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                        RADIO-ACTIVE SUBSTANCES.


                                   BY

                        MDME. SKLODOWSKA CURIE.

         Thesis presented to the Faculté des Sciences de Paris.


                  _Reprinted from the CHEMICAL NEWS._


                           (SECOND EDITION).

                                LONDON:
                        _CHEMICAL NEWS_ OFFICE,
             16, NEWCASTLE STREET, FARRINGDON STREET, E.C.
                                 1904.


                        D. VAN NOSTRAND COMPANY,
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------------------------------------------------------------------------




                        RADIO-ACTIVE SUBSTANCES


                                   BY

                         MDME. SKLODOWSKA CURIE


         Thesis presented to the Faculté des Sciences de Paris.


                  _Reprinted from the CHEMICAL NEWS._


                           (SECOND EDITION).


                                LONDON:
                        _CHEMICAL NEWS_ OFFICE,
             16, NEWCASTLE STREET, FARRINGDON STREET, E.C.
                                 1904.

                        D. VAN NOSTRAND COMPANY,
                               NEW YORK.




                                LONDON:
 PRINTED BY EDWIN JOHN DAVEY, 16, NEWCASTLE STREET, FARRINGDON STREET,
                                  E.C.

------------------------------------------------------------------------

  [_Reprinted from the_ CHEMICAL NEWS, _1903, Vol. 88, p. 85 et seq._]




                        RADIO-ACTIVE SUBSTANCES.




                             INTRODUCTION.


The object of the present work is the publication of researches which I
have been carrying on for more than four years on radio-active bodies. I
began these researches by a study of the phosphorescence of uranium,
discovered by M. Becquerel. The results to which I was led by this work
promised to afford so interesting a field that M. Curie put aside the
work on which he was engaged, and joined me, our object being the
extraction of new radio-active substances and the further study of their
properties.

Since the commencement of our research we thought it well to hand over
specimens of the substances, discovered and prepared by ourselves, to
certain physicists, in the first place to M. Becquerel, to whom is due
the discovery of the uranium rays. In this way we ourselves facilitated
the research by others besides ourselves on the new radio-active bodies.
At the termination of our first publications, M. Giesel, in Germany,
also began to prepare these substances, and passed on specimens of them
to several German scientists. Finally, these substances were placed on
sale in France and Germany, and the subject growing in importance gave
rise to a scientific movement, such that numerous memoirs have appeared,
and are constantly appearing on radio-active bodies, principally abroad.
The results of the various French and foreign researches are necessarily
confused, as is the case with all new subjects in course of
investigation, the aspect of the question becoming modified from day to
day.

From the chemical point of view, however, one point is definitely
established:—_i.e._, the existence of a new element, strongly
radio-active, viz., radium. The preparation of the pure chloride of
radium and the determination of the atomic weight of radium form the
chief part of my own work. Whilst this work adds to the elements
actually known with certainty a new element with very curious
properties, a new method of chemical research is at the same time
established and justified. This method, based on the consideration of
radio-activity as an atomic property of matter, is just that which
enabled M. Curie and myself to discover the existence of radium.

If, from the chemical point of view, the question that we undertook
primarily may be looked upon as solved, the study of the physical
properties of the radio-active bodies is in full evolution. Certain
important points have been established, but a large number of the
conclusions are still of a provisional character. This is not surprising
when we consider the complexity of the phenomena due to radio-activity,
and the differences existing between the various radio-active
substances. The researches of physicists on these substances constantly
meet and overlap. Whilst endeavouring to keep strictly to the limits of
this work and to publish my individual research only, I have been
obliged at the same time to mention results of other researches, the
knowledge of which is indispensable.

I desired, moreover, to make this work an inclusive survey of the actual
position of the question.

I indicate at the end the particular questions with which I am specially
concerned, and those which I investigated in conjunction with M. Curie.

I carried on the work in the laboratories of the School of Physics and
Chemistry in Paris, with the permission of Schützenberger, late Director
of the School, and M. Lauth, actual Director. I take this opportunity of
expressing my gratitude for the kind hospitality received in this
school.


                             _Historical._

The discovery of the phenomena of radio-activity is connected with
researches followed, since the discovery of the Röntgen rays, upon the
photographic effects of phosphorescent and fluorescent substances.

The first tubes for producing Röntgen rays were without the metallic
anticathode. The source of the Röntgen rays was the glass surface
impinged upon by the cathode rays; this surface was at the same time
actively fluorescent. The question then was whether the emission of
Röntgen rays necessarily accompanied the production of fluorescence,
whatever might be the cause of the latter. This idea was first
enunciated by M. Henri Poincaré.

Shortly afterwards, M. Henry announced that he had obtained photographic
impressions through black paper by means of phosphorescent zinc
sulphide. M. Niewenglowski obtained the same phenomenon with calcium
sulphide exposed to the light. Finally, M. Troost obtained strong
photographic impressions with zinc sulphide artificially phosphorescent
acting across black paper and thick cardboard.

The experiences just cited have not been reproduced, in spite of
numerous attempts to this end. It cannot therefore be considered as
proved that zinc sulphide and calcium sulphide are capable of emitting,
under the action of light, invisible rays which traverse black paper and
act on photographic plates.

M. Becquerel has made similar experiments on the salts of uranium, some
of which are fluorescent.

He obtained photographic impressions through black paper with the double
sulphate of uranium and potassium.

M. Becquerel at first believed that this salt, which is fluorescent,
behaved like the sulphides of zinc and calcium in the experiments of MM.
Henry, Niewenglowski, and Troost. But the conclusion of his experiments
showed that the phenomenon observed was in no way related to the
fluorescence. It is not necessary that the salt should be fluorescent;
further, uranium and all its compounds, fluorescent or not, act in the
same manner, and metallic uranium is the most active. M. Becquerel
finally found that by placing uranium compounds in complete darkness,
they continue acting on photographic plates through black paper for
years. M. Becquerel allows that uranium and its compounds emit peculiar
rays—uranium rays. He proved that these rays can penetrate thin metallic
screens, and that they discharge electrified bodies. He also made
experiments from which he concluded that uranium rays undergo
reflection, refraction, and polarisation.

The work of other physicists (Elster and Geitel, Lord Kelvin, Schmidt,
Rutherford, Beattie, and Smoluchowski) confirms and extends the results
of the researches of M. Becquerel, with the exception of those relating
to the reflection, refraction, and polarisation of uranium rays, which
in this respect behave like Röntgen rays, as has been recognised first
by Mr. Rutherford and then by M. Becquerel himself.




                               CHAPTER I.
     RADIO-ACTIVITY OF URANIUM AND THORIUM. RADIO-ACTIVE MINERALS.


_Becquerel Rays._—The uranium rays discovered by M. Becquerel act upon
photographic plates screened from the light; they can penetrate all
solid, liquid, and gaseous substances, provided that the thickness is
sufficiently reduced; in passing through a gas, they cause it to become
a feeble conductor of electricity.

These properties of the uranium compounds are not due to any known
cause. The radiation seems to be spontaneous; it loses nothing in
intensity, even on keeping the compounds in complete darkness for
several years; hence there is no question of the phosphorescence being
specially produced by light.

The spontaneity and persistence of the uranium radiation appear as a
quite unique physical phenomenon. M. Becquerel kept a piece of uranium
for several years in the dark, and he has affirmed that at the end of
this time the action upon a photographic plate had not sensibly altered.
MM. Elster and Geitel made a similar experiment, and also found the
action to remain constant.

I measured the intensity of radiation of uranium by the effect of this
radiation on the conductivity of air. The method of measurement will be
explained later. I also obtained figures which prove the persistence of
radiation within the limits of accuracy of the experiments.

For these measurements a metallic plate was used covered with a layer of
powdered uranium; this plate was not otherwise kept in the dark; this
precaution, according to the experimenters already quoted, being of no
importance. The number of measurements taken with this plate is very
great, and they actually extend over a period of five years.

Some researches were conducted to discover whether other substances were
capable of acting similarly to the uranium compounds. M. Schmidt was the
first to publish that thorium and its compounds possess exactly the same
property. A similar research, made contemporaneously, gave me the same
result. I published this not knowing at the time of Schmidt’s
publication.

We shall say that uranium, thorium, and their compounds emit _Becquerel
rays_. I have called _radio-active_ those substances which generate
emissions of this nature. This name has since been adopted generally.

In their photographic and electric effects, the Becquerel rays
approximate to the Röntgen rays. They also, like the latter, possess the
faculty of penetrating all matter. But their capacity for penetration is
very different; the rays of uranium and of thorium are arrested by some
millimetres of solid matter, and cannot traverse in air a distance
greater than a few centimetres; this at least is the case for the
greater part of the radiation.

The researches of different physicists, and primarily of Mr. Rutherford,
have shown that the Becquerel rays undergo neither regular reflection,
nor refraction, nor polarisation.

The feeble penetrating power of uranium and thorium rays would point to
their similarity to the secondary rays produced by the Röntgen rays, and
which have been investigated by M. Sagnac, rather than to the Röntgen
rays themselves.

For the rest, the Becquerel rays might be classified as cathode rays
propagated in the air. It is now known that these different analogies
are all legitimate.


              _Measurement of the Intensity of Radiation._

[Illustration: FIG. 1.]

The method employed consists in measuring the conductivity acquired by
air under the action of radio-active bodies; this method possesses the
advantage of being rapid and of furnishing figures which are comparable.
The apparatus employed by me for the purpose consists essentially of a
plate condenser, A B (Fig. 1). The active body, finely powered, is
spread over the plate B, making the air between the plates a conductor.
In order to measure the conductivity, the plate B is raised to a high
potential by connecting it with one pole of a battery of small
accumulators, P, of which the other pole is connected to earth. The
plate A being maintained at the potential of the earth by the connection
C D, an electric current is set up between the two plates. The potential
of plate A is recorded by an electrometer, E. If the earth connection be
broken at C, the plate A becomes charged, and this charge causes a
deflection of the electrometer. The velocity of the deflection is
proportional to the intensity of the current, and serves to measure the
latter.

But a preferable method of measurement is that of compensating the
charge on plate A, so as to cause no deflection of the electrometer. The
charges in question are extremely weak; they may be compensated by means
of a quartz electric balance, Q, one sheath of which is connected to
plate A and the other to earth. The quartz lamina is subjected to a
known tension, produced by placing weights in a plate, π; the tension is
produced progressively, and has the effect of generating progressively a
known quantity of electricity during the time observed. The operation
can be so regulated that, at each instant, there is compensation between
the quantity of electricity that traverses the condenser and that of the
opposite kind furnished by the quartz. In this way, the quantity of
electricity passing through the condenser for a given time, _i.e._, the
_intensity of the current_, can be measured _in absolute units_. The
measurement is independent of the sensitiveness of the electrometer.

In carrying out a certain number of measurements of this kind, it is
seen that radio-activity is a phenomenon capable of being measured with
a certain accuracy. It varies little with temperature; it is scarcely
affected by variations in the temperature of the surroundings; it is not
influenced by incandescence of the active substance. The intensity of
the current which traverses the condenser increases with the surface of
the plates. For a given condenser and a given substance the current
increases with the difference of potential between the plates, with the
pressure of the gas which fills the condenser, and with the distance of
the plates (provided this distance be not too great in comparison with
the diameter). In every case, for great differences of potential the
current attains a limiting value, which is practically constant. This is
the _current of saturation_, or _limiting current_. Similarly, for a
certain sufficiently great distance between the plates the current
hardly varies any longer with the distance. It is the current obtained
under these conditions that was taken as the measure of radio-activity
in my researches, the condenser being placed in air at atmospheric
pressure.

I append curves which represent the intensity of the current as a
function of the field established between the plates for two different
plate distances. Plate B was covered with a thin layer of powdered
metallic uranium; plate A, connected with the electrometer, was provided
with a guard-ring.

[Illustration: FIG. 2.]

[Illustration: FIG. 3.]

Fig. 2 shows that the intensity of the current becomes constant for high
potential differences between the plates. Fig. 3 represents the same
curves on another scale, and comprehends only relative results for small
differences of potential. At the origin, the curve is rectilinear; the
ratio of the intensity of the current to the difference of potential is
constant for weak forces, and represents the initial conduction between
the plates. Two important characteristic constants of the observed
phenomenon are therefore to be recognised:—(1) The _initial conduction_
for small differences of potential; (2) the _limiting current_ for great
potential differences. The limiting current has been adopted as the
measure of the radio-activity.

Besides the difference of potential established between the two plates,
there exists between them an electromotive force of contact, and these
two sources of current combine their effects; for this reason, the
absolute value of the intensity of the current changes with the sign of
the external difference of potential. In every case, for considerable
potential differences, the effect of the electromotive force of contact
is negligible, and the intensity of the current is therefore the same
whatever be the direction of the field between the plates.

The investigation of the conductivity of air and other gases subjected
to the action of Becquerel rays has been undertaken by several
physicists. A very complete research upon the subject has been published
by Mr. Rutherford.

The laws of the conductivity produced in gases by the Becquerel rays are
the same as those found for the Röntgen rays. The mechanics of the
phenomenon appear to be the same in both cases. The theory of ionisation
of the gases by the action of the Röntgen or Becquerel rays agrees well
with the observed facts. This theory will not be put forward here. I
will merely record the results to which they point:—

Firstly, the number of ions produced per second in the gas is considered
proportional to the energy of radiation absorbed by the gas.

Secondly, in order to obtain the limiting current relatively to a given
radiation, it is necessary, on the one hand, to cause complete
absorption of this radiation by the gas by employing a sufficient mass
of it; on the other hand, it is necessary for the production of the
current to use all the ions generated by establishing an electric field
of such strength that the number of the ions which recombine may be a
negligible fraction of the total number of ions produced in the same
time, most of which are carried by the current to the electrodes. The
strength of the electric field necessary to give this result is
proportional to the amount of ionisation.

According to the recent researches of Mr. Townsend, the phenomenon is
more complex when the pressure of the gas is low. At first the current
appears to approach to a constant limiting value with increasing
difference of potential; but after a certain point has been reached, the
current begins again to increase with the field, and with very great
rapidity. Mr. Townsend ascribes this increase to a new ionisation
produced by the ions themselves when, under the action of the electric
field, they acquire a velocity such that a molecule of gas encountering
one of them becomes broken down into its constituent ions. A strong
electric field and a low pressure are favourable to the production of
this ionisation by ions already present, and, as soon as the action is
set up, the intensity of the current increases uniformly with the field
between the plates. The limiting current could, therefore, only be
obtained under conditions of ionisation of which the intensity does not
exceed a certain value, and in such a manner that saturation corresponds
to fields in which, from multiplicity of ions, ionisation can no longer
take place. This condition has occurred in my experiments.

The order of magnitude of the saturation currents obtained with uranium
compounds is 10^{–11} ampères for a condenser in which the plates have a
diameter of 8 c.m., and are at a distance of 3 c.m. Thorium compounds
give rise to currents of the same order of magnitude, and the activity
of the oxides of uranium and thorium is very similar.


       _Radio-activity of the Compounds of Uranium and Thorium._

The following are the figures I obtained with different uranium
compounds. I have represented the intensity of the current in ampères by
the letter _i_:—

                                                    _i_ × 10^{11}.

      Metallic uranium (containing a little carbon)            2·3
      Black oxide of uranium, U_{2}O_{5}                       2·6
      Green oxide of uranium, U_{3}O_{4}                       1·8
      Hydrated uranic acid                                     0·6
      Uranate of sodium                                        1·2
      Uranate of potassium                                     1·2
      Uranate of ammonium                                      1·3
      Uranium sulphate                                         0·7
      Sulphate of uranium and potassium                        0·7
      Nitrate of uranium                                       0·7
      Phosphate of copper and uranium                          0·9
      Oxysulphide of uranium                                   1·2

The thickness of the layer of the uranium compound used has little
effect, provided that the layer is uniform. The following illustrate
this point:—

                           Thickness of layer.     _i_ × 10^{11}.
                                  M.m.
      Uranium oxide                0·5                  2·7
      Uranium oxide                3·0                  3·0
      Ammonium uranate             0·5                  1·3
      Ammonium uranate             3·0                  1·4

It may be concluded from this that the absorption of uranium rays by the
substance which generates them is very great, since the rays proceeding
from deep layers produce no significant effect.

The figures I obtained with thorium compounds enable me to state:—

Firstly, that the thickness of the layer used has considerable effect,
especially in the case of the oxide.

Secondly, that the action is only regular if a sufficiently thin layer
is used (_e.g._, 0·25 m.m.). On the contrary, when a thick layer of the
substance is used (6 m.m.), the figures obtained vary between two
extreme limits, especially in the case of the oxide:—

                           Thickness of layer.     _i_ × 10^{11}.
                                  M.m.
      Thorium oxide    0·25                        2·2
      Thorium oxide    0·5                         2·5
      Thorium oxide    2·5                         4·7
      Thorium oxide    3·0                         5·5 (mean)
      Thorium oxide    6·0                         5·5 (mean)
      Thorium sulphate 0·25                        0·8 (mean)

There is here some cause of irregularities which do not exist in the
case of the uranium compounds. The figures obtained for a layer of oxide
6 m.m. thick varied between 3·7 and 7·3.

The experiments that I made on the absorption of uranium and thorium
rays showed that those of thorium are more penetrating than those of
uranium, and that the rays emitted by the oxide of thorium in a thick
layer are more penetrating than those emitted by a thin layer of the
same. The following figures (p. 13) give the fraction of the radiation
transmitted by a sheet of aluminium 0·01 thick.

With the uranium compounds, the absorption is the same whatever be the
compound used, which leads to the conclusion that the rays emitted by
the different compounds are of the same nature.

       Radio-active substance.        Fraction of radiation transmitted
                                                by the sheet.
 Uranium                                                            0·18
 Uranium oxide, U_{2}O_{5}                                          0·20
 Uranate of ammonium                                                0·20
 Phosphate of uranium and copper                                    0·21
 Thorium oxide of thickness 0·25 m.m.                               0·38
 Thorium oxide of thickness 0·5  m.m.                               0·47
 Thorium oxide of thickness 3·0  m.m.                               0·70
 Thorium oxide of thickness 0·60 m.m.                               0·70
 Thorium sulphate           0·25 m.m.                               0·38

The characteristics of the thorium radiation have formed the subject of
very complete publications. Mr. Owens has demonstrated that a uniform
current is only obtained after some time has elapsed, with an enclosed
apparatus, and that the intensity of the current is greatly reduced
under the influence of a current of air (which does not occur with the
compounds of uranium). Mr. Rutherford has made similar experiments, and
has explained them by the proposition that thorium and its compounds
produce, besides the Becquerel rays, another _emanation_, composed of
extremely minute particles, which remain radio-active for some time
after their emission, and are capable of being swept along by a current
of air.

The characteristics of the thorium radiation, which have reference to
the thickness of the layer employed and to the action of air currents,
have an intimate connection with the phenomenon of the _radio-activity
induced, and of its propagation from place to place_. This phenomenon
was observed for the first time with radium, and will be described
later.

The radio-activity of thorium and uranium compounds appears as an
_atomic property_. M. Becquerel has already observed that all uranium
compounds are active, and had concluded that their activity was due to
the presence of the element uranium; he also demonstrated that uranium
was more active than its salts. I have investigated, from this point of
view, the compounds of thorium and uranium, and have taken a great many
measurements of their activity under different conditions. The result of
all these determinations shows the radio-activity of these substances to
be decidedly an atomic property. It seems to depend upon the presence of
atoms of the two elements in question, and is not influenced by any
change of physical state or chemical decomposition. The chemical
combinations and mixtures containing uranium or thorium are active in
proportion to the amount of the metal contained, all inactive material
acting as inert bodies and absorbing the radiation.


            _Is Atomic Radio-activity a general Phenomenon?_

As I have said above, I made experiments to discover whether substances
other than compounds of uranium and thorium were radio-active. I
undertook this research with the idea that it was scarcely probable that
radio-activity, considered as an atomic property, should belong to a
certain kind of matter to the exclusion of all other. The determinations
I made permit me to say that, for chemical elements actually considered
as such, including the rarest and most hypothetical, the compounds I
investigated were always at least 100 times less active in my apparatus
than metallic uranium.

The following is a summary of the substances experimented upon, either
as the element or in combination:—

1. All the metals or non-metals easily procurable, and some, more rare,
pure products obtained from the collection of M. Etard, at the Ecole de
Physique et de Chimie Industrielles de la Ville de Paris.

2. The following rare bodies:—Gallium, germanium, neodymium,
praseodymium, niobium, scandium, gadolinium, erbium, samarium, and
rubidium (specimens lent by M. Demarçay), yttrium, ytterbium (lent by M.
Urbain).

3. A large number of rocks and minerals.

Within the limits of sensitiveness of any apparatus, I found no simple
substance, other than uranium and thorium, possessing atomic
radio-activity. It will be suitable to add a few words here concerning
phosphorus. White moist phosphorus, placed between the plates of the
condenser, causes the air between the plates to conduct. However, I do
not consider this body radio-active in the same manner as thorium and
uranium. For, under these conditions, phosphorus becomes oxidised and
emits luminous rays, whilst uranium and thorium compounds are
radio-active without showing any chemical change which can be detected
by any known means. Further, phosphorus is not active in the red
variety, nor in a state of combination.

In a recent work, M. Bloch has demonstrated that phosphorus, undergoing
oxidation in air, gives rise to slightly motile ions, which make the air
conduct, and cause condensation of aqueous vapour.

Uranium and thorium are elements which possess the highest atomic
weights (240 and 232); they occur frequently in the same minerals.


