spring waters, 516

 Zeeman
   action of magnetic field on light, 77

 Zeleny
   velocity of ions, 42 _et seq._
   difference of velocity of ions, 45
   potential gradient between electrodes, 65

 Zerban and Hofmann
   connection of activity of thorium with uranium, 29

 Zinc Sulphide
   scintillations produced in by α rays, 158
   cause of luminosity of, 160, 549

CAMBRIDGE: PRINTED BY JOHN CLAY, M.A. AT THE UNIVERSITY PRESS.




                       CAMBRIDGE PHYSICAL SERIES.


    =Conduction of Electricity through Gases.= By J. J. THOMSON, D.Sc.,
    LL.D., Ph.D., F.R.S., Fellow of Trinity College and Cavendish
    Professor of Experimental Physics. Demy 8vo. viii + 568 pp. 16_s._

CONTENTS.

    I. Electrical Conductivity of Gases in a normal state.

    II. Properties of a Gas when in the conducting state.

    III. Mathematical Theory of the Conduction of Electricity through a
    Gas containing Ions.

    IV. Effect produced by a Magnetic Field on the Motion of the Ions.

    V. Determination of the Ratio of the Charge to the Mass of an Ion.

    VI. Determination of the Charge carried by the Negative Ion.

    VII. On some Physical Properties of Gaseous Ions.

    VIII. Ionisation by Incandescent Solids.

    IX. Ionisation in Gases from Flames.

    X. Ionisation by Light. Photo-Electric Effects.

    XI. Ionisation by Röntgen Rays.

    XII. Becquerel Rays.

    XIII. Spark Discharge.

    XIV. The Electric Arc.

    XV. Discharge through Gases at Low Pressures.

    XVI. Theory of the Discharge through Vacuum Tubes.

    XVII. Cathode Rays.

    XVIII. Röntgen Rays.

    XIX. Properties of Moving Electrified Bodies.

    Supplementary Notes.

    Index.

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    =A Treatise on the Theory of Solution, including the Phenomena of
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    =Electricity and Magnetism=: an Elementary Text-book, Theoretical
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 ● Transcriber’s Notes:
    ○ Inconsistent spelling and hyphenation were made consistent only
      when a predominant form was found in this book.
    ○ Text that was in italics is enclosed by underscores (_italics_).

      Text that was in bold face is enclosed by equals signs (=bold=).

    ○ Footnotes have been moved to follow the chapters in which they are
      referenced.
    ○ Formulae in the text have been rendered in text form, and in some
      cases there are unusual conventions used. The convention
      name^{expression} means that the expression is a subscript to the
      name, and name_{expression} means that expression is a subscript
      to the name. Many parentheses are added to allow an expression to
      be transcribed on one line.