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THE GASES OF THE ATMOSPHERE

[Illustration]

[Illustration: STEPHEN HALES.]




                               THE GASES
                                  OF
                            THE ATMOSPHERE

                                  THE
                      HISTORY OF THEIR DISCOVERY


                                  BY
                        WILLIAM RAMSAY, F.R.S.
         PROFESSOR OF CHEMISTRY IN UNIVERSITY COLLEGE, LONDON


                            WITH PORTRAITS


                                London

                        MACMILLAN AND CO., LTD.
                      NEW YORK: THE MACMILLAN CO.
                                 1896


                         _All rights reserved_

        “Modern discoveries have not been made by large
      collections of facts, with subsequent discussion,
      separation, and resulting deduction of a truth thus
      rendered perceptible. A few facts have suggested an
      _hypothesis_, which means a _supposition_ proper
      to explain them. The necessary results of this
      supposition are worked out, and then, and not till
      then, other facts are examined to see if their
      ulterior results are found in Nature.”
      --DE MORGAN, _A Budget of Paradoxes_, ed. 1872, p. 55.




PREFACE


The discovery of new elementary gas in the atmosphere in 1894 aroused
much interest, and public attention has again been directed to the
air, which was, for many centuries, a fruitful field for speculation
and conjecture. The account of this discovery, communicated to the
Royal Society in January 1895, was, however, necessarily couched in
scientific language; and many matters of interest to the chemist and
physicist were written in an abbreviated style, in the knowledge that
the passages describing them would be easily understood by the experts
to whom the communication was primarily addressed. But persons without
any special scientific training have frequently expressed to me the
hope that an account of the discovery would be published, in which the
conclusions drawn from the physical behaviour of argon should be
accompanied by a full account of the reasoning on which they are based.
An endeavour to fulfil this request is to be found in the following
pages. And as the history of the discovery of the better known
constituents of the atmosphere is of itself of great interest, and
leads up to an acquaintance with the new stranger, who has so long been
with us incognito, an effort has here been made to tell the tale of the
air in popular language.




               CONTENTS

               CHAPTER I                                  PAGE
  THE EXPERIMENTS AND SPECULATIONS OF BOYLE, MAYOW,
       AND HALES                                            1

               CHAPTER II
  “FIXED AIR” AND “MEPHITIC AIR”--THEIR DISCOVERY
       BY BLACK AND BY RUTHERFORD                          38

               CHAPTER III
  THE DISCOVERY OF “DEPHLOGISTICATED AIR” BY
       PRIESTLEY AND BY SCHEELE--THE OVERTHROW
       OF THE PHLOGISTIC THEORY BY LAVOISIER               68

               CHAPTER IV
  “PHLOGISTICATED AIR” INVESTIGATED BY CAVENDISH--HIS
       DISCOVERY OF THE COMPOSITION OF WATER              119

               CHAPTER V
  THE DISCOVERY OF ARGON                                  146

               CHAPTER VI
  THE PROPERTIES OF ARGON                                 181

               CHAPTER VII
  THE POSITION OF ARGON AMONG THE ELEMENTS                216




                LIST OF PORTRAITS

  STEPHEN HALES                    _Frontispiece_

  ROBERT BOYLE             _To face page_      8

  JOHN MAYOW                     "            17

  JOSEPH BLACK                   "            48

  DANIEL RUTHERFORD              "            62

  JOSEPH PRIESTLEY               "            72

  ANTOINE AUGUSTE LAVOISIER      "           102

  HON. HENRY CAVENDISH           "           121




CHAPTER I

THE EXPERIMENTS AND SPECULATIONS OF BOYLE, MAYOW, AND HALES


To tell the story of the development of men’s ideas regarding the
nature of atmospheric air is in great part to write a history of
chemistry and physics. This history is an attractive and varied one:
in its early stages it was expressed in the quaint terms of ancient
mythology, while in its later developments it illustrates the advantage
of careful experimental inquiry. The human mind is apt to reason from
insufficient premisses; and we meet with many instances of incorrect
conclusions, based upon experiment, it is true, but upon experiment
inadequate to support their burden. Further research has often proved
the reasoning of the Schoolmen to be futile; not indeed from want of
logical method, but because important premisses had been overlooked.

Among the errors which misled the older speculators, three stand out
conspicuously. These are--

First, _The confusion of one gas with another_. Since gases are for the
most part colourless, and always transparent, they make less impression
on the senses than liquids or solids do. It was difficult to believe in
the substantiality of bodies which could not be seen, but the existence
of which had to be inferred from the testimony of other senses; indeed,
in certain instances only by the sense of touch, for many gases
possess neither smell nor taste. This peculiarity led, in past ages,
to the notion that air possessed a semi-spiritual nature; that its
substantiality was less than that of other objects more accessible to
our senses. We meet with a relic of this view in words still in common
use. Thus the Greek words =πνέω=, I blow, and =πνεῦμα=, a spirit or
ghost, are closely connected; in Latin we have _spiro_, I breathe, and
_spiritus_, the human spirit; in English, the words _ghost_ and _gust_
are cognate. And the same connection can be traced in similar words in
many other languages.

Our sense of smell is affected by extremely minute traces of gases
and vapours--traces so small as to be unrecognisable by any other
method of perception, direct or indirect. A piece of musk retains
its fragrant odour for years, and the most delicate balance fails to
detect any appreciable loss of weight in it. We are capable of smelling
gases only: liquids and solids, if introduced into the nostrils,
irritate the olfactory nerves, but do not stimulate them so as to
incite the sense of smell; yet the admixture of a minute trace of some
odorous vapour with air appears entirely to change its properties.
The effect of inhaling such air, although sometimes pleasant, is very
different from the sensation produced by pure inodorous air, and such
admixtures were in olden times naturally taken to be air modified in
its properties. But such modifications are obviously almost infinite
in number, for varieties of scent are excessively numerous; and it
was therefore perhaps deemed useless to attempt to investigate such a
substance as air, whose properties could change in so inexplicable and
mysterious a manner. Owing, therefore, to its elusive and, as it were,
semi-spiritual properties, and to its unexpected changes of character,
it was long before its true nature was discovered. It had not escaped
observation that “air” obtained by distilling animal and vegetable
matter, or by the action of acids on iron and zinc, differed from
ordinary air by being inflammable; but such “airs” were regarded as
atmospheric air, modified in some manner, as it is modified when
perfumed. And “airs” escaping from fermenting liquids, or produced by
the action of acids on carbonates, were neglected. For long no attempt
was made to catch them; and the frothing and bubbling were regarded
as a species of boiling, as is still seen in the use of our word
“fermentation” (_fervere_, to boil).

Second, _Erroneous ideas regarding the phenomena of combustion_. While
it was recognised that a burning candle was extinguished if placed in
a confined space, its extinction was attributed not to the absence of
air, but to the impossibility of the escape of flame. Indeed, flame was
regarded as possessing the same semi-spiritual, semi-material nature as
air. Together with earth and water, air and flame or fire formed the
four elementary principles of the Ancients; and all substances--stones,
metals, animals, and vegetables--were regarded as partaking of the
properties of these elements, and often as being constituted of the
latter in varying proportions, according as they were cold and dry
(earth), cold and moist (water), hot and moist (air), or hot and dry
(fire). It is not within the scope of this book to enter into details
regarding such ancient views. Those who are interested in the matter
will find them expounded in Kopps’ _History of Chemistry_, Rodwell’s
_Dawn of Chemistry_, E. von Mayer’s _History of Chemistry_, and in
other similar works. But we shall be obliged to consider the later
developments of such ideas in the _phlogistic theory_, by means of
which all chemical changes connected with combustion were interpreted
from the latter part of the seventeenth to the end of the eighteenth
century. With erroneous views regarding the nature of combustion, and
ignorance as to the part played by the atmosphere in the phenomena of
burning, the true nature of air was undiscoverable.

Third, _The lack of attention to gain or loss of weight_. It was in
past times not recognised that nothing could be created and nothing
destroyed. In popular language, a candle is destroyed when it is
burned, nothing visible being produced from it. The products, we now
know, are gaseous and invisible, and possessed of greater weight than
the unburnt candle; but for want of careful experiment, it was formerly
concluded that the candle, when burnt, was annihilated. The formation
of a cloud in a cloudless sky; the growth of vegetables in earth,
from which, apparently, they did not derive their substance; and the
reputed growth of metalliferous lodes in mines--these were all adduced
as proofs of the creative power of Nature. With such ideas, therefore,
the necessity of controlling the gain or loss of material during
experiment, by determining gain or loss of weight, did not appear
imperative; and hence but few quantitative experiments were made, and
little importance was attached to these few. It had, for example,
long been noticed that certain metals gained weight when burned and
converted into a “calx,” or, as we should now say, a metallic oxide,
but such gain in weight was not regarded as of any consequence. When
considered in relation to the supposed loss of “phlogiston” suffered
by a metal on being converted into a calx, it was explained by the
hypothesis that phlogiston possessed “levity”--the antithesis of
gravity--and that the calx weighed more than the metal, owing to its
having lost a principle which was repelled instead of being attracted
by the earth.

Among the most remarkable early attempts to elucidate the true nature
of air, we meet with one by the Hon. Robert Boyle, who published about
the middle of the seventeenth century his _Memoirs for a General
History of the Air_. Boyle was one of the most distinguished scientific
men of his own, or indeed of any, age, and in his spirit of calm
philosophical inquiry he was far in advance of his contemporaries.
He was born in the early part of the year 1626, in Ireland, whither
his father, Richard Boyle, had emigrated at the age of twenty-two.
Boyle’s mother, daughter of Sir Geoffrey Fenton, principal Secretary
of State for Ireland, died while he was still a child. Yet she must
have lived in the recollection of her son Robert, for he wrote: “To
be such parents’ son, and not their eldest, was a happiness that
our Philarethes (himself) would mention with great expressions of
gratitude; his birth so suiting his inclinations and designs, that had
he been permitted an election, his choice would scarce have altered
God’s discernment.”

In those days of early development, Boyle had finished his school-days
at Eton by his twelfth year. He informs us that he devoured books
omnivorously, and could hardly be induced to join in games. The next
six years of his life he spent on the Continent with his elder brother;
and on his father’s death, which happened when he was abroad, he
returned to England, and settled at Stalbridge, in Dorsetshire, where
he had inherited a manor. Here he passed most of his life in great
retirement, with only an occasional visit to London; for though he
lived through troublous times, he avoided politics. Indeed, he is
known only to have appeared once on a public platform, and that was in
defence of the Royal Society, then in its infancy, from attacks made
upon it by some too scrupulously loyal Churchmen.

[Illustration: ROBERT BOYLE.]

Boyle did not confine his attention exclusively to scientific pursuits:
he interested himself deeply in theology, and published numerous tracts
on religious subjects. He wrote with equal ease in English, French,
and Latin, and his books appeared simultaneously in the first and last
of these languages. His researches are remarkable for their wide range
and for the boldness of his conceptions. But Boyle, ingenious though
he was, was unable to fathom the mystery of atmospheric air. His views
regarding it are succinctly stated by him in his _Memoirs for a General
History of the Air_, and in the same work he sums up the views of the
Ancients. His words are:

       “The Schools teach the air to be a warm and moist
     element, and consequently a simple and homogeneous body.
     Many modern philosophers have, indeed, justly given up
     this elementary purity in the air, yet few seem to think
     it a body so greatly compounded as it really appears to
     be. The atmosphere, they allow, is not absolutely pure,
     but with them it differs from true and simple air only as
     turbid water from clear. Our atmosphere, in my opinion,
     consists not wholly of purer aether, or subtile matter
     which is diffused thro’ the universe, but in great number
     of numberless exhalations of the terraqueous globe; and the
     various materials that go to compose it, with perhaps some
     substantial emanations from the celestial bodies, make up
     together, not a bare indetermined feculancy, but a confused
     aggregate of different effluvia. One principal sort of
     these effluvia in the atmosphere I take to be saline, which
     float variously among the rest in that vast ocean; for they
     seem not to be equally mixed therein, but are to be found
     of different kinds, in different quantities and places,
     in different seasons.... Many men talk much of a volatile
     nitre in the air, as the only salt wherewith that fluid is
     impregnated. I must own the air, in many places, seems to
     abound in corpuscles of a nitrous nature; but I don’t find
     it proved by experiments to possess a volatile nitre. In
     all my experiments upon salt-peter, I found it difficult
     to raise that salt by a gentle heat; and spirits of nitre,
     which is drawn by means of a vehement one, has quite
     different properties from crude nitre, or the supposed
     volatile kind in the air, for ’tis exceeding corrosive.”[1]

Boyle then proceeds to collect and comment on the effluvia from
volcanoes and from decaying vegetables and animals, and proposes tests
for the presence of such ingredients. He even attributes the darkening
of silver chloride to its being a reagent for certain salts present
in air at one time and not at another, and draws attention to the
sulphurous smell produced by “thunder.” Farther on (p. 61) he writes:

“The generality of men are so accustomed to judge of things by their
senses, that because the air is invisible they ascribe but little to
it, and think it but one remove from nothing. And this fluid is even
by the Schoolmen considered only as a receptacle of visible bodies,
without exerting any action on them unless by its manifest qualities,
heat and moisture; tho’, for my part, I allow it other faculties,
and among them, such as are generative, maturative, and corruptive;
and that, too, in respect not only of animals and bodies of a light
texture, but even of salts and minerals.”

In another place (p. 17) he states:

       “I conjecture that the atmospherical air consists of
     three different kinds of corpuscles: the first, those
     numberless particles which, in the form of vapours or
     dry exhalations, ascend from the earth, water, minerals,
     vegetables, animals, etc.; in a word, whatever substances
     are elevated by the celestial or subterraneal heat, and
     thence diffused into the atmosphere. The second may be
     yet more subtile, and consist of those exceedingly minute
     atoms the magnetical effluvia of the earth, with other
     innumerable particles sent out from the bodies of the
     celestial luminaries, and causing, by their impulse, the
     idea of light in us. The third sort is its characteristic
     and essential property, I mean permanently elastic parts.”

Boyle also relates experiments designed to “produce what appears to be
air”; and he describes the production, by the action of oil-of-vitriol
on steel filings, of “air” (now known as hydrogen) which possessed
the property of elasticity; although he failed to notice its
inflammability. He further obtained carbon dioxide by the fermentation
of raisins, and probably also hydrogen chloride in the gaseous form
by breaking a bulb containing “some good spirit-of-salt” in a vacuous
receiver.

The result of shrewd reasoning power, applied, however, to imperfect
observations, is well illustrated by the following passages:

       “For tho’, by reason of its great thinness and of its
     being, in its usual state, devoid both of taste and smell,
     air seems wholly unfit to be a menstruum [or solvent]; yet
     it may have a dissolving, or at least a consuming, power
     on many bodies, especially such as are peculiarly disposed
     to admit its operations. For the air has a great advantage
     by the vast quantity of it that may come to work, in
     proportion to the bodies exposed thereto.... Thus we find a
     rust on copper that has been long exposed to the air.”[2]

Boyle, shortly after, describes the production of “an efflorescence of
a vitriolic nature” on marcasite (or sulphide of iron) which has been
exposed to the air; and he relates that the “ore of alum, robb’d of its
salt, will in tract of time recover it by being exposed to the air, as
we are assured by the experienced _Agricola_.”

To account for such actions, and for combustion, he proceeds (p. 81):

       “The difficulty we find in keeping flame and fire
     alive, tho’ but for a little time, without air, renders
     it suspicious that there may be dispersed thro’ the rest
     of the atmosphere some odd substance, either of a solar,
     astral, or other foreign nature; on account whereof the
     air is so necessary to the subsistance of flame.... It
     also seems by the sudden wasting or spoiling of this
     fine substance, whatever it be, that the bulk of it is
     but very small in proportion to the air it impregnates
     with its vertue; for after the extinction of the flame,
     the air in the receiver was not visibly alter’d; and for
     ought I could perceive by several ways of judging, the air
     retained either all, or at least the far greatest part, of
     its elasticity; which I take to be its most genuine and
     distinguishing property. And this undestroyed springyness
     of the air, with the necessity of fresh air to the life of
     hot animals, suggest a great suspicion of some vital
     substance, if I may so call it, diffused thro’ the air;
     whether it be a volatile nitre, or rather some anonymous
     substance, sidereal or subterraneal; tho’ not improbably of
     kin to that which seems so necessary to the maintenance of
     the other flames.”

The experimental part of Boyle’s work relates to the oxidation of
cuprous to cupric compounds, with the change of colour from brown to
blue or green, either in ammoniacal or in hydrochloric acid solution;
and he goes so far as to prove that two ounces of marcasites broken
into small lumps, and kept in a room “freely accessible to the air,
which was esteemed to be very pure,” for somewhat less than seven
weeks, gained above twelve grains by oxidation.

In his _Memoirs for a General History of the Air_, Boyle draws up a
programme of research, of the carrying out of which, however, there is
no record. He proposes (p. 23):

  “1.  To produce air by fermentation in well clos’d receivers.

      “To produce air by fermentation in sealed glasses.

      “To separate air from liquors by boiling.

      “To separate air from liquors by the air-pump.

      “To produce air by corrosion, especially with spirit of vinegar.

      “To separate air by animal and sulphureous dissolvants.

      “To obtain air in an exhausted receiver by burning-glasses
       and red-hot irons.

      “To produce air out of gunpowder and other nitrous bodies.

  “2.  To examine the produced aerial substances by their preserving
       or reviving animals, flame, fire, the light of rotten wood,
       and of fish.

      “To examine it by its elasticity, and the duration thereof.

      “To do the same by its weight, and its elevating the fumes
       of liquors.”

We shall all agree that if Boyle had successfully carried out such
experiments, our knowledge of the true nature of air would have come
quite a century before it did. Some of these experiments were indeed
made by John Mayow, his contemporary, whose work and speculations we
shall now proceed to consider.

John Mayow was born in the parish of St. Dunstan, London, in 1645. His
family was originally Cornish, having come from Bree, in Cornwall. He
entered Wadham College, Oxford, at the early age of sixteen, and was
shortly afterwards made a probationer-fellow of All Souls’ College.
After the usual three years of study, he took his degree in Law; but
not being attracted by the legal profession, he turned his attention
to medicine, and became a medical practitioner at Bath, where he lived
during the fashionable season. When not more than twenty-three years of
age, he wrote two essays on Respiration, ascribing the inflation of the
lungs to the action of the intercostal muscles. These “Tractatus duo”
were published in 1668. Some years later he produced the treatise on
which his fame rests; it is entitled “Tractatus quinque medico-physici,
quorum primus agit de sal-nitro et spiritu nitro-aëreo; secundus, de
respiratione; tertius, de respiratione foetus in utero et ovo; quartus,
de motu musculari, et spiritibus animalibus; ultimus, de rhachitide;
studio Joh. Mayow, LL.D. & Medici, nec non Coll. Omn. Anim. in Univ.
Oxon. Socii. Oxonii e Theatro Sheldoniano, An. Dom. MDCLXXIV.”
The work was dedicated to Sir Henry Coventry. It was inserted in
an abridged form in the _Philosophical Transactions_ of the Royal
Society, some time after its publication, but received only scant
recognition, for the fame of Newton and Boyle overshadowed the
labours of less well-known investigators. And Mayow did not live to
press his discoveries on the attention of his contemporaries, for he
died in 1679, five years after the publication of his tracts, in his
thirty-fourth year. Little is known of Mayow’s domestic life, save
that he married shortly before his death. His scientific work proves
that if he had been granted the usual span of life, his extraordinary
genius would have furthered the knowledge of the true explanation
of the nature of air, and its function in supporting combustion and
respiration, and that his views would have been accepted more than
a century before Lavoisier--with fuller knowledge, and with the
scientific position which at once gained a hearing--forced precisely
similar doctrines upon the attention of the scientific world.

[Illustration: John Mayow.]

Mayow was a contemporary of Boyle, and frequently made use of Boyle’s
experiments in support of the theories which he advanced. Curiously
enough, while Boyle seems to have read Mayow’s work, he does not appear
to have been favourably impressed by his conclusions. Boyle, at the age
of fifty-two, had doubtless formed his own opinions, and was unwilling
that they should be disturbed by the speculations, well founded though
they were, of so young a man. And shortly after Mayow’s death, the
views of Becher, one of his contemporaries, expounded and made definite
by Stahl, regarding the nature of combustion, were universally received.

After Lavoisier’s theories had overthrown these false views, attention
was again directed to Mayow’s tracts by Johann Andreas Scherer, in
a work published at Vienna in 1793, and also by Dr. Yeats in 1798.
Scherer gives a careful analysis of Mayow’s work, somewhat altering the
order of his paragraphs, with a paraphrase in German of the Latin text,
which he quotes in full. Yeats’ treatise is more especially concerned
with the medical aspect of Mayow’s work, although it also deals with
the purely chemical portion at considerable length. In the following
account of Mayow’s researches, free use has been made of both of these
works, as well as of his own “Tracts.”

Mayow’s contributions to the chemistry of the atmosphere may be
classified thus:--

1. The atmosphere consists of particles of two kinds of gases at least:
one of these, termed “nitro-aerial particles,” is necessary for the
support of life and for the combustion of inflammable bodies; while the
other, left after this constituent has been removed, is incapable of
supporting either life or combustion. The portion which is necessary
for life enters, during respiration, into the blood. It is the chief
cause of motion in animals and in plants.

2. These “nitro-aerial particles” are also present in saltpetre or
nitre, as can be shown by mixing inflammable substances, such as
sulphur and charcoal, with nitre to form gunpowder, filling a tube
with the powder, and, after setting it on fire, immediately plunging
the open end of the tube under water. The sulphur and charcoal will be
as completely consumed as if burned in the open air. Such combustion
might, however, be ascribed to a “sulphureous” constituent in
saltpetre; by “sulphureous” is to be understood combustible, for those
substances capable of burning were imagined to contain a “sulphur”
which gave them that property. That nitre does not contain such
“sulphur” can be shown by exposing it alone to heat, when no change
takes place, except fusion. Besides, nitre is compounded of “spirit
of nitre” or nitric acid and pure alkali, neither of which contains a
combustible sulphur; hence the particles of fire-air must be present in
nitre in no small amount. But it is probable that it is the spirit of
nitre which contains such fire-air particles, because, as will be shown
later, they are not present in the alkali.

One difficulty occurs to Mayow. How is it that so large a quantity
of gas as is necessary to support combustion can be contained in a
relatively small bulk of saltpetre? He tries whether a solution of
saltpetre evolves air-bubbles when placed in a vacuum, and finds that
it effervesces less than pure water does. He also prepares saltpetre
by mixing nitric acid and alkali in a vacuum; a brisk effervescence
occurs, and the dried-up salt is ordinary saltpetre. Hence saltpetre
cannot contain elastic air. Mayow consequently draws a distinction
between “air” and “air-particles.”

The residue left after the “fire-air,” or _spiritus igneo-aerius_, has
been removed from ordinary air by breathing or by combustion is proved
to be lighter than the fire-air itself; because a mouse dies sooner if
kept at the top of air in a confined bell-jar than at the bottom; and
a candle goes out sooner. Here the conclusion is right, although the
reason given is wrong; for it is the temperature of the respired air
which makes it rise, and not the fact that it is specifically lighter
than the oxygen.

Metallic antimony gains in weight when it is set on fire by a lens,
and burns; if this gain in weight, Mayow remarks, is not due to the
absorption of nitro-aerial particles and to the fire, it is difficult
to say to what it is due.

The reason why substances burn so violently in nitre compared with air,
is because of the proximity of the fire-air particles; and these are
evidently due to the nitric acid, because the residue--the alkali--if
mixed with sulphur and inflamed, does not produce ignition.