                        _Radio-active Minerals._

I have examined many minerals in my apparatus; certain of them gave
evidence of radio-activity, _e.g._, pitchblende, thorite, orangite,
fergusonite, cleveite, chalcolite, autunite, monazite, &c. The following
is a table giving in ampères the intensity, _i_, of the current obtained
with metallic uranium and with different minerals:—

                                              │_i_ × 10^{11}.
           Uranium                            │2·3
           Pitchblende from Johanngeorgenstadt│8·3
           Pitchblende from Joachimsthal      │7·0
           Pitchblende from Pzibran           │6·5
           Pitchblende from Cornwallis        │1·6
           Cleveite                           │1·4
           Chalcolite                         │5·2
           Autunite                           │2·7
           ───────────────────────────────────┼──────────────
           Various thorites                   │0·1
                            〃                 │0·3
                            〃                 │0·7
                            〃                 │1·3
                            〃                 │1·4
           ───────────────────────────────────┼──────────────
           Orangite                           │2·0
           Monazite                           │0·5
           Xenotime                           │0·03
           Æschynite                          │0·7
           ───────────────────────────────────┼──────────────
           Fergusonite (two samples)          │0·4
                            〃                 │0·1
           ───────────────────────────────────┼──────────────
           Samarskite                         │1·1
           ───────────────────────────────────┼──────────────
           Niobite (two samples)              │0·1
                            〃                 │0·3
           ───────────────────────────────────┼──────────────
           Tantalite                          │0·02
           Carnotite                          │6·2

The current obtained with orangite (native oxide of thorium) varied
greatly with the thickness of the layer. By increasing this thickness
from 0·25 m.m. to 6 m.m. the current increased from 1·8 to 2·3.

All the minerals which showed radio-activity contained uranium or
thorium: their activity is therefore not surprising, but the intensity
of the action in certain cases is unexpected. Thus pitchblendes (ores of
uranium oxide) are found which are four times as active as metallic
uranium. Chalcolite (double phosphate of copper and uranium) is twice as
active as uranium. Autunite (phosphate of uranium and calcium) is as
active as uranium. These facts do not accord with previous conclusions,
according to which no mineral should be so active as thorium or uranium.

To throw light on this point, I prepared artificial chalcolite by the
process of Debray, starting with the pure products. The process consists
in mixing a solution of uranium nitrate with a solution of copper
phosphate in phosphoric acid and warming to 50° or 60°. After some time,
crystals of chalcolite appear in the liquid.

Chalcolite thus obtained possesses a perfectly normal activity, given by
its composition; it is two and a-half times less active than uranium.

It therefore appeared probable that if pitchblende, chalcolite, and
autunite possess so great a degree of activity, these substances contain
a small quantity of a strongly radio-active body, differing from uranium
and thorium and the simple bodies actually known. I thought that if this
were indeed the case, I might hope to extract this substance from the
ore by the ordinary methods of chemical analysis.




                              CHAPTER II.
                          METHOD OF RESEARCH.


The results of the investigation of radio-active minerals, announced in
the preceding chapter, led M. Curie and myself to endeavour to extract a
new radio-active body from pitchblende. Our method of procedure could
only be based on radio-activity, as we know of no other property of the
hypothetical substance. The following is the method pursued for a
research based on radio-activity:—The radio-activity of a compound is
determined, and a chemical decomposition of this compound is effected;
the radio-activity of all the products obtained is determined, having
regard to the proportion in which the radio-active substance is
distributed among them. In this way, an indication is obtained, which
may to a certain extent be compared to that which spectrum analysis
furnishes. In order to obtain comparable figures, the activity of the
substances must be determined in the solid form well dried.


                     _Polonium, Radium, Actinium._

The analysis of pitchblende with the help of the method just explained,
led us to the discovery in this mineral of two strongly radio-active
substances, chemically dissimilar:—Polonium, discovered by ourselves,
and radium, which we discovered in conjunction with M. Bémont.

_Polonium_ from the analytical point of view, is analogous to bismuth,
and separates out with the latter. By one of the following methods of
fractionating, bismuth products are obtained increasingly rich in
polonium:—

1. Sublimation of the sulphides _in vacuo_; the active sulphide is much
more volatile than bismuth sulphide.

2. Precipitation of solutions of the nitrate by water; the precipitate
of the basic nitrate is much more active than the salt which remains in
solution.

3. Precipitation by sulphuretted hydrogen of a hydrochloric acid
solution, strongly acid; the precipitated sulphides are considerably
more active than the salt which remains in solution.

_Radium_ is a substance which accompanies the barium obtained from
pitchblende; it resembles barium in its reactions, and is separated from
it by difference of solubility of the chlorides in water, in dilute
alcohol, or in water acidified with hydrochloric acid. We effect the
separation of the chlorides of barium and radium by subjecting the
mixture to fractional crystallisation, radium chloride being less
soluble than that of barium.

A third strongly radio-active body has been identified in pitchblende by
M. Debierne, who gave it the name of _actinium_. Actinium accompanies
certain members of the iron group contained in pitchblende; it appears
in particular allied to thorium, from which it has not yet been found
possible to separate it. The extraction of actinium from pitchblende is
a very difficult operation, the separations being as a rule incomplete.

All three of the new radio-active bodies occur in quite infinitesimal
amount in pitchblende. In order to obtain them in a more concentrated
condition, we were obliged to treat several tons of residue of the ore
of uranium. The rough treatment was carried out in the factory; and this
was followed by processes of purification and concentration. We thus
succeeded in extracting from thousands of kilogrms. of crude material a
few decigrammes of products which were exceedingly active as compared
with the ore from which they were obtained. It is obvious that this
process is long, arduous, and costly.

Other new radio-active bodies have been notified since the termination
of our work. M. Giesel, on the one hand, and MM. Hoffmann and Strauss on
the other, have announced the probable existence of a radio-active body
similar to lead in its chemical properties. At present only a few
samples of this substance have been obtained.

Radium is, so far, the only member of the new radio-active substances
that has been isolated as the pure salt.


                         _Spectrum of Radium._

It was of the first importance to check, by all possible means, the
hypothesis, underlying this work, of new radio-active elements. In the
case of radium, spectrum analysis was the means of confirming this
hypothesis.

M. Demarçay undertook the examination of the new radio-active bodies by
the searching methods which he employs in the study of photographic
spark spectra.

The assistance of so competent a scientist was of the greatest value to
us, and we are deeply grateful to him for having consented to take up
this work. The results of the spectrum analysis brought conviction to us
when we were still in doubt as to the interpretation of the results of
our research.

The first specimens of fairly active barium chloride containing radium,
examined by M. Demarçay, exhibited together with the barium lines a new
line of considerable intensity and of wave-length λ = 381·47 µµ in the
ultra-violet. With the more active products prepared subsequently,
Demarçay saw the line 381·47 µµ more distinctly; at the same time other
new lines appeared, and the intensity of the new lines was comparable
with that of the barium lines. A further concentration furnished a
product for which the new spectrum predominated, and the three strongest
barium lines, alone visible, merely indicated the presence of this metal
as an impurity. This product may be looked upon as nearly pure radium
chloride. Finally, by further purification, I obtained an exceedingly
pure chloride, in the spectrum of which the two chief barium lines were
scarcely visible.

The following is a list, according to Demarçay, of the principal radium
lines for the portion of the spectrum included between λ = 500·0 and λ =
350·0 µµ. The intensity of each line is represented by a figure, the
strongest being marked 16:—

                             λ.   Intensity.
                           482·63         10
                           472·69          5
                           469·98          3
                           469·21          7
                           468·30         14
                           464·19          4
                           460·03          3
                           453·35          9
                           443·61          8
                           434·06         12
                           381·47         16
                           364·96         12

All the lines are clear and narrow, the three lines 381·47, 468·30,
434·06 are strong, and equal the most intense of those actually known.
Two well-marked misty bands are also visible in the spectrum. The first,
which is symmetrical, extends from 463·10 to 462·19, with a maximum at
462·75. The second, which is stronger, fades towards the ultra-violet;
it begins, sharply defined, at 446·37, and passes through a maximum at
445·52; the region of the maximum extends as far as 445·34, then a
nebulous band, gradually fading, extends about as far as 439.

In the least refrangible part, not photographed in the spark spectrum,
the only significant line is 566·5 (approx.), much more feeble, however,
than 482·63.

The general aspect of the spectrum is that of the metals of the alkaline
earths; these metals are known to have well-marked line spectra with
certain nebulous bands.

According to Demarçay, the position of radium may be among the bodies
possessing the most sensitive spectrum reaction. I also have concluded
from the work of concentration, that in the first specimen examined,
which showed clearly the line 3814·7, the proportion of radium must have
been very small (perhaps about 0·02 per cent). Nevertheless, an activity
fifty times as great as that of metallic uranium is required in order to
distinguish clearly the principal radium line in the spectra
photographed. With a sensitive electrometer the radio-activity of a
substance only 1/100 of that of metallic uranium can be detected. It is
clear that, in order to detect the presence of radium, the property of
radio-activity is several thousand times more sensitive than the
spectrum reaction.

Bismuth containing polonium and thorium containing actinium, both very
active, examined by Demarçay, have so far each only yielded bismuth and
thorium lines.

In a recent publication, M. Giesel, who is occupied in preparing radium,
states that radium bromide gives a carmine flame colouration. The flame
spectrum of radium contains two beautiful red bands, one line in the
blue-green, and two faint lines in the violet.


            _Extraction of the New Radio-active Substances._

The first stage of the operation consists in extracting barium with
radium from the ores of uranium, also bismuth with polonium and the rare
earths containing actinium from the same. These three primary products
having been obtained, the next step is in each case to endeavour to
isolate the new radio-active body. This second part of the treatment
consists of a process of fractionation. The difficulty of finding a very
perfect means of separating closely allied elements is well known;
methods of fractionation are therefore quite suitable. Besides this,
when a mere trace of one element is mixed with another element, no
method of complete separation could be applied to the mixture, even
allowing that such a method was known; in fact, one would run the risk
of losing the trace of the material to be separated.

The particular object of my work has been the isolation of radium and
polonium. After working for several years, I have so far only succeeded
in obtaining the former.

Pitchblende is an expensive ore, and we have given up the treatment of
it in large quantities. In Europe the extraction of this ore is carried
out in the mine of Joachimsthal, in Bohemia. The crushed ore is roasted
with carbonate of soda, and the resulting material washed, first with
warm water and then with dilute sulphuric acid. The solution contains
the uranium, which gives pitchblende its value. The insoluble residue is
rejected. This residue contains radio-active substances; its activity is
four and a-half times that of metallic uranium. The Austrian Government,
to whom the mine belongs, presented us with a ton of this residue for
our research, and authorised the mine to give us several tons more of
the material.

It was not very easy to apply the methods of the laboratory to the
preliminary treatment of the residue in the factory. M. Debierne
investigated this question, and organised the treatment in the factory.
The most important point of his method is the conversion of the
sulphates into carbonate by boiling the material with a concentrated
solution of sodium carbonate. This method avoids the necessity of fusing
with sodium carbonate.

The residue chiefly contains the sulphates of lead and calcium, silica,
alumina, and iron oxide. In addition nearly all the metals are found in
greater or smaller amount (copper, bismuth, zinc, cobalt, manganese,
nickel, vanadium, antimony, thallium, rare earths, niobium, tantalum,
arsenic, barium, &c.). Radium is found in this mixture as sulphate, and
is the least soluble sulphate in it. In order to dissolve it, it is
necessary to remove the sulphuric acid as far as possible. To do this,
the residue is first treated with a boiling concentrated soda solution.
The sulphuric acid combined with the lead, aluminium, and calcium
passes, for the most part, into solution as sulphate of sodium, which is
removed by repeatedly washing with water. The alkaline solution removes
at the same time lead, silicon, and aluminium. The insoluble portion is
attacked by ordinary hydrochloric acid. This operation completely
disintegrates the material, and dissolves most of it. Polonium and
actinium may be obtained from this solution; the former is precipitated
by sulphuretted hydrogen, the latter is found in the hydrates
precipitated by ammonia in the solution separated from the sulphides and
oxidised. Radium remains in the insoluble portion. This portion is
washed with water, and then treated with a boiling concentrated solution
of carbonate of soda. This operation completes the transformation of the
sulphates of barium and radium into carbonates. The material is then
thoroughly washed with water, and then treated with dilute hydrochloric
acid, quite free from sulphuric acid. The solution contains radium as
well as polonium and actinium. It is filtered and precipitated with
sulphuric acid. In this way the crude sulphates of barium containing
radium and calcium, of lead, and of iron, and of a trace of actinium are
obtained. The solution still contains a little actinium and polonium,
which may be separated out as in the case of the first hydrochloric acid
solution.

From one ton of residue 10 to 20 kilogrms. of crude sulphates are
obtained, the activity of which is from thirty to sixty times as great
as that of metallic uranium. They must now be purified. For this purpose
they are boiled with sodium carbonate and transformed into the
chlorides. The solution is treated with sulphuretted hydrogen, which
gives a small quantity of active sulphides containing polonium. The
solution is filtered, oxidised by means of chlorine, and precipitated
with pure ammonia. The precipitated hydrates and oxides are very active,
and the activity is due to actinium. The filtered solution is
precipitated with sodium carbonate. The precipitated carbonates of the
alkaline earths are washed and converted into chlorides. These chlorides
are evaporated to dryness, and washed with pure concentrated
hydrochloric acid. Calcium chloride dissolves almost entirely, whilst
the chloride of barium and radium remains insoluble. Thus, from one ton
of the original material about 8 kilogrms. of barium and radium chloride
are obtained, of which the activity is about sixty times that of
metallic uranium. The chloride is now ready for fractionation.


                              _Polonium._

As I said above, by passing sulphuretted hydrogen through the various
hydrochloric acid solutions obtained during the course of the process,
active sulphides are precipitated, of which the activity is due to
polonium. These sulphides chiefly contain bismuth, a little copper and
lead; the latter metal occurs in relatively small amount, because it has
been to a great extent removed by the soda solution, and because its
chloride is only slightly soluble. Antimony and arsenic are found among
the oxides only in the minutest quantity, their oxides having been
dissolved by the soda. In order to obtain the very active sulphides, the
following process was employed:—The solutions made strongly acid with
hydrochloric acid were precipitated with sulphuretted hydrogen; the
sulphides thus precipitated are very active, and are employed for the
preparation of polonium; there remain in the solution substances not
completely precipitated in presence of excess of hydrochloric acid
(bismuth, lead, antimony). To complete the precipitation, the solution
is diluted with water, and treated again with sulphuretted hydrogen,
which gives a second precipitate of sulphides, much less active than the
first, and which have generally been rejected. For the further
purification of the sulphides, they are washed with ammonium sulphide,
which removes the last remaining traces of antimony and arsenic. They
are then washed with water and ammonium nitrate, and treated with dilute
nitric acid. Complete solution never occurs; there is always an
insoluble residue, more or less considerable, which can be treated
afresh if it is judged expedient. The solution is reduced to a small
volume and precipitated either by ammonia or by excess of water. In both
cases the lead and the copper remain in solution; in the second case, a
little bismuth, scarcely active at all, remains also in solution.

The precipitate of oxides or basic nitrates is subjected to
fractionation in the following manner:—The precipitate is dissolved in
nitric acid, and water is added to the solution until a sufficient
quantity of precipitate is formed; it must be borne in mind that
sometimes the precipitate does not at once appear. The precipitate is
separated from the supernatant liquid, and re-dissolved in nitric acid,
after which both the liquids thus obtained are re-precipitated with
water, and treated as before. The different fractions are combined
according to their activity, and concentration is carried out as far as
possible. In this way is obtained a very small quantity of a substance
of which the activity is very high, but which, nevertheless, has so far
only shown bismuth lines in the spectroscope.

There is, unfortunately, little chance of obtaining the isolation of
polonium by this means. The method of fractionation just described
presents many difficulties, and the case is similar with other wet
processes of fractionation. Whatever be the method employed, compounds
are readily formed which are absolutely insoluble in dilute or
concentrated acids. These compounds can only be re-dissolved by reducing
them to the metallic state, _e.g._, by fusion with potassium cyanide.
Considering the number of operations necessary, this circumstance
constitutes an enormous difficulty in the progress of the fractionation.
This obstacle is the greater because polonium, once extracted from the
pitchblende, diminishes in activity. This diminution of activity is
slow, for a specimen of bismuth nitrate containing polonium only lost
half its activity in eleven months.

No such difficulty occurs with radium. The radio-activity remains
throughout an accurate gauge of the concentration; the concentration
itself presents no difficulty, and the progress of the work from the
start can be constantly checked by spectral analysis.

When the phenomena of induced radio-activity, which will be discussed
later on, were made known, it seemed obvious that polonium, which only
shows the bismuth lines and whose activity diminishes with time, was not
a new element, but bismuth made active by the vicinity of radium in the
pitchblende. I am not sure that this opinion is correct. In the course
of my prolonged work on polonium, I have noted chemical effects, which I
have never observed either with ordinary bismuth or with bismuth made
active by radium. These chemical effects are, in the first place, the
extremely ready formation of insoluble compounds, of which I have spoken
above (especially basic nitrates), and, in the second place, the colour
and appearance of the precipitates obtained by adding water to the
nitric acid solution of bismuth containing polonium. These precipitates
are sometimes white, but more generally of a more or less vivid yellow,
verging on red.

The absence of lines other than those of bismuth does not necessarily
prove that the substance only contains bismuth, because bodies exist
whose spectrum reaction is scarcely visible.

It would be necessary to prepare a small quantity of bismuth containing
polonium in as concentrated a condition as possible, and to examine it
chemically, in the first place determining the atomic weight of the
metal. It has not yet been possible to carry out this research on
account of the difficulties of a chemical nature already mentioned.

If polonium were proved to be a new element, it would be no less true
that it cannot exist indefinitely in a strongly radio-active condition,
at least when extracted from the ore. There are therefore two aspects of
the question:—First, whether the activity of polonium is entirely
induced by the proximity of substances themselves radio-active, in which
case polonium would possess the faculty of acquiring atomic activity
permanently, a faculty which does not appear to belong to any substance
whatever; second, whether the activity of polonium is an inherent
property, which is spontaneously destroyed under certain conditions, and
persists under certain other conditions, such as those which exist in
the ore. The phenomenon of atomic activity induced by contact is still
so little understood, that we lack the ground on which to formulate any
opinion on the matter.

(NOTE.—A work has recently appeared on polonium by M. Marckwald. He
plunges a small rod of pure bismuth into a hydrochloric acid solution of
the bismuth extracted from the pitchblende residue. After some time the
rod becomes coated with a very active deposit, and the solution now
contains only inactive bismuth. M. Marckwald also obtains a very active
deposit by adding tin chloride to a hydrochloric acid solution of
radio-active bismuth. From this he concludes that the active element is
allied to tellurium, and gives it the name of _radiotellurium_. This
active substance of M. Marckwald seems identical with polonium, from its
behaviour, and from the easily absorbed rays it emits. The choice of a
new name for this substance is futile in the present state of the
question).


             _Preparation of the Pure Chloride of Radium._

The method by which I extracted pure radium chloride from barium
chloride containing radium consists in first subjecting the mixture of
the chlorides to fractional crystallisation in pure water, then in water
to which hydrochloric acid has been added. The difference in solubility
of the two chlorides is thus made use of, that of radium being less
soluble than that of barium.

At the beginning of the fractionation, pure distilled water is used. The
chloride is dissolved, and the solution raised to boiling-point, and
allowed to crystallise by cooling in a covered capsule. Beautiful
crystals form at the bottom, and the supernatant, saturated solution is
easily decanted. If part of this solution be evaporated to dryness, the
chloride obtained is found to be about five times less active than that
which has crystallised out. The chloride is thus divided into two
portions, A and B—portion A being more active than portion B. The
operation is now repeated with each of the chlorides A and B, and in
each case two new portions are obtained. When the crystallisation is
finished, the less active fraction of chloride A is added to the more
active fraction of chloride B, these two having approximately the same
activity. Thus there are now three portions to undergo afresh the same
treatment.

The number of portions is not allowed to increase indefinitely. The
activity of the most soluble portion diminishes as the number increases.
When its activity becomes inconsiderable, it is withdrawn from the
fractionation. When the desired number of fractions has been obtained,
fractionation of the least soluble portion is stopped (the richest in
radium), and it is withdrawn from the remainder.

A fixed number of fractions is used in the process. After each series of
operations, the saturated solution arising from one fraction is added to
the crystals arising from the following fraction; but if after one of
the series the most soluble fraction has been withdrawn, then, after the
following series, a new fraction is made from the most soluble portion,
and the crystals of the most active portion are withdrawn. By the
successive alteration of these two processes, an extremely regular
system of fractionation is obtained, in which the number of fractions
and the activity of each remains constant, each being about five times
as active as the subsequent one, and in which, on the one hand, an
almost inactive product is removed, whilst, on the other, is obtained a
chloride rich in radium. The amount of material contained in these
fractions gradually diminishes, becoming less as the activity increases.

At first six fractions were used, and the activity of the chloride
obtained at the end was only 0·1 that of uranium.

When most of the inactive matter has been removed, and the fractions
have become small, one fraction is removed from the one end, and another
is added to the other end consisting of the active chloride previously
removed. A chloride richer in radium than the preceding is thus
obtained. This system is continued until the crystals obtained are pure
radium chloride. If the fractionation has been thoroughly carried out,
scarcely any trace of the intermediate products remain.

At an advanced stage of the fractionation, when the quantity of material
in each fraction is small, the separation by crystallisation is less
efficacious, the cooling being too rapid and the volume of the solution
to be decanted too small. It is then advisable to add water containing a
known quantity of hydrochloric acid; this quantity may be increased as
the fractionation proceeds.