3. All acids contain fire-air particles, for acids have great
similarity to each other. This is shown as follows:--Antimony made into
a calx by the sun’s rays with a burning-glass gives the same calx as
when it is evaporated repeatedly with nitric acid and converted into
“Bezoar-mineral,” _i.e._ oxide of antimony. And iron-rust obtained from
sulphide of iron appears to be formed by the union of the fire-air
particles with the metallic “sulphur” of the iron.

It has up till now been believed that sulphuric acid is an ingredient
of common sulphur. But this is unlikely, for sulphur has a sweetish,
and not an acid taste. Moreover, quite a different substance from
a vitriol (or sulphate) is obtained by melting together alkali and
sulphur; and no effervescence takes place during its preparation.
Sulphur, too, is precipitated out of the “liver of sulphur” (potassium
persulphide) by the addition of sulphuric acid. Now, were sulphuric
acid contained in sulphur, it would hinder the union of the sulphur
with the alkali.

It is to be noticed that the volatile sulphuric acid, from the
combustion of sulphur, is produced in the following way:--“The flame
of the burning sulphur consists, like every other flame, in the
violent motion of the sulphur particles with that of the nitro-aerial
particles; hence the sulphur particles, at first solid, become sharp
and acid, and probably form the ordinary ‘spirit of sulphur’ (sulphuric
acid). If this be not so, I know not in what manner this acid can be
produced; for, as has been shown, it is very improbable that it
previously existed in the mass of the sulphur before its deflagration.
Such a change also, in all probability, takes place in pyrites, when
it is converted to green vitriol; because pyrites yields sulphur on
distillation; and the green vitriol on distillation gives sulphuric
acid, leaving red colcothar (iron oxide) behind.”

Similarly, nitre appears to be a triple salt, formed by the union of
the fiery part of air with a salt-like substance existing in the
earthy material, together forming nitric acid; and this added to earthy
salts (alkali) yields ordinary nitre. “I have tried to show that
all acids consist of certain saline particles rendered fluid by the
nitro-aerial particles.”

4. Boyle has shown that a flame is extinguished more rapidly in
a vacuous space than in a confined space containing air; this is
obviously due to absence of nourishment in the air, rather than to
its choking by its own vapours; for in the vacuous vessel there is
evidently more space for such noxious vapours than in the air-filled
vessel, and yet the flame is more rapidly extinguished. Moreover, no
combustible matter can be kindled in a vacuum by means of a
burning-glass. But it must not be concluded that this fire-air
constitutes the whole of ordinary air; because a candle goes out in air
confined in a glass while a large quantity of air is still contained in
it.

While gunpowder burns owing to the fire-air particles which it
contains, and requires no sustenance from external air, the combustion
of vegetables is supported partly by the igno-aerial particles which
they themselves contain, partly by those of the external air.

Air which has supported combustion loses to some extent its elasticity
(_i.e._ diminishes in volume), as shown by the burning of a candle in
air confined over water. This is to be ascribed partly to actual loss
of elasticity, partly to the absorption of the fire-air. The loss of
volume amounts to about three per cent of the whole quantity of air
taken.

All this is exceedingly clear, and in accordance with our modern views,
but Mayow’s mind is somewhat confused with reference to flame and heat,
since he imagined that the diminution of the volume of air in which
combustible substances have been burned is due to the escape of heat;
and inasmuch as a rise of temperature was known to increase the volume
of air, so a loss of heat should, in his opinion, produce the opposite
effect. The fire-air particles are apparently regarded as a sort of
compound of heat with matter (as indeed in a certain sense they are);
and by combustion or by respiration both are removed. The loss of
volume is to be explained by the removal of both from the air, and the
gain in weight by the union of the matter with the combustible body,
such as antimony.

Such is a brief account of Mayow’s views on the nature of atmospheric
air. But the tale would be incomplete without mention of the fact that
he prepared a gas by the action of nitric acid on iron, viz. nitric
oxide, which, when introduced into ordinary air confined over water,
decreased its volume; and he found that further admission of nitric
oxide produced no further diminution in the volume of the air. A very
little more, and he would have recognised in this a means of analysing
air, and depriving it wholly of its oxygen. He goes so far as to
speculate that a compound is formed between the nitric oxide and the
oxygen, but the solubility of gases in water appears not to have struck
him as important. He notices, however, that the combination of the two
gases is attended by rise of temperature, and is in so far analogous to
combustion.

It would lead us too far to consider in detail Mayow’s theories of
fermentation and of respiration. Suffice it to say that he ascribes the
production of animal heat to the consumption of his fire-air particles
by the animal, and remarks that the pulse is heightened by respiration.
This view was in opposition to that held by his contemporaries, viz.
that the purpose of respiration was to cool the blood.

It is impossible to avoid being impressed with the clearness and
justice of Mayow’s inferential reasoning. All that was wanting was the
discovery of oxygen and carbon dioxide, and the identification of the
first with his fire-air, and of the second with one of the products
of combustion. But these discoveries were not made until a century
after his death. Had he lived, there can be little doubt that, unless
discouraged by the want of appreciation with which his ideas were
received, he would have continued to labour in the fruitful fields from
which he had already reaped so rich a harvest.

Before leaving the seventeenth century, it is perhaps fitting to
mention the name of Jean Rey, a French physician, who wrote in 1630
concerning the gain in weight of tin and lead when calcined. While Rey
exhibited some leaning towards the modern methods of experimentation,
he still lay fettered in the bonds of mediæval scholasticism. In
discussing the weight of air and fire, he finds occasion to consider
the question whether a vacuum can exist. His words are so quaint that
they are worth quoting: “It is quite certain that in the bounds of
Nature a vacuum, which is nothing, can find no place. There is no power
in Nature from which nothing could have made the universe, and none
which could reduce the universe to nothing: that requires the same
virtue. Now the matter would be otherwise if there could be a vacuum.
For if it could be here, it could also be there; and being here and
there, why not elsewhere? and why not everywhere? Thus the universe
could reach annihilation by its own forces; but to Him alone who could
make it is the glory of being able to compass its destruction.” And
since air cannot be drawn down by a vacuum, it must descend by virtue
of its own weight when it fills a hole. And hence, as air has weight,
tin and lead gain in weight when they combine with air. It will be
admitted that this is very inferior to the speculations and deductions
of Boyle and Mayow.

The next stage in the history of our subject is the consideration
of the work of Stephen Hales and of Joseph Priestley. Both of these
philosophers were essentially experimentalists. While both discovered
gases and prepared them in a more or less pure state, Hales had no
theory to guide him, and concluded as the result of his researches
that air was possessed of “a chaotic nature”; for he did not recognise
his gases as different kinds of matter, but supposed them all to be
modified air. Priestley, on the other hand, was an adherent of the
theory of phlogiston, and interpreted all his experiments by its help.
Hales was a country clergyman, interested in botany, and undertook
researches on air in order to gain knowledge of the growth and
development of plants. Priestley was also a divine, who amused himself
with experiments during the intervals of composing sermons or writing
controversial pamphlets on disputed doctrines. Both possessed the
experimental faculty, and both employed it to good purpose.

Hales’ chief work is entitled “Statical Essays, containing Vegetable
Staticks; or an account of Statical Experiments on the Sap in
Vegetables, being an Essay towards a Natural History of Vegetation:
of use to those who are curious in the Culture and Improvement of
Gardening, etc.: Also, a specimen of an attempt to analyse the air by a
great Variety of Chymiostatical Experiments, which were read at several
meetings before the Royal Society. By Stephen Hales, D.D., F.R.S.,
Rector of Farringdon, Hampshire, and Minister of Teddington, Middlesex.”

In his “Introduction” Hales reveals his method of research. The
determination of weight and volume was at that date especially
necessary; for want of numerical data the experimental researches of
the time were of a somewhat vague character, and it often happened that
the conclusions drawn from them were incorrect. Hence it is with a
feeling of satisfaction that we read (vol. i. p. 2):--

“And since we are assured that the all-wise Creator has observed the
most exact proportions of _number_, _weight_, and _measure_ in the
make of all things, the most likely way, therefore, to get any insight
into the nature of those parts of the creation which come within our
observation must in all reason be to number, weigh, and measure. And
we have much encouragement to pursue this method of searching into
the nature of things, from the great success which has attended any
attempts of this kind.” For God has “comprehended the dust of the earth
in a measure, and weighed the mountains in scales, and the hills in a
balance.”

From experiments on the rise of sap in plants, many of them very
ingenious and well adapted to secure their end, and which are still
regarded by botanists as classic, Hales noticed that a quantity of
air was inspired by plants. In order to ascertain the composition and
amount of this air, the process of distillation was resorted to; for
Hales remarks: “That elasticity is no immutable property of air is
further evident from these experiments; because it were impossible for
such great quantities of it to be confined in the substances of animals
and vegetables, in an elastick state, without rending their constituent
parts with a vast explosion” (Preface, p. viii.). Hence, concluding
that the air absorbed by plants and animals could be recovered by their
distillation, Hales proceeded to distil a great number of substances of
animal and vegetable origin, such as hogs’ blood, tallow, a
fallow-deer’s horn, oystershell, oak, wheat, peas, amber, tobacco,
camphor, aniseed oil, honey, beeswax, sugar, Newcastle coal, earth,
chalk, pyrites, a mixture of salt and bone-ash, of nitre and bone-ash,
tartar, compound aquafortis, and a number of other substances. He
collected the “air” in each case over water, and gave numerical data to
show what proportion the air bore by weight to the substance from which
it had been obtained. He even tried to compare the weight of ordinary
air with that of air from distilled tartar; but his experiment led to
no positive conclusion, because of the crudeness of his appliances. The
compressibility or “elasticity” of the air from tartar, however, was
found to be identical with that of common air.

Hales does not appear to have made any special experiments on the
properties of his various airs, by trying whether they supported
combustion, whether they were themselves combustible, etc. We see from
this list that he had under his hands mixtures of hydrocarbons, carbon
dioxide, probably sulphur dioxide, hydrochloric acid and ammonia (both,
however, dissolving in water as they were formed), oxides of nitrogen,
possibly chlorine, and, as minium or red-lead was one of the substances
he tried, oxygen in a more or less pure state. It must be remembered
that in all cases the gas obtained was mixed with the air originally
present in the retort. He next proceeded to produce “air” by the
fermentation of grain, of raisins, and of other fruits; this “air”
obviously was carbon dioxide more or less pure.

It is curious to note here that he anticipated Lord Kelvin in devising
a sounding-lead which should register the depth of the sea by the
compression of air, the distance to which the air had receded along the
tube being shown by the entry of treacle. He successfully carried out a
sounding by means of his apparatus.

The next series of experiments related to the generation of “air” by
the action of acids on metals. _Aqua-regia_ and gold, _aqua-regia_
and antimony, _aquafortis_ and iron, dilute oil-of-vitriol and iron,
yielded gases which contracted on standing in contact with water.
This, in the case of the oxides of nitrogen, is to be ascribed to
their reacting with the oxygen of the air accidentally present in the
receiver; but in the last case Hales noticed that the gas absorbed in
cold weather was re-evolved on rise of temperature, as one would expect
with hydrogen.

These experiments led him to investigate the action of certain mixtures
on ordinary air. Thus a mixture of spirits of hartshorn (or ammonia)
with iron filings absorbed 1½ cubic inches of air, and one with
copper filings, twice as much. Further, a mixture of iron filings and
brimstone absorbed in two days no less than 19 cubic inches of air.

But it is disappointing to find that, in spite of all the experimental
facts which Hales accumulated, he was unable to make use of them. The
prejudice in favour of the unity and identity of all these “airs” was
too great for him to overcome. True, he sometimes theorises a little,
as for example when he remarks (p. 285):--“If fire was a particular
kind of body inherent in sulphur (_i.e._ combustible matter of all
kinds), as _Mr. Homberg_, _Mr. Lemery_, and some others imagine, then
such sulphureous bodies, when ignited, should rarefy and dilute all
the circumambient air; whereas it is found by many of the preceding
experiments, that acid sulphureous fuel constantly attracts and
condenses a considerable part of the circumambient elastick air: an
argument that there is no fire endued with peculiar properties inherent
in sulphur; and also that the heat of fire consists principally in the
brisk vibrating action and re-action between the elastick repelling air
and the strongly attracting acid sulphur, which sulphur in its Analysis
is found to contain an inflammable oil, an acid salt, a very fix’d
earth, and a little metal.”

Enough has now been said to give a fair idea of Stephen Hales’
researches. It will suffice if his conclusions be stated in his own
words (p. 314):--

“Thus, upon the whole, we see that air abounds in animal, vegetable,
and mineral substances; in all which it bears a considerable part; if
all the parts of matter were only endued with a strongly attracting
power, whole nature would then immediately become one unactive cohering
lump; wherefore it was absolutely necessary, in order to the actuating
and enlivening this vast mass of attracting matter, that there should
be everywhere intermix’d with it a due proportion of strongly repelling
elastick particles, which might enliven the whole mass, by the
incessant action between them and the attracting particles; and since
these elastick particles are continually in great abundance reduced by
the power of the strong attracters, from an elastick to a fixt state,
it was therefore necessary that these particles should be endued with a
property of resuming their elastick state, whenever they were
disengaged from that mass in which they were fixt, that thereby this
beautiful frame of things might be maintained in a continual round of
the production and dissolution of animal and vegetable bodies.

“The air is very instrumental in the production and growth of animals
and vegetables, both by invigorating their several juices while in an
elastick active state, and also by greatly contributing in a fix’d
state to the union and firm connection of several constituent parts of
those bodies, viz. their water, salt, sulphur, and earth. This band of
union, in conjunction with the external air, is also a very powerful
agent in the dissolution and corruption of the same bodies; for it
makes one in every fermenting mixture; the action and re-action of the
aerial and sulphureous particles is, in many fermenting mixtures, so
great as to excite a burning heat, and in others a sudden flame; and it
is, we see, by the like action and re-action of the same principles, in
fuel and the ambient air, that common culinary fires are produced and
maintained.

“Tho’ the force of its elasticity is so great as to be able to bear a
prodigious pressure, without losing that elasticity, yet we have, from
the foregoing Experiments, evident proof that its elasticity is easily,
and in great abundance destroyed; and is thereby reduced to a fixt
state by the strong attraction of the acid sulphureous particles which
arise either from fire or from fermentation; and therefore _elasticity_
is not an _essential immutable property_ of air-particles; but they
are, we see, easily changed from an elastick to a fixt state, by the
strong attraction of the acid, sulphureous, and saline particles
which abound in air. Whence it is reasonable to conclude that our
atmosphere is a _Chaos_, consisting not only of elastick, but also
of unelastick air-particles, which in plenty float in it, as well as
the sulphureous, saline, watery, and earthy particles, which are no
ways capable of being thrown off into a permanently elastick state,
like those particles which constitute true permanent air. Since, then,
air is found so manifestly to abound in almost all natural bodies;
since we find it so operative and active a principle in every chymical
operation; since its constituent parts are of so durable a nature, that
the most violent action of fire or fermentation cannot induce such an
alteration of its texture as thereby to disqualify it from resuming,
either by the means of fire or fermentation, its former elastick state;
unless in the case of vitrification, when, with the vegetable Salt and
Nitre in which it is incorporated, it may, perhaps, some of it, with
other chymical principles, be immutably fixt,--since then this is the
case, may we not with good reason adopt this now fixt, now volatile
_Proteus_ among the chymical principles, and that a very active one, as
well as acid sulphur; notwithstanding it has hitherto been overlooked
and rejected by chymists, as no way intitled to that denomination?”

This quotation shows us how little Mayow’s shrewd reasoning and
well-devised experiments had impressed the thinkers of his age. While
Hales quotes frequently from Boyle’s and Newton’s works, his reference
to Mayow is meagre; nor does he adopt any one of Mayow’s conclusions.
One would have thought that, having prepared so many gases by means
of apparatus well adapted to their purpose, and having observed that
certain substances introduced into air produced contraction, he would
have drawn the conclusion that such “airs” were essentially different
kinds of matter. But the “Proteus” was too much for him; and he left
the subject practically in the same state of “Chaos” in which he found
it.

[1] _Memoirs for a General History of the Air_; Shaw’s Abridgment of
    Boyle’s works, edition 1725, vol. iii. p. 26.

[2] “Suspicions about some hidden qualities of the Air,” _ibid._ p. 77.




CHAPTER II

“FIXED AIR” AND “MEPHITIC AIR”--THEIR DISCOVERY BY BLACK AND
BY RUTHERFORD


Before relating the history of the discoveries of Black, Rutherford,
and Priestley, it will be appropriate to give an account of a theory
which professed to explain the phenomena of combustion, and with it the
conversion of metals into calces, and the reduction of these calces to
the reguline or metallic state. Like other theories, it was slow in
developing. Its germ is to be traced to the writings of Johann Baptist
van Helmont of Brabant, Seigneur of Merode, Royenboch, Oorshot, and
Pellines, who was born in Brussels in 1577. He adopted a fantastical
creation of Paracelsus, the _archaeus_, a kind of demon which, by means
of fermentation, draws together all the particles of matter. Believing
that water was the true principle and origin of everything (for he had
succeeded in producing a willow tree, weighing 164 lbs., from water
alone, the earth in which it grew having neither gained nor lost
appreciably in weight), he conceived that it was acted on by a
_ferment_ or principle pre-existing in the seed developed by it, and
exhaling an odour by which the archaeus was attracted. Water undergoing
the action of this ferment developed a vapour, to which van Helmont
gave the name of “gas.” A “gas” was a substance intermediate between
spirit and matter, and the word was probably derived from _Geist_, the
common German word for spirit. Another word introduced by him to denote
the life-principle of the stars was _Blas_, connected probably with
_blasen_, to blow, and our English word _blast_.

It is curious to notice how the idea of an _archaeus_ survived down to
later times under the name of a “life-principle”--a conception that all
organic substances must necessarily owe their origin to life itself,
and not to the usual chemical and physical transformations.

Van Helmont was acquainted with various kinds of gases, as appears
from his treatise “De Flatibus.” His _gas sylvestre_ was evolved from
fermenting liquors, and he knew that it was formed during the
combustion of charcoal, and also that it was present in the Grotto del
Cane near Naples. He was likewise acquainted with combustible gases,
which he named _gas pingue_, _gas siccum_, or _gas fuliginosum_.

These principles of van Helmont’s apparently suggested to his
successors, Becher and Stahl, the notion of a principle inherent in
every combustible substance, which was lost during combustion. The
development of this--the phlogistic--theory is almost wholly due to the
latter chemist, and indeed it is difficult to trace Becher’s share in
it.

George Ernest Stahl was born at Anspach in 1660; he studied and
graduated in medicine at Halle, and in 1694 he was appointed second
professor of medicine at that University, where he continued
to teach for twenty-two years. His most important work was his
_Fundamenta chymiae dogmaticae et experimentale_. His theoretical
views are contained in the last part of this work. He there treats
of _zymotechnia_, or fermentation; _halotechnia_, or the production
of salts; and _pyrotechnia_, or the doctrine of combustion. It is
the last of these sections which gives an account of the doctrine of
_phlogiston_.

The fundamental conception of this doctrine is that all combustible
bodies are compounds. During combustion one of these constituents,
common to all, was dissipated and escaped, while the other, sometimes
an acid, sometimes an earthy powder or calx, remained behind. Thus
sulphur and phosphorus, when burnt, give acids; and the metals form
_calces_. Non-combustible substances, such as lime, were imagined to
be _calces_, and it was supposed that if phlogiston were restored
to them, they too would be converted into metals. This combustible
principle was thought to be inherent in all combustible bodies
whatsoever; it corresponds in kind with the “sulphur” of more ancient
writers, but differs from the latter inasmuch as no very precise ideas
were entertained of the identity of the “sulphur” which conferred on
the substances containing it as a constituent, or possessing it as a
property, their power of combustion. It was also made more definite by
Stahl that substances capable of burning or conversion into calces are
compounds containing phlogiston in combination with other substances.

Stahl can hardly be credited with more than the invention of the term
“phlogiston,” and with bringing the subject in a clear and definite
form before his contemporaries. For Stahl wrote in 1720; and we find
Mayow, in 1674, entering into an elaborate argument to prove that
sulphuric acid is not contained in sulphur, but that it is produced
by the union of the sulphur with his fire-air particles. But Stahl
amplified the doctrine which Mayow had controverted, in pointing
out that if such substances as phosphorus, sulphur, or metals are
heated, they burn, and are changed into phosphoric acid, sulphuric
acid, or “calces”; and reciprocally, if phosphoric acid, sulphuric
acid, or a calx such as that of tin or lead, is heated with matter
rich in phlogiston, such as charcoal, pitcoal, sugar, flour, etc.,
phlogiston is restored to the burnt substance, and the original
material, phosphorus, sulphur, tin, or lead, is reproduced. The idea
at once captivated the minds of the chemists of that age, who received
it with approbation, and devised experiments designed to extend the
applications of the theory and to confirm its truth.

Substances were not supposed always to be completely deprived of
phlogiston by combustion. Indeed, if the phlogiston were removed
wholly, or nearly so, it was by no means easy to restore it. Thus
the calx of zinc, or of iron, which was regarded as nearly devoid of
phlogiston, is difficult to reduce to the metallic state by ignition
with substances rich in phlogiston, such as coal or charcoal. The
addition of phlogiston alters the appearance of the substance as
regards colour or metallic lustre, and these vary according to the
proportion of phlogiston present.

There existed no very definite idea regarding the appearance or
properties of phlogiston itself. Becher’s name for it was _terra
pinguis_, and it was represented by Becher and by Stahl as a dry
substance of an earthy nature, consisting of very fine particles, which
were capable of being set into violent motion; this idea was derived
partly from the fact that combustion is usually accompanied by flame,
which was supposed to be produced by the motion of the particles of the
body, communicated to it by the phlogiston.

It must not be forgotten that at this time it was perfectly well known
that metals gain weight on calcination. Jean Rey was quite aware of
this, and Boyle relates an experiment to show that tin gains weight
when converted into calx; and it will be remembered that Mayow made
experiments on the ignition of antimony by the aid of a burning-glass,
and rightly conjectured that the substance produced was the same as
that formed by treating it with nitric acid, and subsequent ignition.
Boyle’s view was that calx of tin was a compound of _tin_ and _heat_;
Mayow’s more correct view was that calx of antimony was a compound of
_antimony_ and _fire-air_. But in spite of these well-proved facts,
the adherents of the theory of phlogiston ignored them, and it does
not appear to have occurred to Becher or to Stahl that they were
inconsistent with their theories.

When this difficulty was stated, which was not until a much later
date, a lame explanation of a metaphysical nature, and in itself
contradictory, was all that could be offered. It was that phlogiston
is endowed with the contrary of gravity or weight, _i.e._ levity or
absolute lightness. This means, of course, that it is repelled by the
earth. But if repelled by matter, how comes it that it enters into
combination with matter? For it could not remain united if its property
were to repel and not to attract. Notwithstanding this, however, the
idea satisfied some as to the gain in weight which metals undergo in
changing into calces.

It is indeed astonishing that men of such great ability and acumen as
Black and Cavendish should have so long lain under the yoke of this
absurd theory. It is probable that, in the case of these two great
chemists, they stated their results in terms of the theory, partly
because they were content to express the facts to which they wished
to call attention in this manner, partly because they were not in
a position to replace the theory by a more rational one. It is not
easy to revolutionise a language, even though its vocabulary be a
restricted one. The object of writing is to convey thoughts to others;
and it is certainly more convenient to make use of terms understood by
others, even if they only imperfectly convey the meaning which it is
desired to express, than to attempt a revolution which will probably
be unsuccessful, and even if successful, will at all events take time.
It is not so difficult to understand Priestley’s attitude, which
we shall have to consider later; for Priestley was first of all an
experimentalist, and was captivated more by the acquisition of a new
fact than by assigning to that fact its proper place in the cosmogony
of nature.