The advantage gained thus consists in increasing the quantity of the
solution, the solubility of the chlorides being less in water acidified
with hydrochloric acid than in pure water. By using water containing
much acid, excellent separations are effected, and it is only necessary
to work with three or four fractions.

The crystals, which form in very acid solution, are elongated needles,
those of barium chloride having exactly the same appearance as those of
radium chloride. Both show double refraction. Crystals of barium
chloride containing radium are colourless, but when the proportion of
radium becomes greater, they have a yellow colouration after some hours,
verging on orange, and sometimes a beautiful pink. This colour
disappears in solution. Crystals of pure radium chloride are not
coloured, so that the colouration appears to be due to the mixture of
radium and barium. The maximum colouration is obtained for a certain
degree of radium present, and this fact serves to check the progress of
the fractionation.

I have sometimes noticed the formation of a deposit composed of crystals
of which one part remained uncoloured, whilst the other was coloured,
and it seems possible that the colourless crystals might be sorted out.

The fractional precipitation of an aqueous solution of barium chloride
by alcohol also leads to the isolation of radium chloride, which is the
first to precipitate. This method, which I first employed, was finally
abandoned for the one just described, which proceeds with more
regularity. I have, however, occasionally made use of precipitation by
alcohol to purify radium chloride which contains traces of barium
chloride. The latter remains in the slightly aqueous alcoholic solution,
and can thus be removed.

M. Giesel, who, since the publication of our first researches, has been
preparing radio-active bodies, recommends the separation of barium and
radium by fractional crystallisation in water from a mixture of the
bromides. I can testify that this method is advantageous, especially in
the first stages of the fractionation.


            _Determination of the Atomic Weight of Radium._

In the course of my work I determined at intervals the atomic weight of
the metal contained in specimens of barium chloride containing radium.
With each newly obtained product I carried the concentration as far as
possible, so as to have from 0·1 grm. to 0·5 grm. of material containing
most of the activity of the mixture. From this small quantity I
precipitated with alcohol or with hydrochloric acid some milligrams of
chloride for spectral analysis. Thanks to his excellent method, Demarçay
only required this small quantity of material to obtain the photograph
of the spark spectrum. I made an atomic weight determination with the
product remaining.

I employed the classic method of weighing as silver chloride the
chlorine contained in a known weight of the anhydrous chloride. As
control experiment, I determined the atomic weight of barium by the same
method, under the same conditions, and with the same quantity of
material, first 0·5 grm. and then 0·1 grm. The figures obtained were
always between 137 and 138. I thus saw that the method gives
satisfactory results, even with a very small quantity of material.

The first two determinations were made with chlorides, of which one was
230 times and the other 600 times as active as uranium. These two
experiments gave the same figure as the experiment with the pure barium
chloride. There was therefore no hope of finding a difference except by
using a much more active product. The following experiment was made with
a chloride, the activity of which was about 3500 times as great as that
of uranium; and this experiment enabled me, for the first time, to
observe a small but distinct difference; I found, as the mean atomic
weight of the metal contained in this chloride, the number 140, which
showed that the atomic weight of radium must be higher than that of
barium. By using more and more active products, and obtaining spectra of
radium of increasing intensity, I found that the figures obtained rose
in proportion, as is seen in the following table (p. 28).

The figures of column A must only be looked upon as a rough estimate.
The calculation of the activity of strongly radio-active bodies is
difficult, for many reasons which will be discussed later.

          A.         │ M.
         3500        │140   Spectrum of radium faint.
         4700        │141
         7500        │145·8 Spectrum of radium strong, but that of
                     │        barium predominating.
 ────────────────────┼──────────────────────────────────────────────────
 Order of Magnitude, │173·8 The two spectra of almost equal intensity.
   10^6 ...          │
          〃          │225   Only a trace of barium present.

A represents the activity of the chloride, that of uranium being unity;
M the atomic weight found.

At the termination of the processes described above, I obtained, in
March, 1902, 0·12 grm. of radium chloride, of which Demarçay made the
spectral analysis. This radium chloride, in the opinion of Demarçay, was
fairly pure; its spectrum, however, showed the three principal barium
lines with considerable intensity. I made four successive estimations of
the chloride, the results of which as follows:—

              Anhydrous radium chloride. Silver chloride.  M.
           I. 0·1150                     0·1130           220·7
          II. 0·1140                     0·1119           223·0
         III. 0·11135                    0·1086           222·8
          IV. 0·10925                    0·10645          223·1

I then re-purified this chloride, and obtained a much purer substance,
in the spectrum of which the two strongest barium lines were very faint.
Given the sensitiveness of the spectrum reaction of barium, Demarçay
estimated that the purified chloride contained only the merest traces of
barium, incapable of influencing the atomic weight to an appreciable
extent. I made three determinations with this perfectly pure radium
chloride. The results were as follows:—

              Anhydrous radium chloride. Silver chloride.  M.
           I.          0·09192               0·08890      225·3
          II.          0·08936               0·08627      225·8
         III.          0·08839               0·08589      224·0

The mean of these numbers is 225. They were calculated in the same way
as the preceding ones by considering radium as a bivalent element, the
chloride having the formula RaCl_{2}, and taking for silver and chlorine
the values Ag = 107·8, Cl = 35·4.

Hence the atomic weight of radium is Ra = 225.

The weighings were made with a Curie aperiodic balance, perfectly
regulated, accurate to the twentieth of a milligrm. This direct reading
balance permits of very rapid weighing, a condition which is essential
in the case of the anhydrous chlorides of radium and barium, which
gradually absorb moisture, in spite of the presence of desiccating
substances in the balance. The bodies to be weighed were placed in a
platinum crucible; this crucible had been long in use, and its weight
did not vary the tenth part of a milligrm. during the course of one
operation.

The hydrated chloride obtained by crystallisation was placed in the
crucible and heated till converted into the anhydrous chloride. When the
chloride has been kept for several hours at 100° its weight becomes
constant, and does not change even if the temperature is raised to 200°.
The anhydrous chloride thus obtained constitutes, therefore, a perfectly
definite body.

The following is a series of determinations on this point. The chloride
(100 m.g.) is dried in the oven at 55°, and placed in a desiccator over
anhydrous phosphoric acid; it then gradually loses weight, which proves
that it still contains moisture; in the course of twelve hours the loss
was 3 m.g. The chloride is replaced in the stove, and the temperature
raised to 100°. During this process, the chloride lost 6·3 m.g. in
weight. After being left three hours fifteen minutes in the oven, it
lost 2·5 m.g. more. The temperature was maintained for forty-five
minutes between 100° and 120°, which caused a loss of weight of 0·1 m.g.
Then after being kept for thirty minutes at 125°, the chloride showed no
diminution in weight. Then, however, after thirty minutes at 150°, it
lost 0·1 m.g. Finally, after being heated for four hours at 200°, it
lost 0·15 m.g. During these operations the crucible varied from 0·05
m.g.

After each determination of the atomic weight, the radium was converted
into the chloride in the following manner:—To the solution containing
the weighed radium nitrate and excess of silver nitrate was added pure
hydrochloric acid; the silver chloride was filtered off; the solution
was evaporated to dryness several times with excess of pure hydrochloric
acid. In this way the nitric acid is entirely removed.

The precipitated silver chloride was always radio-active and
phosphorescent. In determining the amount of silver contained in it, I
satisfied myself that no ponderable amount of radium had been carried
down with it out of the solution. The method I pursued was to reduce the
silver chloride precipitated in the crucible by hydrogen generated from
dilute hydrochloric acid and zinc; after washing, the crucible was
weighed with the metallic silver contained in it.

I made another experiment which showed that the weight of radium
chloride regenerated was the same as that before beginning the
operation.

These verifications are not so reliable as direct experiments; but they
serve to indicate the absence of any significant error.

From its chemical properties, radium is an element of the group of
alkaline earths, being the member next above barium.

From its atomic weight also, radium takes its place in Mendeleeff’s
table after barium with the alkaline earth metals, in the row which
already contains uranium and thorium.


                 _Characteristics of the Radium Salts._

The salts of radium, chloride, nitrate, carbonate, and sulphate,
resemble those of barium, when freshly prepared, but they gradually
become coloured.

All the radium salts are luminous in the dark.

In their chemical properties, the salts of radium are absolutely
analogous to the corresponding salts of barium. However, radium chloride
is less soluble than barium chloride; the solubility of the nitrates in
water is approximately the same.

The salts of radium are the source of a spontaneous and continuous
evolution of heat.


              _Fractionation of Ordinary Barium Chloride._

We have endeavoured to determine whether commercial barium chloride
contains small quantities of radium chloride, which escape detection
with the means of estimation at our command. For this purpose we
fractionated a great quantity of commercial barium chloride, in the hope
of thus concentrating the trace of radium chloride if such were present.

Fifty kilos. of commercial barium chloride were dissolved in water; the
solution was precipitated by hydrochloric acid free from sulphuric acid,
which yielded 20 kilos. of the precipitated chloride. This was dissolved
in water and partially precipitated by hydrochloric acid, which gave 8·5
kilos. of precipitated chloride. This chloride was fractionated by the
method used for the barium chloride containing radium; and at the end of
the process, 10 grams of chloride were obtained, corresponding to the
least soluble part. This chloride showed no radio-activity; it therefore
contained no radium; this substance is, consequently, absent from the
ores of barium.




                              CHAPTER III.
             RADIATION OF THE NEW RADIO-ACTIVE SUBSTANCES.


              _Methods of Investigation of the Radiation._

In order to investigate the radiation emitted by radio-active bodies,
any one of the properties of this radiation can be utilised. Thus the
action of the rays on photographic plates may serve, or their property
of ionisation of the air, which renders it a conductor, or their
capacity for causing fluorescence of certain bodies. Henceforth, in
speaking of these different methods of working, I shall use the
expressions radiographic method, electrical method, fluoroscopic method.

The first two have been used from the beginning in the study of uranium
rays; the fluoroscopic method can only be applied in the case of the new
bodies which are strongly radio-active, for the feebly active bodies
such as uranium and thorium produce no appreciable fluorescence. The
electrical method is the only one which serves for exact determinations
of intensity; the other two are specially adapted for giving qualitative
results, and only furnish rough approximations. The results obtained
with the three methods just considered are not strictly comparable the
one with the other. The sensitive plate, the gas which is ionised, the
fluorescent screen, are in reality receivers, which absorb the energy of
the radiation, and transform it into another kind of energy, chemical
energy, ionic energy, or luminous energy. Each receiver absorbs a
fraction of the radiation, which depends essentially upon its nature.
Later on, we shall see that the radiation is complex, that the fractions
of the radiation absorbed by the different receivers may differ among
themselves both quantitatively and qualitatively. Finally, it is neither
evident, nor even probable, that the energy absorbed is entirely
transformed by the receiver into the form that we wish for observation;
part of this energy may be transformed into heat, into the evolution of
secondary radiations which may or may not assist in the production of
the observed phenomenon, into chemical action which differs from that
under observation, &c., and here also the effective action of the
receiver, with reference to the end we have in view, depends essentially
upon the nature of that receiver.

Let us compare two radio-active substances, one containing radium and
the other polonium, and which show an equal degree of activity in the
condenser of Fig. 1. If each is covered with a thin leaf of aluminium,
the second appears considerably less active than the first, and the same
is the case when they are placed under the same fluorescent screen, if
the latter is of sufficient thickness, or is placed at a certain
distance from the two radio-active bodies.


                         _Energy of Radiation._

Whatever be the method of research employed, the energy of radiation of
the new radio-active substances is always found to be considerably
greater than that of uranium and thorium. Thus it is that, at a short
distance, they act instantaneously upon a photographic plate, whereas an
exposure of twenty-four hours is necessary when operating with uranium
and thorium. A fluorescent screen is vividly illuminated by contact with
the new radio-active bodies, whilst no trace of luminosity is visible
with uranium and thorium. Finally, the ionising action upon air is
considerably stronger in the ratio of 10^6 approximately. But it is,
strictly speaking, not possible to estimate the _total intensity of the
radiation_, as in the case of uranium, by the electrical method
described at the beginning (Fig. 1). With uranium, for example, the
radiation is almost completely absorbed by the layer of air between the
plates, and the limiting current is reached at a tension of 100 volts.
But the case is different for strongly radio-active bodies. One portion
of the radiation of radium consists of very penetrating rays, which
penetrate the condenser and the metallic plates, and are not utilised in
ionising the air between the plates. Further, the limiting current
cannot always be obtained for the tensions supplied; for example, with
very active polonium the current remains proportional to the tension
between 100 and 500 volts. Therefore the experimental conditions which
give a simple interpretation are not realised, and, consequently, the
numbers obtained cannot be taken as representing the measurement of the
total radiation; they merely point to a rough approximation.


                   _Complex Nature of the Radiation._

The researches of various physicists (MM. Becquerel, Meyer and von
Schweidler, Giesel, Villard, Rutherford, M. and Mdme. Curie) have proved
the complex nature of the radiation of radio-active bodies. It will be
convenient to specify three kinds of rays, which I shall denote,
according to the notation adopted by Mr. Rutherford, by the letters α,
β, γ.

I. The α-rays are very slightly penetrating, and appear to constitute
the principal part of the radiation. These rays are characterised by the
laws by which they are absorbed by matter. The magnetic field acts very
slightly upon them, and they were formerly thought to be quite
unaffected by the action of this field. However, in a strong magnetic
field, the α-rays are slightly deflected; the deflection is caused in
the same manner as with cathode rays, but the direction of the
deflection is reversed; it is the same as for the canal rays of the
Crookes tubes.

II. The β-rays are less absorbable as a whole than the preceding ones.
They are deflected by a magnetic field in the same manner and direction
as cathode rays.

III. The γ-rays are penetrating rays, unaffected by the magnetic field,
and comparable to Röntgen rays.

[Illustration: FIG. 4.]

Consider the following imaginary experiment:—Some radium, R, is placed
at the bottom of a small deep cavity, hollowed in a block of lead, P
(Fig. 4). A sheaf of rays, rectilinear and slightly expanded, streams
from the receptacle. Let us suppose that a strong uniform magnetic field
is established in the neighbourhood of the receptacle, normal to the
plane of the figure and directed towards the back. The three groups of
rays, α, β, γ, will now be separated. Then rather faint γ-rays continue
in their straight path without a trace of deviation. The β-rays are
deflected in the manner of cathode rays, and describe circular paths in
the plane of the figure. If the receptacle is placed on a photographic
plate, A C, the portion, B C, of the plate which receives the β-rays is
acted upon. Lastly, the α-rays form a very intense shaft which is
slightly deflected, and which is soon absorbed by the air. These rays
describe in the plane of the figure a path of great curvature, the
direction of the deflection being the reverse of that with the β-rays.

If the receptacle is covered with a thin sheet of aluminium (0·1 m.m.
thick), the α-rays are suppressed almost entirely, the β-rays are
lessened, and the γ-rays do not appear to be absorbed to any great
extent.


                    _Action of the Magnetic Field._

We have seen that the rays emitted by radio-active bodies have many
properties common to cathode rays and to Röntgen rays. Cathode rays, as
well as Röntgen rays, ionise the air, act on photographic plates, cause
fluorescence, undergo no regular deflection. But the cathode rays differ
from Röntgen rays in being deflected from their rectilinear path by the
action of the magnetic field, and in the transportation of charges of
negative electricity.

The fact that the magnetic field acts upon the rays emitted by
radio-active substances was discovered almost simultaneously by MM.
Giesel, Meyer and von Schweidler, and Becquerel. These physicists
observed that the rays of radio-active substances are deflected by the
magnetic field in the same manner and direction as the cathode rays;
their observations were in relation to the β-rays.

M. Curie demonstrated that the radiation of radium comprises two groups
of quite distinct rays, of which one is readily deflected by the
magnetic field (β-rays), whilst the other seems to be unaffected by the
action of this field (α- and γ-rays).

M. Becquerel did not find that the specimens of polonium prepared by us
emitted rays of the cathode kind. On the contrary, he first noticed the
effect of the magnetic field on a specimen of polonium prepared by
himself. None of the polonium prepared by us ever gave rise to rays of
the cathode order.

The polonium of M. Giesel only gives rise to these rays when recently
prepared, and it is probable that the emission is due to the phenomenon
of induced radio-activity of which we shall speak later.

The following are experiments which prove that one portion of the
radiation of radium, and one portion only, consists of easily deflected
rays (β-rays). These experiments were done according to the electrical
method.

The radio-active body A (Fig. 5) sends forth radiations in the direction
A D between the plates P and P′. The plate P is now at a potential of
500 volts, plate P′ is connected to an electrometer and to a quartz
electric piezometer. The intensity of the current passing through the
air under the influence of the radiations is measured. The magnetic
field can be established at will perpendicular to the plane of the
figure over the whole region E E E E. If the rays are deflected, even
slightly, they no longer pass between the plates, and the current is
suppressed. The region of the passage of the rays is surrounded with
masses of lead, B, B′, B″, and by the armatures of the electro-magnet;
when the rays are deflected, they are absorbed by the masses of lead B
and B′.

[Illustration: FIG. 5.]

The results obtained depend essentially on the distance, A D, of the
radiating substance, A, from the condenser at D. If the distance A D is
great enough (greater than 7 c.m.), most of the radium rays (90 to 100
per cent) arriving at the condenser are deflected and suppressed for a
field of 2500 units. These are the β-rays. If the distance A D is less
than 65 m.m., a smaller part of the rays are deflected by the action of
the field; this portion is also entirely deflected by a field of 2500
units, and the proportion of the rays suppressed is not increased by
increasing the field from 2500 to 7000 units.

The proportion of the rays not suppressed by the field increases with
decrease of the distance, A D, between the radiating body and the
condenser. For small distances, the rays which can be easily deflected
form a very small fraction of the total radiation. The penetrating rays
are therefore, for the most part, deviable rays of the cathode order
(β-rays).

Under the experimental conditions just described, the action of the
magnetic field on the α-rays could not be well observed for the fields
employed. The chief radiation, apparently undergoing no deflection,
observed at a short distance from the radiating source, consisted of
α-rays; the undeflected radiation observed at a greater distance
consisted of γ-rays.

If an absorbing lamina (aluminium or black paper) is placed in the path
of the bundle of rays, those which pass through are nearly all deflected
by the field in such a way that, with the aid of the screen and the
magnetic field, almost all the radiation is suppressed in the condenser,
the remainder being due to the γ-rays, the proportion of which is small.
The α-rays are absorbed by the screen.

An aluminium plate of 1/100 m.m. thickness is sufficient for the
suppression of almost all the rays not readily deflected when the
substance is far enough from the condenser; for smaller distances (34
m.m. and 51 m.m.) two pieces of this aluminium foil are necessary to
give the same result.

Similar determinations were made with four substances containing radium
(chlorides or carbonates) of very different activity; analogous results
were obtained.

It may be remarked that, in all cases, the penetrating rays deflected by
the magnet (β-rays) form only a small fraction of the total radiation;
they influence but slightly the determinations in which the whole
radiation is made use of to produce conductivity of the air.

The radiation emitted by polonium may be studied by the electrical
method. When the distance, A D, of the polonium from the condenser is
varied, no current is observed at first while the distance is fairly
great; on nearing the polonium, the radiation suddenly becomes manifest
with great intensity; the current then increases uniformly whilst
approaching the polonium, but the magnetic field produces no appreciable
effect under these conditions. The radiation of polonium is apparently
limited in space, and does not pass into the air beyond a kind of sheath
surrounding the substance to a thickness of several centimetres.

The interpretation of the experiments I have just described must be
accompanied by some important general reservations. In speaking of the
proportion of the rays deflected by the magnet, I refer only to that
portion of the radiation capable of causing a current in the condenser.
In employing the fluorescent action of the Becquerel rays, or their
action on photographic plates, the proportion would probably be
different—a measure of intensity having, as a rule, no meaning except
for the method of measurement adopted.

The rays of polonium are α-rays. In the experiments just described, I
observed no action of the magnetic field upon them, but the experimental
conditions were such that a slight deflection would pass unnoticed.

The experiments made by the radiographic method confirmed the preceding
results. Taking radium as the source of radiation, and receiving the
impression on a plate parallel to the primitive shaft and normal to the
field, a very clear print is obtained of two shafts separated by the
action of the field, the one deflected, the other not deflected. The
β-rays constitute the deflected beam; the α-rays, being very slightly
deflected, are not to be distinguished from the undeflected bundle of
the γ-rays.


                          _Deflected β-Rays._

The experiments of M. Giesel and MM. Meyer and von Schweidler showed
that the radiation of the radio-active bodies is, in part at least,
deflected by a magnetic field, and that this deflection resembles that
of the cathode rays. M. Becquerel investigated the action of the field
on the rays by the radiographic method. The experimental arrangement was
that of Fig. 4. The radium was placed in the lead receptacle, P, and
this receptacle was placed on the sensitive face of a photographic
plate, A C, covered with black paper. The whole was placed between the
poles of an electro-magnet, the magnetic field being normal to the plane
of the figure.

If the field is directed to the back of this plane, the part B C of the
plate is acted upon by rays which, after having described circular
paths, return to the plate and strike it at a right angle. These rays
are β-rays.

M. Becquerel has demonstrated that the impression consists of a wide
diffused band, a continuous spectrum indeed, showing that the sheaf of
deviable rays emitted by the source is formed of an infinite number of
radiations unequally deflected. If the gelatin of the plate be covered
with different absorbent screens (paper, glass, metals), one portion of
the spectrum is suppressed, and it is found that the rays most deflected
by the magnetic field—otherwise those which have the smallest radius of
curvature—are the most completely absorbed. With each screen, the
impression on the plate begins at a certain distance from the source of
radiation, this distance being proportional to the absorptive power of
the screen.


                    _Charge of the Deflected Rays._

The cathode rays are, as shown by M. Perrin, charged with negative
electricity. Further, according to the experiments of M. Perrin and M.
Lenard, they are capable of carrying their charge through the metallic
envelopes connected to earth and through isolating screens. At every
point where the cathode rays are absorbed, there is a continuous
evolution of negative electricity. We have proved that the same is the
case for the deflected β-rays of radium. _The deviable β-rays of radium
are charged with negative electricity._

(NOTE.—Let the radio-active substance be placed on one of the plates of
a condenser, this plate being connected to earth; the second plate is
connected to an electrometer, it receives and absorbs the rays emitted
by the substance. If the rays are charged, a continuous flow of
electricity into the electrometer should be observed. In this
experiment, carried out in air, we were not able to detect a charge
accompanying the rays, but such an experiment is not delicate. The air
between the plates being caused by the rays to conduct, the electrometer
is no longer isolated, and can only respond to charges if these be
sufficiently strong. In order that the α-rays may not interfere with the
experiment, they may be suppressed by covering the source of radiation
with a thin metallic screen. We repeated this experiment, without more
success, by causing the rays to pass through the interior of a Faraday
cylinder in connection with the electrometer).

According to the preceding experiments, it was evident that the charge
of the rays of the radiating body employed was a weak one.

In order to fix a feeble evolution of electricity upon the conductor
which absorbs the rays, this conductor should be completely insulated;
this is effected by screening it from the air, either by placing it in a
tube with a very perfect vacuum, or by surrounding it with a good solid
dielectric. We employed the latter arrangement.

[Illustration: FIG. 6.]

A conducting disc, M M (Fig. 6), is connected by the wire, _t_, to the
electrometer; the disc and wire are completely enveloped by the
insulating substance _i i i i_; the whole is again surrounded with the
metallic covering, E E E E, which is in electric connection with the
earth. The insulator, _p p_, and the metallic envelope are very thin
upon one of the faces of the disc. This face is exposed to the radiation
of the barium and radium salt, R, placed outside in a lead receptacle.
The rays emitted by the radium penetrate the metallic envelope and the
insulating lamina, _p p_, and are absorbed by the metallic disc, M M.
The latter then becomes the source of a continuous evolution of negative
electricity, as determined by the electrometer, and is measured by means
of a quartz piezometer.

The current thus created is very weak. With very active barium-radium
chloride, forming a layer of 2·5 sq. c.m. in area, and of 0·2 c.m. in
thickness, a current of magnitude 10^{–11} ampères is obtained, the rays
utilised having traversed, before being absorbed by the disc M M, a
thickness of aluminium of 0·01 m.m., and a thickness of ebonite of 0·3
m.m.

We used successively lead, copper, and zinc for the disc M M, ebonite
and paraffin for the insulator; the results obtained were the same.

The current diminishes with increasing distance from the source of
radiation, R, also when a less active product is used.

We obtained the same results again when the disc M M is replaced by a
Faraday cylinder filled with air, and covered outside with insulating
material. The opening of the cylinder, closed by the thin insulating
plate, _p p_, was opposite the radiating source.

[Illustration: FIG. 7.]

Finally, we made the inverse experiment, which was to place the lead
receptacle with the radium in the centre of the insulating material and
in connection with the electrometer (Fig. 7), the whole being surrounded
with the metallic covering connected to earth.

Under these conditions, it is evident from the electrometer that the
radium has a positive charge equal in magnitude to the negative charge
of the former experiment. The radium rays penetrate the thin dielectric
plate, _p p_, and leave the conductor inside carrying with them negative
electricity.

The α-rays of radium do not interfere in these experiments, being almost
completely absorbed by a very thin layer of matter. The method just
described is not suitable for the study of the charge of the rays of
polonium, these rays very slightly penetrating. We observed no
indication of any charge in the case of polonium, which gives rise to
α-rays only; but, for the reason just given, no conclusion can be drawn
from this.

Thus, in the case of the deflected β-rays of radium, as in the case of
cathode rays, the rays carry a charge of electricity. But, hitherto, the
existence of electric charges uncombined with matter has been unknown.
In the study of the emission of the β-rays of radium, we are therefore
led to make use of the theory which is in vogue for the study of cathode
rays. In this ballistic theory, formulated by Sir William Crookes, since
developed and completed by Prof. J. J. Thomson, the cathode rays consist
of extremely minute particles, which are hurled from the cathode with
great velocity, and which are charged with negative electricity. We
might similarly conceive that radium sends into space negatively
electrified particles.

A specimen of radium, enclosed in a solid thin perfectly insulated
envelope, should become spontaneously charged to a very high potential.
By the ballistic hypothesis the potential would increase until the
potential difference of the surrounding conductors became sufficient to
hinder the ejection of the electrified particles and to cause their
return to the source of radiation.

We have performed an experiment on these lines. A specimen of very
active radium was enclosed for some time in a glass vessel. In order to
open the vessel, we made a trace on the glass with a glass cutter.
Whilst so doing, we clearly heard the report of a spark, and upon
examining the vessel with a magnifying glass, we observed that the glass
had been pierced by a spark at the spot where it had been weakened by
the scratch. The phenomenon produced is comparable to the rupture of the
glass of an overcharged Leyden jar.

The same phenomenon occurred with another glass. Further, at the moment
of the passing of the spark, M. Curie, who was holding the glass, felt
the electric shock of discharge in his fingers.

Certain kinds of glass have good insulating properties. If the radium is
enclosed in a sealed glass vessel, well insulated, it is to be expected
that, at a given moment, the vessel will be spontaneously perforated.

_Radium is the first example of a body which is spontaneously charged
with electricity._


  _Action of the Electric Field upon the Deflected β-Rays of Radium._

The β-rays of radium, being analogous to the cathode rays, should be
deflected by an electric field in a manner similar to the latter;
_i.e._, as would a particle of matter negatively charged and hurled into
space with a great velocity. The existence of such a deflection has been
demonstrated both by M. Dorn and M. Becquerel.

Let us consider the case of a ray which traverses the space situated
between the two plates of a condenser. Suppose the direction of the ray
parallel to the plates: when an electric field is established between
the latter, the ray is subjected to the action of this uniform field
along its whole path in the condenser l. By reason of this action the
ray is deflected towards the positive plate and describes the arc of a
parabola; on leaving the field, it continues its path in a straight
line, following the tangent to the arc of the parabola at the point of
exit. The ray can be received on a photographic plate perpendicular to
its original direction. Observations are taken of the impression
produced on the plate when the field is zero, and when it has a known
value, and from that is deduced the value of the deflection, δ, which is
the distance of the points in which the new direction of the ray and its
original direction meet a common plane perpendicular to the original
direction. If h is the distance of this plane from the condenser,
_i.e._, at the edge of the field, we have, by a simple calculation,—

                           _e_F_l_ (_l_/2 + _h_)
                       δ = —————————————————————;
                                   _mv_^2

_m_ being the mass of the moving particles, _e_ its charge, _v_ its
velocity, and F the strength of the field.

The experiments of M. Becquerel enable him to assign a value approaching
to δ.


 _Relation of the Charge to the Mass for a Particle Negatively Charged
                          Emitted by Radium._

When a material particle having a mass _m_ and a negative charge _e_, is
projected with a velocity _v_ into a uniform magnetic field
perpendicular to its initial velocity, this particle describes, in a
plane normal to the field and passing through its initial velocity, an
arc of a circle of radius ρ, so that—H being the strength of the
field—we have the relation—

                                  _m_
                             Hρ = ——— _v_.
                                  _e_

If, for the same ray, the deflection, δ, and the radius of curvature, ρ,
be measured in a magnetic field, values could be found from these two
experiments for the ratio _e_/_m_ and for the velocity, _v_.

The experiments of M. Becquerel threw the first light upon this subject.
They gave for the ratio _e_/_m_ a value approximately equal to 10^7
absolute electro-magnetic units, and for v a magnitude of 1·6 × 10^{10}.
These values are of the same order of magnitude as those of the cathode
rays.

Accurate experiments have been made on the same subject by M. Kaufmann.
This physicist subjected a narrow beam of radium rays to the
simultaneous action of an electric field and a magnetic field, the two
fields being uniform and having a similar direction, normal to the
original direction of the beam. The impression produced on a plate
normal to the primitive beam and placed beyond the limits of the field
with reference to the source, has the form of a curve, each point of
which corresponds to one of the original beam. The most penetrating and
least deflected rays are at the same time those with the greatest
velocity.

It follows from the experiments of M. Kaufmann, that for the radium
rays, of which the velocity is considerably greater than that of the
cathode rays, the ratio _e_/_m_ decreases, while the velocity increases.

According to the researches of J. J. Thomson and Townsend, we may assume
that the moving particle, which constitutes the ray, possesses a charge,
_e_, equal to that carried by an atom of hydrogen during electrolysis,
this charge being the same for all the rays. We are therefore led to the
conclusion that the mass of the particle, _m_, increases with increase
of velocity.

These theoretical considerations lead to the idea that the inertia of
the particle is due to its state of charge during motion, the velocity
of an electric charge in motion being incapable of modification without
expenditure of energy. To state it otherwise, the inertia of the
particle is of electro-magnetic origin, and the mass of the particle
is—in part at least—a virtual mass or an electro-magnetic mass. M.
Abraham goes further, and assumes that the mass of the particle is
entirely an electro-magnetic mass. If, according to this hypothesis, the
value of this mass, _m_, be calculated for a known velocity, _v_, we
find that _m_ approaches infinity when _v_ approaches the velocity of
light, and that _m_ approaches a constant value when the velocity, _v_,
is much less than that of light. The experiments of M. Kaufmann are in
agreement with the results of this theory, the importance of which is
great because it foreshadows the possibility of establishing mechanical
bases upon the dynamical of little particles of matter charged in a
state of motion.

These are the figures obtained by M. Kaufmann for _e_/_m_ and _v_.

           _e_/_m_          _v_c.m./sec. │
   Electro-magnetic units.               │
   1·865 × 10^7            0·7  × 10^{10}│For cathode rays (Simon).
   ──────────────────────────────────────┼───────────────────────────
   1·31  × 10^7            2·36 × 10^{10}│For radium rays (Kaufmann).
   1·17  × 10^7            2·48 × 10^{10}│             〃
   0·97  × 10^7            2·59 × 10^{10}│             〃
   0·77  × 10^7            2·72 × 10^{10}│             〃
   0·63  × 10^7            2·83 × 10^{10}│             〃

M. Kaufmann concludes, from comparison of his experiments with the
theory, that the limiting value of the ratio _e_/_m_ for radium rays of
relatively small velocity would be the same as the value _e_/_m_ for
cathode rays.

The most complete experiments of M. Kaufmann were made with a minute
quantity of pure radium chloride, with which we provided him.

According to M. Kaufmann’s experiments, certain β-rays of radium possess
a velocity very near to that of light. These rapid rays seem to possess
great penetrating capacity towards matter.


            _Action of the Magnetic Field upon the α-Rays._

In a recent work, Mr. Rutherford announced that, in a powerful electric
or magnetic field, the α-rays of radium are slightly deflected, in the
manner of particles positively electrified and possessing great
velocity. Mr. Rutherford concludes from his experiments that the
velocity of the α-rays is of the order of magnitude 2·5 × 10^9c.m./sec.
and that the ratio _e_/_m_ for these rays is of the order of magnitude 6
× 10^3, which is 10^4 times as great as for the deflected β-rays. We
shall see later that these conclusions of Mr. Rutherford are in
agreement with the properties already known of the α-radiation, and that
they account, in part at least, for the law of absorption of this
radiation.

The experiments of Mr. Rutherford have been confirmed by M. Becquerel.
M. Becquerel has further demonstrated that polonium rays behave in a
magnetic field like the α-rays of radium, and that, for the same field,
they seem to have the same curvature as the latter.

It also appears from M. Becquerel’s experiments that the α-rays do not
form a magnetic spectrum, but act rather like a homogeneous radiation,
all the rays being equally deflected.


    _Action of the Magnetic Field on the Rays of other Radio-active
                              Substances._

We have just seen that radium gives off α-rays comparable to the tube
rays, β-rays comparable to cathode rays, and γ-rays which are
penetrating and not deflected. Polonium gives off α-rays only. Amongst
the other radio-active substances, actinium seems to behave like radium,
but the study of its radiation has not yet advanced so far as in the
case of radium. As regards the faintly radio-active bodies, we know
to-day that uranium and thorium give rise to α-rays as well as β-rays
(Becquerel, Rutherford).


           _Proportion of β-Rays in the Radiation of Radium._

As I have already mentioned, the proportion of β-rays increases with
increase of distance from the source of radiation. These rays never
occur alone, and for great distances the presence of γ-rays is always
discernible. The presence of very penetrating, undeflected rays in the
radiation of radium was first observed by M. Villard. These rays
constitute only a small portion of the radiation measured by the
electrical method, and their presence escaped our notice in our first
experiments, so that we believed falsely that the radiation at great
distances contained only rays capable of deflection.

The following are the numerical results obtained with experiments made
by the electrical method with an apparatus similar to that of Fig. 5.
The radium was only separated from the condenser by the surrounding air.
I shall indicate by the letter _d_ the distance from the source of
radiation to the condenser. The numbers of the second line represent the
current subsisting when the magnetic field is acting, supposing the
current obtained with no field equal to 100 for each distance. These
numbers may be considered as giving the percentage of the total α- and
γ-rays, the deflection of the α-rays having been scarcely observable
with the conditions employed.

At great distances there are no α-rays, and the undeflected radiation is
therefore of the γ kind only.

Experiments made at short distances:—

           _d_, in centimetres             3·4  5·1  6·0  6·5
           Percentage of undeflected rays 74   56   33   11

Experiments made at long distances with a product considerably more
active than that which was used for the preceding series:—

            _d_, in centimetres             14  30 53 80 98
            Percentage of undeflected rays  12  14 17 14 16
            ───────────────────────────────────────────────
            _d_                            124 157
            Percentage of undeflected rays  14  11

It is thus evident that after a certain distance the proportion of
undeflected rays in the radiation is approximately constant. These rays
probably all belong to the γ species.

The following is another series of experiments in which the radium was
enclosed in a very narrow glass tube, placed below the condenser and
parallel to the plates. The rays emitted traversed a certain thickness
of glass and air before entering the condenser:—

    _d_, in centimetres            2·5  3·3  4·1  5·9  7·5  9·6 11·3
    Percentage rays not deflected 33   33   21   16   14   10    9
    ────────────────────────────────────────────────────────────────
    _d_                           13·9 17·2
    Percentage rays not deflected  9   10

As in the preceding experiments, the number of the second line
approximate to a constant value, when the distance _d_ increases, but
the limit is reached for smaller distances than in the preceding series,
because the α-rays have been more completely absorbed by the glass than
the β- and γ-rays.

The following experiment shows that a thin sheet of aluminium (0·01 m.m.
thick) absorbs principally α-rays. The product being placed 5 c.m. from
the condenser, the proportion of rays other than β, when the magnetic
field is acting, is about 71 per cent. When the same substance is
covered with the sheet of aluminium, the distance remaining the same,
the radiation transmitted is found to be almost totally deflected by the
magnetic field, the α-rays having been absorbed by the aluminium. The
same result is obtained when paper is used as the absorbing screen.

The greatest part of the radiation of radium consists of α-rays, which
are probably emitted principally by the superficial layer of the
radiating matter. When the thickness of the layer of radiating matter is
varied, the intensity of the current increases with this thickness; the
increase is not proportional to the thickness for the whole of the
radiation; it is, moreover, more considerable for the β-rays than for
the α-rays, so that the proportion of β-rays increases with the
thickness of the active layer. The source of radiation being placed at a
distance of 5 c.m. from the condenser, it is found that for a thickness
equal to 0·4 m.m. of the active layer, the total radiation is given by
the number 28, and the proportion of the β-rays is 29 per cent. By
making the layer 2 m.m. thick, _i.e._, five times as thick, a total
radiation equal to 102, and a proportion of β-rays equal to 45 per cent
are obtained. The total radiation which exists at this distance has
therefore been increased in the ratio of 3·6, and the β-radiation has
become five times as strong.

The preceding experiments were made by the electrical method. When the
radiographic method is used, certain results seem to be in contradiction
with what precedes. In the experiments of M. Villard, a beam of radium
rays, subjected to the action of the magnetic field, was received on to
a pile of photographic plates. The undeflected and penetrating γ-beam
passed through all the plates, leaving its trace on each. The deflected
β-beam produced an impression on the first plate only. This beam
appeared therefore to contain no rays of great penetration.

On the contrary, in our experiments a beam which is propagated in the
air contains at the greatest distances accessible to observation about
9/10 of β-rays, and the same is the case when the source of radiation is
enclosed in a little sealed glass vessel. In M. Villard’s experiments,
these deflected and penetrating β-rays did not affect the photographic
plates beyond the first, because they are to a great extent diffused in
all directions by the first solid obstacle encountered, and no longer
form a beam. In our experiments the rays given off by radium and
transmitted through the glass of the vessel were also probably scattered
by the glass, but the vessel being very small would itself act as a
source of β-rays at its surface, and we were able to follow the course
of the latter to a great distance from the vessel.

The cathode rays of Crookes tubes can only traverse very thin screens
(aluminium screens of 0·01 m.m. thickness). A beam of rays striking the
screen normally is scattered in all directions; but the diffusion
becomes less with diminishing thickness of the screen, and for very thin
screens the emerging beam is practically the prolongation of the
incident beam.

The deflected β-rays of radium behave in a similar manner, but the
transmitted beam experiences, for the same thickness of screen, a much
slighter modification. According to the experiments of M. Becquerel, the
very readily deflected β-rays of radium (those with a relatively small
velocity) are powerfully scattered by an aluminium screen of thickness
0·1 m.m.; but the penetrating and less deflected rays (rays of the
cathode kind of great velocity) pass through this screen without being
sensibly diffused, whatever be the inclination of the screen to the
direction of the beam. The β-rays of great velocity penetrate without
diffusion a much greater thickness of paraffin (several centimetres),
and in this the curvature of the beam produced by the magnetic field can
be traced. The thicker the screen, and the more absorbent the material
of which it is composed, the greater is the modification of the
deflected primitive beam, because, with increasing thickness of screen,
diffusion occurs progressively among fresh groups of rays of increasing
penetration.

The β-rays of radium experience a diffusion in passing through the air,
which is very marked for readily deflected rays, but which is much
slighter than that produced by equal thicknesses of solid substances.
For this reason, the β-rays traverse long distances in the air.


      _Penetrating Power of the Radiation of Radio-active Bodies._

Since the beginning of the researches on radio-active bodies,
investigations of the absorption produced by different screens upon the
rays given off by these bodies have been carried on. In a previous paper
on this subject I gave figures (quoted at the beginning of this work)
representing the penetrating power of uranium and thorium rays. Mr.
Rutherford has made a special study of the radiation of uranium, and
proved it to be heterogeneous. Mr. Owens has arrived at the same results
for thorium rays. When the discovery of strongly radio-active bodies
immediately followed upon this, the penetrating power of their rays was
also studied by various physicists (Becquerel, Meyer and von Schweidler,
Curie, Rutherford). The first observations brought to light the
complexity of the radiation, which seems to be a general phenomenon, and
common to the radio-active bodies. In them we have sources which give
rise to a variety of radiations, each of which has a power of
penetration proper to itself.

Radio-active bodies emit rays which are propagated both in the air and
_in vacuo_. The propagation is rectilinear; this fact is proved by the
distinctness and shape of the shadows formed by interposing bodies
opaque to the radiation between the source and the sensitive plate or
fluorescent screen which serves as receiver, the source being of small
magnitude in comparison with its distance from the receiver. Various
experiments demonstrating the rectilinear propagation of uranium,
radium, and polonium rays have been made by M. Becquerel.

It is interesting to know the distance that rays can travel in air. We
have found that radium emits rays which can be detected in the air at a
distance of several metres from the source. In certain of our electrical
determinations, the action of the source upon the air of the condenser
made itself felt at a distance of between 2 and 3 metres. We have also
obtained fluorescent effects and radiographic impressions at similar
distances. The experiments are not easily carried out, except with very
intense radio-active sources, because, independently of the absorption
by the air, the action upon a given receiver varies inversely as the
square of the distance from a source of small dimensions. This
radiation, which travels a long distance in the case of radium,
comprises rays of the cathode kind and rays which are undeflected;
however, the deflected rays predominate, according to the results of the
experiments already mentioned. The greater part of the radiation
(α-rays) is, on the contrary, limited in air to a distance of about 7
c.m. from the source.

I made several experiments with radium enclosed in a little glass
vessel. The rays emerging from the vessel, after traversing a certain
space of air, were received in a condenser, which served to measure
their ionising capacity by the usual electrical method. The distance,
_d_, from the source to the condenser was varied, and the current of
saturation, _i_, obtained in the condenser was measured. The following
are the results of one of the series of determinations:—

                 _d_, c.m. _i._  _i_ × _d_^2 × 10^{–3}.
                        10 127             13
                        20  38             15
                        30  17·4           16
                        40  10·5           17
                        50   6·9           17
                        60   4·7           17
                        70   3·8           19
                       100  1·65           17

After a certain distance, the intensity of radiation varies inversely as
the square of the distance from the condenser.

The radiation of polonium is only propagated in air to a distance of a
few centimetres (4 to 6 c.m.) from the source of radiation.