The influence of the phlogistic theory on the knowledge of the nature
of air was of such a kind as to retard its progress. For how could
that knowledge be furthered, when the most active constituent of air
was represented by a negation? It may be said that it is easy to be
wise after the event,--in this case the discovery of oxygen; but here
was a theory which was in contradiction to many known facts, and which
furnished but a lame explanation of phenomena, and which had been
anticipated by another theory, subsequently proved to be correct. Its
sole support was the authority of its inventors or adapters, and the
deeply-ingrained notions of centuries. We may read from it a lesson
that it is wiser to seek out facts which test and prove a theory rather
than those which support it, and we may learn for the hundredth time
the folly of relying on authority, however ancient and associated with
famous names it may be. This was happily expressed by Boyle when he
wrote:[3] “For I am wont to judge of opinions as of coins: I consider
much less in any one that I am to receive, whose inscription it bears,
than what metal ’tis made of. ’Tis indifferent enough to me whether
’twas stamped many years or ages since, or came but yesterday from the
mint. Nor do I regard how many or how few hands it has passed through,
provided I know by the touchstone whether or no it be genuine, and
does or does not deserve to have been current. For if, on due proof,
it appears to be good, its having been long, and by many, received for
such, will not tempt me to refuse it. But if I find it counterfeit,
neither the prince’s image nor superscription, nor the multitude of
hands it has passed through, will engage me to receive it. And one
disfavouring trial, well made, will much more discredit it with me
than all these spurious things I have named can recommend it.”

It has been necessary to enter at some length into the nature of the
phlogistic theory, because the discoveries of the time were expressed
in its language. The _fire-air_ or _vital air_ of Mayow was termed
dephlogisticated air; _i.e._ air wholly deprived of the power of
burning, or air more capable of supporting combustion than ordinary
air; while airs capable of burning were supposed to be more or less
highly charged with phlogiston; indeed, at one time, it was imagined
that hydrogen was phlogiston itself.

It is to Joseph Black that the discovery of carbon dioxide, that
constituent of air first to be definitely recognised, if we except
Mayow’s early work, is generally ascribed. But we must remember that
it had been prepared by Becher and by Hales, and had been doubtless
obtained in an impure state by many others. It will be seen that
Black’s work was so complete, and established the identity of this
gas in so definite a manner, that his right to be named as its true
discoverer can hardly be questioned.

Black was born near Bordeaux in 1728. His father, a wine-merchant,
was originally a native of Belfast, being descended from a Scottish
family which had been settled there for some time. When twelve years
of age. Black returned to Belfast, and received his education in the
local grammar-school, afterwards proceeding to the University of
Glasgow in 1746, at the age of eighteen. He was a pupil of Dr. Cullen,
then Lecturer on Chemistry at the College there, who is mentioned
by Professor Thomas Thomson, in his _History of Chemistry_, as an
excellent and instructive lecturer. Black intended to choose the career
of medicine, and he indeed practised occasionally as a medical man
during the greater part of his life.

[Illustration: JOSEPH BLACK.]

He began his medical studies in Edinburgh in the year 1751, and in
1755 he published, as his thesis for the degree of M.D., the work
which has rendered his name famous. It appears that as early as 1752
he had been occupied with investigations on quicklime, which was
then attracting attention as a remedy for urinary calculi. Opinion
was divided regarding its virtue. In a manuscript copy of notes of
Black’s lectures, which the author is so fortunate as to possess,
he mentions that his attention was directed to the subject through
the rival views of Drs. Alston and Whytt. It was not long before he
proved that, in opposition to the commonly received notion, quicklime
had gained nothing from the fire in which it was made, but that the
limestone used for its preparation had lost nearly half its weight in
becoming caustic. He also attempted successfully to trap the escaping
gas, and again placed it in presence of lime, confining it over water.
Instead of any escape of material when the lime became mild, “nothing
escapes--the cup rises considerably by absorbing air.” And in his
notes, a few pages farther on, he compares the loss of weight undergone
by limestone on being calcined, with its loss on being dissolved in
muriatic acid. These experiments appear from his journal to have been
made before November 1752.

His thesis was not published, however, until 1755. Immediately after,
in 1756, he succeeded Dr. Cullen as Professor in Glasgow, where he
remained until 1766. During these ten years he began and made great
progress with his well-known researches on the heat of fusion of ice,
and the heat of vaporisation of water, or, as he termed them, the
“latent heats” of water and of steam. In 1766, Dr. Cullen was appointed
Professor of Medicine in the University of Edinburgh, and Black again
succeeded him as Professor of Chemistry. There he lectured until 1797,
when he retired from public life; he died as peacefully as he had
lived, in 1799, in the seventy-first year of his age. Thomson, who
relates these particulars, was one of his last students; he writes:--“I
never listened to any lectures with so much pleasure as to his; and it
was the elegant simplicity of his manner, the perfect clearness of his
statements, and the vast quantity of information which he contrived in
this way to communicate, that delighted me.... His illustrations were
just sufficient to answer completely the object in view, and no more.”

Black’s original thesis for his degree was entitled _Experiments upon
Magnesia Alba, Quicklime, and other Alcaline Substances_. It was
published in 1755, and several times reprinted. It is now to be had in
a convenient form as one of the “Alembic Club Reprints.”

It was the custom in those days to administer alkalies as a remedy
for urinary calculi; and about the year 1750 lime-water was tried as
a substitute. Opinion was divided as regarded its efficacy; and it
was with the view of preparing a better remedy that Black undertook
researches on magnesia alba. Black prepared magnesia from “bittern,”
which remains in the pans after the crystallisation of salt from
sea-water, and also from Epsom salts, “which is evidently composed
of _magnesia_ and the vitriolic acid.” The magnesia is thrown down
from the sulphate as carbonate, by the addition of pearl ashes, at
the temperature of ebullition, the soluble product being “vitriolated
tartar,” or potassium sulphate. He describes how “magnesia is quickly
dissolved with effervescence or explosion of air, by the acids of
vitriol, nitre, and of common salt, and by distilled vinegar,” and
gives an account of the properties of the sulphate, nitrate, chloride,
and acetate. He subsequently heated this magnesia, and found that it
lost “a remarkable proportion of its weight in the fire,” and his
“attempts were directed to the investigation of this volatile part.”
The residue in the retort did not effervesce on the addition of acids;
hence the volatile part had been driven away by the heat. “Chemists
have often observed, in their distillations, that part of the body has
vanished from their senses, notwithstanding the utmost care to retain
it; and they have always found, upon further inquiry, that subtile part
to be air, which, having been imprisoned in the body, under a solid
form, was set free, and rendered fluid and elastic by the fire. We may
safely conclude that the volatile matter lost in the calcination of
_magnesia_ is mostly air; and hence the calcined _magnesia_ does not
emit air, or make an effervescence when mixed with acids.”

Magnesia, thus freed from “air” by ignition, was dissolved in “spirit
of vitriol” and thrown down with an alkali. Its weight was nearly equal
to that which it possessed before calcination, and it again effervesced
with acids. “The air seems to have been furnished by the alkali, from
which it was separated by the acid; for Dr. Hales has clearly proved
that alkaline salts contain a large quantity of fixed air, which they
emit in great abundance when joined to a pure acid. In the present
case, the alkali is really joined to an acid, but without any visible
emission of air: and yet the air is not retained in it; for the neutral
salt, into which it is converted, is the same in quantity, and in
every other respect, as if the acid employed had not been previously
saturated with magnesia, but offered to the alkali in its pure
state, and had driven the air out of it in their conflict. It seems,
therefore, evident that the air was forced from the alkali by the acid,
and lodged itself in the _magnesia_.”

After an account of some experiments showing that magnesia is not
identical with lime or with alumina, he proceeds:--“It is sufficiently
clear that the calcareous earths in their native state, and that the
alkalis and _magnesia_ in their ordinary condition, contain a large
quantity of fixed air; and this air certainly adheres to them with
considerable force, since a strong fire is necessary to separate it
from _magnesia_, and the strongest is not sufficient to expel it
entirely from fixed alkalis, or take away their power of effervescing
with acid salts.

“These considerations led me to conclude that the relation between
fixed air and alkaline substances was somewhat similar to the relation
between these and acids: that as the calcareous earths and alkalis
attract acids strongly, and can be saturated with them, so they also
attract fixed air, and are, in their ordinary state, saturated with it;
and when we mix an acid with an alkali, or with an absorbent earth,
that the air is then set at liberty, and breaks out with violence;
because the alkaline body attracts it more weakly than it does the
acid, and because the acid and air cannot both be joined to the same
body at the same time.... Crude lime was therefore considered as a
peculiar acrid earth, rendered mild by its union with fixed air; and
quicklime as the same earth, in which, by having separated the air, we
discover that acrimony or attraction for water, for animal, vegetable,
and for inflammable substances.”

The solubility of slaked lime in water is next discussed. If a solution
of lime “be exposed to the open air, the particles of quicklime which
are nearest the surface gradually attract the particles of fixed air
which float in the atmosphere.”

Black next points out that, on mixing _magnesia alba_ with lime-water,
the air leaves the magnesia and joins itself to the lime; and as both
magnesia and calcium carbonate are insoluble in water, the water is
left pure. Similarly quicklime deprives alkalies of their air and
renders them caustic. And it follows that if caustic alkali be added
to a salt of magnesia or of lime, it will separate the magnesia or the
calcareous earth from the acid, in a condition free from “air” but
combined with water.

In order to show that the “air” which exists in combination with lime
or alkalies is not the air which is contained in solution in water,
lime-water was placed under an air-pump, along with an equal quantity
of pure water; on making a vacuum, an approximately equal amount of
air was evolved from each. “Quicklime, therefore, does not attract air
when in its most ordinary form, but is capable of being joined to one
particular species only, which is dispersed through the atmosphere,
either in the shape of an exceedingly subtile powder, or more probably
in that of an elastic fluid. To this I have given the name of _fixed
air_, and perhaps very improperly; but I thought it better to use a
word already familiar in philosophy than to invent a new name, before
we be more fully acquainted with the nature and properties of this
substance, which will probably be the subject of my further inquiry.”

The next proceeding was to render “mild alkali” caustic by means of
lime, and to determine that nearly the same amount of acid is required
to saturate the caustic alkali as to saturate the mild alkali from
which the caustic alkali had been prepared. On exposure to air for
a fortnight, the caustic alkali again became mild, owing to its
absorption of fixed air. Careful experiments were made to prove that
such caustic alkali contains no lime, and does not therefore owe its
causticity and corrosive properties to the presence of that ingredient.
The volatile alkali (ammonium carbonate) was also rendered caustic, and
Black “obtained an exceedingly volatile and acrid spirit, which neither
effervesced with acids nor altered in the least the transparency of
lime-water; and although very strong was lighter than water, and
floated upon it like spirit of wine.”

After a description of some unsuccessful attempts to render mild
alkalies caustic by heat alone (_i.e._ to expel carbon dioxide from
potassium carbonate), Black examines the action of the “sedative salt”
or boracic acid on mild alkalies, by rubbing them together in presence
of some water. At first there is no effervescence, but on adding
successive quantities of boracic acid, brisk effervescence finally
takes place, borax being formed. “This phenomenon may be explained by
considering the fixed alkalis as not perfectly saturated with air ...
if they expel a small quantity of air from some of the salt, this air
is at the same time absorbed by such of the contiguous particles as are
destitute of it.” And on “exposing a small quantity of a pure vegetable
fixed alkali (carbonate of soda) to the air, in a broad and shallow
vessel, for the space of two months,” crystals were obtained, which
possessed a milder taste than that of ordinary salt of tartar, which
effervesced with acids more violently than usual, and which could not
be mixed with the smallest portion of boracic acid without emitting
a sensible quantity of air (hydrogen sodium carbonate). It therefore
follows that such alkaline substances have an attraction for fixed
air; and this was proved by mixing _magnesia alba_ in fine powder with
caustic alkali, and shaking for some time. The magnesia was converted
into the variety which did not effervesce with acids, and the alkali
was rendered mild, like a solution of salt of tartar. These are the
principal results of Black’s researches, and he concludes with a table
of affinity of acids for fixed alkali, calcareous earth, volatile
alkali, and magnesia, contrasting it with the affinity possessed by
fixed air for the same bases.

It was the habit of the Scottish students to pass down notes taken
during the lectures of their professors from one generation to another.
As the lectures were generally read, and not delivered _extempore_,
the process resulted in an almost verbatim report of the actual words
of the lecturer. One of these copies of lectures, bearing the date
1778, gives an account of the experiments which have been described,
in words almost identical with those used in the thesis of 1755. Black
appears to have shown his class this air, made, however, according to
Hales’ plan, by heating magnesium carbonate in a bent gun-barrel, and
collected over water in the usual way. He demonstrated its weight by
pouring it from one vessel to another, and showed that it extinguished
the flame of a candle. He mentions also that in 1752 he discovered that
this air is the same as choke-damp, and that it is fatal to animal
life. He speaks of the Grotto del Cane, and observes that fixed air
is produced by fermentation, and by the burning of charcoal, and showed
to his class experiments in which air from each source is shaken with
lime-water, giving a turbidity of carbonate. The well-known experiment
of inspiring air through lime-water, which, owing to the small amount
of carbonic anhydride it contains, does not produce a turbidity, and
expiring through lime-water, showing the formation of carbon dioxide in
the lungs, is described and performed. He next describes Cavendish’s
experiments on the solubility of fixed air and its density, and
researches by Dr. Brownrigg and Dr. Gahn of Sweden on its occurrence
in mineral waters. He also explains how calcareous petrifactions are
produced by the escape of fixed air from water, which then deposits its
dissolved calcium carbonate, present in solution as bicarbonate. The
deposit of iron from chalybeate waters is ascribed to the same cause,
and the explanation is attributed to Mr. Lane.

“Upon the whole,” these manuscript notes relate, “this sort of air
is quite distinct from common air, though it is commonly mixed with
it in small quantity.” “With regard to its origin, when treating of
inflammable substances and metals I shall consider this more
completely. I shall now only hint that it is a vital air, changed by
some matter, seemingly the principle of inflammability. This appears
from several phenomena when an animal or burning body is enclosed
with a certain quantity of this air, until it is changed as much as
possible.” The air is diminished in volume by the breathing of the
animal or by the burning of the candle. And Dr. Priestley has found
that “growing vegetables had the power of restoring this sort of air
to common or vital air again, which must be by their taking away some
matter which it had received from the burning body or animal.”

Black’s account of fixed air and its properties is the first example we
possess of a clear and well-reasoned series of experimental researches,
where nothing was taken on trust, but everything was made the subject
of careful quantitative measurement. It was not long since Hales had
pronounced air to be a chaotic mixture of effluvia. Black showed that
common air contains a small amount of fixed air, and that fixed air
must be considered as a fluid differing in many of its properties
from common air, especially in its being absorbed by quicklime and by
alkalies. It must be remembered that at that time carbon was not
recognised as an element; and hence, though Black knew that fixed air
was a product of the combustion of charcoal, he did not attribute it
to the union of carbon with oxygen, although the sentence quoted above
closely approaches to the truth.

The discovery of nitrogen was next in the order of time. It was made by
Daniel Rutherford, a pupil of Black’s, and at his instigation, and its
description formed a thesis for his degree of Doctor of Medicine.

Daniel Rutherford was born at Edinburgh on November 3rd, 1749. He was
the son of a medical man, Dr. John Rutherford, one of the founders
of the Medical School in that city. He was educated at Edinburgh
University, and after graduating in Arts, became a medical student,
taking his degree of M.D. in 1772. His diploma was obtained on 12th
September. He then travelled for three years in England, France, and
Italy, and in 1775 he returned to his native town, where he practised
his profession. In 1786 he succeeded Dr. John Hope in the Chair of
Botany in his University, but he did not on that account resign his
practice. He was president of the Royal College of Physicians of
Edinburgh from 1796 to 1798. During the greater part of his life he
suffered from gout; he died in 1819, at the age of seventy.

Rutherford does not seem to have pursued the study of chemistry
further: his duties led him into other fields. His genial, pleasant
face, seen in the portrait by Raeburn, shows him to have possessed a
happy disposition; and he is said to have maintained until his death
his friendship with Black, and his interest in the progress which
science was then rapidly making.

The title of Rutherford’s dissertation, of which I have been able to
find a copy only in the British Museum, is _Dissertatio Inauguralis
de aere fixo dicto, aut mephitico_. It was published at Edinburgh in
1772, seventeen years after Black’s memorable dissertation on Fixed
Air. As will be seen shortly, it precedes Priestley’s and Scheele’s
writings by a year or two. Evidently Black had noticed that a residue
was left after the combustion of carbonaceous bodies in air, and
absorption of the fixed air produced by the combustion, and had
suggested to Rutherford, then a student of his, the advantage of
further investigating the matter, and ascertaining the properties of
the residual gas.

[Illustration: DANIEL RUTHERFORD.]

Rutherford begins his essay with an apt quotation from Lucretius:--

    Denique res omnes debent in corpore habere
    Aëra, quandoquidem rara sunt corpora et aër
    Omnibus est rebus circumdatus appositusque.

He next proceeds to define the atmosphere as a pellucid thin fluid,
in which clouds float and vapours rise. Its necessity for animal
and vegetable life is acknowledged by all. It possesses weight and
elasticity. It can be fixed by other bodies; but the air obtained from
them by distillation differs from ordinary vital, salubrious air, and
is often termed mephitic or poisonous.

After acknowledging his debt to his illustrious preceptor Black, he
proceeds to quote from the latter to the effect that mephitic or fixed
air is the air which proves fatal to animals and extinguishes fire;
which is easily absorbed by quicklime and by alkaline salts; which
occurs in the Grotto del Cane, and in mineral waters; and which is
produced during exhalation from the lungs, by combustion, and during
certain kinds of fermentation. Its density, compared with that of
ordinary air, is as 15½ or 16 to 9; hence it can be kept for some
time in an open glass, and a candle lowered into it is extinguished. It
has an agreeable taste and smell; and it changes the colour of syrup of
violets from blue to purple. It prevents putrefaction, but putrefied
bodies are not made fresh by it. It possesses the power of combining
with lime, which acquires new properties as the result of its action.
Rutherford then recalls Black’s experiments on lime and on magnesia,
pointing out how these bases absorb fixed air, and how it can be
recovered from them and from its compounds with alkalies, sometimes by
heat, and always by the action of acids.

Rutherford next describes experiments which show that a mouse, placed
in atmospheric air, and left till dead, diminishes the volume of the
air by one-tenth; and that the residual air, on treatment with alkali,
loses one-eleventh of its volume. The residue extinguishes the flame of
a candle; but tinder continues to smoulder in it for a short time. It
is thus proved that after the whole of the fixed air has been withdrawn
by alkalies, the residue is still incapable of supporting life and
combustion.

Some burning bodies deprive air of its “salubrity” more easily than
others. The phosphorus of urine continues to glow in air in which a
candle has ceased to burn, or in which charcoal has burned until it is
extinguished. Even after the absorption of all fixed air by alkalies,
phosphorus burns, emitting clouds of the dry acid of phosphorus, which
can be absorbed by lime-water.

“It therefore appears that pure air is not converted into mephitic air
by force of combustion, but that this air rather takes its rise or is
thrown out from the body thus resolved. And from this it is permissible
to draw the conclusion that that unwholesome air is composed of
atmospheric air in union with, and, so to say, saturated with,
phlogiston. And this conjecture is confirmed by the fact that air which
has served for the calcination of metals is similar, and has clearly
taken away from them their phlogiston.” Such air differs from the air
evolved from metals by the action of acids, which is more thoroughly
impregnated with phlogiston; and also from that from decaying flesh,
which is a mixture of mephitic air and combustible air.

He proceeds:--“I had intended to add something regarding the
composition of mephitic air, and to seek for a reason for its
unwholesome effects, but I have not been able to find out anything
with certainty. Certain experiments appear to show, however, that it
consists of atmospheric air in union with phlogistic material; for
it is never produced except from bodies which abound in inflammable
parts: the phlogiston appears to escape from such bodies when they
become converted into the calces of metals. I say from phlogistic
material, because, as already mentioned, pure phlogiston, in
combination with common air, can be seen to yield another kind of air
[viz. hydrogen].... I have lately heard that Priestley believes that
vegetables growing in mephitic air dispel its noxious ingredients, or,
as it were, extract them, and restore its original wholesomeness; and
that mephitic air, added to air from putrid flesh, partly mitigates its
unwholesome character. But I have been unable to try such experiments.”

We see, then, that Rutherford’s claims to the discovery of nitrogen
amount to this:--he removed the oxygen from ordinary air by
combustibles such as charcoal, phosphorus, or a candle; and having got
rid of the carbon dioxide, in those cases when it was formed, by alkali
or lime, he obtained a residue, now known as nitrogen. His view of the
nature of this gas, in the phlogistic language of the time, was that
the burning bodies had given up some of their “phlogistic material” to
the air, which was thus altered. Nitrogen was “phlogisticated air,”
even though incombustible; hydrogen, too, was phlogisticated air, but
air produced by the union of pure phlogiston with atmospheric air.
The step taken by Rutherford, under Black’s guidance, was an advance,
though not a great one, in the development of the theory of the true
nature of air; and he may be well credited with the discovery of
nitrogen.

[3] _A Free Inquiry into the Vulgar Notion of Nature_; Prefatory
    remarks.




CHAPTER III

THE DISCOVERY OF “DEPHLOGISTICATED AIR” BY PRIESTLEY AND BY
SCHEELE--THE OVERTHROW OF THE PHLOGISTIC THEORY BY LAVOISIER


We have seen that Stephen Hales must have prepared oxygen, among the
numerous gases and mixtures of gases which he extracted from various
substances; for, among the many materials which he heated, one was
red-lead. The red-lead of that day, however, must have contained
carbonate, because, as we shall see, Priestley always obtained a
mixture of oxygen and carbon dioxide from that source. In the account
of his researches, Hales only incidentally mentions the collection of
gas from minium; and he appears to have made no experiments with the
object of ascertaining its properties.

The discovery of oxygen was made nearly simultaneously by Priestley and
Scheele, though it appears from the recent publication of Scheele’s
laboratory notes by Baron Nordenskjöld that Scheele had in reality
anticipated Priestley by about two years. His researches, however,
were not published until a year after Priestley had given to the world
an account of his experiments. Priestley had no theory to defend; his
experiments were undertaken in an almost haphazard manner, probably
as a relaxation. “For my own part,” he says,[4] “I will frankly
acknowledge that, at the commencement of the experiments recited in
this section, I was so far from having formed any hypothesis that led
to the discoveries made in pursuing them, that they would have appeared
very improbable to me had I been told of them; and when the decisive
facts did at length obtrude themselves upon my notice, it was very
slowly, and with great hesitation, that I yielded to the evidence of
my senses.” On the other hand, Scheele was engaged in forming a theory
of the nature of fire. He writes:[5]--“I perceived the necessity of a
knowledge of fire, because without this it is impossible to make any
experiment; and without fire or heat, it is impossible to utilise
the action of any solvent. I began, therefore, to dismiss from my
mind all explanations of fire, and undertook a series of experiments
in order to gain as full knowledge as possible of these lovely
phenomena. I ere long found, however, that it was not possible to form
any correct opinion concerning the appearances which fire exhibits,
without a knowledge of the air. After a series of experiments, I saw
that air really is concerned in the mixture termed fire, and that it
is a constituent of flame and sparks. I learned, moreover, that such
a treatise on fire as this could not be compiled with thoroughness
without also taking air into consideration.”

Scheele’s views concerning fire need not be mentioned here; but his
researches on air are so methodical and so complete as to command our
entire admiration. They remind us of those of Mayow, and had the latter
lived a little longer, they would not improbably have been carried out
by him. Since, however, Priestley had the advantage of priority of
publication, we shall commence with an account of his researches.