In the case of the absorption of radiations by solid screens, we find
another fundamental difference between radium and polonium. Radium emits
rays capable of penetrating great thicknesses of solid matter, _e.g._,
several centimetres of lead or of glass. The rays which have passed
through a great thickness of a solid body are extremely penetrating, and
it is practically impossible to absorb them entirely by any material
whatever. But these rays form only a small fraction of the total
radiation, the greater part of which is absorbed by a slight thickness
of solid matter.

Polonium emits rays which are readily absorbed, and which can only pass
through extremely thin screens.

The following are figures showing the absorption produced by an
aluminium lamina of thickness 0·01 m.m. This lamina was placed above and
almost in contact with the substance. The direct radiation and that
transmitted by the aluminium were measured by the electrical method
(apparatus of Fig. 1); the current of saturation was practically
obtained in every case. I have represented the activity of the radiating
body by _a_, that of uranium being unity.

                                            _a._         Fraction of
                                                          radiation
                                                         transmitted.
 Chloride of barium and radium                      57             0·32
 Bromide of barium and radium                       43             0·30
 Chloride of barium and radium                    1200             0·30
 Sulphate of barium and radium                    5000             0·29
 Sulphate of barium and radium                  10,000             0·32
 Metallic bismuth and polonium                                     0·22
 Compounds of uranium                                              0·20
 Compounds of thorium in a thin layer                              0·38

We see that radium compounds of different nature and activity give very
similar results, as I have already pointed out in the case of uranium
and thorium compounds at the beginning of this work. We see also that,
taking into account the whole of the radiation, and with a given
absorbent screen, the different radio-active bodies can be arranged in
the following decreasing order of penetrating power:—Thorium, radium,
polonium, uranium.

These results are similar to those which have been published by Mr.
Rutherford.

Mr. Rutherford also finds that the order is the same when air is the
absorbent substance. But it is probable that this order has no absolute
value, and would not be maintained independently of the nature and
thickness of the screen. Experiment shows, indeed, that the law of
absorption is very different for polonium and radium, and that, for the
latter, the absorption of the rays of each of the three groups must be
considered separately.

Polonium is particularly well adapted to the study of α-rays, because
the specimens which we possess emit no other kind of rays. I made a
preliminary series of experiments with extremely active recently
prepared specimens of polonium. I found the absorbability of the rays to
increase with increase of thickness of the matter traversed. This
singular law of absorption is contrary to that known for other kinds of
radiation.

I employed for this research our apparatus for the determination of
electrical conductivity arranged in the following manner:—

The two plates of a condenser, P P and P′ P′ (Fig 8), are horizontally
disposed in a metallic box, B B B B, connected to earth. The active
body, A, placed in a thick metallic box, C C C C, connected with the
plate P′ P′, acts upon the air of the condenser across a metallic sheet,
T; the rays which pass through the sheet are alone utilised for
producing the current, the electric field being limited by the sheet.
The distance, A T, of the active body from the sheet may be varied. The
field between the plates is established by means of a battery. By
placing in A upon the active body different screens, and by adjusting
the distance A T, the absorption of rays which travel long or short
distances in the air may be determined.

[Illustration: FIG. 8.]

The following are the results obtained with polonium:—

For a certain value of the distance A T (4 c.m. and more), no current
passes; the rays do not penetrate the condenser. When the distance A T
is diminished, the appearance of the rays in the condenser is manifested
somewhat suddenly, a weak current changing to one of considerable
strength for a slight diminution of distance; the current then increases
regularly as the active body continues to approach the sheet T.

When the active body is covered with a sheet of aluminium 1/100 m.m.
thick, the absorption produced by the lamina becomes greater, the
greater the distance A T.

If a second similar lamina of aluminium be placed upon the first, each
absorbs a fraction of the radiation it receives, and this fraction is
greater for the second lamina than for the first.

In the following table I have represented in the first line the
distances in centimetres between the polonium and the sheet T; in the
second line the percentage of the rays transmitted by a sheet of
aluminium; in the third line the percentage of the rays transmitted by
two sheets of the same aluminium:—

  Distance A T                                 3·5 2·5 1·9  1·45  0·5
  Percentage of rays transmitted by one lamina 0   0   5   10    25
  Percentage of rays transmitted by two laminæ 0   0   0    0     0·7

In these experiments the distance of the plates, P and P′, was 3 c.m. We
see that the interposition of the aluminium screen diminishes the
intensity of the radiation to a greater degree at further distances than
at nearer distances.

This effect is still more marked than the preceding figures seem to
indicate. For a distance of 0·5 c.m. 25 per cent represents the mean
penetration for all the rays which pass beyond this distance. If, for
example, only those rays between 0·5 c.m. and 1 c.m. be comprehended,
the penetration would be greater. And if the plate P be placed at a
distance of 0·5 c.m. from P′ the fraction of the radiation transmitted
by the aluminium lamina (for A T = 0·5 c.m.) is 47 per cent, and through
two laminæ it is 5 per cent of the original radiation.

I have recently performed a second series of experiments with these same
specimens of polonium, the activity of which was considerably
diminished, the interval of time between the two series of experiments
being three years.

In the former experiments, polonium nitrite was used; in the latter, the
polonium was in the state of metallic particles obtained by fusing the
nitrite with potassium cyanide.

I found that the radiation of polonium had preserved its essential
characteristics, and I discovered new results. The following, for
different values of the distance A T, are the fractions of the radiation
transmitted by a screen composed of four superposed very thin leaves of
beaten aluminium.

       Distance A T, in centimetres                  0  1·5  2·6
       Percentage of rays transmitted by the screen 76 66   39

I also found that the fraction of the radiation absorbed by a given
screen increases with the thickness of the material already traversed by
the radiation, but this only occurs after the distance A T has reached a
certain value. When this distance is zero (the polonium being in contact
with the sheet, either outside or inside the condenser), it is observed
that with several similar superposed screens, each absorbs the same
fraction of the radiation it receives; otherwise expressed, the
intensity of the radiation diminishes therefore according to an
exponential law as a function of the thickness of the material
traversed, as in the case of homogeneous radiation transmitted by the
lamina without changing its nature.

The following numerical results are given with reference to these
experiments:—

For a distance A T equal to 1·5 c.m. a thin aluminium screen transmits
the fraction 0·51 of the radiation it receives when acting alone, and
the fraction 0·34 of the radiation it receives when it is preceded by
another similar screen.

On the contrary, for a distance A T equal to zero, the same screen
transmits in both the cases considered the same fraction of the
radiation it receives, and this fraction is equal to 0·71; it is
therefore greater than in the preceding case.

The following numbers indicate for a distance A T equal to 0 and for a
succession of thin superposed screens, the ratio of the radiation
transmitted to the radiation received for each screen:—

   Series of nine very thin copper   Series of seven very thin aluminium
               leaves.                             leaves.
                0·72                                0.69
                0·78                                0.94
                0·75                                0.95
                0·77                                0.91
                0·70                                0.92
                0·77                                0.93
                0·69                                0.94
                0·79
                0·68

Taking into account the difficulties of the manipulation of very thin
screens and of the superposition of screens in contact, the numbers of
each column may be looked upon as constant; the first number only of the
aluminium column indicates a greater absorption than that indicated by
the following numbers.

The α-rays of radium behave similarly to the rays of polonium. These
rays may be investigated almost isolated by deflecting to one side the
β-rays with the magnetic field; the γ-rays seem of slight importance in
comparison with the α-rays. The operation can only be carried on at some
distance from the source of radiation. The following are the results of
an experiment of this kind. The fraction of the radiation transmitted by
a lamina of aluminium 0·01 m.m. thick is measured; this screen was
placed always in the same position, above and at a little distance from
the source of radiation. With the apparatus of Fig. 5, the current
produced in the condenser for different values of the distance A D is
observed, both with and without the screen:—

      Distance A D                                    6·0 5·1  3·4
      Percentage of rays transmitted by the aluminium 3   7   24

The rays which travel furthest in the air are those most absorbed by the
aluminium. There is therefore a great similarity between the absorbable
α-rays of radium and the rays of polonium.

The deflected β-rays and the undeflected penetrating γ-rays are, on the
contrary, of a different nature. The experiments, notably of MM. Meyer
and von Schweidler, clearly show that, considering the radiation of
radium as a whole, the penetrating power of this radiation increases
with the thickness of the material traversed, as is the case of Röntgen
rays. In these experiments the α-rays produce scarcely any effect, being
for the most part suppressed by very thin absorbent screens. Those which
penetrate are, on the one hand, β-rays more or less scattered; on the
other hand, γ-rays, which appear similar to Röntgen rays.

The following are the results of some of my experiments on the subject:—

The radium is enclosed in a glass vessel. The rays, which emerge from
the vessel, traverse 30 c.m. of air, and are received upon a series of
glass plates, each of thickness 1·3 m.m.; the first plate transmits 49
per cent of the radiation it receives, the second transmits 84 per cent
of the radiation it receives, the third transmits 85 per cent of the
radiation it receives.

In another series of experiments the radium was enclosed in a glass
vessel placed 10 c.m. from the condenser which received the rays. A
series of similar screens of lead each 0·115 m.m. thick were placed on
the vessel.

The ratio of the radiation transmitted to the radiation received is
given for each of the successive screens by the following numbers:—

              0·40 0·60 0·72 0·79 0·89 0·92 0·94 0·94 0·97

For a series of four screens of lead, each of which was 1·5 m.m. thick,
the ratio of the radiation transmitted to the radiation received was
given for the successive screens by the following numbers:—

                          0·09 0·78 0·84 0·82

The results of these experiments show that when the thickness of the
lead traversed increases from 0·1 m.m. to 6 m.m., the penetrating power
of the radiation increases.

I found that, under the experimental conditions mentioned, a screen of
lead 1·8 c.m. thick transmits 2 per cent of the radiation it receives; a
screen of lead 5·3 c.m. thick transmits 0·4 per cent of the radiation it
receives. I also found that the radiation transmitted by a thickness of
lead of 1·5 m.m. consists largely of rays capable of deflection (cathode
order). The latter are therefore capable of traversing not only great
distances in the air, but also considerable thicknesses of very
absorbent solids, such as lead.

In investigating with the apparatus of Fig. 2 the absorption exercised
by an aluminium screen 0·01 m.m. thick upon the total radiation of
radium, the screen being always placed at the same distance from the
radiating body, and the condenser being placed at a variable distance, A
D, the results obtained are the sum of those due to the three groups of
the radiation. At a long distance the penetrating rays predominate, and
the absorption is slight; at a short distance the α-rays predominate,
and the absorption becomes less with nearer approach to the substance;
for an intermediate distance the absorption passes through a maximum and
the penetration through a minimum.

  Distance A D                                 7·1  6·5  6·0  5·1  3·4
  Percentage of rays transmitted by aluminium 91   82   58   41   48

Certain experiments made in connection with absorption always
demonstrate a certain similarity between the α-rays and the β-rays. Thus
it was that M. Becquerel discovered that the absorbent action of a solid
screen upon the β-rays increases with the distance of the screen from
the source, such that if the rays are subjected to a magnetic field, as
in Fig. 4, a screen placed in contact with the source of radiation
allows a larger portion of the magnetic spectrum to be in evidence than
does the same screen placed upon the photographic plate. This variation
of the absorbent effect of the screen with the distance of the screen
from the source is similar to that which occurs with the α-rays; this
has been verified by MM. Meyer and von Schweidler, who operated by means
of the fluoroscopic method; M. Curie and I observed the same fact when
working by the electrical method. However, when the radium is enclosed
in a glass tube and placed at a distance from the condenser, which is
itself enclosed in a thin aluminium box, it becomes a matter of
indifference whether the screen be placed against the source or against
the condenser; the current obtained is the same in both cases.

The investigation of the α-rays led me to the reflection that these rays
behave like projectiles having a certain initial velocity, and which
lose their force on encountering obstacles. These rays, moreover, travel
by rectilinear propagation, as has been shown by M. Becquerel in the
following experiment:—Polonium emitting rays was placed in a very narrow
straight cavity hollowed in a sheet of cardboard. Thus a linear source
of radiation was produced. A copper wire, 1·5 m.m. in diameter, was
placed parallel and opposite to the source at a distance of 4·9 m.m.
Beyond was placed a parallel photographic plate at a distance of 8·65
m.m. After an exposure of ten minutes, the geometric shadow of the wire
was perfectly reproduced, with a narrow penumbra corresponding to the
size of the source. The same experiment succeeded equally well when a
double leaf of beaten aluminium was placed against the wire, through
which the rays must pass.

There are therefore rays capable of giving perfect geometric shadows.
The experiment with the aluminium shows that these rays are not
scattered in traversing the screen, and that this screen does not give
rise to any noticeable extent to secondary rays similar to the secondary
rays of the Röntgen rays.

The experiments of Mr. Rutherford show that the projectiles which
constitute the α-rays are deflected by a magnetic field, as if they were
positively charged. The deflection in a magnetic field becomes less as
the product _mv_/_e_ becomes greater; _m_ being the mass of the
particle, _v_ its velocity, and _e_ its charge. The cathode rays of
radium are but slightly deflected, because their velocity is enormous;
they are, on the other hand, very penetrating, because each particle has
a very small mass together with a great velocity. But particles which,
with an equal charge and a less velocity, have a greater mass, would be
also only slightly influenced by the action of the field, and would give
rise to very absorbable rays. From the results of Mr. Rutherford’s
experiments, this seems to take place in the case of the α-rays.

The penetrating γ-rays appear to be of quite another nature and similar
to Röntgen rays.

We have now seen how complex a phenomenon is the radiation of
radio-active bodies. The difficulties of investigation are increased by
the question as to whether the radiation undergoes a merely selective
absorption on the part of the material, or whether a more or less
radical transformation.

Little is so far known with regard to this question. If the radiation of
radium be regarded as containing rays both of the cathode and Röntgen
species, it might be expected to undergo transformations in traversing
screens. It is known:—Firstly, that cathode rays emerging from a Crookes
tube through an aluminium window are greatly scattered by the aluminium;
and, further, that the passage through the screen entails a diminution
of the velocity of the rays. In this way, cathode rays with a velocity
equal to 1·4 × 10^{10} c.m. lose 10 per cent of their velocity in
passing through 0·01 m.m. of aluminium. Secondly, cathode rays on
striking an obstacle give rise to the production of Röntgen rays.
Thirdly, Röntgen rays, on striking a solid obstacle, give rise to the
production of _secondary rays_, which partly consist of cathode rays.

The existence, by analogy, of all these preceding phenomena may
therefore be predicted for the rays of radio-active substances.

In investigating the transmission of polonium rays through a screen of
aluminium, M. Becquerel observed neither the production of secondary
rays nor any transformation into cathode rays.

I endeavoured to demonstrate a transformation of the rays of polonium by
using the method of interchangeable screens. Two superposed screens,
E_{1} and E_{2}, being traversed by the rays, the order in which they
are traversed should be immaterial if the passage through the screens
does not transform the rays; if, on the contrary, each screen transforms
the rays during transmission, the order of the screens is of moment. If,
for example, the rays are transformed into more absorbable rays in
passing through lead, and no such effect is produced by aluminium, then
the system lead-aluminium will be more opaque than the system
aluminium-lead; this takes place with Röntgen rays.

My experiments show that this phenomenon is produced with the rays of
polonium. The apparatus employed was that of Fig. 8. The polonium was
placed in the box, C C C C, and the absorbing screens, of necessity very
thin, were placed upon the metallic sheet T.

    Screens employed.          Thickness.       │   Current observed.
                                  M.m.          │
 Aluminium               0·01                   │ 17·9
 Brass                   0·005                  │           〃
 ───────────────────────────────────────────────┼───────────────────────
 Brass                   0·005                  │  6·7
 Aluminium               0·01                   │           〃
 ───────────────────────────────────────────────┼───────────────────────
 Aluminium               0·01                   │150
 Tin                     0·005                  │           〃
 ───────────────────────────────────────────────┼───────────────────────
 Tin                     0·005                  │125
 Aluminium               0·01                   │           〃
 ───────────────────────────────────────────────┼───────────────────────
 Tin                     0·005                  │ 13·9
 Brass                   0·005                  │           〃
 ───────────────────────────────────────────────┼───────────────────────
 Brass                   0·005                  │  4·4
 Tin                     0·005                  │           〃

The results obtained prove that the radiation is modified in passing
through a solid screen. This conclusion accords with the experiments in
which, of two similar superposed metallic screens, the first is less
absorbent than the second. From this it is probable that the
transforming action of a screen increases with the distance of the
screen from the source. This fact has not been verified, and the nature
of the transformation has not been studied in detail.

I repeated the same experiments with a very active salt of radium; the
result was negative. I only observed insignificant variations in the
intensity of the radiation transmitted with interchange of the order of
the screens. The following systems of screens were experimented with:—

                    Aluminium, thickness 0·55  m.m.
                    Aluminium, thickness 0·55  m.m.
                    Aluminium, thickness 0·55  m.m.
                    Aluminium, thickness 1·07  m.m.
                    Aluminium, thickness 0·55  m.m.
                    Aluminium, thickness 1·07  m.m.
                    Aluminium, thickness 0·15  m.m.
                    Aluminium, thickness 0·15  m.m.
                    Aluminium, thickness 0·15  m.m.
                    Platinum, thickness  0·01  m.m.
                    Lead, thickness      0·1   m.m.
                    Tin, thickness       0·005 m.m.
                    Copper, thickness    0·05  m.m.
                    Brass, thickness     0·005 m.m.
                    Brass, thickness     0·005 m.m.
                    Platinum, thickness  0·01  m.m.
                    Zinc, thickness      0·05  m.m.
                    Lead, thickness      0·1   m.m.

The system lead-aluminium was slightly more opaque than the system
aluminium-lead, but the difference was not great.

Thus, I was unable to discover an appreciable transformation of the rays
of radium. However, in various radiographic experiments, M. Becquerel
observed very intense effects due to scattered or secondary rays,
emitted by solid screens which received radium rays. Lead seemed to be
the most active substance in this respect.


        _Ionising Action of Radium Rays on Insulating Liquids._

M. Curie has pointed out that radium rays and Röntgen rays act upon
liquid dielectrics as upon air, imparting to them a certain electrical
conductivity. The experiment was carried out in the following manner
(Fig. 9):—

The experimental liquid is placed in a metal vessel, C D E F, into which
a thin copper tube, A B, is plunged; these two pieces of metal serve as
electrodes. The outer vessel is maintained at a known potential, by
means of a battery of small accumulators, one pole of which is connected
to earth. The tube, A B, is connected to the electrometer. When a
current traverses the liquid the electrometer is kept at zero by means
of a quartz electrical piezometer, which gives the strength of the
current. The copper tube, M N M′ N′, connected to earth, serves as a
guard tube, preventing the passage of the current through the air. A
bulb containing the radium-barium salt may be placed at the bottom of
the tube, A B; the rays act on the liquid after having penetrated the
glass of the bulb and the sides of the metal tube. The radium may also
be allowed to act by placing the bulb beneath the side, D E.

In working with Röntgen rays the course of the rays is through side D E.

The increase of conductivity by the action of the radium rays or the
Röntgen rays seems to be produced in the case of all liquid dielectrics;
but in order to determine this increase, the conductivity of the liquid
itself must be so slight as not to mask the effect of the rays.

M. Curie obtained results of the same order of magnitude with both
radium rays and Röntgen rays.

When investigating with the same apparatus the conductivity of air or of
another gas under the action of the Becquerel rays, the intensity of the
current obtained is found to be proportional to the difference of
potential between the electrodes, as long as the latter does not exceed
a few volts; but at higher tensions, the intensity of the current
increases less and less rapidly, and the saturation current is
practically attained for a tension of 100 volts.

Liquids examined with the same apparatus and the same radio-active body
behave differently; the intensity of the current is proportional to the
tension when the latter varies between 0 and 450 volts, and when the
distance between the electrodes does not exceed 6 m.m.

[Illustration: FIG. 9.]

The figures of the following table multiplied by 10^{–11} give the
conductivity in megohms per c.c.:—

                         Carbon bisulphide 20
                         Petroleum ether   15
                         Amylene           14
                         Benzine            4
                         Liquid air         1·3
                         Vaseline oil       1·6

We may, however, assume that liquids and gases behave similarly, but
that, in the case of liquids, the current remains proportional to the
tension up to a much higher limit than in the case of gases. It
therefore seemed probable that the limit of proportionality could be
lowered by using a much more feeble radiation, and this idea was
verified by experiment. The radio-active body employed was 150 times
less active than that which had served for the previous experiments. For
tensions of 50, 100, 200, 400 volts, the intensities of the current were
represented respectively by the numbers 109, 185, 255, 335. The
proportionality was no longer maintained, but the current showed great
variation when the difference of potential was doubled.

Some of the liquids examined are nearly perfect insulators when
maintained at a constant temperature and when screened from the action
of the rays. Such are liquid air, petroleum ether, vaseline oil, and
amylene. It is therefore very easy to study the effect of the rays.
Vaseline oil is much less sensitive to the action of the rays than is
petroleum ether. This fact may have some relation to the difference in
volatility which exists between these two hydrocarbons. Liquid air,
which has boiled for some time in the experimental vessel, is more
sensitive to the action of the rays than that newly poured in; the
conductivity produced by the rays is one-fourth as great again in the
former case. M. Curie has investigated the action of the rays upon
amylene and upon petroleum ether at temperatures of +10° and –17°. The
conductivity due to the radiation diminishes by one-tenth of its value
only, in passing from 10° to –17°.

In the experiments in which the temperature of the liquid is varied, the
temperature of the radium may be either that of the surrounding
atmosphere or that of the liquid; the same result is obtained in both
cases. This leads to the conclusion that the radiation of radium does
not vary with the temperature, and remains unaltered even at the
temperature of liquid air. This fact has been verified directly by
measurements.


  _Various Effects and Applications of the Ionising Action of the Rays
                  Emitted by Radio-active Substances._

The rays of the new radio-active substances have a strongly ionising
action upon air. By the action of radium _the condensation of
supersaturated water vapour_ can be easily induced, just as happens by
the action of cathode rays and Röntgen rays.

Under the influence of the rays emitted by the new radio-active
substances, _the distance of discharge between two metallic conductors
for a given difference of potential is increased_; to put it otherwise,
the passage of the spark is facilitated by these rays.

In causing conductivity, by the action of radio-active bodies, in the
air in the neighbourhood of two metallic conductors, one of which is
connected to earth and the other to a well-insulated electrometer, the
electrometer is seen to be permanently deflected, which gives a measure
of the electromotive force of the battery formed by the air and the two
metals (electromotive force of contact of the two metals, when they are
separated by air). This method of measurement was employed by Lord
Kelvin and his students, the radiating body being uranium; a similar
method had been previously employed by M. Perrin, who was using the
ionising action of Röntgen rays.

Radio-active bodies may be employed in the study of atmospheric
electricity. The active substance is enclosed in a little box of thin
aluminium fixed at the extremity of a metal wire connected with the
electrometer. The air is made to conduct in the neighbourhood of the end
of the wire, and the latter adopts the potential of the surrounding air.
Radium thus replaces, with advantage, the flames or the apparatus of
running water of Lord Kelvin, till now in general use for the
investigation of atmospheric electricity.


                  _Fluorescent and Luminous Effects._

The rays emitted by the new radio-active bodies cause fluorescence of
certain substances. M. Curie and myself first discovered this phenomenon
when causing polonium to act upon a layer of barium platinocyanide
through aluminium foil. The same experiment succeeds yet more easily
with barium containing radium. When the substance is strongly
radio-active the fluorescence produced is very beautiful.

A large number of bodies are capable of becoming phosphorescent or
fluorescent by the action of the Becquerel rays. M. Becquerel studied
the effect upon the uranium salts, the diamond, &c. M. Bary has
demonstrated that the salts of the metals of the alkalis and alkaline
earths, which are all fluorescent under the action of luminous rays and
Röntgen rays, are also fluorescent under the action of the rays of
radium. Paper, cotton, glass, &c., are all caused to fluoresce in the
neighbourhood of radium. Among the different kinds of glass, Thuringian
glass is specially luminous. Metals do not seem to become luminous.

Barium platinocyanide is most conveniently used when the radiation of
the radio-active bodies is to be investigated by the fluoroscopic
method. The effect of the radium rays may be followed at distances
greater than 2 m. Phosphorescent zinc sulphide is made extremely
luminous, but this body has the inconvenient property of preserving its
luminosity for some time after the action of the rays has ceased.

The fluorescence produced by radium may be observed when the fluorescent
screen is separated from the radium by absorbent screens. We were able
to observe the illumination of a screen of barium platinocyanide across
the human body. However, the action is incomparably greater when the
screen is placed immediately in contact with the radium, being separated
from it by no solid screen at all. All the groups of rays appear capable
of producing fluorescence.

In order to observe the action of polonium, the substance must be placed
close to the fluorescent screen, without the intervention of a solid
screen, unless the latter be extremely thin.

The luminosity of fluorescent substances exposed to the action of
radio-active bodies diminishes with time. At the same time the
fluorescent substance undergoes a transformation. The following are
examples:—

Radium rays transform barium platinocyanide into a brown, less luminous
variety (an action similar to that produced by Röntgen rays, and
described by M. Villard). Uranium sulphate and potassium sulphate are
similarly altered. The changed barium platinocyanide is partially
regenerated by the action of light. If the radium be placed beneath a
layer of barium platinocyanide spread on paper, the platinocyanide
becomes luminous; if the system be kept in the dark, the platinocyanide
becomes changed, and its luminosity diminishes considerably. But if the
whole be exposed to light, the platinocyanide is partially regenerated,
and if the whole is replaced in darkness the luminosity reappears with
vigour. By means of a fluorescent body and a radio-active body, we have
therefore obtained a system which acts as a phosphorescent body capable
of long duration of phosphorescence.

Glass made fluorescent by the action of radium becomes coloured brown or
violet. At the same time its fluorescence diminishes. If the glass thus
changed be warmed, it is decolorised, and when this occurs the glass
becomes luminous. The glass has now regained its fluorescent property in
the same degree as before the transformation.

Zinc sulphide, which has been exposed for a sufficient length of time to
the action of radium, gradually becomes used up, and loses its
phosphorescent property, whether under the action of radium or that of
light.

The diamond becomes phosphorescent under the action of radium, and may
thus be distinguished from paste imitations, which have only a very
faint luminosity.

All the barium-radium compounds _are spontaneously luminous_. The dry
anhydrous halogen salts emit a particularly intense light. This
illumination cannot be seen in broad daylight, but it is easily visible
in the twilight or by gas-light. The light emitted may be strong enough
to read by in the dark. The light emitted emanates from the entire body
of the product, whilst in the case of a common phosphorescent body, the
light emanates specially from the portion of the surface illuminated.
Radium products lose much of their luminosity in damp air, but they
regain it on drying (Giesel). There is apparently conservation of
luminosity. After many years no sensible modification is produced in the
luminosity of feebly active products, kept in the dark in sealed tubes.
In the case of very active and very luminous radium-barium chloride, the
light changes colour after several months; it becomes more violet and
loses in intensity; at the same time the product undergoes
transformations; on re-dissolving the salt in water and drying it
afresh, the original luminosity is restored.

Solutions of barium-radium salts, which contain a large proportion of
radium, are equally luminous; this fact may be observed by placing the
solution in a platinum capsule, which not being itself luminous permits
of the faint luminosity of the solution being seen.

When a solution of a barium-radium salt contains crystals deposited in
it, these crystals are luminous at the bottom of the solution, and much
more so than the solution itself, so that they alone appear luminous.

M. Giesel has made a preparation of barium-radium platinocyanide. When
this salt is newly crystallised, it has the appearance of ordinary
barium platinocyanide and is very luminous. But gradually the salt
becomes spontaneously coloured, taking a brown tint, the crystals at the
same time becoming dichroic. In this state the salt is much less
luminous, although its radio-activity is increased. The radium
platinocyanide, prepared by M. Giesel, changes still more rapidly.

Radium compounds are the first example of self-luminous bodies.


              _Evolution of Heat by the Salts of Radium._

MM. Curie and Laborde have recently discovered that _the salts of radium
are the source of a spontaneous and continuous evolution of heat_. This
evolution has the effect of keeping the salts of radium at a temperature
higher than that of their surroundings; an excess of temperature of 1·5°
has been observed. This excess of temperature is dependent upon the
thermal insulation of the body. MM. Curie and Laborde have determined
the amount of heat produced in the case of radium. They found that the
output is of the order of magnitude of 100 calories per grm. of radium
per hour. One grm.-atom (225 grm.) of radium give rise in one hour to
22,500 cal., a quantity of heat comparable to that produced by the
combustion of 1 grm.-atom (1 grm.) of hydrogen. So great an evolution of
heat can be explained by no ordinary chemical reaction, more
particularly as the condition of the radium remains unaffected for
years. The evolution of heat might be attributed to a slow
transformation of the radium atom. If this were the case, we should be
led to conclude that the quantities of energy generated during the
formation and transformation of the atoms are considerable, and that
they exceed all that is so far known.


      _Chemical Effects produced by the New Radio-active Bodies._

_Colourations._—The radiations of strongly radio-active bodies are
capable of causing certain chemical reactions. The rays emitted by
radium products exercise colouring actions upon glass and porcelain.

The colouration of glass, generally brown or violet, is very deep; it is
produced in the body of the glass, and remains after removal of the
radium. All glasses become coloured after a longer or a shorter interval
of time, and the presence of lead is not essential. This fact may be
compared to that recently observed of the colouration of the glass of
vacuum tubes, after having been long in use for the production of
Röntgen rays.

M. Giesel has demonstrated that the crystallised halogen salts of the
alkali metals become coloured under the influence of radium, as under
the action of cathode rays. M. Giesel points out that similar
colourations are obtained when the salts of the alkalis are exposed to
sodium vapour.

I investigated the colouration of a collection of glasses of known
composition, kindly lent me for the occasion by M. Le Chatelier. I
observed no great variety in the colouration. It is generally brown,
violet, yellow, or grey. It appears to be associated with the presence
of the alkali metals.

With the pure crystallised alkali salts more varied and more vivid
colours are obtained; the salt, originally white, becomes blue, green,
yellow, brown, &c.

M. Becquerel has discovered that yellow phosphorus is transformed into
the red variety by the action of radium.

Paper is changed and coloured by the action of radium. It becomes
brittle, scorched, and, finally, resembles a colander perforated with
holes.

Under some circumstances there is a production of ozone in the
neighbourhood of very active compounds. Rays emerging from a sealed jar
containing radium do not produce ozone in the air they pass through. On
the contrary, a strong odour of ozone is detected when the jar is
opened. In a general way, ozone is produced in the air when the latter
is in direct contact with the radium. Communication by a channel, even
if extremely narrow, suffices; it appears as if the production of ozone
is associated with the propagation of induced radio-activity, of which
we shall speak later.

Radium compounds appear to change with lapse of time, doubtless under
the action of their own radiation. It was seen above that crystals of
barium-radium chloride, which are colourless when formed, become
gradually coloured first yellow or orange, then pink; this colouration
disappears in solution. Barium-radium chloride generates oxygen
compounds of chlorine; the bromide those of bromine. These slow changes
generally manifest themselves some time after the preparation of the
solid product, which at the same time changes in form and colour,
becoming yellow or violet. The light emitted also becomes more violet.

A solution of a radium salt evolves hydrogen (Giesel).

Pure radium salts seem to undergo the same changes as those containing
barium. However, crystals of the chloride, deposited in acid solution,
do not become sensibly coloured after some time has elapsed, whereas
crystals of barium-radium chloride, rich in radium, become deeply
coloured.

_Production of Thermo-luminosity._—Certain bodies, such as fluorite,
become luminous when heated; they are thermo-luminescent. Their
luminosity disappears after some time, but the capacity of becoming
luminous afresh through heat is restored to them by the action of a
spark, and also by the action of radium. Radium can thus restore to
these bodies their thermo-luminescent property. Fluorite when heated
undergoes a change, which is accompanied by the emission of light. If
the fluorite is afterwards subjected to the action of radium, an inverse
change occurs, which is also accompanied by an emission of light.

An absolutely similar phenomenon occurs when glass is exposed to radium
rays. Here also a change is produced in the glass while luminous from
the effect of the radium rays; this change shows itself in the
colouration which appears and gradually increases. If the glass is
afterwards heated, the inverse change takes place, the colour
disappears, and this phenomenon is accompanied by production of light.
It appears very probable that we have here a change of a chemical
nature, and the production of light is associated with this change. This
phenomenon may be general. It might be that the production of
fluorescence by the action of radium and the luminosity of radium
compounds is of necessity associated with some chemical or physical
change in the substance emitting the light.

_Radiographs._—The radiographic action of the new radio-active bodies is
very marked. However, the method of operating should be very different
with polonium and radium. Polonium acts only at very short distances,
and its action is considerably weakened by solid screens; it is
practically annihilated by means of a screen of slight thickness (1 m.m.
of glass). Radium acts at considerably greater distances. The
radiographic action of radium rays may be observed at more than 2 m.
distance in air, even when the active product is enclosed in a glass
vessel. The rays acting under these conditions belong to the β- and
γ-groups. Owing to the differences in transparency of different
materials to the rays, radiographs of different objects may be obtained,
as in the case of Röntgen rays. Metals are, as a rule, opaque, with the
exception of aluminium, which is very transparent. There is no
noteworthy difference of transparency between flesh and bone. The
operation may be carried on at a great distance and with a source of
very small dimensions; and very delicate radiographs are thus produced.
The beauty of the radiograph is enhanced by deflecting to one side the
β-rays, by means of a magnetic field, and utilising only the γ-rays. The
β-rays, in traversing the object to be radiographed, undergo a certain
amount of diffusion, and thus cause a slight fog. In suppressing them, a
longer time of exposure is necessary, but better results are obtained.
The radiograph of an object, such as a purse, requires one day with a
radiating source composed of several centigrams of a radium salt,
enclosed in a glass vessel, and placed at a distance of 1 m. from the
sensitive plate, in front of which the object is placed. If the source
is at a distance of 20 c.m. from the plate, the same result is obtained
in one hour. In the immediate vicinity of the source of radiation, a
sensitive plate is instantaneously acted upon.


                        _Physiological Effects._

Radium rays exert an action upon the epidermis. This has been observed
by M. Walkhoff and confirmed by M. Giesel, since also by MM. Becquerel
and Curie.

If a celluloid or thin indiarubber capsule containing a very active salt
of radium be placed upon the skin, and be left thus for some time, a
redness is produced upon the skin, either immediately or at the end of
some time, which is longer in proportion as the action is weaker; this
red spot appears in the place which has been exposed to the action; the
local change in the skin appears and acts like a burn. In certain cases
a blister is formed. If the exposure was of long duration, an ulceration
is produced which is long in healing. In one experiment, M. Curie caused
a relatively weak radio-active product to act upon his arm for ten
hours. The redness appeared immediately, and later a wound was caused
which took four months to heal. The epidermis was locally destroyed, and
formed again slowly and with difficulty, leaving a very marked scar. A
radium burn with half-an-hour’s exposure appeared after fifteen days,
formed a blister and healed in fifteen days. Another burn, caused by an
exposure of only eight minutes, occasioned a red spot which appeared two
months after, its effect being quite insignificant.

The action of radium upon the skin can take place across metal screens,
but with weakened effect.

The action of radium upon the skin has been investigated by Dr. Daulos,
at the Hospital of St. Louis, as a process of treating certain
affections of the skin, similar to the treatment with the Röntgen rays
or the ultra-violet rays. In this respect radium gives encouraging
results; the epidermis partially destroyed by the action of the radium
is renewed in a healthy condition. The action of radium is more
penetrating than that of light, and its use is easier than that of light
or of Röntgen rays. The study of the conditions of application is of
necessity rather lengthy, because the effect of the application does not
at once appear.

M. Giesel has observed the action of radium upon plant leaves. The
leaves thus treated turn yellow and wither away.

M. Giesel has also discovered the action of radium rays upon the eye. If
a radio-active substance be placed in the dark in the vicinity of the
closed eye or of the temple, a sensation of light fills the eye. This
phenomenon has been studied by MM. Himstedt and Nagel. These physicists
have demonstrated that the centre of the eye is rendered fluorescent by
the action of radium, and this explains the sensation of light
experienced. Blind people whose retina is intact are sensitive to the
action of radium, whilst those whose retina is diseased do not
experience any sensation of luminosity.

Radium rays either arrest or hinder the development of colonies of
microbes, but this action is not very intense.

M. Danysz has recently demonstrated the ready action of radium upon the
marrow and brain. After one hour’s exposure paralysis of the animals
experimented upon occurred, and the latter usually died in a few days.


               _Influence of Temperature upon Radiation._

There is so far but little information regarding the manner of variation
of the radiation of radio-active bodies with temperature. We know,
however, that radiation subsists at low temperatures. M. Curie placed a
glass tube containing barium-radium chloride in liquid air. The
luminosity of the radio-active body persisted under these conditions. At
the moment, indeed, of removing the tube from the cold bath, it appears
more luminous than at the ordinary temperature. At the temperature of
liquid air radium continues to cause fluorescence in the sulphates of
uranium and potassium. M. Curie has verified, by electrical
determinations, that the radiation, measured at a certain distance from
the source, possesses the same intensity whether the radium be at the
temperature of the atmosphere or of liquid air. In these experiments the
radium was placed at the bottom of a tube closed at one end. The rays
emerged from the tube at the open end, traversed a certain space in the
air, and were received into a condenser. The action of the rays upon the
air of the condenser was determined both on leaving the tube in the air
and on surrounding it to a certain height with liquid air. The same
result was obtained in both cases.

The radio-activity of radium persists at high temperatures.
Barium-radium chloride after being fused (towards 800°) is radio-active
and luminous. However, prolonged heating at a high temperature has the
effect of temporarily lowering the radio-activity of the body. This
decrease is very considerable; it may constitute 75 per cent of the
total radiation. The decrease is less in proportion for the absorbable
rays than for the penetrating rays, which are to some extent suppressed
by heating. In time the radiation of the product regains the intensity
and composition that it possessed before heating; this occurs after the
lapse of about two months from the occasion of heating.




                              CHAPTER IV.
   COMMUNICATION OF RADIO-ACTIVITY TO SUBSTANCES INITIALLY INACTIVE.


During the course of our researches on radio-active substances M. Curie
and I have observed that every substance which remains for some time in
the vicinity of a radium salt becomes itself radio-active. In our first
publication on this subject, we confined ourselves to proving that the
radio-activity thus acquired by substances initially inactive is not due
to the transference of radio-active particles to the surface of these
substances. This is proved beyond dispute by all the experiments which
will be here described, and by the laws according to which the
radio-activity excited in naturally inactive bodies disappears when the
latter are removed from the influence of radium.

We have given the name of _induced radio-activity_ to the new phenomenon
thus discovered.

In the same publication, we indicated the essential characteristics of
induced radio-activity. We excited screens of different substances by
placing them in the neighbourhood of solid radium salts, and we
investigated the radio-activity of these screens by the electrical
method. We observed the following facts:—

1. The activity of a screen exposed to the action of radium increases
with the time of exposure, approaching to a definite limit according to
an asymptotic law.

2. The activity of a screen which has been excited by the action of
radium, and which is afterwards withdrawn from its action, disappears in
a few days. This induced activity approaches zero as a function of the
time, following an asymptotic law.

3. Other things being equal, the radio-activity induced by the same
radium product upon different screens is independent of the nature of
the screen. Glass, paper, metals, all acquire the same degree of
activity.

4. The radio-activity induced in one screen by differing radium products
has a limiting value which rises with the increased activity of the
product.

Shortly afterwards, Mr. Rutherford published a research, which showed
that compounds of thorium are capable of producing the phenomenon of
induced radio-activity. Mr. Rutherford discovered for this phenomenon
the same laws as those just enunciated, besides this additional
important fact, that bodies charged with negative electricity become
more active than others. Mr. Rutherford also observed that air passed
over thorium oxide preserves a notable conductivity for about ten
minutes. Air in this condition communicates induced radio-activity to
inactive substances, especially to those negatively charged. Mr.
Rutherford explains his experiences by the supposition that compounds of
thorium, particularly the oxide, give rise to a _radio-active emanation_
capable of being carried by air currents and charged with positive
electricity. This emanation would be the origin of induced
radio-activity. M. Dorn has repeated, with salts of barium containing
radium, the experiments of Mr. Rutherford with thorium oxide.

M. Debierne has shown that actinium causes, to a marked degree, induced
activity of bodies placed in its vicinity. As in the case of thorium,
there is a considerable carriage of activity by air currents.

Induced radio-activity has various aspects, and irregular results are
obtained when the activity of a substance in the neighbourhood of radium
is excited in free air. MM. Curie and Debierne have observed, however,
that the phenomenon is quite regular when operating in a closed vessel;
they therefore investigated induced activity in a closed space.


                _Activity Induced in an Enclosed Space._

The active material is placed in a little glass jar, _a_, open at O
(Fig. 11), in the centre of a closed space. Several plates, A, B, C, D,
E, placed in the enclosure become radio-active after one day’s exposure.
The activity is the same whatever be the nature of the plate, for equal
dimensions (lead, copper, aluminium, glass, ebonite, wax, cardboard,
paraffin). The activity of one face of one of the plates is greater in
proportion to the amount of free space about this face.

If the preceding experiment be repeated with the jar, _a_, completely
closed, no activity is induced.

The radiation of radium does not directly affect the production of
induced radio-activity. For this reason, in the preceding experiment the
plate D, screened from the radiation by a lead plate of thickness, P P,
is made as active as B and E.

Radio-activity is transmitted by the air by degrees from the radiating
body to the body to be excited. It can even be transmitted to a distance
by very narrow capillary tubes.

Induced radio-activity is both more intense and more regular if the
solid radium salt be replaced by an aqueous solution of the same.

Liquids are capable of acquiring induced radio-activity. For example,
pure water may be rendered active by placing it with a solution of a
radium salt within an enclosure.

[Illustration: FIG. 11]

Certain substances become luminous when placed in an active enclosure
(phosphorescent and fluorescent bodies, glass, paper, cotton, water,
salt solutions). Phosphorescent zinc sulphide is particularly brilliant
under the circumstances. The radio-activity of these luminous bodies is,
however, the same as that of a piece of a metal or other body which is
excited under the same conditions without becoming luminous.

Whatever be the substance made active in a closed vessel, this substance
acquires an activity which increases with length of time until it
attains a _limiting value_, always the same, for the same material and
the same experimental arrangement.

_The limit of induced radio-activity is independent of the nature and
pressure of the gas inside the active enclosure_ (air, hydrogen,
carbonic acid).

_The limit of induced radio-activity for the same enclosure depends only
on the quantity of radium present_ in the state of solution, and is
apparently proportional to it.


   _Part played by Gases in the Phenomena of Induced Radio-activity._

_Emanation._—The gases present in an enclosure containing a solid salt
or a solution of a salt of radium are radio-active. This radio-activity
persists when the gas is drawn off with a tube and collected in a
test-tube. The sides of the test-tube become themselves radio-active,
and the glass of the test-tube is luminous in the dark. The activity and
luminosity of the test-tube finally completely disappear, but very
gradually, and a month afterwards radio-activity may still be detected.