       *       *       *       *       *

Joseph Priestley was born in 1733 at Fieldheads, about six miles from
Leeds. His father, a maker and dresser of woollen cloth, lost his
wife when his son Joseph was about six years of age; and being poor,
his sister, Mrs. Keighley, offered to bring up the boy. The early
associations of the lad were closely connected with dissent; and after
some time spent at a public school in the neighbourhood, he was sent,
in 1752, to the Academy at Daventry, in which he was trained for the
ministry. There he gained some knowledge of mechanics and metaphysics,
and also acquired some acquaintance with Chaldee, Syriac, and Arabic,
besides being a competent French and German scholar. After leaving the
Academy, he settled at Needham in Suffolk, as assistant in a small
meeting-house, where his income was not over £30 a year. His views
were, however, too liberal for his hearers; and after some years he
moved to Nantwich in Cheshire, where he preached and also taught a
school. Here his income was improved, though still miserably small;
yet he managed to buy some books, a small air-pump, and an electrical
machine. He subsequently removed to Warrington, being employed there
in teaching and in literary work; among his writings was a _History of
Electricity_, which first brought him into notice, and which procured
for him the degree of LL.D. of Edinburgh, thus giving him a right
to the title of Doctor, by which he was always afterwards known. At
Warrington, too, he married. We next find him being asked in 1767
to take the pastorship of Millhill Chapel at Leeds, a call which he
accepted. The chapel was next door to a brewery, and this circumstance
first induced him to take up the subject of the chemistry of gases,
which has made his name famous. Here too he published his _History of
Discoveries relative to Light and Colours_. After six years spent at
Leeds, he became librarian to the Earl of Shelburne (afterwards Marquis
of Lansdowne) and travelled with him on the Continent. While with Lord
Shelburne he published the first three volumes of _Experiments on
Air_, and carried out investigations which were recorded in a fourth
volume, published after his removal to Birmingham. After some years
spent in this way, he was pensioned off, and settled as minister of
a meeting-house in Birmingham, where he employed his time partly
in theological controversy, and partly in prosecuting researches in
chemistry. He published during this period another three volumes giving
a description of his experiments on air, and communicated several
papers to the _Philosophical Transactions_ of the Royal Society, of
which he had been made a Fellow. Towards the year 1790 he was so
unfortunate as to attack Burke’s book on the French Revolution; and
this had the effect of rousing popular opinion against him, more
especially that of the local clergy, whose political views he had
frequently opposed. During the riots which took place at Birmingham
in 1791, his house was burned, and he was obliged to escape to London
under an assumed name. After some years spent in the charge of a
meeting-house at Hackney, he left England for America. His opinions,
though by no means uncommon at the present day, were so antagonistic to
those of his English contemporaries that he was cut by his Fellows of
the Royal Society, and he therefore resigned his Fellowship. And this
feeling was in no way lessened by the action of the French Government
of the time, which made him a Citizen of the Republic, and even chose
him as a member of their Legislative Assembly. Arriving in America in
1795, he was well received, and settled at Northumberland, not far from
Philadelphia. There he died in 1804.

[Illustration: JOSEPH PRIESTLEY.]

In Priestley’s work on gases he employed the form of apparatus which
had been used by Mayow a century before. Such apparatus is indeed
generally used now: the flasks with bent delivery-tubes, the Wolff’s
bottles with two necks, and the pneumatic trough filled with water or
mercury were his chief utensils. By means of such apparatus, gases
can be collected in a state of comparative purity: they can be easily
transferred from one vessel to another, and substances which it is
desired to submit to their action can be readily introduced. Scheele,
on the other hand, employed less convenient methods: his gases were
generally collected in bladders, and their transference to bottles
must have been attended with the introduction of atmospheric air.
Scheele’s method was to allow a certain amount of gas to escape from
the generating flask in order to expel air; an empty bladder was then
tied over the neck, and the gas entered the bladder. When he wished to
transfer the gas to a bottle, the bladder was tied at some distance
from the neck, and its loose open end was secured by a string round the
neck of a bottle full of water. The string confining the gas was then
untied, and the bottle was inverted; the water ran into the bladder and
was replaced by gas. A cork was also enclosed in the bladder, and it was
possible to push this cork into the neck of the bottle and re-tie the
string which confined the gas; and then, by loosing the string which
secured the bottle to the bladder, the full bottle could be conveyed
away. This process is obviously a clumsy one, although in Scheele’s
hands it yielded splendid results; and the methods which Priestley had
borrowed from Mayow have attested their superiority by their survival.

The first gas which Priestley investigated was “nitrous gas,” or, as it
is now named, nitric oxide. It had previously been prepared by Mayow
(see p. 25) by the action of nitric acid on iron; and Mayow had made
the important observation that when it was introduced into ordinary air
confined over water, the volume of the air was decreased, and a rise
of temperature occurred. But Mayow did not apply his discovery to the
analysis of air, though he rightly conjectured that the reason of the
decrease in volume of the latter was due to combination between the
nitric oxide and his “fire-air particles.” It was left for Priestley to
rediscover this fact, and to apply it to the analysis of air, or, as he
expressed it, to the determination of its “goodness.”

Priestley’s use of a mercurial trough enabled him to collect and
investigate various kinds of airs, among others “marine acid air” or
gaseous hydrogen chloride, a gas differing entirely in properties
from ordinary air. This made his mind familiar with the thought
that different kinds of air exist, not necessarily modifications
of atmospheric air. He had previously from his experiments come to
the conclusion that “atmospheric air is not an unalterable thing,
for that the phlogiston with which it becomes loaded from bodies
burning in it, and animals breathing it, and various other chemical
processes, so far alters and depraves it, as to render it altogether
unfit for inflammation, respiration, and other purposes to which it is
subservient; and I had discovered that agitation in water, the process
of vegetation, and probably other natural processes, by taking out the
superfluous phlogiston, restore it to its natural purity. But I own I
had no idea of the possibility of going any farther in this way, and
thereby procuring air purer than the best common air.”

On the 1st of August 1774, Priestley heated by means of a burning-glass
red oxide of mercury. This was produced by heating mercury until it
oxidised, and therefore had been untouched by acids, or by any
substance which could have “imparted phlogiston” to atmospheric air.
The resulting air was insoluble in water, and supported combustion
better than common air, for a candle burned more brightly, and a piece
of red-hot wood sparkled in it. This air he also produced from “red
precipitate,” the product of heating nitrate of mercury; and at the
same time from red-lead, or minium. It differed from “modified nitrous
air,” in which a candle also burns brightly, inasmuch as shaking with
water the gases produced after a candle had burned for some time in
it did not deprive it of its power of supporting combustion; nor did
it diminish the bulk of common air, as the nitrous air does in some
degree. Priestley here refers to a mixture obtained by distilling
nitrates, which is essentially a mixture of nitric peroxide with
oxygen. A candle burns in such a mixture, depriving the nitric peroxide
of part of its oxygen, and converting it into nitric oxide mixed with
nitrogen. Nitric oxide, deprived of the excess of peroxide by shaking
with water, with which the peroxide reacts and is absorbed, is no
longer capable of supporting the combustion of a candle; and when added
to ordinary air it combines with its oxygen, again forming nitric
peroxide, which in its turn is absorbed by water.

Priestley’s experiments were performed at intervals from August 1774
till March 1775, and at that date it occurred to him to mix with
his dephlogisticated air some nitric oxide over water; absorption
took place, and he concluded that he might assume his new air to be
respirable. And what surprised him especially was, that even after
addition of nitric oxide and agitation with water, the residue still
supported the combustion of a candle. A mouse, too, lived half an
hour in the new air, and revived after being removed; whereas similar
experiments with an equal volume of common air had shown that, after
respiring it for a quarter of an hour, a mouse was indisputably dead.
Even after the mouse had breathed it for so long a time, it was still
capable of supporting the combustion of a candle; and this induced
him to add more nitric oxide to the respired air, when he found that
a further contraction occurred. He reintroduced the same unfortunate
mouse into the remainder of the air--a portion to which nitric oxide
had not been added--when it lived for another half-hour, and was quite
vigorous when withdrawn.

Subsequent experiments with nitric oxide showed that air from red
precipitate or from “_mercurius calcinatus_” (red oxide of mercury
in each case, although prepared in different ways) was “between four
and five times as good as common air.” He proceeds:[6]--“Being now
satisfied with respect to the _nature_ of this new species of air,
viz. that being capable of taking more phlogiston from nitrous air,
it therefore originally contains less of this principle, my next
inquiry was, by what means it comes to be so pure, or, philosophically
speaking, to be so much _dephlogisticated_.” He therefore went on to
heat the various oxides of lead, but without any special results worth
chronicling. On moistening red-lead with nitric acid, however, and
distilling the mixture, he obtained, in successive operations, air
which was “five times as good” as common air. This process formed lead
nitrate, which on distillation yielded nitric peroxide and oxygen; the
gas was, of course, collected over water, which absorbed the peroxide,
allowing pure oxygen to pass. He found that red-lead was not the only
“earth” which produced this effect; but that “flowers of zinc” (zinc
oxide), chalk, slaked lime, and other substances also gave a gas,
when distilled with nitric acid, which was “better” than common air.
In some cases he broke up nitric acid by heat into water, nitric
peroxide, and oxygen; in others he heated nitrates. His conclusion
is: “_Atmospherical air_, or the thing we breathe, _consists of the
nitrous acid and earth_, with so much phlogiston as is necessary to
its elasticity; and likewise so much more as is required to bring it
from its state of perfect purity to the mean condition in which we find
it.”[7]

When such experiments were made by heating nitrates in a gun-barrel,
“phlogisticated air” was obtained. This was nitrogen, for the iron had
reduced the oxides of the latter, and combining with their oxygen, had
formed nitrogen; moreover, it had absorbed to a greater or less extent
the oxygen simultaneously produced.

Having concluded that respirable air was a compound of nitrous acid,
phlogiston, and earth, Priestley endeavoured to ascertain what was the
nature of this earth. He concludes “that the _metallic earths_, if free
from phlogiston, are the most proper, and next to them the _calcareous
earths_.”

“Dephlogisticated air may be procured from any kind of earth with which
the spirit of nitre will unite.” A few quantitative experiments would
surely have refuted this erroneous conclusion. Those which he attempted
to make were very crude. A bladder (of which he does not give the
capacity) was filled with

  Phlogisticated air, and weighed 7 dwts. 15 grs.
  Nitrous air          "     "    7   "   16  "
  Common air           "     "    7   "   17  "
  Dephlogisticated air "     "    7   "   19  "

He concludes (taking into consideration that inflammable air is
very light) “that the less phlogiston any kind of air contains, the
heavier it is; and the more phlogiston it contains, the lighter it
is.”[8] Strange that this should not have led to the rejection of the
phlogistic hypothesis!

Priestley had the curiosity to breathe his “good” air. He says: “My
reader will not wonder that, after having ascertained the superior
goodness of dephlogisticated air by mice living in it, and the other
tests above mentioned, I should have the curiosity to taste it myself.
I have gratified that curiosity by breathing it, drawing it through a
glass syphon, and by this means I reduced a large jar full of it to the
standard of common air. The feeling of it to my lungs was not sensibly
different from that of common air, but I fancied that my breast felt
peculiarly light and easy for some time afterwards. Who can tell but
that in time this pure air may become a fashionable article in luxury?
Hitherto only two mice and myself have had the privilege of breathing
it.”[9]

It will be seen from this account that Priestley’s work was to some
extent that of an amateur. He performed experiments, often without any
definite object; and he was not always successful in devising theories.
As before remarked, his chemical pursuits were to him a recreation, and
were undertaken during the intervals of his necessary work. His mind
was therefore not given over to them alone; and this is to be seen from
the character of his writings. His style is a delightfully familiar
one: he exposes his inmost thoughts with perfect frankness, and his
writings are therefore very readable.--We have now to compare his work
with that of his contemporary, Scheele, whose mission in life was that
of a chemist; and the reader will be interested in noting the different
points of view which these two eminent discoverers adopted.

       *       *       *       *       *

Carl Wilhelm Scheele was born on the 9th of December 1742 in Stralsund,
the capital of Swedish Pomerania, where his father was a merchant and
a burgess. He was the seventh of eleven children. After receiving his
education, partly in a private school, partly in the public school
(_gymnasium_) at Stralsund, he was apprenticed at the age of fourteen
to the apothecary Bauch in Gothenburg. In those days an apothecary was
in large measure a manufacturer as well as a retailer of drugs. He had
to prepare his medicines in a pure state from very impure materials, as
well as to mix them in order to carry out prescriptions; and, indeed,
he himself often, as sometimes happens still, ventured to prescribe in
mild cases. Scheele’s master taught him such methods, and in addition
instructed him in the use of the chemical symbols in vogue at that
date; these he afterwards freely employed in his manuscripts, and this
renders them exceedingly difficult to decipher. There still exists a
catalogue of the drugs his master kept; many of them are of a
fantastic nature, such as “ointment of vipers”, “human brain prepared
without heat”, etc.; but among them were many of the well-known salts
of metals, and the commoner acids, besides phosphorus, sulphur,
rock-crystal, some ores, and some carbon compounds; for example,
benzoic acid and camphor. There was a fair chemical library, which
included the works of Boerhaave and Lemery, and his master devoted much
pains to his instruction. In a letter to Scheele’s father, however, he
expressed a fear that too great devotion to study and experimental work
would undermine the health of a growing lad.

In 1765 the business was sold, and Scheele obtained a situation in
Malmö with an apothecary named Kjellström. His master testified that
he had extraordinary application and ability, and related that he was
in the habit of criticising all that he read, saying of one statement,
“This may be the case”; of another, “This is wrong”; of a third, “I
shall look into this.” His memory was prodigious: he is said never to
have forgotten anything which he had read relating to his favourite
subject. He took little interest in anything else, and both his
employers appear to have encouraged him to the utmost in his favourite
pursuit. In 1768 he left Malmö for Stockholm; but here the exigencies
of his duties interfered with his leisure for experimentation. While
there, in conjunction with his friend Retzius, he discovered tartaric
acid, which up till then had never been separated from tartar, its
potassium salt. Here too he made investigations on the acid of
fluor-spar (hydrofluoric acid); but finding his time too greatly
occupied with routine work, he took a situation at Upsala, the seat
of the largest university of Sweden, in 1770. At that time Bergman
was Professor of Chemistry there, and Linnaeus occupied the Chair of
Botany; both had then achieved a wide reputation. With Bergman he soon
established close relations, and Retzius wrote that it was difficult
to say which was pupil and which teacher. While at Upsala he wrote his
great work on _Fire and Air_, which we shall shortly have to consider.
From his laboratory notes it appears that before 1773 he had obtained
oxygen by the ignition of silver carbonate, red mercuric oxide, nitre,
magnesium nitrate, and from a mixture of arsenic acid and manganese
dioxide. Here too he discovered chlorine, and made researches on
manganese, arsenic, and baryta. In 1775 he was elected a member of the
Royal Swedish Academy of Sciences, an honour which much improved
his social status. In the same year he became manager of a business
at Köping, where he passed the rest of his days, in spite of urgent
appeals to engage in more remunerative work; indeed, he was strongly
pressed to go to Berlin, and also, it is said, to London, for his
publications had led to his recognition as one of the greatest chemists
of the age. His book on _Fire and Air_ was not published for some
years after the manuscript had been in the printer’s hands. We learn
from his letters that he was much afraid of being anticipated in his
discoveries, as indeed events showed that he had reason to be.

From his letters and from the verdict of his contemporaries, Scheele
is depicted as an amiable and honourable man, singularly free from
vanity and selfishness. Unfortunately no portrait of him has survived.
His last memoir on the action of sunlight on nitric acid was published
in 1786; he died suddenly at the early age of forty-three in May of
that year, two days after his marriage to Sara Margaretha Pohl. His
devotion to science had told on his health, and his death was caused by
a complication of diseases. Yet he was during his life, as after his
death, regarded as one of the greatest of chemists: his great
knowledge, extraordinary aptitude in experimenting, and high
intellectual powers placed him among the foremost men of science of his
day.

Near the beginning of his _Treatise on Air and Fire_,[10] Scheele
defines air. It is that fluid invisible substance which we continually
breathe; which surrounds the whole surface of the earth, is very
elastic, and possesses weight. “It is always filled with an astonishing
quantity of all kinds of exhalations, which are so finely divided in
it that they are scarcely visible, even in the sun’s rays.”[11] It
also contains another elastic substance resembling air, termed aerial
acid by Bergman (identical with Black’s fixed air). Since atmospheric
air has not been completely converted into fixed air by admixture of
foreign materials, “I hope I do not err if I assume as many kinds of
air as experiment reveals to me. For when I have collected an elastic
fluid, and observe concerning it that its expansive power is increased
by heat and diminished by cold, while it still uniformly retains its
elastic fluidity, but also discover in it properties and behaviour
different from those of common air, then I consider myself justified
in believing that this is a peculiar kind of air. I say that air thus
collected must retain its elasticity even in the greatest cold, because
otherwise an innumerable multitude of varieties of air would have to be
assumed, since it is very probable that all substances can be converted
by excessive heat into a vapour resembling air.”[12]

After defining the properties characteristic of air, namely, its
power of supporting combustion, its diminution by one third or one
quarter during the combustion of any substance which does not produce
any fluid resembling air, its insolubility in water, its power of
supporting life, and the fact of its being favourable to the growth
of plants, Scheele demonstrates that air must consist of at least
two elastic fluids. This he proves by exposing it to “liver of
sulphur” (polysulphide of potassium), when six parts out of twenty
were absorbed. He obtained the same result by employing a solution
of sulphur in caustic potash, and also by polysulphide of calcium,
prepared by boiling lime-water with sulphur, and by means of yellow
sulphide of ammonium. Nitric oxide, “the nitrous air which arises
on the dissolution of metals in nitrous acid,” produces a similar
contraction, and so also do oil of turpentine and “drying oils” in
general. Dippel’s animal oil, obtained by distilling bones, and ferrous
hydroxide, produced from “vitriol of iron” and “caustic ley,” or
ferrous sulphate and caustic potash, may also be used as absorbents;
as may also iron filings moistened with water, a solution of iron in
vinegar, and a solution of cuprous chloride. “In none of the foregoing
kinds of air can a candle burn or the smallest spark glow.”

He accounts for these results by the theory that all such absorbents
contain phlogiston, which is attracted by the air, and, combining with
it, diminishes its bulk. The alkalies and lime attract the vitriolic
acid of the sulphides used, and the air attracts the phlogiston. “But
whether the phlogiston which was lost by the substances was still
present in the air left behind in the bottle, or whether the air which
was lost had united and fixed itself with the materials, such as liver
of sulphur, oils, etc., are questions of importance.”[13] The conclusion
that such air, which had received phlogiston and had contracted in
volume, ought to be specifically heavier than common air was, however,
rudely dissipated by experiment. The air must therefore contain two
fluids, one of which does not manifest the least attraction for
phlogiston, while the other is peculiarly disposed to such attraction.
“But where this latter kind of air has gone to, after it has united
with the inflammable substance, is a question which must be decided by
further experiments, and not by conjectures.”[14]

To decide this question, Scheele burned in air substances such as
phosphorus, which do not produce by their combustion any kind of “air.”
The result was that the air lost 9 volumes out of an original 30, or
about one-third of its bulk. A flame of hydrogen burning in air caused
it to lose one-fifth of its volume. On burning a candle, some spirits
of wine, or some charcoal, in a confined quantity of air, very little,
if any, diminution of volume was noticed; but on shaking the air with
milk of lime, contraction ensued, but not to the same extent as when
phosphorus was burnt in it. This greatly puzzled Scheele; we now know
that such combustibles are not able to remove all the oxygen, but that
they are extinguished when only a portion of each has entered into
combination. Here, again, however, his memory comes to his help, for he
says, “It is known that one part of aerial acid mixed with ten parts
of ordinary air extinguishes fire; and there are here in addition,
expanded by the heat of the flame and surrounding the latter, the
watery vapours produced by the destruction of those oily substances. It
is these two elastic fluids, separating themselves from such a flame,
which present no small hindrance to the fire which would otherwise burn
much longer, especially since there is here no current of air by means
of which they can be driven away from the flame. When the aerial acid
is separated from this air by milk of lime, then a candle can burn in
it again, though only for a very short time.”[15] Thus the question was
correctly solved. Scheele’s acumen led him at once to make experiments
admirably adapted to discover the true reason; he was not turned aside
by any imaginary difficulties, but went straight to the point. He next
burned sulphur in confined air, and found little alteration of volume,
but on shaking with clear lime-water, absorption took place, and
one-sixth of the air was removed. “The lime-water was not in the least
precipitated in this case, an indication that sulphur gives out no
aerial acid during its combustion, but another substance resembling
air; this is the volatile acid of sulphur, which occupies again the
empty space produced by the union of the inflammable substance with
air.”[16]

The next set of experiments were devised “to prove that ordinary
air, consisting of two kinds of elastic fluids, can be compounded
again, after these have been separated from one another by means of
phlogiston.”

“I have already stated that I was not able to find again the lost air.
One might indeed object that the lost air remains in the residual
air which can no more unite with phlogiston; for, since I have found
that it is lighter than ordinary air, it might be believed that the
phlogiston, united with this air, makes it lighter, as appears to
be known already from other experiments. But since phlogiston is a
substance, which always presupposes some weight, I much doubt whether
such hypothesis has any foundation.”[17] He had formerly conjectured
that hydrogen, the “air” obtained by the action of vitriol on zinc,
might be phlogiston; “still, other experiments are contrary to this.”

Scheele next directs attention to acid of nitre, and points out that
when prepared in absence of organic material, it is nearly colourless;
but that if phlogiston be given to it, it becomes red. At the end of a
distillation of pure nitre with pure sulphuric acid, however, red fumes
are produced: “Where does the acid now obtain its phlogiston? There is
the difficulty.”

He collected some of this “red air” in a bladder containing milk of
lime, to prevent its corrosive action; and having tried whether the
resulting gas, which was now no longer red, would support combustion,
“the candle began to burn with a large flame, whereby it gave out such
a bright light that it was sufficient to dazzle the eyes. I mixed one
part of this air with three parts of that air in which fire would
not burn; I had here an air which was like the ordinary air in every
respect. Since this air is necessarily required for the origination of
fire, and makes up about the third part of our common air, I shall call
it after this, for the sake of shortness, Fire-air; but the other air,
which is not in the least serviceable for the fiery phenomena, I shall
designate after this with the name already known, Vitiated air.”[18] How
history repeats itself! Here is Scheele, in 1772, reproducing Mayow’s
name “fire-air particles” for the same substance of which Mayow had
inferred the existence a century before, and which he had pointed out
as being present in the acid of nitre, as well as in common air.

This air is not a “dry acid of nitre converted into elastic vapours,”
for it does not produce nitre with alkalies; moreover, it can be
prepared from substances which have nothing in common with nitre, no
compound of nitre having been used during their preparation. Scheele
next describes experiments proving that “fire-air” is produced by the
distillation of black oxide of manganese with concentrated oil of
vitriol, or with the “phosphorus acid of urine” (phosphoric acid), by
distilling nitrate of magnesium, made by dissolving the “white magnesia
employed in medicine” (magnesium carbonate) in _aquafortis_ (nitric
acid), or by distilling “mercurial nitre” (mercuric nitrate). The
cheapest and the best method of producing “fire-air” is to distil
purified nitre in a glass retort. But Scheele also obtained it from
“calx of silver” (silver carbonate) prepared from silver nitrate and
“alkali of tartar” (potassium carbonate); during this process he got
aerial acid, which had been present originally in the alkali of tartar;
but it was easily removed by means of milk of lime. Similarly, “calx of
gold,” obtained from a solution of gold with “alkali of tartar,” gave
“fire-air” when heated; but no aerial acid, for that air escapes during
the precipitation of the “calx.” The brown-red precipitate obtained by
adding “alkali of tartar” to “corrosive sublimate” (potassium carbonate
to mercuric chloride, giving a basic carbonate of mercury and potassium
chloride) yielded a mixture of fire-air and aerial acid when heated.
But if the “calx of mercury” had been prepared by means of the “acid
of nitre,” or in modern language by heating mercuric nitrate, a pure
“fire-air,” unmixed with “aerial acid,” was the product. And lastly,
arsenic acid, when heated, gave ordinary white arsenic together with
“fire-air.”