Since the beginning of our researches, M. Curie and I have, by heating
pitchblende, extracted a strongly radio-active gas, but, as in the
preceding experiment, the activity of this gas finally completely
vanished.

We could discern no new ray in the spectrum of this gas; this was
therefore not a case of a new radio-active gas, and we understood later
that it was the phenomenon of induced radio-activity.

Thus, for thorium, radium, and actinium induced radio-activity is
progressively propagated through the gases, from the radiating body to
the walls of the enclosure containing it, and the exciting principle is
carried away with the gas itself, when the latter is extracted from the
enclosure.

When the radio-activity of radium compounds is measured by the
electrical method by means of the apparatus of Fig. 1, the air between
the plates is itself radio-active; however, on passing a current of air
between the plates, there is no observable lowering of the intensity of
the current, which proves that the radio-activity distributed in the
space between the plates is of little account in comparison with that of
the radium itself in the solid state.

It is quite otherwise with thorium. The irregularities which I observed
in determining the radio-activity of the thorium compounds arose from
the fact that at this point I was working with a condenser open to the
air; the least air current caused a considerable change in the intensity
of the current, because the radio-activity dispersed in the space in the
vicinity of the thorium is considerable as compared with the
radio-activity of the substance.

This effect is still more marked in the case of actinium. A very active
compound of actinium appears much less active when a current of air is
passed over the substance.

The radio-active energy is therefore contained in the gas in a special
form. Mr. Rutherford suggests that radio-active bodies generate an
_emanation_ or gaseous material which carries the radio-activity. In the
opinion of M. Curie and myself, the generation of a gas by radium is a
supposition which is not so far justified. We consider the emanation as
radio-active energy stored up in the gas in a form hitherto unknown.


   _Dissipation in Free Air of the Induced Activity of Solid Bodies._

A solid body, which has been excited by radium in an enclosed space for
a sufficient length of time, and which has then been removed from the
enclosure, parts with its activity in free air according to an
exponential law, which is the same for all bodies represented by the
following formula:—

      I = I_{0}(_a__e_^{–_t_/θ_{1}} – (_a_ – 1)_e_^{–_t_/θ_{2}}).

I_{0} being the initial intensity of the radiation at the moment of
withdrawing the plate from the enclosure; I, the intensity after time,
_t_; _a_ is a numerical coefficient, a = 4·20; θ_{1} and θ_{2} are time
constants, θ_{1} = 2420 secs., θ_{2} = 1860 secs. After two or three
hours this law becomes practically a simple exponential, and the effect
of the second exponential upon the value of I is negligible. The law of
dissipation is therefore such that the intensity of radiation becomes
diminished to one-half its value in twenty-eight minutes. This final law
may be considered as characteristic for the dissipation in an unconfined
air space of the activity induced in solid bodies by radium.

Solid bodies excited by actinium lose their activity in the open air,
according to an exponential law similar to the preceding, the
dissipation being, however, rather slower.

Solid bodies, made active by thorium, lose their activity much more
slowly; the intensity of the radiation is reduced to one-half in eleven
hours.


_Dissipation of Activity in a Confined Space. Velocity of Destruction of
                            the Emanation._

An enclosure, made active by radium and then removed from its influence,
loses its activity by a law which is much less rapid than that of
dissipation in the open air. The experiment may be carried out with a
glass tube, rendered active internally by placing it for some time in
contact with a solution of a salt of radium. The tube is then sealed in
the flame, and the intensity of radiation emitted by the walls of the
tube is measured while the dissipation takes place.

The law of dissipation is an exponential law. It is given very
accurately by the formula.

                          I = I_{0}_e_^{t/θ}.

 I_{0} = initial intensity of radiation.

 I     = intensity of radiation at time, _t_.

 θ     = a time constant, θ = 4·970 × 10^5 secs.

The intensity of the radiation is reduced to one-half in four days.

This law of dissipation is absolutely invariable whatever be the
experimental conditions (dimensions of enclosure, nature of the walls,
nature of the gas within the enclosure, duration of action, &c.). The
law of dissipation remains the same for any temperature between –180°
and +450°. The law is therefore altogether characteristic.

In these experiments it is the radio-active energy accumulated in the
gas that maintains the activity of the walls. If the gas be withdrawn
and a vacuum caused in the enclosure, we have found that dissipation of
activity at once occurs in the rapid method; the intensity of radiation
being reduced to one-half in twenty-eight minutes. The same result is
obtained when ordinary air is substituted for the active air in the
enclosure.

The law of dissipation with reduction of intensity of radiation to
one-half in four days, is therefore characteristic of the disappearance
of radio-active energy accumulated in the gas. By making use of the
expression adopted by Mr. Rutherford, the emanation from radium may be
said to disappear spontaneously as a function of the time, with
reduction to one-half in four days.

The emanation from thorium is of another kind, and disappears much more
rapidly. The intensity of radiation diminishes to one-half in about one
minute ten seconds.

The emanation from actinium disappears still more rapidly; reduction to
one-half takes place in a few seconds.


    _Variation of Activity of Liquids rendered Active and of Radium
                              Solutions._

Any liquid whatever becomes radio-active when placed in an active
confined space. On being removed and left freely exposed to the air, the
liquid rapidly loses its activity, imparting it to the gas and solid
bodies surrounding it. If a liquid thus made active be placed in a
closed flask, it loses its activity much more slowly; the latter being
reduced in intensity to one-half in four days, just as would a gas under
similar circumstances. This fact may be explained by assuming that the
radio-active energy is stored in liquids in exactly the same form as in
gases (in the form of an emanation).

A solution of a radium salt behaves in a somewhat similar manner. At
first, it is a remarkable fact that the solution of a radium salt placed
for some time in a confined space is no more active than pure water
placed in a vessel in the same enclosure, when the equilibrium of
activity is established. If the radium solution be removed from the
enclosure and left standing in the air in a wide-necked vessel, the
activity spreads itself into space, and the solution becomes nearly
inactive, though still containing radium. If this solution be now
enclosed in a stoppered flask, it gradually regains, in about a
fortnight, a maximum of activity, which may be considerable. On the
other hand, a liquid made active, but not containing radium, does not
regain its activity in a closed flask after having been exposed to the
atmosphere.


                      _Theory of Radio-activity._

The following is, according to MM. Curie and Debierne, a very general
theory which allows of the co-ordination of the results of the
investigation of induced radio-activity, which results I have just
stated, and which constitute facts apart from any hypothesis.

It may be said that each atom of radium acts as a constant and
continuous source of energy, without actually defining the origin of
this energy. The radio-active energy which accumulates in the radium
tends to become dissipated in two different ways:—(1) By radiation (rays
both charged and uncharged with electricity); (2) by conduction, _i.e._,
by gradual transmission to surrounding bodies in a medium of gases and
liquids (production of an emanation and transformation into induced
radio-activity).

The loss of radio-active energy, both by radiation and by conduction,
increases with the amount of energy accumulated in the radio-active
body. The system is necessarily in equilibrium when the double loss of
which I have just made mention compensates the constant gain due to the
action of radium. This manner of regarding the subject is similar to
that in use for calorific phenomena. If in the interior of any body
there is, owing to any cause, a continuous and constant evolution of
heat, the heat accumulates in the body and the temperature rises until
the loss of heat by radiation and conduction is in equilibrium with the
constant gain of heat.

In general, except under certain special circumstances, the activity is
not propagated through solid bodies. When a solution is kept in a sealed
tube, the loss by radiation alone takes place, and the radiating
activity of the solution is of a higher degree.

If, on the contrary, the solution stands in an open vessel, the loss of
activity by conduction becomes considerable, and when the state of
equilibrium is attained, the radiating activity of the solution is very
feeble.

The radiating activity of a solid radium salt left exposed to the air
does not sensibly diminish, because the propagation of activity by
conduction not taking place through solid bodies, it is a very thin
superficial layer only that produces induced radio-activity. The
solution, however, of the same salt produces much more intense phenomena
of induced radio-activity. With a solid salt the radio-active energy
accumulates in the salt, and is dissipated chiefly by radiation. On the
other hand, when the salt has been for several days in aqueous solution,
the radio-active energy is divided between the salt and the water, and
if separated by distillation the water carries with it a large portion
of the activity, and the solid salt is much less active (ten or fifteen
times) than before solution. Afterwards the solid salt gradually regains
its original activity.

The preceding theory may be yet further defined by supposing the
radio-activity of radium itself to be produced through the medium of the
radio-active energy emitted in the form of an emanation.

Each atom of radium may be considered as a constant and continuous
source of emanation. At the same moment that this form of energy is
produced, it undergoes a progressive transformation into radio-active
energy of the Becquerel radiation. The velocity of this transformation
is proportional to the quantity of the emanation accumulated.

When a radium solution is placed within an enclosure, the emanation is
able to expand into the enclosure and to spread out over the walls. Here
it is, therefore, that it is transformed into a radiation, the solution
giving off but few Becquerel rays; the radiation is, in some sort,
_externalised_. On the other hand, with solid radium, the emanation not
being able to escape readily, accumulates, and is transformed into the
Becquerel radiation on the spot; this radiation therefore acquires a
higher value.

If this theory of radio-activity were general, we should have to say
that all radio-active bodies give rise to an emanation. Now this
emission has been confirmed in the case of radium, thorium, and
actinium; with the latter in particular the emission is enormous, even
in the solid state. Uranium and polonium do not seem to emit any
emanation, though they generate Becquerel rays. These bodies produce no
induced radio-activity in an enclosed space, as do the radio-active
bodies mentioned before. This fact is not in absolute contradiction to
the preceding theory. If uranium and polonium were to emit emanations
which become destroyed with very great rapidity, it would be very
difficult to observe the carriage of such emanations by the air and the
effects of induced radio-activity produced by them upon neighbouring
bodies. Such a hypothesis is not improbable, since the times required
for certain quantities of the emanations of radium and thorium to
diminish to one-half are in the proportion of 5000 to 1. We shall see,
moreover, that, under certain conditions, uranium can excite induced
activity.


               _Another Form of Induced Radio-activity._

According to the law of dissipation in the open air of the activity
induced by radium in solid bodies, the activity after one day is almost
imperceptible.

Certain bodies, however, form exceptions; such are celluloid, paraffin,
caoutchouc, &c. When these bodies have been acted upon to a sufficient
degree, they lose their activity more slowly than the law can account
for, and it is often fifteen or twenty days before the activity becomes
imperceptible. These bodies appear to have the property of becoming
charged with radio-active energy in the form of an emanation; they
afterwards lose it gradually, causing induced radio-activity in the
vicinity.


            _Induced Radio-activity with Slow Dissipation._

There is yet another form of induced radio-activity, which appears to be
produced in all bodies which have been kept for months in an active
enclosure. When these bodies are removed from the enclosure their
activity at first diminishes to a very low value according to the
ordinary law (diminution to one-half in half-an-hour); but when the
activity has fallen to about 1/20,000 of the initial value, it
diminishes no further, or at least it is dissipated very slowly,
sometimes even increasing in amount. We have sheets of copper,
aluminium, and glass which still retain a residual activity after six
months.

These phenomena of induced radio-activity appear to be of a different
kind from the ordinary ones, and show a much slower process of
evolution.

A considerable time is also necessary both for the production and
dissipation of this form of induced radio-activity.


   _Radio-activity Induced upon Substances in Solution with Radium._

When a radio-active ore containing radium is treated, with the object of
extracting the radium, chemical separations are effected, after which
the radio-activity is confined entirely to one of the products. In this
way active products, which may be several hundred times as active as
uranium, are separated from totally inactive products, such as copper,
antimony, arsenic, &c. Certain other bodies (iron, lead) were never
separated in an entirely inactive state. As these active bodies are
concentrated, the case is no longer the same; each chemical separation
no longer furnishes absolutely inactive products; all the resulting
products of a separation are active in varying degrees.

After the discovery of induced radio-activity, M. Giesel was the first
to attempt to excite activity in ordinary inactive bismuth by keeping it
in solution with very active radium. He thus obtained radio-active
bismuth, and from this he concluded that the polonium extracted from
pitchblende was probably bismuth made active by the vicinity of the
radium contained in the pitchblende.

I have also prepared active bismuth by keeping bismuth in solution with
a very active radium salt.

The difficulties of this experiment consist in the extreme precautions
which must be taken to remove all traces of radium from the solution. If
we realise what an infinitesimal quantity of radium suffices to produce
very considerable radio-activity in 1 grm. of material, it is difficult
to believe in the possibility of sufficiently washing and purifying the
active product. Each purification causes a diminution of activity of the
product, whether this be due to removal of traces of radium or that the
induced radio-activity is, under these circumstances, not proof against
chemical reactions.

The results I obtained appear, however, to establish with certainty the
fact that the activity is produced and persists after the radium is
removed. On fractionating the nitrate of my active bismuth by
precipitation with water from the nitric acid solution, I found that
after careful purifying it fractionated like polonium, the most active
portion being precipitated first.

If the purification is not complete the opposite occurs, showing that
traces of radium still remain. I thus obtained active bismuth which from
the manner of fractionation showed great purity and which was 2000 times
as active as uranium. This bismuth diminishes in activity with lapse of
time. But another portion of the same product, prepared with the same
precautions, and fractionating in the same manner, preserves its
activity without diminution for actually a period of about three years.

This activity is 150 times as great as that of uranium.

I have also prepared active lead and silver by leaving them in solution
with radium. Generally induced radio-activity obtained in this way
scarcely lessens with lapse of time, but it does not as a rule withstand
many successive chemical changes of the active body.

M. Debierne made active barium by placing it in solution with actinium.
This barium remains active after several chemical reactions, its
activity being therefore a somewhat stable atomic property. Active
barium chloride fractionates like barium-radium chloride, the more
active portions being the least soluble in water and dilute hydrochloric
acid. The dry chloride is spontaneously luminous: its Becquerel
radiation is similar to that of barium-radium chloride. M. Debierne has
prepared an active barium chloride 1000 times as active as uranium. This
barium, however, had not acquired all the characteristics of radium, for
it showed none of the strongest radium lines in the spectroscope.
Further, its activity diminished on standing, and after three weeks it
had become one-third of its original value.

There is a wide field for research upon the radio-activity induced in
substances in solution with active bodies. It appears that, according to
the conditions of experiment, more or less stable forms of induced
atomic radio-activity may be obtained. The radio-activity induced under
these circumstances is perhaps identical with that form, which
dissipates slowly, obtained by prolonged exposure at a distance in an
active enclosure. We have reason to enquire to what degree induced
radio-activity affects the chemical nature of the atom, and if it is
able to modify the chemical properties of the latter, either temporarily
or permanently.

The chemical investigation of bodies excited at a distance is rendered
difficult by the fact that the induced activity is limited to a very
thin superficial layer, and that, consequently, only a very small
proportion of the material has been affected.

Induced radio-activity also results from leaving certain substances in
solution with uranium. The experiment succeeded in the case of barium.
If, as was done by M. Debierne, sulphuric acid be added to a solution
containing uranium and barium, the precipitate of barium sulphate
acquires radio-activity, and, at the same time, the uranium salt loses
part of its activity. M. Becquerel found, after repeating this
experiment several times, that almost inactive uranium was obtained.
This might lead to the opinion that a radio-active body differing from
uranium had been separated from the latter, its presence producing
radio-activity in uranium. This, however, is not the case, for after
some months the uranium regains its original activity; the precipitated
barium sulphate, on the contrary, loses what it acquired.

A similar phenomenon is observed with thorium. Mr. Rutherford
precipitated a solution of a salt of thorium with ammonia; he separated
off the solution and evaporated it to dryness. He thus obtained a small
very active residue, and the precipitated thorium was observed to be
less active than before. This active residue, to which Mr. Rutherford
gives the name of _thorium_ X, loses its activity after a time, whilst
the thorium regains its original activity.

It appears, then, that concerning induced radio-activity all bodies do
not behave in a similar manner, and that certain of them are much more
readily excited than others.


 _Dissemination of Radio-active Particles and Induced Radio-activity of
                            the Laboratory._

In making investigations of strongly radio-active bodies, particular
precautions must be observed for obtaining delicate determinations. The
different objects used in the chemical laboratory and those used for
physical experiments soon acquire radio-activity, and act upon
photographic plates through black paper. Dust particles, the air of the
room, clothing, all become radio-active. The air of the room becomes a
conductor. In our laboratory the evil has become acute, and we no longer
have any apparatus properly insulated.

Special precautions must therefore be taken to avoid as much as possible
the dissemination of active dust particles, and to avoid also the
phenomena of induced activity.

The objects employed in chemistry should never be brought into the room
where physical research is carried on, and as far as possible should be
avoided any unnecessary keeping of active substances in this room.
Before beginning our researches we were in the habit, in the case of
electrical experiments, of making a connection between the different
parts of the apparatus by insulated metallic wires, protected by metal
cylinders connected to earth, which screened the wires from all outside
electrical forces. In the investigation of radio-active bodies this
arrangement is quite defective; the air being a conductor there is
incomplete insulation between the thread and the cylinder, and the
inevitable electromotive force of contact between the thread and the
cylinder tends to produce a current through the air, and to cause a
deflection of the electrometer. We now screen all the wires from the air
by placing them inside cylinders filled with paraffin or other
insulating material. It would also be advantageous in these
investigations to make use of carefully enclosed electrometers.


  _Activity Induced Outside the Influence of Radio-active Substances._

Attempts were made to produce induced radio-activity outside the action
of radio-active bodies.

M. Villard subjected to the action of the cathode rays a piece of
bismuth placed as anticathode in a Crookes tube; the bismuth was thus
rendered active to a very slight degree, for it required an exposure of
eight days to obtain a photographic impression.

Mr. MacLennan has exposed different salts to the action of cathode rays,
afterwards warming them slightly. The salts then acquired the property
of neutralising bodies positively charged.

Studies of this kind are of great interest. If, by using known physical
agents, it were possible to create a considerable radio-activity in
bodies originally inactive, we might hope thence to discover the cause
of the spontaneous radio-activity of certain substances.


 _Variations of Activity of Radio-active Bodies. Effects of Solution._

The activity of polonium, as I have said above, diminishes with time.
This diminution is slow, and does not take place at the same rate with
different specimens. A sample of bismuth-polonium nitrate lost half its
activity in eleven months, and 95 per cent in thirty-three months. Other
specimens have evidenced similar diminution.

A specimen of metallic bismuth containing polonium was prepared from the
nitrite, its activity after preparation being 100,000 times that of
uranium. The metal is now only a body of medium radio-activity (2000
times that of uranium). Its radio-activity is determined at intervals.
In six months it has lost 67 per cent of its activity.

The loss of activity does not seem to be facilitated by chemical action.
In rapid chemical changes no considerable loss of activity has in
general taken place.

In contrast to that which occurs with polonium, radium salts possess a
permanent radio-activity which evidences no appreciable diminution after
many years.

A freshly prepared radium salt in the solid state does not at first
possess an activity of constant strength. Its activity increases from
the time of preparation until it attains a practically constant limiting
value after about one month. The opposite is the case for a solution.
When freshly prepared the solution is very active, but when left exposed
to the air it rapidly loses activity, and finally reaches a limiting
activity which may be considerably less than the original. These
variations of activity were first observed by M. Giesel. They are easily
accounted for by the emanation theory. The diminution of the activity of
the solution corresponds to the loss of the emanation which escapes into
space; this diminution is much less when the solution is contained in a
sealed tube. A solution which has lost its activity in air recovers a
greater activity in a sealed tube. The time of increase of the activity
of the salt which, after solution, has been recently obtained in the
solid state, is that during which the emanation is being newly stored in
the solid radium.

The following are some experiments on this subject:—

A solution of barium-radium chloride left exposed to the air for two
days becomes 300 times less active.

A solution is enclosed in a stoppered vessel; the vessel is opened, the
solution poured into a dish, and the activity determined:—

                 Activity immediately determined 67
                 Activity after two hours        20
                 Activity after two days          0·25

A solution of barium-radium chloride, which has been kept open to the
air, is enclosed in a sealed glass tube, and the radiation of this tube
determined. The following results were observed:—

                 Activity determined immediately    27
                 Activity determined after 2 days   61
                 Activity determined after 3 days   70
                 Activity determined after 4 days   81
                 Activity determined after 7 days  100
                 Activity determined after 11 days 100

The initial activity of a solid salt after preparation is feeble in
proportion as the time of solution was long. A greater proportion of
activity is then transmitted to the solvent. The following figures give
the initial activity with a chloride whose limiting activity is 800, and
which was kept for a given time in solution; the salt was afterwards
dried, and its activity immediately determined:—

   Limiting activity                                              800
   Initial activity after solution and immediate evaporation      440
   Initial activity after the salt has remained dissolved 5 days  120
   Initial activity after the salt has remained dissolved 18 days 130
   Initial activity after the salt has remained dissolved 32 days 114

During this experiment the dissolved salt was placed in a vessel merely
covered with a watch-glass.

I made with the same salt two solutions which I kept in sealed tubes for
thirteen months; one of these solutions was eight times the strength of
the other:—

 Initial activity of the salt in concentrated solution after
   evaporation                                                       200
 Initial activity of the salt in dilute solution after evaporation   100

The loss of activity of the salt is therefore greater when the amount of
solvent is greater, the radio-active energy transmitted to the liquid
having a greater volume of liquid to saturate and a greater space to
fill. The two specimens of the same salt, which thus had a different
initial activity, further increased in activity at very different rates
at first; at the end of one day they had the same activity, and the
increase of activity now continued in the same manner for both till the
limit was reached.

When the solution is dilute the loss of activity by the salt is very
rapid, as is shown by the following experiments:—Three equal portions of
the same radium salt are dissolved in equal quantities of water. The
first solution (_a_) is allowed to stand in contact with the air for one
hour, and is then evaporated. The second solution (_b_) has a current of
air passed through it for one hour, and is then evaporated. The third
solution (_c_) is left exposed to the air for thirteen days, and then
evaporated to dryness. The initial activity of each of the three salts
is:—

                         For portion _a_ 145·2
                         For portion _b_ 141·6
                         For portion _c_ 102·6

The limiting activity of the same salt is about 470. We thus see that
the greatest part of the effect was produced at the end of one hour.
Further, the air current bubbling through solution _b_ for one hour
produced little effect. The proportion of salt in solution was about 0·5
per cent.

Radio-active energy in the form of an emanation is propagated with
difficulty from solid radium in air; it experiences the same resistance
to propagation from solid radium in a liquid. When radium sulphate is
shaken with water for a whole day, its activity after the operation is
practically the same as that of a portion of the same sulphate left
exposed to air.

On placing the radium salt in a vacuum, all the emanation capable of
displacement is withdrawn. However, the radio-activity of a radium
chloride kept _in vacuo_ for six days was not sensibly affected by the
operation. This experiment shows that the radio-activity of the salt is
principally due to the radio-active energy generated within the
particles, and which is unaffected by the vacuum.

The loss of activity that radium undergoes when in solution is
relatively greater for the penetrating rays than for the absorbable
rays. The following are examples of this:—

A radium chloride which had reached its limit of activity, 470, was
dissolved and left in solution for one hour; it was then evaporated, and
its initial radio-activity determined by the electrical method. The
total initial radiation was found to be equal to the fraction 0·3 of the
total limiting radiation. If the determination of the intensity of
radiation be made by covering the active body with an aluminium screen
0·01 m.m. thick, the initial radiation which traverses this screen is
found to be only the fraction 0·17 of the limiting radiation traversing
the same screen.

When the salt has been thirteen days in solution, the total initial
radiation is found to be the fraction 0·22 of the total limiting
radiation, and is 0·13 of the limiting radiation after traversing 0·01
m.m. of aluminium.

In the two cases the ratio of the initial radiation after solution to
the limiting radiation is 1·7 times as great for the total radiation as
for the radiation which has traversed 0·01 m.m. of aluminium.

It must further be mentioned that on evaporating the product after
solution, it is impossible to avoid a certain period of time during
which the product is in an intermediate condition, neither entirely
solid nor entirely liquid. Neither can one avoid warming the product to
remove the water quickly.

For these two reasons it is scarcely possible to determine the true
initial activity of the product passing from solution to the solid
state. In the experiments just quoted, equal quantities of the active
bodies were dissolved in the same quantity of water, and the solutions
were then evaporated to dryness under conditions as identical as
possible, and without heating above 120° or 130°.

I investigated the law according to which the activity of a solid radium
salt increases, from the moment in which the salt is obtained dry after
solution to the moment in which it reaches its limit of activity. In the
tables which follow, the intensity of radiation, I, is represented as a
function of the time, the limiting intensity being supposed equal to
100, and the time being reckoned from the moment at which the product
was dried. Table I. (Fig. 12, Curve I.) refers to the total radiation.
Table II. (Fig. 12, Curve II.) refers only to the penetrating rays (rays
which have traversed 3 c.m. of air and 0·01 m.m. of aluminium).

                      TABLE I.           TABLE II.
                     Time.      I.      Time.       I.
                     Days.              Days.
                              0  21              0  1·3
                              1  25              1 19
                              3  44              3 43
                              5  60              6 60
                             10  78             15 70
                             19  93             23 86
                             33 100             46 94
                             67 100

[Illustration: FIG. 12.]

I made several other series of determinations of the same kind, but they
do not absolutely agree with one another, although the general character
of the curves obtained remains the same. It is difficult to obtain very
regular results. It may, however, be remarked that the acquisition of
activity requires more than one month for its production, and that the
most penetrating rays are the most deeply affected by solution.

The initial intensity of the radiation which is able to traverse 3 c.m.
of air and 0·01 m.m. of aluminium is only 1 per cent of the limiting
intensity, whilst the initial intensity of the total radiation is 21 per
cent of the total limiting radiation.

A radium salt which has been dissolved and recently evaporated to
dryness possesses the same power of causing induced activity (and,
consequently, of allowing the escape of an emanation), as a specimen of
the same salt which, after having been prepared in the solid state, has
remained in this condition long enough to have attained its limiting
radio-activity. The radiant activity of these two products is, however,
quite different; the former is, for example, five times less active than
the latter.


        _Variations of the Activity of Radium Salts on Heating._

When a radium compound is heated, it gives off an emanation and loses
activity. The more intense and the more prolonged the heating, the
greater is the loss of activity. Thus, on heating a radium salt for one
hour to 130°, it loses 10 per cent of its total radiation; on the other
hand, heating for ten minutes to 400° produces no apparent effect.
Heating to redness for several hours destroys 77 per cent of the total
radiation.

The loss of activity on heating is more considerable for the penetrating
than for the absorbable rays. Thus, heating for several hours destroys
about 77 per cent of the total radiation, but the same amount of heating
destroys nearly the whole (99 per cent) of the radiation that traverses
3 c.m. of air and 0·01 m.m. of aluminium. If barium-radium chloride be
kept fused for several hours (towards 800°) 98 per cent of the radiation
capable of traversing 0·3 m.m. of aluminium is destroyed. The
penetrating rays may be considered as no longer in existence after
intense and prolonged heating.

When a radium salt has lost part of its activity by heating, the
diminution is not lasting; the activity of the salt is spontaneously
regenerated at the ordinary temperature, and approaches a certain
limiting value. I have observed the curious fact that this limit is
higher than the limiting activity of the salt before being heated—this,
at least, is the case with the chloride. I give examples of this:—A
specimen of barium-radium chloride which, after having been prepared in
the solid state, has long since attained its limiting activity,
possesses a total radiation represented by the number 470, and a
radiation capable of traversing 0·01 m.m. of aluminium, represented by
the number 157. This, specimen is heated to redness for several hours.
Two months after the heating it attains its limit of activity with a
total radiation equal to 690, and a radiation through 0·01 m.m. of
aluminium, equal to 227. The total radiation and the radiation
transmitted by aluminium are therefore increased respectively in the
ratios 690/470 and 227/156. These two ratios are practically equal to
one another, and are equivalent to 1·45.

A specimen of radium-barium chloride which, after having been prepared
in the solid state, has attained a limiting activity of 62, is
maintained in a state of fusion for some hours; the fused product is
then powdered. The product regains a new limiting activity equal to 140,
which is twice as great as that to which it was able to attain when
prepared in the solid state without having been sensibly heated during
evaporation.

I have investigated the law of increase of activity of radium compounds
after heating. The following are the results of two series of
determinations:—The figures of Table I. and II. represent the intensity
of the radiation (I) as a function of time, the limiting intensity being
supposed equal to 100, and the time being reckoned from the close of the
heating. Table I. (Fig. 13, Curve I.) refers to the total radiation of a
specimen of barium-radium chloride. Table II. (Fig. 13, Curve II.)
relates to the penetrating radiation of a specimen of barium-radium
sulphate, the intensity of the radiation which traversed 3 c.m. of air
and 0·01 m.m. of aluminium having been determined. The two products were
subjected to a bright red heat for seven hours.

                     TABLE I.            TABLE II.
                   Time.       I.       Time.       I.
                   Days.                Days.
                0              16·2  0               0·8
                0·6            25·4  0·7            13
                1              27·4  1              18
                2              38    1·9            26·4
                3              46·3  6              46·2
                4              54   10              55·5
                6              67·5 14              64
               10              84   18              71·8
               24              95   27              81
               57             100   36              91
                                    50              95·5
                                    57              99
                                    84             100

I made several other series of determinations, but the results did not
agree well.

The effect of heating does not persist when the heated radium compound
is dissolved. Of two specimens of the same radium compound of activity
1800, one was strongly heated and its activity thereby reduced to 670.
The two portions being now dissolved and left in solution for twenty
hours, their initial activities in the solid state were 460 for the not
heated portion and 420 for the heated one; the difference between the
two portions was therefore not considerable. But if the two products do
not remain for a sufficient length of time in solution—if, for example,
they are evaporated to dryness, immediately after solution—the not
heated product is much more active than the heated one; a certain time
is necessary in the dissolved state for the effects of heating to
disappear. A product of activity 3200 was heated, and its activity
thereby reduced to 1030. This product and a similar portion which had
not been heated were simultaneously dissolved, and the two then
immediately evaporated to dryness. The initial activity was 1450 for the
not heated portion, and 760 for the heated one.

[Illustration: FIG. 13.]

In the case of solid radium salts the capacity for exciting induced
radio-activity is largely affected by heating. During heating radium
compounds give off a larger amount of emanation than at the ordinary
temperature; but on being cooled to the ordinary temperature, not only
is their radio-activity much less than before heating, but their
capacity for inducing activity is much diminished. During the time that
follows the heating, the radio-activity of the product increases, and
may even exceed the original value. The induction capacity is also
partially re-established; however, after prolonged heating to redness,
this capacity is almost entirely destroyed without spontaneous
re-appearance afterwards. The induction capacity may be restored to the
radium salt by dissolving it in water, and drying it in the oven at a
temperature of 120°. This seems to have the effect of leaving the salt
in a peculiar physical condition, in which the emanation is given off
with much less facility than is the case with the same solid product not
heated to a high temperature, and it follows naturally that the salt
attains a higher limit of activity than that which it possessed before
heating. To transform the salts into the physical condition proper to it
before heating, it suffices to dissolve it and to evaporate it to
dryness without heating it above 150°.

The following are numerical examples of the above:—

I represent by _a_ the limit of induced activity produced in a closed
vessel upon a plate of copper by a specimen of barium-radium carbonate
of activity 1600.

Suppose _a_ = 100 for the not heated product. We find—

                     1 day after heating  _a_ =  3·3
                     4 days after heating _a_ =  7·1
                    10 days after heating _a_ = 15
                    20 days after heating _a_ = 15
                    37 days after heating _a_ = 15

The radio-activity of the product had diminished 90 per cent by heating,
but one month afterwards the original value was regained.

The following is an experiment of the same kind made with a
barium-radium chloride of activity 3000. The induction capacity is
determined in the same manner as before.

For the product not heated _a_ = 100.

Induction capacity of the product after being heated to redness for
three hours:—

  2 days after heating                                               2·3
  5 days after heating                                               7·0
 11 days after heating                                               8·2
 18 days after heating                                               8·2
 Induction capacity of the unheated substance which has been
   dissolved and then dried at 150°                                 92
 Induction capacity of the heated substance which has been
   dissolved and then dried at 150°                                105


  _Theory of Interpretation of the Causes of Variations of Activity of
            Radium Salts after Solution and after Heating._

The facts previously indicated may be in part explained by the theory
according to which the energy of radium is produced in the form of an
emanation, which is then transformed into the energy of radiation. When
a radium salt is dissolved, the emanation produced by it spreads beyond
the solution, and causes radio-activity outside the source from which it
proceeds; when the solution is evaporated, the solid salt obtained is
but slightly active, because it contains only a small amount of
emanation. Gradually the emanation is accumulated in the salt, the
activity of which rises to a limiting value, which is reached when the
production of the emanation by the radium compensates the loss by
external emission and by local transformation into Becquerel rays.

When a radium salt is heated, the external emission of the emanation is
greatly increased, and the phenomena of induced radio-activity are more
intense than when the salt is at the ordinary temperature. But when the
salt returns to the ordinary temperature it is exhausted, as is the case
after being dissolved, and contains but a small amount of emanation, its
activity having become greatly reduced. Gradually the emanation
accumulates afresh in the solid salt, and the radiation increases.

It may be said that radium gives rise to a constant generation of any
emanation—part of which escapes to the exterior, the remainder being
transformed in the radium itself into Becquerel rays. When radium is
raised to a red heat, it loses the greater part of its capacity to cause
the induction of activity; otherwise stated, the evolution of the
emanation is lessened. Consequently, the proportion of the emanation
utilised in the radium itself should be greater, and the substance
attains a higher limit of radio-activity.

We will endeavour to establish theoretically the law of rise of activity
of a solid radium salt which has been dissolved or has been heated. We
will assume that the intensity of radiation of radium is, at each
instant, proportional to the quantity of emanation, _q_, present in the
radium. We know that the emanation is spontaneously destroyed according
to a law such that, at each instant—

             _q_ = _q__{0}_e_^{–_t_/θ} . . . . . . . . . 1,

_q__{0} being the amount of the emanation at the moment of starting the
observation, and θ the time constant, equal to 4·97 × 10^5 secs.

Now let Δ be the evolution of the emanation by radium, a quantity which
I will assume constant. Let us consider what would occur if no emanation
were escaping to the exterior. The emanation generated would then be
completely utilised by the radium for the production of the radiation.
We have from Formula 1—

             _dq_/_dt_ = –(_q__{0}/θ)_e_^{–_t_/θ} = –_q_/θ;

and consequently, in the state of equilibrium, the radium would contain
a certain quantity of emanation, Q, such that—

                          Δ = Q/θ . . . . . 2,

and the radiation of the radium would then be proportional to Q.

Let us suppose the radium placed in the circumstances under which it
gives off the emanation to the exterior; this is obtained by dissolving
the radium compound or by heating it. The equilibrium will be disturbed,
and the activity of the radium diminished. But as soon as the cause of
the loss of emanation has been abolished (the body being restored to the
solid state or the heating having ceased), the emanation is accumulated
afresh in the radium and we have a period during which the evolution, Δ,
surpasses the velocity of destruction, _q_/θ. We then have—

                  _dq_/_dt_ = Δ – _q_/θ = (Q – _q_)/θ,

from which—

              (_d_/_dt_)(Q – _q_) = –(Q – _q_)/θ,

                Q – _q_ = (Q – _q__{0})e^{–_t_/θ}      3,

_q__{0} being the amount of emanation present in the radium at time _t_
= 0.

According to Formula 3, the excess of the quantity of emanation, Q,
contained by the radium in a state of equilibrium above the quantity,
_q_, contained at a given moment, decreases as a function of the time
according to an exponential law, which is also the law of the
spontaneous disappearance of the emanation. The radiation of radium
being proportional to the amount of emanation, the excess of the
intensity of the limiting radiation above the actual intensity should
decrease as a function of the time by the same law; the excess should
thus diminish to one-half in about four days.

The preceding theory is incomplete, since the loss of emanation to the
exterior has been neglected. It is also difficult to determine the
manner in which this acts as a function of the time. In comparing the
results of experiment with those of this incomplete theory, there is
found to be no satisfactory agreement; the conviction is, however,
retained that the theory in question is partially true. The law by which
the excess of limiting activity above the actual activity diminishes to
one-half in four days represents approximately the course of the renewal
of activity after heating for ten days. In the case of the renewal of
activity after solution, the same law appears to hold approximately for
a certain period of time, which begins two or three days after
evaporation to dryness and continues for ten or fifteen days. The
phenomena are otherwise complex; the theory sketched out does not
explain the reason of the suppression of the penetrating rays in greater
proportion than the absorbable rays.


          NATURE AND CAUSE OF THE PHENOMENA OF RADIO-ACTIVITY.

From the beginning of research upon the radio-active bodies, and when
the properties of these bodies were yet hardly known, the spontaneity of
their radiation presented itself as a problem having the greatest
interest for physicists. To-day we have advanced considerably in the
understanding of radio-active bodies, and are able to isolate one of
very great power, viz., radium. With the object of making use of the
remarkable properties of radium, a profound investigation of the rays
emitted by radio-active bodies is indispensable; the various groups of
rays under investigation present points of similarity with the groups of
rays existing in Crookes tubes: cathode rays, Röntgen rays, canal rays.
The same groups of rays are found in the secondary radiation produced by
Röntgen rays, and in the radiation of bodies which have acquired
radio-activity by induction.

But if the nature of the radiation is actually better known, the cause
of this spontaneous radiation remains a mystery, and the phenomena
always presents itself to us as a profound and wonderful enigma.

The spontaneously radio-active bodies, and in the first place radium,
are sources of energy. The evolution of energy, to which they give rise,
is manifested by Becquerel radiation, by chemical and luminous effects,
and by the continuous generation of heat.

The question often arises as to whether energy is created within the
radio-active bodies themselves, or whether it is borrowed by them from
external sources. No one of the numerous hypotheses arising from these
two points of view has yet received experimental confirmation.

The radio-active energy may be assumed to have been initially
accumulated and then gradually dissipated, as happens in the case of
long continued phosphorescence. We imagine the evolution of radio-active
energy to correspond to a transformation of the nature of the atom of
the active body; the fact of the continuous generation of heat by radium
speaks in favour of this hypothesis. The transformation may be assumed
to be accompanied by a loss of weight and by an emission of material
particles constituting the radiation. The source of energy may yet be
sought in the energy of gravitation. Finally, we may imagine that space
is constantly traversed by radiations yet unknown, which are arrested in
their course by radio-active bodies and transformed into radio-active
energy.

Many reasons are adduced for and against these different views, and most
often attempts at experimental verifications of the conclusions drawn
from these hypotheses have given negative results. The radio-active
energy of uranium and radium apparently neither becomes exhausted nor
varies appreciably with lapse of time. Demarçay examined
spectroscopically a specimen of pure radium chloride after a five
months’ interval, and observed no change in the spectrum. The principal
barium line, which was visible in the spectrum indicating the presence
of a trace of barium, had not increased in intensity during the
interval, showing therefore that there was no transformation of radium
into barium to an appreciable extent.

The variations of weight announced by M. Heydweiller in radium compounds
cannot yet be looked upon as established facts.

Elster and Geitel found that the radio-activity of uranium is not
affected at the bottom of a mine-shaft 850 m. deep; a layer of earth of
this thickness would therefore not affect the hypothetical primary
radiation which would be excited by the radio-activity of uranium.

We have determined the radio-activity of uranium at midday and at
midnight, thinking that if the hypothetical primary radiation had its
origin in the sun it would be partly absorbed in traversing the earth.
The experiment showed no difference in the two determinations.


                             _Conclusions._

I will define, in conclusion, the part I have personally taken in the
researches upon radio-active bodies.

I have investigated the radio-activity of uranium compounds. I have
examined other bodies for the existence of radio-activity, and found the
property to be possessed by thorium compounds. I have made clear the
atomic character of the radio-activity of the compounds of uranium and
thorium.

I have conducted a research upon radio-active substances other than
uranium and thorium. To this end I investigated a large number of
substances by an accurate electrometric method, and I discovered that
certain minerals possess activity which is not to be accounted for by
their content of uranium and thorium.

From this I concluded that these minerals must contain a radio-active
body different from uranium and thorium, and more strongly radio-active
than the latter metals.

In conjunction with M. Curie, and subsequently with MM. Curie and
Bémont, I was able to extract from pitchblende two strongly radio-active
bodies—polonium and radium.

I have been continuously engaged upon the chemical examination and
preparation of these substances. I effected the fractionations necessary
to the concentration of radium, and I succeeded in isolating pure radium
chloride. Concurrently with this work, I made several atomic weight
determinations with a very small quantity of material, and was finally
able to determine the atomic weight of radium with a very fair degree of
accuracy. The work has proved _that radium is a new chemical element_.
Thus the new method of investigating new chemical elements, established
by M. Curie and myself, based upon radio-activity, is fully justified.

I have investigated the law of absorption of polonium rays, and of the
absorbable rays of radium, and have demonstrated that this law of
absorption is peculiar and different from the known laws of other
radiations.

I have investigated the variation of activity of radium salts, the
effect of solution and of heating, and the renewal of activity with
time, after solution or after heating.

In conjunction with M. Curie, I have examined different effects produced
by the new radio-active substances (electric, photographic, fluorescent,
luminous colourations, &c.).

In conjunction with M. Curie, I have established the fact that radium
gives rise to rays charged with negative electricity.

Our researches upon the new radio-active bodies have given rise to a
scientific movement, and have been the starting-point of numerous
researches in connection with new radio-active substances, and with the
investigation of the radiation of the known radio-active bodies.

------------------------------------------------------------------------


                                 RADIUM
                               AND OTHER
                         RADIO-ACTIVE ELEMENTS.

  A Popular account of Radium and other Radio-active Elements treated
                            experimentally.

   Including a number of Radium Photographs and other Illustrations.

               By LEONARD A. LEVY and HERBERT G. WILLIS.

                  PRICE 2s. 6d. net; post free 2s. 9d.


                               CONTENTS.

INTRODUCTION.—Chap. I. The Discovery of Radium; its Extraction.—Chap.
II. The Radium Emanation.—Chap. III. Physical Effects (Electrical and
Magnetic).—Chap. IV. Physical Effects (continued).—Chap. V. Chemical
Effects.—Chap. VI. Actions on Animal Life and Living Tissues.—Chap. VII.
Other Radio-active Elements.—Appendix.—Experimental Apparatus.

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                  *       *       *       *       *

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                          TRANSCRIBER’S NOTES


 1. Silently corrected typographical errors and variations in spelling.
 2. Anachronistic, non-standard, and uncertain spellings retained as
      printed.
 3. Enclosed italics font in _underscores_.
 4. Enclosed bold font in =equals=.
 5. Superscripts are denoted by a caret before a single superscript
      character or a series of superscripted characters enclosed in
      curly braces, e.g. M^r. or M^{ister}.
 6. Subscripts are denoted by an underscore before a series of
      subscripted characters enclosed in curly braces, e.g. H_{2}O.





End of Project Gutenberg's Radio-Active Substances, by Sklodowska Curie