This fire-air was completely absorbed by “liver of sulphur” (a
polysulphide of potassium, formed by heating together potassium
carbonate and sulphur); and a mixture of four parts of “fire-air” with
fourteen parts of “vitiated air” lost the whole of its fire-air on
standing for fourteen days in contact with liver of sulphur. Dippel’s
animal oil, and burning phosphorus, charcoal, and sulphur, all absorbed
“fire-air”--completely if it was pure, incompletely if it was mixed
with “vitiated air”; in short, the identity of “fire-air” prepared from
calces, etc., with that in ordinary air was completely established.

As “vitiated air” is lighter than ordinary air, it follows that
“fire-air” must be heavier; and experiment proved this to be the case.

To completely disprove the possible contention that nitre was necessary
for the production of “fire-air,” some “calx of mercury” (or red
oxide), which had been prepared by boiling mercury for a long time
in contact with air, was heated. The products were metallic mercury
and “fire-air”. “This is a remarkable circumstance, that the fire-air
which had previously removed from the mercury its phlogiston in a slow
calcination, gives the same phlogiston up to it again, when the calx is
simply made red-hot.”[19] Is it not remarkable that the true explanation
should not have forced itself upon Scheele’s mind, which was so acute,
and so capable of forming true deductions?

The next set of experiments dealt with the phenomena of respiration. A
rat, confined in air until it died, polluted the air with one-thirtieth
of aerial acid. Respiration from Scheele’s own lungs had the same
effect. A few flies, bees, and caterpillars also polluted the air in
the same way. Peas, roots, herbs, and flowers all converted about
one-fourth part of ordinary air into “aerial acid”. “These are
accordingly strange circumstances, that the air is not noticeably
absorbed by animals endowed with lungs, contains in it very little
aerial acid, and yet extinguishes fire. On the other hand, insects and
plants alter the air in exactly the same way, but still they convert
the fourth part of it into aerial acid.”[20] And so he makes experiments
which prove that it is the fire-air which is converted into “aerial
acid” by peas; and that “fire-air” is absorbed by fresh blood, and
acquires no aerial acid from it. And, further, he was able to breathe
fire-air for a long time, especially if a “handful of potashes” was put
into the bladder. A couple of large bees, confined in “fire-air,” along
with milk of lime, consumed practically the whole of the air in eight
days. But plants, confined in “fire-air,” along with milk of lime,
would not grow; however, they yielded a little aerial acid. Scheele is
again puzzled here by the circumstance that the blood and the lungs
have not the same action on air as insects and plants, inasmuch as the
former convert it into vitiated air, and the latter into aerial acid.
We now know that air will not support life of warm-blooded animals when
the oxygen falls below a certain not very small amount, while insects
appear to be capable of exhausting the oxygen to a great extent; and it
is probable that the plants, under the unnatural circumstances in which
they were placed, gave off a considerable amount of carbon dioxide.
Scheele’s explanation in terms of phlogiston is not successful. He
wrote:--“I am inclined to believe that fire-air consists of a subtle
acid substance united with phlogiston, and it is probable that all
acids derive their origin from fire-air. Now if this air penetrates
into plants, these must attract the phlogiston, and consequently the
acid, which manifests itself as aerial acid, must be produced.”[21]
This is reversing what may be termed the true explanation on the basis
of the phlogistic theory. For Scheele supposes that oxygen contains
phlogiston, and by losing it, yields carbon dioxide. On the other hand,
the consistent explanation would be that carbon is carbonic acid plus
phlogiston, and that when it burns it loses phlogiston and becomes
carbonic acid again. We see how confused the phlogistic ideas became
after the discovery of oxygen, and how ripe the time was for Lavoisier
to formulate the views which are now universally accepted.

In the concluding sections of his treatise Scheele describes
experiments which prove the solubility of “fire-air” in water; he
mentions a convenient test for free oxygen in solution, viz. a mixture
of ferrous sulphate and lime, which turns dark green, and finally
rust-coloured, when added to water containing oxygen; and he shows that
water is deprived of oxygen by the presence of a leech, kept in it for
two days.

It is impossible not to recognise in Scheele one of the most acute
intellects and able experimenters whom the world has ever seen. And
although we cannot but feel surprise that his discoveries did not lead
him to take the step of renouncing the hypothesis of phlogiston, it
must be borne in mind that the doctrine was surrounded with the halo of
old age, and sanctioned by many names of great repute in their time.
We shall see later that Cavendish, one of the greatest of English
chemists, on weighing the rival theories, decided in favour of the
phlogistic hypothesis. The actual escape of flame, a visible entity,
from a burning substance, may have had much to do with this decision;
and the uncertainty concerning the nature of heat, and the doubt
whether it was not a form of imponderable matter, may have led both
Scheele and Cavendish to retain the older views. It was Lavoisier who
first dared to throw off the shackles of tradition; and this he did
before oxygen had been discovered, as early as 1772.

       *       *       *       *       *

Antoine Auguste Lavoisier was born in Paris on the 26th of August 1743.
His father was wealthy, and spared no expense on his education. In his
twenty-first year he obtained a gold medal from the Academy of Sciences
for an essay on the best method of lighting the streets of Paris, but
it was some years before he made definite choice of his subject. He
published memoirs relating to geology and to mathematics, before the
fame of Black’s and Priestley’s discoveries reached him and induced
him to turn his attention to scientific chemistry. Lavoisier’s life
was divided between his researches and the performance of public
duties. In his twenty-fifth year he was elected a Member of the French
Academy of Sciences, and, somewhat later, became its treasurer. He
drew up numerous reports for the Government on questions on the
borderland of Science and Technology; for example, on the preparation
of paper for bills, which would not admit of forgery; on experimental
agriculture; and on the manufacture of gunpowder. In 1771 he married
Marie Anna Pierette Paulze, the daughter of a “fermier-général” or
collector of Government revenue; and after his death, she became the
wife of Count Rumford, another distinguished scientific man. Made a
“fermier-général” himself, it was during his tenure of this office that
Lavoisier was accused--along with others holding similar positions--of
misappropriating revenue moneys, with the result that, under the
dictatorship of the infamous Robespierre, he and twenty-eight of those
who held like office were guillotined publicly, on the 8th of May
1794. It is stated that Lavoisier’s last plea, presented by Hallé--for
permission to finish a research--was refused by Coffinhal, with the
brutal phrase, “La Republique n’a pas besoin de savants; il faut que la
justice suive son cours.” Within twenty-four hours the execution took
place.

Lavoisier was a tall, handsome man, with a remarkably pleasing face. He
possessed great influence, and used it all for good.

The first account which we possess of Lavoisier’s revolutionary ideas,
for revolutionary they were then deemed, was in a sealed note, placed
in the hands of the Secretary of the Academy on the 1st of November
1772. It is to the following effect:--

       “About eight days ago, I discovered that sulphur, when
     burned, instead of losing weight, gains weight; that is
     to say, from one pound of sulphur much more than one
     pound of vitriolic acid is produced, not counting the
     moisture gained from the air. Phosphorus presents the same
     phenomenon. This increase of weight is due to a great
     quantity of air which becomes fixed during the combustion,
     and which combines with the vapours. This discovery, which
     I confirmed by experiments which I regard as decisive,
     led me to think that what is observed in the combustion
     of sulphur and phosphorus might likewise take place with
     respect to all the bodies which augment in weight by
     combustion and calcination; and I was persuaded that the
     gain of weight in calces of metals proceeded from the
     same cause. Experiment fully confirmed my conjectures. I
     effected the reduction of litharge in closed vessels with
     Hales’ apparatus, and I observed that at the moment of the
     passage of the calx into the metallic state, there was a
     disengagement of air in considerable quantity, and that
     this air formed a volume at least a thousand times greater
     than that of the litharge employed. As this discovery
     appears to me to be one of the most interesting which has
     been made since the time of Stahl, I thought it expedient
     to secure to myself the property, by depositing the present
     note in the hands of the Secretary of the Academy, to
     remain secret till the period when I shall publish my
     experiments.”

                                            LAVOISIER.

       “PARIS, _11th November 1772_.”

[Illustration: ANTOINE AUGUSTE LAVOISIER.]

There is no account in Hales’ work of his reducing litharge in closed
vessels. It is to be presumed that Lavoisier heated in a retort a
mixture of litharge and charcoal, and that the air which he speaks of
was a mixture of oxides of carbon. This account does not inform us of
Lavoisier’s views on combustion, but merely shows the date at which he
had first obtained what he supposed were results new to science. We
recognise that Mayow had anticipated him in this.

It was not until Priestley, when dining with him in the autumn of
1774 (being in Paris with Lord Shelburne at the time), had informed
Lavoisier of his discovery of “dephlogisticated” air, that the ideas
of the latter upon the subject became precise. Priestley’s own words
are:--“Having made the discovery some time before I was in Paris, in
the year 1774, I mentioned it at the table of Mr. Lavoisier, when most
of the philosophical people of the city were present, saying that it
was a kind of air in which a candle burned much better than in common
air, but I had not then given it any name. At this all the company,
and Mr. and Mrs. Lavoisier as much as any, expressed great surprise.
I told them I had gotten it from _precipitate per se_, and also
from _red-lead_. Speaking French very imperfectly, and being little
acquainted with the terms of chemistry, I said _plombe rouge_, which
was not understood till Mr. Macquer said I must mean _minium_.”

Shortly after this, Lavoisier repeated Priestley’s experiments and
confirmed their truth; and this led to the true explanation of
experiments of which an account is given in the _Memoirs_ of the French
Academy for 1774, and which were fundamental in their character. They
referred to the calcination of tin in hermetically-sealed retorts. The
tin was placed in a retort which was heated on a sand-bath until the
metal had melted. The beak of the retort, previously drawn out into a
capillary, was then sealed, the air expelled having been collected and
its weight noted. The retort was then cooled and weighed. It was again
heated, and the temperature was maintained until the calcination of the
tin stopped. With a large retort the calcination was more complete than
when a smaller one was employed, this implying that the degree to which
the calcination proceeded was dependent upon the amount of air present.
After cooling the retort a second time, it was again weighed, when it
was found to have undergone no change of weight. The beak was then
broken, and air entered with a hissing noise. The gain in weight was
now about 10 grains with a large retort. The tin and its calx were next
weighed, and it was found that the gain in weight of the tin was always
equal to the loss of weight of the air in the retort, measured by the
quantity of air which entered on breaking the beak of the retort, less
the air driven out of the retort before hermetically sealing it. From
this Lavoisier concluded that calx of tin is a compound of tin and air.

Lavoisier’s next research, communicated to the Academy in 1775, and
published in 1778, was entitled “On the Nature of the Principle which
combines with Metals during their Calcination, and which increases
their Weight.” In this he describes experiments showing that when
metallic calces are converted into metals by heating with charcoal,
a quantity of fixed air is expelled; and here for the first time he
points out that _fixed air is a compound of carbon with the elastic
fluid contained in the calx_. He then describes the preparation
of oxygen by Priestley’s process of heating red oxide of mercury
(_mercurius precipitatus per se_), and shows that the red oxide, when
heated with charcoal, manifests the properties of a true calx, inasmuch
as metallic mercury is formed, and a large quantity of fixed air is
produced.

His next paper, which appeared in 1777 in the _Mémoires_ of the
Academy, deals with the combustion of phosphorus; and here he
recapitulates Rutherford’s experiments, and shows that one-fifth of
the air disappears, and that the residue, to which he gave the name
“mouffette atmosphérique,” is incapable of supporting combustion. It
will be remembered that Rutherford named this residue “phlogisticated
air,” inasmuch as he imagined it to have absorbed phlogiston from
the burning phosphorus; Scheele, too, had made a similar experiment
with a similar result. From these observations, Lavoisier concluded
that air consists of a mixture or compound of two gases, one capable
of absorption by burning bodies, the other incapable of supporting
combustion.

This paper was immediately followed by another, also published in 1777.
Its title is, “On the Combustion of Candles in Atmospheric Air, and
in Air eminently respirable.” In this memoir he distinguishes between
four kinds of air:--1, Atmospheric air, in which we live and which we
breathe. 2, Pure air, alone fit for breathing, constituting
about one-fourth of atmospheric air, and termed by Priestley
“dephlogisticated air.” 3, Azotic gas, identical with Rutherford’s
“mephitic air,” and of which the properties were then unknown. 4, Fixed
air, which he proposed to call “acide crayeux,” or acid of chalk,
discovered twenty-five years previously by Black.

By this time his theory was well developed. He accounted for the
phenomena of combustion without having recourse to the phlogistic
hypothesis: the calx was produced by the union of the metal with the
active constituent of air; and when carbonaceous material burned,
the carbon united with this same constituent, producing fixed air.
But there were still difficulties in his way: it was known that in
dissolving metals in dilute vitriol or muriatic acid, a combustible and
very light air was evolved; and that the metals were thereby converted
into calces in combination with the respective acids. This fact was not
explained even by the supporters of the phlogistic theory, but it had
the effect of preventing them from accepting Lavoisier’s views. Some
considered that hydrogen and phlogiston were identical, and that on
dissolving a metal the calx was formed by the escape of the phlogiston;
while others had a hazy idea that hydrogen was a compound of water and
phlogiston; but of this more hereafter.

Lavoisier’s objection to such a theory was that the calx was _heavier
than the metal_, and that hydrogen, though light, still possessed
weight.[22] Moreover, he had ascertained that the calces of mercury,
tin, and lead are compounds of these metals with active air, and that
as fixed air is produced by heating such calces with carbon, fixed
air must be a compound of carbon and vital air, or, as he named it,
the “oxygine principle,” inasmuch as its combination with phosphorus,
sulphur, and carbon resulted in the formation of acids (=ὀξύς=,
an acid).

In 1777 he read another memoir, “On the Solution of Mercury in
Vitriolic Acid, and on the Resolution of that Acid into Aeriform
Sulphurous Acid, and into Air eminently respirable.” Priestley had
already shown that this process yielded sulphur dioxide; Lavoisier
carried the temperature higher, and, decomposing the sulphate of
mercury, produced metallic mercury, sulphur dioxide, and oxygen. It
appeared therefore that sulphurous differed from sulphuric acid in
containing a smaller proportion of oxygen.

He also experimented with iron pyrites, and his experiments recall
those of Boyle. Boyle found that “marcasite,” a disulphide of iron, on
exposure to air, gained in weight, while vitriol of iron was formed.
Lavoisier performed the same experiment, not “in a very pure air,” as
Boyle did when he left the pyrites exposed in a quiet dust-free room,
but in a confined quantity of ordinary air; and he found that the air
was rendered incapable of supporting combustion, or, in other words,
its oxygen was removed.

In the same volume of the _Memoirs_ of the Academy for 1778, another
of Lavoisier’s papers--“On Combustion in General”--is to be found. In
this he showed that oxygen gas is the only substance which supports
combustion; that during the burning of combustible substances in air a
portion of the oxygen disappears, and converts the burning substance
into one of two kinds of compounds: either an acid, such as sulphuric
acid from sulphur, phosphoric acid from phosphorus, or carbonic acid
from carbon (for in those days the term “acid” was applied to what we
now term an anhydride); or in the case of metals a calx, or compound of
oxygen with the metal. The processes are analogous, but differ in the
rate at which they take place; for the calcination of metals is a much
slower operation than the combustion of sulphur or phosphorus. It is
the rapidity of the action which leads to actual inflammation. He next
examined and attacked the theory of phlogiston, and maintained that the
existence of phlogiston is purely hypothetical, and quite unnecessary
for the explanation of the phenomena. But his papers were received with
doubt. The change demanded was too great; the trammels of custom were
too firmly bound. He gained no converts.

Until the true nature of hydrogen had been explained, the attack on the
phlogistic theory could not be said to be complete. This combination of
hydrogen and oxygen to form water was first proved by Cavendish. And
as soon as Sir Charles Blagden, in 1783, had communicated Cavendish’s
results to Lavoisier, the latter at once saw their bearing on the new
theory which he was endeavouring to uphold, and perceived how they
would give a final blow to the adherents of the theory of phlogiston.
For it had been frequently adduced as an objection to his new views,
that they were incapable of explaining why hydrogen should be evolved
during the solution of metals in acids, or why it should be absorbed
during the reduction of calces to the metallic state. Lavoisier at once
repeated Cavendish’s experiments on a large scale, and was assisted
on that occasion by Laplace, Sir Charles Blagden also being present.
A considerable quantity of water was produced, and the volumes of the
combining gases were found to be 1 of oxygen to 1·91 of hydrogen.
Shortly after, in conjunction with Meunier, he performed the converse
operation, in decomposing steam by passing it over iron wire heated
to redness in a porcelain tube. The iron withdrew the oxygen from
the water, while the hydrogen passed on and was collected in the
gasholder.

The explanation of the solution of metals in acids was now easy:
it depended on the decomposition of water. While the oxygen united
with the metal to form a calx, the hydrogen was evolved; the calx
dissolved in the acid, forming a salt of the metal. And the operation
of producing hydrogen by the action of steam on red-hot iron met
with an equally simple explanation: the oxygen and iron united to
form an oxide--the ancient _ethiops martial_--while the hydrogen
escaped. The converse took place during the reduction of a calx to the
metallic state by hydrogen. Here the hydrogen seized on the oxygen of
the calx, removed it in the form of water, and the metal was left.
These experiments were due to Cavendish; all that Lavoisier did was
to show the true nature of the phenomena. The opponents of the new
doctrines, Priestley chief among them, did their best to disprove the
view that water was a compound of oxygen and hydrogen. But in vain.
Many of Lavoisier’s opponents had to admit the justice of his views;
and in 1787 De Morveau, Berthollet, and Fourcroy joined Lavoisier in
reconstructing the nomenclature of chemistry on a new basis, which
is substantially that in use at the present day. Black, too, was a
convert, but Priestley and Cavendish remained true to their old
faith, and one of Priestley’s last acts was to publish a defence of
the phlogistic theory. We shall see later how Cavendish carefully
considered the rival theories, and what reasons induced him to cast his
vote for the older one.

Among the numerous memoirs which Lavoisier communicated to the Academy
during the ten years between 1772 and 1782, one still remains to be
mentioned. It was published as early as 1777, but it must be remembered
that many of these memoirs were antedated. It referred to the
respiration of animals; and Lavoisier concluded, on the ground that the
phenomena of respiration are essentially similar to those of combustion
and calcination, that the only portion of the air which supports animal
life is the oxygen. The azote or nitrogen is inhaled along with the
oxygen, but is exhaled unaltered. The oxygen, however, is gradually
converted into carbonic acid; and when a certain amount, but by no
means the whole, has been thus changed, the air becomes unfit for
respiration. If the carbonic acid is withdrawn by means of lime-water
or caustic alkali, the residue is air poor in oxygen, and the azote is
the same as that left after the calcination of metals, or the burning
of a candle, in air.

At the time of his impeachment, Lavoisier was engaged in experiments
on perspiration, along with Séguin. He had nearly finished his
experimental work, but had drawn up no account of it. His request that
his life might be prolonged until he had compiled a statement of his
results was refused; but Séguin, who was fortunately spared, undertook
the task. The facts collected do not, however, bear directly on our
subject, and shall not be further alluded to here.

This account of Lavoisier’s researches would be incomplete without
a reference to his text-book of chemistry, _Traité élémentaire de
Chimie_, in which his views are stated in order, and with great
clearness. The nomenclature current at the time was so cumbrous, that
it was almost, if not quite, impossible for the supporters of the new
theory to express their meaning in an intelligible manner. De Morveau
had suggested a nomenclature for salts; Black, too, had invented one;
but neither of these systems was adapted to represent the new views.
It was partly with the object of avoiding such embarrassment that
Lavoisier wrote his _Treatise_.

He begins with a clear statement of what is generally termed “the
states of matter”--solid, liquid, and gaseous--and points out
that solids and liquids are almost all capable of change into the
aeriform state by the addition of “caloric.” Proceeding next to the
consideration of the nature of air, he shows that it must necessarily
contain all those gases capable of existence at the ordinary
temperature; and he explains how water-vapour must be one of them,
seeing that even though water is a liquid at the ordinary temperature,
it is capable, like many other liquids, of existing as vapour, when
mixed with other gases. He next treats of the analysis of air, and
describes his classical experiment of heating four ounces of mercury
for twelve days in a retort communicating with a bell-shaped receiver,
standing in a mercury trough. Having marked the initial height of the
air in the jar by means of a piece of gummed paper, he found that,
after twelve days’ heating close to the boiling-point, the air had
diminished in volume by about one-sixth, and that the mercury had
become covered with a red deposit of _mercurius calcinatus per se_,
which, when collected, weighed 45 grains. The residual air in the
retort and in the jar was incapable of supporting life or combustion;
but the _red precipitate_, when heated, lost 3½ grains of its
weight, yielding 41½ grains of metallic mercury, while it evolved
7 or 8 cubic inches of oxygen, capable of supporting the combustion
of a candle vividly, and of causing charcoal to burn with a crackling
noise, throwing out sparks. Oxygen was thus successfully separated from
air, and obtained from it in a pure condition for the first time, in a
single series of operations.

In Lavoisier we see a master mind, not only capable of devising and
executing beautiful experiments, but of assimilating those of others,
and deducing from them their true meaning. Although his additions to
the known chemical compounds were few in number, and cannot be compared
with those of Scheele or of Priestley, yet his reasoning in disproof
of the phlogistic theory was so accurate and so exact that it rapidly
secured conviction. With the exceptions already mentioned, almost all
the eminent chemists of the day accepted his conclusions; and one,
Kirwan, who had written a formal treatise in defence of the phlogistic
theory, was so fair-minded, that after his work had been translated
into French and published with comments, he acknowledged that the old
theory was dead, and that truth had conquered.

It will be interesting now to trace Cavendish’s part in developing
the history of the discovery of the constituents of air, and to note
his arguments in favour of the phlogistic theory. Although Cavendish
never publicly acknowledged its insufficiency, yet he had ceased to
occupy himself with chemical problems at the time when its adoption was
universal, and his true opinions have never been recorded.

[4] _Experiments and Observations on Different Kinds of Air_, vol. ii.
    By Joseph Priestley, LL.D., F.R.S. Second edition (1776), p. 29.

[5] _Chemical Treatise on Air and Fire_ (1777), § 3.

[6] _Loc. cit._ p. 46.

[7] _Loc. cit._ p. 55.

[8] _Loc. cit._ p. 94.

[9] _Loc. cit._ p. 102.

[10] The accurate translation of Scheele’s _Treatise_ published by the
     Alembic Club (William F. Clay, 1894) has been made use of here.

[11] § 4.

[12] § 5.

[13] § 16.

[14] § 16.

[15] § 22.

[16] § 23.

[17] § 24.

[18] § 29.

[19] § 80.

[20] § 87.

[21] § 93.

[22] This, as previously remarked, had already been noticed. In
Maquer’s _Élémens de Chymie-pratique_, published in 1752, a work which
ran through many editions, we read (p. 307): “There happens during all
these calcinations, and especially in that of lead, a very strange
phenomenon for which it is very difficult to assign a reason. It is
that those bodies, which lose no small proportion of their substance,
whether by the dissipation of phlogiston, or because part of the
metal is exhaled as vapour, yield _calces_ increased in weight after
calcination; and this increase is by no means inconsiderable....
Physicists and chemists have devised many ingenious systems to account
for this phenomenon, but no one of them is absolutely satisfactory. As
no well-established theory has been devised, we shall not undertake to
attempt an explanation of this singular fact.”




CHAPTER IV

“PHLOGISTICATED AIR” INVESTIGATED BY CAVENDISH--HIS
DISCOVERY OF THE COMPOSITION OF WATER


While Lavoisier was engaged in experiments on oxygen, Cavendish,
too, was devoting his attention to the constituents of air, but in
a somewhat different manner. His early experiments led him to the
discovery of the composition of water; and it has already been pointed
out how necessary a knowledge of the true nature of hydrogen is to the
understanding of the phenomena of combustion. His second paper deals
with the inactive constituent of air, the mephitic portion, now known
as nitrogen or azote. But before considering these, a sketch of his
life will prove of interest.

The Honourable Henry Cavendish was a very singular man, retiring and
uncommunicative to a degree; hence little is known of his early life.
He was the elder son of Lord Charles Cavendish, who was the third son
of the second Duke of Devonshire. His only brother, Frederick, was
also an eccentric, but a very benevolent man, and the two brothers,
though they seldom met, lived on excellent terms with each other. Henry
Cavendish was born at Nice in October 1731. His mother died when he
was two years old. Nothing is known of his childhood and youth, save
that he attended Hackney School from 1742 to 1749, and that he went to
Cambridge in the end of 1749, and remained till 1753, without taking a
degree. After leaving Cambridge, it is supposed that he lived in London
for ten years. It is known that his allowance from his father amounted
to £500 a year, and that his rooms were a set of stables fitted up
for his accommodation. It is probable that this was his own choice,
and that he made use of them chiefly as a laboratory and a workshop.
Although at his father’s death and by the legacy of an aunt he acquired
a large fortune, he never spent more than a fraction of it. He left
more than a million sterling to his relative, Lord George Cavendish;
but they saw each other only once a year, and the interview seldom
lasted more than ten minutes. The writer of his obituary notice, M.
Biot, epigrammatically said:--“Il était le plus riche de tous les
savans, et le plus savant de tous les riches.”

[Illustration: H. Cavendish]

He was a regular attendant at the meetings of the Royal Society, of
which he was made a Fellow in 1760, and was a constant diner at the
Royal Society Club. It is said that he used to talk to his neighbour at
table so long as others did not join in the conversation; but if the
conversation took a general turn, he was silent.

His death took place in February 1810, and was as solitary as his
life. It is related by his servant that Cavendish, on feeling his end
approaching, dismissed him from the room, telling him to come back in
half an hour. He disobeyed instructions, and, being anxious, found some
pretext to enter the room. Cavendish ordered him away in a voice of
displeasure; and on returning, the man found his master dead.

Such a life demands our pity; yet, if an object of human life is to
give pleasure to its possessor, we can hardly say that Cavendish’s was
a failure. Ordinary mortals have a craving for the sympathy of their
fellows; Cavendish appears to have been devoid of any such sensation.
Indeed, his experiments were in many cases not published until long
after they had been made. He appears to have carried on his work for
his own information, and to have been indifferent to the impression
which his labours made on his fellow-men. Yet his inquiries cover a
more extensive field than those of almost any other man of science.
They begin with experiments on arsenic, by which he endeavoured to
determine the difference between the element arsenic and its two
oxides. He held that arsenic acid was more thoroughly “deprived of
phlogiston” than arsenious acid (_i.e._ more highly oxidised); and
on the same occasion he studied the effect of the addition of air to
nitric oxide, produced by the action of nitric acid on the element
arsenic and on arsenious oxide. His next experiments related to heat;
and had he published them, he would doubtless have anticipated Black in
his discovery of latent heat. His paper on “Factitious Airs,” published
in the _Philosophical Transactions_ for 1766, deals with the properties
of hydrogen, carbon dioxide, and the gases produced by the destructive
distillation of organic substances. As we shall see later, he supposed
that hydrogen, generated by the action of acids on metals, came out of
the metal, and was an unknown principle in combination with phlogiston,
if indeed it was not phlogiston itself; and this idea is not absurd,
for many metals, and indeed a very large number of minerals, evolve
hydrogen when heated, the gas having been “occluded” in their pores.

In 1772 he communicated privately to Dr. Priestley the results of a
series of experiments dealing with nitrogen. To prepare it, he passed
air repeatedly over red-hot charcoal, and absorbed the resulting
carbon dioxide in potash. The residue was nitrogen. His description
of it is:--“The specific gravity of this air was found to differ very
little from that of common air; of the two it seemed rather lighter.
It extinguished flame, and rendered common air unfit for making bodies
burn in the same manner as fixed air, but in a less degree, as a candle
which burned about 80 seconds in pure common air, and which went out
immediately in common air mixed with 6/55ths of fixed air, burned about
26 seconds in common air mixed with the same proportion of this burnt
air.”[23] He named it, as usual, “mephitic air,” and it is certain that,
although Cavendish did not publish his results, his discovery was
not later in date than Rutherford’s. Dealing next with the phenomena
observed when that curious fish, the torpedo, produces shocks, he
ascribed them to the discharge of electricity, and he was the first
to distinguish between intensity, or potential, and quantity of
electricity, a distinction now familiar to all.

It was in 1777 that he commenced his beautiful “Experiments on Air,”
the first account of which was published in 1783. They led to the
discovery of the constant quantitative composition of the atmosphere,
of the compound nature of water, and of the composition of nitric acid,
and pointed the way to the recent discovery of argon.

In determining the composition of the atmosphere, Cavendish made use of
nitric oxide in presence of water, as a means of removing oxygen. This
process, originally devised by Mayow, was rediscovered by Priestley,
who employed it to ascertain the “goodness” of various samples of
air; in Cavendish’s hands it became an accurate quantitative method.
The title of his paper, published in the _Philosophical Transactions_
for 1783, is “Of a new Eudiometer.” The term “eudiometer,” signifying
“measurer of goodness,” was devised when it was supposed that ordinary
air presented considerable variations in its power of supporting
respiration and combustion, according to the seasons, and according
to the place from which it was collected. Dr. Ingenhousz had found a
greater absorption when air from near the sea-coast was tested by
Priestley’s method with nitric oxide, than when town-air was employed;
and he ascribed the salubrious nature of sea-air to its being richer
in “vital air.” The Abbé Fontana, too, had made similar experiments,
and had come to similar conclusions. Cavendish modified Fontana’s
apparatus, rendering it capable of giving more accurate results; and
during the last half of the year 1781 he analysed the air collected on
sixty days, some fine, some wet, and some foggy. He also collected air
from different localities, sometimes at Marlborough Street, sometimes
at Kensington, which was then a country village. The results of his
analyses establish as the composition of air, freed from carbon dioxide
by potash:

    79·16 per cent of phlogisticated air (nitrogen).
    20·84 per cent of dephlogisticated air (oxygen).

This result does not differ materially from those obtained by the best
modern analyses, which give, within very small variations:

    79·04 per cent of nitrogen and argon,
    20·96 per cent of oxygen,

after absorption of carbon dioxide, ammonia, and water-vapour.

In the following year, 1784, Cavendish published the first of his great
memoirs, entitled _Experiments on Air_. His experiments were made
principally “with a view to find out the cause of the diminution which
common air is well known to suffer by all the various ways in which it
is phlogisticated, and to discover what becomes of the air thus lost or
condensed.”

Cavendish chose processes for “phlogisticating” air in the course of
which no fixed air should be produced. He therefore avoided the use of
animal and vegetable materials, and confined himself to combustibles,
such as sulphur or phosphorus, to the calcination of metals, the
explosion of inflammable air, and the mixture of nitrous air. He adds
as a suggestion, “Perhaps it may be supposed that I ought to add to
these the electric spark; but I think it much more likely that the
phlogistication of the air, and production of fixed air, in this
process is owing to the burning of some inflammable matter in the
apparatus.” We shall see later what magnificent results arose from this
last mode of “phlogisticating” air.

He begins with an account of a repetition of an experiment of Mr.
Waltire’s, related by Priestley, in which a mixture of hydrogen and air
was exploded in a copper vessel, with the result that they observed
a loss of a few grains in weight; it is also stated by Waltire that
if the explosion took place in a glass vessel, it became dewy, “which
confirmed an opinion he had long entertained, that common air deposits
its moisture by phlogistication.” But Cavendish, using a glass vessel
of much greater capacity than Waltire’s, could remark no change of
weight; and he concluded that 423 measures of hydrogen, or “inflammable
air” as he named it, are “nearly sufficient to completely phlogisticate
1000 of common air, and that the bulk of the air remaining after the
explosion is then very little more than ⅘ths of the common air
employed; so that, as common air cannot be reduced to a much less bulk
than that, by any method of phlogistication, we may safely conclude
that, when they are mixed in this proportion and exploded, almost all
the inflammable air, and about ⅕th part of the common air, lose their
elasticity, and are condensed into the dew which lines the glass.

“The better to examine the nature of this ‘dew,’ 500,000 grain measures
of inflammable air were burnt with about 2½ times that quantity of
common air, and the burnt air made to pass through a glass cylinder 8
feet long and about ¾ of an inch in diameter, in order to deposit the
dew”. “By this means upwards of 135 grains of water were condensed in
the cylinder, which had no taste or smell, and which left no sensible
sediment when evaporated to dryness, neither did it yield any pungent
smell during the evaporation; in short, it seemed pure water”. “And
by this experiment it appears that this dew is plain water, and
consequently that almost all the inflammable and about ⅕th of the
common air are turned into pure water.”

But on firing little by little a mixture of “dephlogisticated air” or
oxygen, obtained from red precipitate (that is, mercuric oxide prepared
by heating the nitrate), with twice its volume of “inflammable air” or
hydrogen, the resulting water was acid to the taste, and on evaporation
with alkali gave a small quantity--about 2 grains--of nitre. Cavendish
suspected that the acid came from the nitrate of mercury in his
red precipitate, and, to test this, procured his oxygen from other
sources--from red-lead and sulphuric acid, and from the leaves of
plants--but still with the same result; nitric acid was formed.
Repeating the experiment so as to have present an excess of hydrogen,
he found that no acid was produced.

“From the foregoing experiments it appears that when a mixture of
inflammable and dephlogisticated air is exploded in such proportion
that the burnt air is not much phlogisticated, the condensed liquor
contains a little acid, which is always of the nitrous kind,
whatever substance the dephlogisticated air is procured from; but
if the proportion be such that the burnt air is almost entirely
phlogisticated, the condensed liquor is not at all acid, but seems pure
water, without any addition whatever; and as, when they are mixed in
that proportion, very little air remains after the explosion, almost
the whole being condensed, it follows that almost the whole of the
inflammable and dephlogisticated air is converted into pure water.” The
quantity of uncombined gas was so small that it must be regarded as an
impurity. “There can be little doubt that it proceeds only from the
impurities mixed with the dephlogisticated and inflammable air, and
consequently that if those airs could be obtained perfectly pure, the
whole would be condensed.”

The next paragraph is interesting. “During the last summer also [of
1781] a friend of mine gave some account of them [these experiments]
to Mr. Lavoisier, as well as of the conclusion drawn from them, that
dephlogisticated air is only water deprived of phlogiston; but at
that time, so far was Mr. Lavoisier from thinking any such opinion
warranted, that, till he was prevailed upon to repeat the experiment
himself, he found some difficulty in believing that nearly the whole of
the two airs could be converted into water.”

And next comes an important deduction. “Phlogisticated air appears
to be nothing else than the nitrous acid united to phlogiston; for
when nitre is deflagrated with charcoal, the acid is almost entirely
converted into this kind of air.” This is the first statement of the
true relation between nitrogen and nitric acid; we should now state the
matter by the expression, “Nitrogen is nothing else than nitric acid
deprived of oxygen.” And the further deduction is made that “it is
well known that nitrous acid is also converted by phlogistication
into nitrous air, in which respect there seems a considerable analogy
between that and the vitriolic acid; for this acid, when united to a
smaller proportion of phlogiston, forms the volatile sulphurous acid
and vitriolic acid air, both of which, by exposure to the atmosphere,
lose their phlogiston, though not very fast, and are turned back
into the vitriolic acid; but when united to a greater proportion
of phlogiston, it forms sulphur, which shows no signs of acidity.”
“In like manner the nitrous acid, united to a certain quantity of
phlogiston, forms nitrous acid and nitrous air, which readily quit
their phlogiston to common air; but when united to a different, in
all probability a larger quantity, it forms phlogisticated air, which
shows no signs of acidity, and is still less disposed to part with its
phlogiston than sulphur.”

But the origin of the acid in water made from inflammable and
dephlogisticated air was still unexplained. To settle this point
Cavendish added to an explosive mixture of oxygen and hydrogen a tenth
of its volume of nitrogen, and found that the water was much more
strongly acid; and if hydrogen was much in excess, a still greater
amount of nitric acid was produced. After relating these experiments he
proceeds:--

“From what has been said there seems the utmost reason to think that
dephlogisticated air is only water deprived of its phlogiston, and
that inflammable air, as was before said, is either phlogisticated
water or else pure phlogiston, but in all probability the former.” In
a foot-note he gives his reason for the choice, viz. that it requires
a red-heat to cause hydrogen and oxygen to combine, while nitrous air
combines with oxygen at the ordinary temperature; now, if hydrogen
were pure phlogiston, one would expect it to combine more readily than
nitrous gas, which has been shown to be a compound of nitric acid with
phlogiston. It seems inexplicable that dephlogisticated air should
refuse to unite at the ordinary temperature with pure phlogiston, when
it is able to extract it from substances with which it has an affinity.
Hence it is unlikely that hydrogen is phlogiston itself.

And a few paragraphs farther on Cavendish very nearly discards the
phlogistic theory by this statement: “Instead of saying air is
phlogisticated or dephlogisticated by any means, it would be more
strictly just to say, it is deprived of, or receives, an addition of
dephlogisticated air; but as the other expression is convenient, and
can scarcely be considered as improper, I shall still frequently make
use of it in the remainder of this paper.”

And now we come to the consideration of Lavoisier’s new theory, and
its rejection in favour of the old one of phlogiston. It is curious to
follow the reasoning which made such an exceptionally acute thinker as
Cavendish deliberately reject the true explanation. Cavendish first
states his results in Lavoisier’s terms:--

“According to this hypothesis, we must suppose that water consists
of inflammable air united to dephlogisticated air; that nitrous air,
vitriolic acid air (sulphur dioxide), and the phosphoric acid are
also combinations of phlogisticated air, sulphur, and phosphorus with
dephlogisticated air; and that the two former, by a further addition
of the same substance, are reduced to the common nitrous and vitriolic
acids; that the metallic calces consist of the metals themselves united
to the same substance, commonly, however, with a mixture of fixed air;
that on exposing the calces of the perfect metals to a sufficient
heat, all the dephlogisticated air is driven off, and the calces are
restored to their metallic form; but as the calces of the imperfect
metals are vitrified by heat, instead of recovering the metallic
form, it should seem as if all the dephlogisticated air could not be
recovered from them by heat alone. In like manner, according to this
hypothesis, the rationale of the production of dephlogisticated air
from red precipitate is, that during the solution of the quicksilver
in the acid and the subsequent calcination, the acid is decompounded,
and quits part of its dephlogisticated air to the quicksilver, whence
it comes over in the form of nitrous air, and leaves the quicksilver
behind united to dephlogisticated air, which, by a further increase
of heat, is driven off, while the quicksilver resumes its metallic
form. In procuring dephlogisticated air from nitre, the acid is also
decompounded; but with this difference, that it suffers some of its
dephlogisticated air to escape, while it remains united to the alkali
itself in the form of phlogisticated nitrous acid. As to the production
of dephlogisticated air from plants, it may be said that vegetable
substances consist chiefly of three different bases, one of which
[hydrogen], when united to dephlogisticated air, forms water; another
[carbon] fixed air; and the third phlogisticated air [nitrogen]; and
that, by means of vegetation, each of these substances are decomposed,
and yield their dephlogisticated air; and that, in burning, they again
acquire dephlogisticated air, and are restored to their pristine form.

“It seems, therefore, from what has been said, as if the phenomena of
nature might be explained very well on this principle, without the help
of phlogiston; and indeed, as adding dephlogisticated air to a body
comes to the same thing as depriving it of its phlogiston and adding
water to it, and as there are perhaps no bodies destitute of water,
and as I know no way by which phlogiston may be transferred from one
body to another, without leaving it uncertain whether water is not
at the same time transferred, it will be very difficult to determine
by experiment which of these opinions is the truest; but as the
commonly-received principle of phlogiston explains all phenomena, at
least as well as Mr. Lavoisier’s, I have adhered to that.”

“Another thing which Mr. Lavoisier endeavours to prove is that
dephlogisticated air is the acidifying principle. From what has been
explained, it appears that this is no more than saying that acids
lose their acidity by uniting to phlogiston, which, with regard to
the nitrous, vitriolic, phosphoric, and arsenical acids, is certainly
true”. “But as to the marine acid and acid of tartar, it does not
appear that they are capable of losing their acidity by any union with
phlogiston.”

Here Cavendish does not consider the question of gain of weight on
loss of phlogiston, or if he does, he must ascribe it to simultaneous
entry of water. And experimental research at that time was not far
enough advanced to enable him to decide finally as to the truth of this
hypothesis.

In his next memoir, read before the Royal Society on June 2nd, 1785,
Cavendish relates experiments on the passage of electric sparks through
air, the experiment having first been tried by Priestley. Priestley
says:[24]--“Lastly, the same effect [_i.e._ the diminution of the volume
of common air], I find, is produced by the _electric spark_, though I
had no expectation of this event when I made the experiment.” And
again:--“At the time of my former publication, I had found that taking
the _electric spark_ in given quantities of several kinds of air had a
very remarkable effect on them, that it diminished common air and made
it noxious, making it deposit its fixed air exactly like any phlogistic
process; from whence I concluded that the electric matter either is or
contains phlogiston.”

Cavendish had mentioned this process casually as one of the methods of
phlogisticating air; in beginning his second paper he says:--“I now
find that, though I was right in supposing the phlogistication of the
air does not proceed from phlogiston communicated to it by the electric
spark, and that no part of the air is converted into fixed air; yet
that the real cause of the diminution is very different from what I
suspected, and depends upon the conversion of phlogisticated air into
nitrous acid.” The apparatus he used was very simple. It consisted
of a glass siphon filled with mercury, each leg dipping into a glass
likewise containing mercury; the air was admitted by a gas-pipette into
the bend of the siphon, and on connecting the mercury in one of the
glasses with a ball placed near the prime conductor of an electric
machine, and the other with the earth, sparks could be made to pass
from the mercury in one limb to that in the other.

The product obtained by passing sparks through air in this manner
turned litmus red, and gave rise to no cloud in lime-water, while the
air was reduced to two-thirds of its original volume; nor did the
lime-water give a precipitate on introducing some fixed air, this
showing that it had been saturated by an acid. It was found, too, that
“soap-lees,” or solution of caustic potash, if present, diminished
the volume more rapidly than did lime-water; and repeated trials
proved that “when five parts of pure dephlogisticated air were mixed
with three parts of common air, almost the whole of the air was made
to disappear.” The nitrate of potassium thus produced caused paper
soaked in it and dried to deflagrate; and it contained no sulphuric
acid. “There is no reason to think that any other acid entered into it
except the nitrous.” But it gave a precipitate with silver nitrate;
and Cavendish, suspecting that this was silver nitrite, prepared some
potassium nitrite by heating the nitrate; on comparing the white
precipitate which this solution gave with silver nitrate with that
obtained from his “soap-lees,” he found them identical. There was
therefore no “muriatic acid” present, which would have yielded chloride
of silver, of appearance somewhat similar to the nitrite.

As it had previously been shown to be probable that phlogisticated
air is nitrous air united with phlogiston, and that nitrous air is
nitric acid united with phlogiston, “we may safely conclude that in
the present experiments the phlogisticated air was enabled, by means
of the electric spark, to unite to, or form a chemical combination
with, the dephlogisticated air, and was thereby reduced to nitrous
acid, which united to the soap-lees and formed a solution of nitre; for
in these experiments the two airs actually disappeared, and nitrous
acid was actually formed in their room”. “A further confirmation of
the above-mentioned opinion is that, as far as I can perceive, no
diminution of air is produced when the electric spark is passed either
through pure dephlogisticated air or through perfectly phlogisticated
air, which indicates a necessity of a combination of these two airs to
produce the acid. Moreover, it was found in the last experiment that
the quantity of nitre procured was the same that the soap-lees would
have produced if saturated with nitrous acid; which shows that the
production of the nitre was not owing to any decomposition of the
soap-lees.”

Nothing more clearly shows the care with which Cavendish reasoned than
these last quotations. No loophole is left unstopped; every precaution
is taken to make the proof as faultless as it is possible for a proof
to be.

But this was not enough. It was necessary for Cavendish to show
that, so far as he could ascertain it experimentally, _all_ the
phlogisticated air was capable of combining with dephlogisticated air
to form nitre. This he next proceeded to do.

“As far as the experiments hitherto published extend, we scarcely know
more of the phlogisticated part of our atmosphere than that it is not
diminished by lime-water, caustic alkalies, or nitrous air; that it
is unfit to support fire or maintain life in animals; and that its
specific gravity is not much less than that of common air; so that
though the nitrous acid, by being united to phlogiston, is converted
into air possessed of these properties, and consequently, though it was
reasonable to suppose that part at least of the phlogisticated air
of the atmosphere consists of this acid united to phlogiston, yet it
might fairly be doubted whether the whole is of this kind, or whether
there are not in reality many different substances confounded together
by us under the name of dephlogisticated air. I therefore made an
experiment to determine whether the whole of a given portion of the
phlogisticated air of the atmosphere could be reduced to nitrous acid,
or whether there was not a part of a different nature from the rest,
which would refuse to undergo that change. The foregoing experiments,
indeed, in some measure decided this point, as much the greatest part
of the air let up into the tube lost its elasticity; yet, as some
remained unabsorbed, it did not appear for certain whether that was of
the same nature as the rest or not. For this purpose I diminished a
similar mixture of dephlogisticated and common air in the same manner
as before, till it was reduced to a small part of its original bulk. I
then, in order to decompound as much as I could of the phlogisticated
air which remained in the tube, added some dephlogisticated air to it,
and continued the spark until no further diminution took place. Having
by these means condensed as much as I could of the phlogisticated
air, I let up some solution of liver of sulphur to absorb the
dephlogisticated air; after which only a small bubble of air remained
unabsorbed, which certainly was not more than 1/120th of the bulk of
the phlogisticated air let up into the tube; so that, if there is
any part of the phlogisticated air of our atmosphere which differs
from the rest, and cannot be reduced to nitrous acid, we may safely
conclude that it is not more than 1/120th part of the whole.” We shall
afterwards see that this is a marvellously close estimate. There is
actually 1/84th part of the supposed nitrogen of the air which will not
combine with oxygen when sparked with it in presence of potash.

But there still remained, in Cavendish’s opinion, one point unproved.
It was still conceivable that the potash might contain some
“inflammable matter” which would diminish the air on sparking, and
therefore oxygen nearly pure was sparked in presence of potash; but
only a very small diminution of volume occurred, owing probably to some
nitrogen present as an impurity in the oxygen. Water was substituted
for potash with the same result; but if litmus was added to the water
the colour was discharged, and lime-water introduced into the tube
gave a cloud, showing that “the litmus, if not burnt, was at least
decompounded, so as to lose entirely its purple colour and to yield
fixed air; so that, though soap-lees cannot be decompounded by the
process, yet the solution of litmus can, and so very likely might the
solutions of many other combustible substances.”

Such are the chemical researches of Cavendish. Of all experimenters
on the subject he was undoubtedly the greatest, though Mayow and
Scheele were near rivals. But his researches were so complete that it
is scarcely possible to criticise. He was not content with partial
results: every point was proved and re-proved, and every possibility
of erroneous conclusion was allowed for. It is curious that he did not
employ the balance to check his results. Had he done so he could not
have remained an adherent of the phlogistic theory. Although, as we
have seen, he was perfectly acquainted with the method in which his
results were interpreted by Lavoisier, he chose the old well-trodden
path leading to the wilderness of distorted facts. Lavoisier tried to
repeat Cavendish’s experiments, but without success; and in 1788 the
last part of his _Experiments on Air_ was published, in which he
recorded the successful repetition by a Committee of the Royal Society
of the conversion of nitrogen into nitric acid by the electric spark in
presence of oxygen and potash.

His remaining papers deal with meteorological and astronomical
subjects. One, published in 1790, refers to the height of a remarkable
aurora seen in 1784; another to the civil year of the Hindoos; and
another to a method for reducing lunar distances. And in 1798 his
famous memoir on the density of the earth appeared. It would be quite
beyond the province of this book to enter into any detail regarding
it; but it may be remarked in passing that the method consisted in
measuring, by means of a torsion balance, the attraction of one leaden
ball for another, and that recent experiments, made with the utmost
refinement, have barely altered the number which he obtained,
5·4, to 5·527.

His last paper, on an improvement in a machine for dividing
astronomical instruments, was published in 1809, the year before his
death.

Nothing has been said here regarding the rival claims of Watt to the
discovery of the composition of water, and little need be said. The
discovery was made by both in 1784, yet Cavendish visited Watt at
Birmingham in 1785, and was apparently on the best of terms with him;
and Watt, as proved by Cavendish’s diary, showed him many of his
devices connected with the steam-engine. There can be no doubt that
Watt had also discovered that when hydrogen and oxygen are exploded
together water is the sole product, but he coupled the phenomenon with
views involving the material nature of heat, or caloric, as it was then
called, which Cavendish repudiated.

Cavendish’s later work was carried out in a villa at Clapham, which
was fitted as a laboratory, workshop, and observatory, but he had
a town-house near the British Museum, at the corner of Gower Street
and Montague Place. He had also a library in Dean Street, Soho, which
was available for any scientific man who chose to present himself. So
singular were Cavendish’s habits that when he wished a book he went to
this house and borrowed it as from a public library, giving a receipt
for it.

Of all men, Cavendish was probably the most singular, but there can be
no question of his extraordinary genius.

[23] _Brit. Assoc. Report_, 1839, p. 64.

[24] _Experiments and Observations on Different Kinds of Air_, vol. i.
     p. 181, and vol. ii. p. 238. Second edition, 1776.




CHAPTER V

THE DISCOVERY OF ARGON


With the advent of Lavoisier’s system of representing the phenomena
of combustion, and the expression in his terms of the various changes
resulting in air when metals are oxidised, and when carbonaceous
substances burn, the investigation of air was abandoned. It was no
longer regarded as a mysterious element, possessed of “chaotic”
properties, but was held to be a mixture of oxygen, nitrogen, and small
quantities of carbon dioxide and water vapour, together with a trace
of ammonia. More exact determinations of the proportion between its
oxygen and so-called nitrogen than Cavendish had made by the nitric
oxide method were carried out in 1804 by Gay-Lussac and Humboldt, by
explosion with measured quantities of hydrogen, according to the method
suggested by Volta; and they concluded, from a large number of analyses
made on specimens collected in all weathers and from various
localities, that 100 volumes of air contained 21 volumes of oxygen and
79 volumes of nitrogen. These experiments, too, led Gay-Lussac to the
conviction that oxygen and hydrogen unite to form water in the exact
proportion of one volume of the former to two volumes of the latter;
and he published, some years later, accounts of numerous experiments
of the same kind, as the result of which he found that, when two gases
combine or react with each other, they do so in some simple number of
volumes; for example, one to one, one to two, or one to three.

The almost constant relation between the volumes of oxygen and nitrogen
in air made it appear not unlikely, in the opinion of some, that air
was a compound, and not a mixture; for the law of combination in
definite proportions had by this time been enunciated by Professor
Thomas Thomson, Dalton’s intimate friend. But between the numbers
21 and 79 there exists no such simple ratio; and, moreover, on
artificially producing air by mixing oxygen and nitrogen, there are
none of the usual phenomena which characterise the formation of a
compound: there is no rise or fall of temperature, nor does the product
differ in any way in properties from the constituents. And in 1846
Bunsen showed that the proportion between oxygen and nitrogen is not a
constant one, but that the oxygen varies between 20·97 and 20·84; the
experimental error did not exceed 0·03 volume, while the difference
found amounted to 0·13 volume. Regnault, Angus Smith, A. R. Leeds, and
von Jolly confirmed these results at later dates, from analyses of air
collected from all parts of the world.

That air contains ammonia was first observed by Scheele. He found that
the stopper of a bottle containing muriatic acid, when exposed to air
became covered with a film or deposit which he recognised to be sal
ammoniac, or ammonium chloride.

The amount of ammonia in atmospheric air is, however, exceedingly
small, and it is best detected in rain-water, which dissolves it; thus
the air is considerably poorer in ammonia after a shower. The ammonia,
small though its proportion is, plays a great part, although not an
exclusive one, in yielding to plants their supply of nitrogen. The
rain, percolating through the soil, leaves the ammonia behind, in some
form of combination; and it is then attacked by the nitrifying ferments
and converted into nitrates, from which the plants derive the nitrogen
which forms part of their substance, in combination with carbon,
oxygen, and hydrogen.

There are also traces of nitric and nitrous acids in air, which are
apparently in combination with ammonia. While the ammonia has been
found to vary between 0·1 and 100 volumes per million volumes of
air--the latter number refers to Manchester streets--nitrous and
nitric acids are present in still smaller amounts; and in spite of
the widespread opinion that ozone is contained in air, its occurrence
is still a matter of dispute. That some powerful oxidising agent
such as ozone or hydrogen peroxide is present appears certain; but
the characteristic test for ozone--the formation of peroxide of
silver on exposure of metallic silver to its influence--has never
been successful. On the other hand, a small quantity of hydrogen
dioxide--also, like water, a compound of oxygen and hydrogen, but one
containing more oxygen than water--appears to be almost constantly
present in air. Its amount is also extremely minute: it does not exceed
one part per million. Its presence in air was discovered by Schönbein.
The atmosphere further contains dust, some of which appears to
consist largely of metallic iron, which is conjectured to be of
extraterrestrial origin--minute meteorites in fact--and also the
spores of micro-organisms; but these spores, however important from
a biological or a sanitary point of view, hardly come within the
scope of the chemical composition of air. They serve to emphasise the
conjectures of Boyle and of Scheele that air may contain “corpuscles”
of all sorts, some in the form of dry exhalations, while other
innumerable particles may be sent out from the celestial luminaries.

       *       *       *       *       *

Up to within the last few years it was supposed that the constituents
of air had all been discovered. But Lord Rayleigh and Professor William
Ramsay have recently found that the supposed nitrogen of the air is
in reality a mixture of nitrogen with a new gaseous element, to which
they have given the name “argon,” on account of its chemical inactivity
(=ἂργον=, idle, inactive).

In his presidential address to Section A of the British Association at
Southampton in 1882, Lord Rayleigh alluded to an investigation which he
had begun on the densities of hydrogen and oxygen, relatively to each
other. The object of the research was to discover whether the atomic
weights of these gases, determinable from their densities and from the
proportions by volume in which they combine, was actually as 1 to 16,
or whether some fractional number was necessary to express the weight
of an atom of oxygen relatively to that of hydrogen. In 1888 his first
account of the determination was published in the _Proceedings_ of the
Royal Society. In 1889 he published a continuation of his first paper,
and in 1892 he gave his final results; the number obtained was 15·882
for the atomic weight of oxygen, calculated from its density, hydrogen
being taken as 1. In 1893 further experiments on densities were
published,[25] those of oxygen and nitrogen being specially considered
with reference to the density of air. He found the weights of one litre
of oxygen, nitrogen, and air to be

     Oxygen          1·42952 grams
    “Nitrogen”       1·25718   "
     Air             1·29327   "

A simple calculation leads to the composition of purified air. The
percentage of oxygen must be 20·941, and that of “nitrogen” 79·059, in
order to give a mixture of which the weight of a litre is 1·29327. Now,
this corresponds with the results of the best analyses, quoted above.
And the accuracy of these determinations of density is confirmed by
this means, as well as by results of other experiments made by Leduc,
von Jolly, and Morley.

But Lord Rayleigh was not content to prepare his gases by one process
only. The oxygen, of which the mean value of the weight of a litre is
given above, was prepared in three different ways: by the electrolysis
of water, by heating chlorates, and by heating potassium permanganate.
The results showed that the only difference which could be detected,
and that an extremely minute one, must be attributed to experimental
error. The actual weights of the contents of his globe were--

    Electrolysis, May 1892               2·6272 grams
          "        "    "                2·6271   "

    Heating chlorates, May  1892         2·6269   "
       "        "      June   "          2·6269   "

    Heating permanganate, January 1893   2·6271   "

These numbers are subject to a deduction of 0·00056, due to the fact
that when the globe was empty of air, its capacity was somewhat
reduced, owing to the external pressure of the atmosphere.

It was next deemed necessary to test whether nitrogen was homogeneous
by preparing it too by several different methods. In the same paper
Lord Rayleigh (p. 146) mentions that nitrogen, prepared from ammonia,
its compound with hydrogen, is somewhat lighter than “atmospheric
nitrogen,” the deficiency in weight amounting to about 1 part in 200.
Now it is evident from inspection of the numbers quoted above, that
the accuracy of the density determination may be trusted to within
1 part in 10,000, and that the balance would detect a discrepancy
one-fiftieth of that observed in the densities of “atmospheric” and
“chemical” nitrogen. In a letter to _Nature_, Lord Rayleigh asked for
suggestions from chemists as to the reason of this curious anomaly, but
his letter went without reply. He himself was inclined to believe that
the difference was due to the decomposition of some of the ordinary
molecules of nitrogen, usually believed to consist of two atoms in
union with each other, into molecules consisting of one atom; and as it
is held that equal numbers of molecules inhabit the same volume,
temperature and pressure being equal, if the total number of molecules
in his globe were increased by the splitting of some double-atom
molecules into single-atom molecules, the effect would be that, owing
to an admixture of some lighter molecules, the density would be
somewhat reduced.

But two other suppositions were entertained as possible. The oxygen
might have been imperfectly removed from the nitrogen derived from the
atmosphere; or, on the other hand, the nitrogen from ammonia might
conceivably have retained traces of hydrogen. In the former case, the
nitrogen would have an increased weight owing to admixture of some
heavier oxygen; in the latter, a diminished weight, due to the presence
of the lighter hydrogen. The first of these suppositions is out of
the question, inasmuch as it would have required that the nitrogen
should contain one-thirtieth of its volume of oxygen, or one-sixth of
that present in air, in order that its density should be raised by one
two-hundredth; for the densities of oxygen and nitrogen are not so very
different. The second supposition was negatived by introducing hydrogen
purposely, and removing it by passing the gas over red-hot copper
oxide, which oxidises the hydrogen to water. This yielded nitrogen of
the same density as that which had not undergone that treatment.

One other possibility was considered: the atmospheric nitrogen might
contain some molecules of greater complexity than two-atom molecules,
say N_{3}-molecules. Now it is known that when oxygen is electrified
by the passage of a rain of small sparks through it, it acquires
new properties: it possesses an odour, and attacks metallic mercury
and silver. And this product, ozone, has been shown to consist of
three-atom molecules of oxygen, by various experiments of which an
account cannot be given here.

It was not inconceivable that if such a “silent electric discharge”
were to be passed through “atmospheric” nitrogen, it might increase the
number of such three-atom molecules, and might render the gas still
denser; or if passed through “chemical” nitrogen, it might increase its
density so as to make it equal to that of “atmospheric” nitrogen. Lord
Rayleigh made such experiments, but without changing the density in the
least: the nitrogen from ammonia or from oxides of nitrogen, which has
been termed “chemical” nitrogen, still remained too light by about one
two-hundredth, and the atmospheric nitrogen still remained too heavy by
the same amount.

At this stage Professor Ramsay asked and received permission to make
some experiments on the nitrogen of the atmosphere, with the view of
explaining its anomalous behaviour. He had several years before made
experiments on the possibility of causing nitrogen and hydrogen to
combine directly, by passing the mixture over heated metals; among
these was magnesium, and although no direct combination to any great
extent was observed, still it was noticed that magnesium was a good
absorbent for nitrogen, when that gas was passed over the red-hot
filings of the metal. This process was therefore applied to the
absorption of “atmospheric” nitrogen, in order to find out whether any
portion of it was different from the rest. The plan adopted was to
heat turnings of magnesium, which can be made very thin and loose, to
redness in a tube of hard glass, in contact with the nitrogen of the
atmosphere, carefully purified from oxygen, which would otherwise have
also combined with the metallic magnesium. As absorption proceeded,
more nitrogen was admitted from a reservoir, and after a certain
quantity had been absorbed, the residual gas was extracted from the
tube by a mercury pump, and weighed.

The amount weighed was very small,--smaller perhaps than had up till
then been thought possible, if accurate results were to be obtained.
But here large differences were to be looked for. Only 40 cubic
centimetres--the twenty-fifth part of a litre--was weighed; and its
weight was only 0·050 gram. But with careful weighing the error should
not exceed one five-hundredth of the amount weighed; and if there were
to be any increase in density, that increase should be expected greatly
to exceed this small fraction.

The first weighing--in May 1894--showed that the nitrogen had increased
in density by reason of the operations, and instead of being fourteen
times as heavy as hydrogen, it was nearly fifteen times as heavy.

[Illustration: FIG. 1.]

The result was encouraging, and led to the probability of the nitrogen
being altered in some way, or of the presence of some new component of
the atmosphere. An experiment was therefore begun on a larger scale,
the atmospheric nitrogen being passed backwards and forwards from
one large glass gasholder A to another B, through a tube filled with
magnesium heated to redness G, to absorb nitrogen; over red-hot copper
oxide (_a_) (_b_), so that any carbonaceous matter such as dust should
be oxidised to carbon dioxide and water; and these, if produced, were
absorbed by placing in the train of tubes, one filled with a mixture
of soda and lime F and I, to absorb any carbon dioxide which might
possibly be formed, and two filled with pentoxide of phosphorus D and
H, to dry the gas, so that water-vapour, carried along with the gas
from the gasholders (which contained water) might be removed before
the gas passed over the red-hot magnesium; for water acts on hot
magnesium, forming oxide of magnesium and hydrogen, and the gas would
have become contaminated with the latter had this precaution not been
taken.

The process was continued for ten days, by which time most of the
nitrogen had become absorbed. The apparatus was then somewhat altered,
so as to make it possible to work with a smaller quantity of gas; but
the tubes destined to absorb nitrogen, hydrogen, etc., were filled with
the same materials as before. In a few days more the volume was reduced
to one-seventh of what it had been when the transference to the smaller
apparatus was made, or about one-eightieth of the original volume of
the atmospheric nitrogen taken.

The gas was then weighed, this time in a larger bulb, the weight being
0·2190 gram; and such is the possibility of precision in weighing on
a good balance, that a difference of one two-thousandth of the whole
weight was detectable. The density of the gas was now found to be 16·1.
At this stage it was still believed that the new gas was an ozone-like
modification of nitrogen, difficult to attack by magnesium. It was
supposed that just as oxygen, when exposed to an electric discharge,
undergoes a cleavage of its molecules, two-atom molecules becoming
one-atom molecules for an instant, which then unite to form three-atom
molecules, so the action of the magnesium on the nitrogen might be to
withdraw one atom of nitrogen from the two-atom molecule, leaving a
single uncombined atom, which might not improbably find two partners,
each of its own kind, to form with them a three-atom molecule--a sort
of nitrogen-ozone, in fact. Hence it was resolved to continue the
absorption with fresh magnesium for a still longer time, in the hope of
its being possible to isolate the three-atom nitrogen molecules. But
it became apparent that the bright metallic magnesium was now not much
attacked; and on estimating the total amount of nitrogen absorbed,
by treating the compound of nitrogen and magnesium with water, and
liberating the nitrogen as ammonia, it appeared that only a small
quantity of magnesium nitride had been formed. The density of this
further purified gas was again determined, when it was found that a
litre now weighed 1·7054 gram, corresponding to a density of 19·086.

A portion of this gas was mixed with oxygen and exposed to a rain of
electric sparks in presence of caustic soda; in fact, Cavendish’s old
plan of causing nitrogen to combine was now resorted to. Contraction
occurred, and on removing the excess of oxygen, the diminution of
volume was found to amount to 15·4 per cent of the original volume
taken. Making the supposition that the gas of density 19 still
contained nitrogen, and allowing for its influencing the density, it
followed that the pure gas should be twenty times as heavy as hydrogen.

A tube such as is usually employed in examining the spectra of gases
at low pressures was next filled with the gas of density 19. Such a
tube, called a Pflücker’s tube, after its inventor, contains wires of
platinum sealed through at each end, where it is about half an inch
in width; the middle portion of the tube is about 3 inches long, and
its bore is a fine capillary. When the platinum wires are connected
with the secondary terminals of a Ruhmkorff’s coil, and the tube is
partially exhausted, a brilliant glow appears in the capillary portion.
If viewed through a glass prism, different gases show different sets
of coloured lines crossing the usual gradation of colours of the
spectrum. Thus hydrogen exhibits three striking lines, one bright red,
one peacock blue, and one violet; nitrogen shows a large number of
somewhat hazy bands, red, orange, yellow, and yellow-green in colour,
besides a number of bands of a violet colour; but the new gas, while
exhibiting the bands characteristic of nitrogen, showed in addition
certain groups of red and green lines which did not appear to belong to
the spectrum of any known gas.

[Illustration: FIG. 2.]

While these experiments were in progress, Lord Rayleigh was occupied in
preparing nitrogen from other sources, and in determining its density;
and in every case it was evident that nitrogen from all sources except
the atmosphere weighed somewhat less than atmospheric nitrogen.
He therefore proceeded to repeat Cavendish’s experiment, and like
Cavendish, he obtained a small residue of gas which would not disappear
on sparking with oxygen, in presence of caustic soda. The sparks, as
they passed, could be observed through a spectroscope (which consists
of an arrangement of prisms and lenses so designed as to examine the
components of the light emitted by the sparks), and he, too, was struck
with the unusual character of the spectrum. His experiments proved,
besides, that the amount of residue was roughly proportional to the
amount of air taken; thus, beginning with 50 cubic centimetres of air,
the residue was 0·32 cubic centimetre; and from 5 cubic centimetres of
air, only 0·06 cubic centimetre of gas was obtained.

These small amounts are not proportional to the quantities of air
taken; but, as will afterwards be seen, the discrepancy is owing to
the solubility of the new gas in water. Still they served to show that
from a comparatively large amount of air, more of the new gas could be
obtained than from a smaller amount.

At this stage the two discoverers joined forces, and letters passed
almost daily between them, describing the results of experiments which
one or other had made. And just prior to the meeting of the British
Association at Oxford in August 1895, it was decided that the proof of
the existence of a new constituent gas in air was sufficiently clear to
render it advisable to make to the Association a short announcement of
the discovery. The statement was received with surprise and interest;
chemists were naturally somewhat incredulous that air, a substance of
which the composition had been so long and so carefully studied, should
yield anything new. One of the audience inquired whether the name of
this new substance had been discovered; as a matter of fact it was then
under consideration.

But it was still conceivable, although improbable, that the new gas was
being produced by the very processes designed for its separation, and
attention was first turned to devising a complete proof of its actual
presence in air. Now it is known that the rates of diffusion of gases
through a narrow opening, or through a number of minute holes, such
as exist in a pipe of porous clay, _e.g._ a tobacco-pipe stem, are in
inverse proportion to the square roots of the densities of the gases.
Oxygen is, in round numbers, sixteen times as dense as hydrogen; the
square roots of 16 and 1 being 4 and 1, it was found by Graham, who
first carefully investigated this subject, that four times as much
hydrogen would pass through a porous diaphragm, in a given time, as
oxygen. The _compound_ of hydrogen and oxygen, however, in the state
of gas, _viz._ steam, is not separated by such a process into its
constituents; it diffuses as such, and since it is nine times as dense
as hydrogen, the relative rates of diffusion of steam and hydrogen are
as 1: √9, or as 1 to 3; that is, for every 3 parts of hydrogen
passing through such a septum, 1 part of steam would pass in the same
time.

An experiment was therefore devised, in which a large quantity of air
was made to stream slowly through a long train of stems of churchwarden
tobacco-pipes, placed inside a glass tube, the latter being closed at
each end, except for the entrance and exit tubes of the tobacco-pipes;
in the encasing glass tube a vacuum was maintained, and the gases,
passing through the walls of the pipe-stems, were pumped off and
discharged. According to what has just been said, these should be the
lighter gases, nitrogen and oxygen, which ought to pass through the
porous stems more quickly than the supposed heavier constituent of air;
while the air issuing from the end of the train of pipes should contain
relatively more of the heavier constituent, and should in consequence
have a greater weight than an equal volume of air. But it was obviously
convenient to remove the oxygen before weighing this sample of altered
air, and this was done in the usual way by passing the mixed gases
over red-hot copper. It was found that such nitrogen was even heavier
than ordinary atmospheric nitrogen; not much, it is true, but still
consistently heavier. The denser constituent could, in fact, be
concentrated by this means. The proof was therefore indubitable that
the new gas existed in air as such.

There is another method of proof, however, which was not left untried.
Experiment showed that the solubility of the new gas in water is
considerably greater than that of nitrogen, although less than that of
oxygen. In 100 volumes of water at the ordinary temperature, about 1·5
volumes of nitrogen will dissolve, about 4·5 volumes of oxygen, and
about 4 volumes of the new gas, to which the name finally chosen for
it, “argon,” may now be applied. Now the proportion in which the
constituents of a mixture of gases will dissolve in a solvent is
conditioned first by their relative solubilities, and second, by their
relative proportion. Thus, if air be considered to be simply a mixture
of 1 volume of oxygen and 4 volumes of nitrogen, the gas extracted from
water which has been shaken with air will have the composition--

     Oxygen    1 x 4·5  =  4·5 volumes
     Nitrogen  4 x 1·5  =  6·0    "

So that the proportion of oxygen to nitrogen in such a mixture of gases
is considerably greater than in air: instead of being approximately 1
to 4, it is nearly 4·5 to 6. The discovery of this law concerning the
composition of the gases dissolved in liquids was due to Dr. Henry, one
of the biographers of Dalton.

The gases can be almost entirely extracted by boiling the water. But to
boil large quantities of water at one operation in a vessel suitable
for collecting the escaping gas is not easy. It is much simpler to
cause the water to pass slowly through a can below which there is a
powerful flame, so that the water in its passage becomes heated to
the boiling-point, and gives off its gas before it escapes. Of course
the gas collected contained oxygen, but this was easily removed by
the usual method of passing it over red-hot copper. The density of
the residual gas was determined, and it was found to be at least as
much greater than that of “atmospheric” nitrogen as the density of
“atmospheric” nitrogen exceeded that from chemical sources. Hence it
was to be concluded that the new constituent of air, argon, was being
concentrated by dissolving air in water, and extracting the dissolved
mixture of gases. A third proof that argon exists in air will be given
farther on.

[Illustration: FIG. 3.]

In order that the properties of the newly-discovered gas, argon, might
be thoroughly investigated, it was necessary to prepare it on a much
larger scale than had hitherto been attempted, and this was carried out
by the two processes for removing the oxygen and nitrogen which have
been already described. Supposing the new gas to have the density 20
compared with oxygen as 16, the density of the atmospheric mixture of
nitrogen and argon compared with that of nitrogen alone shows that air
should, roughly speaking, contain less than one part of argon in one
hundred. Hence, to obtain a litre of argon, it was necessary to work up
a large quantity of atmospheric nitrogen. Now, as has just been said,
there are two ways of doing this. (1) One is to produce an electric
flame between two pieces of stout platinum in air, confined in a large
glass balloon of about 6 litres capacity, over a weak solution of
caustic soda. For this purpose a very powerful rapidly alternating
current is necessary. The latest, and apparently the best, method
of carrying this out, was described by Lord Rayleigh in his Royal
Institution lecture in January 1896. The neck of the balloon is placed
downwards, and connected by means of a glass tube, passing through
a cork which closes the neck, with a rotating fan or paddle-wheel
with curved blades, which forces through the tube a weak solution of
caustic soda; another tube, also entering through the cork, conveys
away the excess of soda to the fan, whence it is again forced into
the balloon. The soda solution makes a fountain in the balloon, and
flows in a uniform stream down its sides, covering its inner surface
with a thin layer of liquid. Through the cork the two electrodes,
with their thick platinum terminals, enter; and there is another tube
besides, which conveys into the balloon a mixture of air and oxygen
in such proportions that they combine completely on exposure to the
flame. The layer of soda solution plays a double part. It prevents the
undue heating of the glass balloon, which otherwise must be sunk in
running water in order to keep it cool; and it exposes a very large
and constantly renewed surface of soda to the nitrous fumes which are
produced by the combination of the nitrogen and the oxygen, and so
removes them as quickly as they are formed. It appears probable that
the union results initially in the formation of nitric oxide, NO, which
then unites partially with oxygen to form some nitrogen peroxide,
NO_{2}. This is absorbed by the soda, giving a mixture of nitrite and
nitrate of sodium, NaNO_{2} and NaNO_{3}. Working in this way, from 7
to 8 litres of mixed gases can be made to combine per hour. The rapidly
alternating current is best obtained by the use of a transformer; and
as the heating effect on the platinum terminals is very great, they
must be made of stout rods.

(2) To prepare a large quantity of argon by the absorption of
atmospheric nitrogen by magnesium is a somewhat tedious process.
The air must be first freed from oxygen by means of red-hot copper,
and the atmospheric nitrogen collected in a gasholder. Long tubes
of combustion-glass tubing, which stands a bright red heat without
becoming deformed, are packed with magnesium turnings and heated to
redness in long gas furnaces, such as are used in organic analyses;
and through these the “atmospheric nitrogen,” dried by passage over
soda-lime and phosphorus pentoxide, is then passed. The magnesium
begins to glow at that end of the tube nearest the entrance, owing to
its combination with nitrogen, and a hot ring is seen to travel
slowly down the tube to the other end, marking the place where such
combustion is in progress. The gas issuing from the tube is collected
in a small gasholder. When one tube of magnesium is exhausted, another
is substituted for it. Each tube is capable of absorbing about seven
litres of nitrogen, so that to obtain a litre of argon about one
hundred litres of “atmospheric nitrogen” must be employed, and about
fourteen tubes of magnesium are required. M. Maquenne, who has prepared
the nitrides of several metals, has stated that a mixture of lime and
magnesium, yielding metallic calcium, is more easily manipulated than
pure magnesium, owing to the absorption of the nitrogen at a lower
temperature. The process has not been tried on a large scale, but if
the temperature of combination of magnesium and nitrogen could be thus
reduced, it would much facilitate the operation, for the greatest care
has to be taken not to overheat the combustion tube, else it softens,
and blows into holes. Porcelain tubes are attacked by the magnesium,
and crack on cooling; and iron tubes are difficult to clean out.

This preliminary operation does not yield pure argon; it merely
removes a large portion of the nitrogen. To free the argon from the
remainder, it is caused to circulate (by means of a specially contrived
mercury-pump, where each drop of mercury in falling down a narrow glass
tube carries before it a small bubble of gas) through tubes containing
red-hot copper, red-hot copper oxide, red-hot magnesium, and cold
soda-lime and phosphoric anhydride. The copper serves to remove traces
of oxygen; the copper oxide yields up its oxygen to any hydrogen or
carbon compound--dust and the like--which may happen to be present;
the soda-lime absorbs any carbon dioxide produced by the combustion of
the carbon compounds, and at the same time partially dries the gas;
while the phosphoric anhydride effectually dries the gas, previous to
its passage over the red-hot magnesium, which in its turn removes the
nitrogen. It is necessary to continue this circulation for several days
before the litre of gas is entirely freed from nitrogen.

It is difficult to choose between these two methods: both are
troublesome, and require a considerable time, but in an ordinary
laboratory the latter is probably the more easily set in operation, for
the former requires a suitable electric current, and power, so as to
rotate the water-fan. Up to the present date, the only sources which
have yielded argon are atmospheric air, gases extracted from mineral
waters or from springs, one meteorite, and a few rare minerals. No
animal or vegetable substance appears to contain it. Experiments were
made in the summer of 1895 by Mr. George MacDonald and Mr. Alexander
Kellas, in order to decide whether argon was a constituent of any
living matter. Some peas were reduced to powder and dried; the carbon
and hydrogen of the peas were burned to carbon dioxide and water
by heating with oxide of copper, and under these circumstances the
nitrogen is evolved in the state of gas. Had argon been contained in
the vegetable, it too would have accompanied the nitrogen. The nitrogen
was then, as usual, absorbed for the most part by means of magnesium,
and the small unabsorbed residue was mixed with oxygen and exposed to
electric sparks for many hours, in presence of caustic soda. There
was _no_ residue left after absorbing the excess oxygen: the gas was
completely removed. Similar experiments carried out on animal tissue
led to a similar conclusion. Two mice were chloroformed, and when dead
they were dried in an oven until all the moisture of their bodies was
completely driven off, and it was possible to reduce them to powder. It
is interesting to note that one of these mice contained 73 per cent of
water, and the other 70·5 per cent. The dried animals yielded about 11
per cent of their weight of nitrogen. Absolutely no residue of gas was
obtained on causing this nitrogen to combine; hence it appears to be a
legitimate conclusion that neither animal nor vegetable tissue contains
any appreciable amount of argon.

But these experiments lead to a further result. They show that
nitrogen, procured from its compounds, when treated in the same way as
atmospheric nitrogen, yields no trace of argon. And it must therefore
be taken as proved without doubt that argon is actually present in the
atmosphere as such, and is not produced by any process to which the
nitrogen has been submitted in order to extract it.

This point having been settled, the actual percentage of argon in
atmospheric air next invited inquiry. It is by no means very easy to
absorb quantitatively the whole of the nitrogen from an accurately
measured sample of air, for small gains and losses are apt to occur.
It is necessary to keep the air out of contact with water as much as
possible, because argon, being more soluble than nitrogen, dissolves
in larger proportional amount in the water, and is thereby partially
removed. The air was therefore entirely manipulated over mercury. The
processes were like those previously employed: most of the nitrogen was
removed with magnesium, and the residue was freed from all nitrogen by
sparking with oxygen. Experiments directed to this end were carried
out by Mr. Kellas in Professor Ramsay’s laboratory, and independently
by M. H. Schloesing in Paris. The results were identical. “Atmospheric
nitrogen” consists of pure nitrogen mixed with 1·186 per cent of its
volume of argon.

It is now possible, knowing the percentage of argon in atmospheric
nitrogen and its density (19·94), to calculate whether Lord Rayleigh’s
determinations of the density of atmospheric nitrogen were correct.
The weight of one litre of pure nitrogen is 1·2511 gram, and of argon,
1·7818 gram; hence a litre of a mixture of 98·814 volumes of nitrogen
with 1·186 volume of argon must possess the weight 1·2574 gram. The
actual number found by Lord Rayleigh was 1·2572 gram, which is almost
exactly identical with the number calculated.

Mineral waters, as a rule, contain small quantities of argon mixed
with oxygen, nitrogen, carbon dioxide, and in some cases sulphuretted
hydrogen and helium, a gas of which more hereafter. The waters actually
examined were the Bath waters, which contain much nitrogen, a little
argon, and a trace of helium; the Buxton waters, containing nitrogen
and a little argon; the water from “Allhusen’s Well,” Middlesborough,
which evolved gas of an inflammable nature consisting mainly of
nitrogen, but also containing marsh-gas, and argon to the extent
of O·4 per cent; water from boiling springs in Iceland evolved gas
containing somewhat more argon than air does, viz. 1·14 per cent; and
lastly, water from the Harrogate sulphur springs yielded a gas largely
consisting of a mixture of sulphuretted hydrogen, carbon dioxide
and nitrogen, but giving also an appreciable amount of argon. Such
determinations show that argon is not merely confined to the atmosphere
above the earth, but that it penetrates the earth and is contained in
subterraneous water. These results have been obtained by Lord Rayleigh,
Professor Ramsay, Mr. Travers, and Mr. Kellas.[26]

Similar experiments have been made by Dr. Bouchard in Paris[27] on
effervescing waters from Cauterets in the Pyrenees. One of those
springs yielded a mixture of nitrogen with a small amount of argon
and helium; another yielded only nitrogen and argon; while a third
gave nitrogen and helium. Such are, up to the present, the sources
of argon. It has been several times stated that the element helium,
which is closely allied to argon in its physical properties, and in
its inertness, is a normal constituent of our atmosphere, although in
very small amount. This, however, is not the case. For the argon of the
atmosphere has been very carefully examined for helium in two ways.
First, Lord Rayleigh dissolved a large quantity of argon in water,
leaving only a minute bubble undissolved. Now while 1000 parts of water
dissolve 40 parts of argon, they dissolve only 7 parts of helium; and
if helium were present, it should be found in the residue. But careful
spectroscopic examination failed to reveal the characteristic lines
of helium. Next, Professor Ramsay and Dr. Collie diffused argon
fractionally; and as the densities of argon and helium are very
different (2 and 20), the helium should have been visible in the
first portion which diffused. There was no trace to be detected.
But, further, they submitted this portion of argon to a discharge of
electricity for several hours in a vacuum-tube provided with platinum
electrodes. This process, as they proved, carries out the helium
with the platinum which is splashed on to the sides of the tube. The
argon was then removed by pumping it off in the cold. On heating the
splashed-off platinum, no helium spectrum could be observed. In a
similar experiment, in which a very minute trace of helium had been
added to the argon, there was no difficulty in detecting the helium on
heating the tube. The conclusion therefore follows that no helium is
present in our atmosphere.

It is, besides, exceedingly improbable that helium should be present.
For, as shown by Dr. Johnstone Stoney, the rate of motion of a molecule
of hydrogen is so rapid that, on finding its way to the confines of the
atmosphere, it would escape, and travel through space until it found a
planet of sufficient attractive force to hold it. The same is true of
helium. Were helium present in the atmosphere, it would ultimately
leave us for the sun, or for some planet of much greater mass than the
earth. This conception of Dr. Stoney’s tallies with the observation
that the moon, a planet of small mass, is devoid of an atmosphere, and
that the sun, a body with a mass 300,000 times as great as the earth,
shows in its chromosphere the spectra both of hydrogen and of helium
with great brilliancy.

It is now of interest to inquire what are the properties of argon and
how it is related to other elements.

[25] _Proc. Roy. Soc._ vol. liii. p. 134.

[26] _Proc. Roy. Soc._ vol. lix. p. 68.
     _Phil. Trans._ vol. clxxxvi. p. 227.

[27] _Compt. rend._ vol. cxxi. p. 394.




CHAPTER VI

THE PROPERTIES OF ARGON


The density of a gas is one of its most characteristic and important
properties. Avogadro’s law, which postulates that equal volumes of
gases, at equal temperature and pressure, contain equal numbers of
molecules, renders it possible to compare the weights of the molecules
by determining the relative weights of the gases. Thus, as the ratio
between the densities of nitrogen and oxygen is 7 to 8, a single
molecule of nitrogen, the smallest portion which can exist in freedom,
uncombined with other elements, is ⅞ths of the weight of a single
molecule of oxygen. Hence a determination of the density of argon leads
directly to a knowledge of the relative weight of a single molecule of
this gas.

But with what should the density of argon be compared? What gas must
serve as the standard of density? To answer this question, it is
necessary to give a short sketch of the development of chemical theory
regarding the atomic weights of elements and their relative volumes.

Dalton proposed to adopt as the unit of atomic weight the weight of the
lightest atom, namely, that of hydrogen. Taking, for example, water as
one substance containing hydrogen, its percentage composition by weight
is--

     Hydrogen      11·11 per cent
     Oxygen        88·88    "

If the smallest portion of water capable of free existence contains
one atom of hydrogen and one of oxygen, then placing the weight of an
atom of hydrogen as unity, the weight of an atom of oxygen is eight
times as great. And although we do not know the absolute weight of
any single atom, we are justified in supposing that an atom of oxygen
is eight times as heavy as an atom of hydrogen. But have we any right
to make the assumption that a molecule of water contains one atom of
each element? Dalton came to the conclusion that this supposition was
a justifiable one; but there are strong reasons against it. We have
already seen that Cavendish discovered approximately, and that
Gay-Lussac and Humboldt determined accurately, that when hydrogen and
oxygen unite to form water, two volumes of the former combine with one
of the latter. Now it appears improbable on the face of it that any
given volume of hydrogen should contain only half as many particles as
an equal volume of oxygen; and it is still more improbable, when we
take into consideration (1) Boyle’s discovery that if the pressure on a
gas be increased, the volume of the gas, whatever it may be, diminishes
in like proportion; and (2) Gay-Lussac’s and Dalton’s discovery, that
all gases, when equally raised in temperature, expand equally. It would
be very remarkable if one gas, containing twice as many particles in
unit volume as another, should show exactly similar behaviour towards
pressure and temperature. Hence it appeared not unreasonable to suppose
that the composition of water was expressed by one particle of oxygen
in union with two particles of hydrogen. (The word “particle” is here
used in the meaning of “small portion”; such particles may be molecules
or they may be atoms.)

When steam is formed by the union of hydrogen with oxygen, it has a
volume equal not to the sum of the volumes of the hydrogen and the
oxygen, but to two-thirds of the sum, or equal to that of the hydrogen
alone, or twice that of the oxygen. And as steam, like hydrogen and
oxygen, follows Boyle’s and Gay-Lussac’s laws, it must be supposed
that in the steam there are as many particles as in the hydrogen from
which it was formed. But the particles of steam must necessarily be
more complex than those of the hydrogen, inasmuch as the steam contains
oxygen as well as hydrogen.

These difficulties may, however, be easily overcome by the following
supposition, which was first formulated by Avogadro in 1811. The
ordinary particles of hydrogen and of oxygen are complex, each
containing at least two atoms, or smaller particles, which usually
exist in combination with each other, or with atoms of some other
element. Two volumes of hydrogen, therefore, contain twice as many
particles as one volume of oxygen; to such particles the name
“molecules” is now universally applied. And as these molecules are
themselves each made up of two smaller particles, now termed “atoms,”
there exist in two volumes of hydrogen twice as many atoms as in one
volume of oxygen. On combination, the atoms in the molecules of
hydrogen and oxygen rearrange themselves, so that two atoms of hydrogen
and one atom of oxygen combine to form a molecule of water-vapour,
containing three atoms. The steam now contains as many molecules as did
the hydrogen before combination; but whereas the molecules of hydrogen
originally consisted of two atoms each, the molecules of steam contain
three atoms. It is this which causes the contraction from three volumes
to two when hydrogen and oxygen molecules exchange partners in forming
water molecules.

Of course the difficulty would meet with an equally good explanation if
it were supposed that the hydrogen molecules and the oxygen molecules
each contained four atoms, or eight atoms; but there is no need to
increase the complexity of the molecule, and the assumption that
these molecules are “diatomic” completely serves the purpose. The
composition of water is therefore believed to be two atoms of hydrogen
in combination with one atom of oxygen; and when hydrogen and oxygen
unite to form water, a transaction similar to an exchange of partners
is supposed to occur; the atoms of hydrogen and oxygen are imagined to
leave their partners of like kind, and to rearrange themselves so that
groups of atoms, or molecules, each containing two atoms of hydrogen
and one of oxygen, are formed. To such an arrangement the formula
H_{2}O is applied, while ordinary hydrogen molecules may be represented
as H_{2}, and molecules of oxygen as O_{2}.

It has been shown already (p. 150) how Lord Rayleigh obtained the
number 15·882 for the density of oxygen compared with that of hydrogen.
To determine the atomic weights of elements, the usual process has been
to analyse their oxides, for only a few elements form compounds with
hydrogen. Thus the analysis of copper oxide yields the numbers--

     Copper      79·96 per cent
     Oxygen      20·04    "

And as no compound of copper and hydrogen is known which lends itself
to analysis, the atomic weight of copper is necessarily referred to
that of oxygen. If the atomic weight of hydrogen be taken as unity,
that of oxygen, from Lord Rayleigh’s determination, must be 15·882,
because, in comparing the weights of equal volumes of the gases, a
comparison is made of the weights of equal numbers of molecules; and as
it is reasonable to suppose that each molecule of hydrogen and of
oxygen contains two atoms, the number 15·882 represents the weight of
an atom of oxygen compared with that of an atom of hydrogen taken as 1.
But this number has not been regarded as sufficiently established by
experiment. Other observers (for the importance of this ratio has been
acknowledged since the beginning of the century) have obtained results
differing from that given above, although not to any great extent. And
as it is a matter of indifference what basis or standard be taken for
atomic weights, which represent only relative numbers, it is common
to accept the atomic weight of oxygen as 16, in which case that of
hydrogen, if Lord Rayleigh’s determination of its density be regarded
as accurate, would be 1·0074. Hence if we place the atomic weight of
oxygen as 16, that of copper would be 63·34. And as with copper, so
with most other elements. It is very seldom that the atomic weight of
an element has been directly compared with that of hydrogen; it is, in
fact, almost always ascertained by analysis of its chloride, bromide,
or oxide; and the atomic weights of chlorine and bromine have been very
carefully compared with that of oxygen. There is, besides, another
convenience in accepting 16 as the atomic weight of oxygen: it is that
many atomic weights are then represented by whole numbers instead of by
fractions; thus, sulphur has the atomic weight 32, if oxygen be made
16, whereas, if it were 15·882, the atomic weight of sulphur would be
31·764, a number much more difficult to remember.

We see then that it is convenient to refer the density of argon
to oxygen taken as 16. The density obtained by Professor Ramsay
in February 1895, using a globe of small capacity (only 160 cubic
centimetres), was 19·94; exactly the same result was given by Lord
Rayleigh’s experiments in June 1895 on argon prepared by means of the
electric discharge, with a balloon of much greater capacity, which
held over two litres of gas. Now as a molecule of oxygen consists of
two atoms, the weight of a molecule is twice the atomic weight, or 32;
and as a given volume of argon must contain as many molecules as the
same volume of oxygen, the weight of a molecule of argon must be twice
19·94, or 39·88.

But this gives no information regarding the relative weight of an
_atom_ of argon. To ascertain this important quantity two methods
may be chosen. One is to make compounds of the element, and this
will be first considered. Since an atom of an element is defined
as the smallest amount which can exist in combination, then, if
numerous compounds of an element be examined, that one which contains
proportionally the least amount of the element may be regarded as
containing an atom, unless there are reasons to the contrary. For
example, reverting to the former instance of water, the relative
proportions by weight of oxygen and hydrogen are, in round numbers, 16
to 2. Reasons have already been given showing why its formula should
be H_{2}O and not HO; its molecule must contain two atoms of hydrogen.
But another compound of oxygen and hydrogen is known in which the
proportions are 16 parts by weight of oxygen to 1 part by weight of
hydrogen. Here also there are reasons for believing that this compound,
hydrogen peroxide, contains two atoms of hydrogen; whence it follows
that it must contain two atoms of oxygen, or 32 parts by weight to
2 parts by weight of hydrogen, and must therefore have the formula
H_{2}O_{2}. No other compound of oxygen and hydrogen is known; and it
may be stated briefly that no compound of oxygen with any element
whatever is known in which less than 16 parts by weight enters,
compared, of course, with the atomic weight of the other element or
elements in the compound. Hence 16 is accepted on this ground as the
atomic weight of oxygen.

If now it were possible to prepare compounds of argon, similar
reasoning might be applied to them, and that compound containing least
argon would be regarded as indicating its atomic weight. Many attempts
were therefore made to induce argon to enter into combination. And
the consistent failure of these attempts led to the choice of the
name “argon” or “idle” for the newly discovered element. The methods
employed to prepare argon free from nitrogen, namely, by exposing the
mixed gases to the action of oxygen in a discharge of electric sparks,
and by passing them over red-hot magnesium, show that it cannot be
induced to combine with one of the most electro-negative of elements,
oxygen, and one of the most electro-positive, magnesium. It also
refuses to combine with hydrogen or with chlorine when sparked with
these gases; nor is it absorbed or altered in volume by passage through
a red-hot tube along with the vapours of phosphorus, sulphur,
tellurium, or sodium. Red-hot caustic soda, or a red-hot mixture of
soda and lime, which attacks the exceedingly refractory metal platinum,
was without action on argon. The combined influence of oxygen and an
alkali in the shape of fused potassium nitrate or red-hot peroxide of
sodium was also without effect. Gold would, however, have resisted
such action, but it would have been attacked by the next agent tried,
viz. persulphide of sodium and calcium. This mixture was exposed at a
red-heat to a current of argon, again without result. Nascent chlorine,
or chlorine at the moment of liberation, obtained from a mixture of
nitric and hydrochloric acids, and from permanganate of potassium
and hydrochloric acid, was without action. A mixture of argon with
fluorine, the most active of all the elements, was exposed to a rain
of electric sparks by M. Moissan, the distinguished chemist who first
succeeded in preparing large quantities of fluorine in a pure state,
without his observing any sign of chemical combination.

An attempt was also made to cause argon to combine with carbon by
making an electric arc between two rods of carbon in an atmosphere of
argon. It was at first believed that combination had taken place, for
expansion occurred, the final volume of gas being larger than the
volume taken; but subsequent experiments have shown that the expansion
was due to the formation of some oxide of carbon from the oxygen
adhering to the carbon rods. On absorption of this oxide by the usual
absorbent, a mixture of cuprous chloride and ammonia, the argon was
recovered unchanged.

M. Berthelot, the celebrated French chemist, has stated that, on
exposing argon mixed with benzene vapour to a rain of electric sparks,
he has succeeded in causing argon to combine. Its volume certainly
decreases, but whether this decrease is to be attributed to true
combination or not is very doubtful. The benzene is converted into
a resinous mass, which coats the walls of the tube; and it is not
improbable that the argon may be dissolved, or even mechanically
retained, in the resinous deposit. Helium, a gas closely resembling
argon in properties, may be made to enter into a similar combination
with metallic platinum, if combination it can be called; but the amount
absorbed in both cases is extremely minute, and the gas is evolved
unchanged on heating the resin or the metal.

Professor Ramsay has also made experiments on the action of a silent
electric discharge upon a mixture of argon with the vapour of carbon
tetrachloride; the latter decomposes, giving, not a resin, but crystals
of hexachlorobenzene and free chlorine; but the volume of the argon was
unchanged. It was all recovered without loss. Lastly, the rare elements
titanium and uranium have been heated to redness in a current of argon
with no alteration or absorption of the gas. In short, all likely
agents have been tried as absorbents for argon, but in no case has any
true chemical combination been obtained.

These failures to produce compounds make it impossible to gain any
knowledge regarding the atomic weight of argon by a study of its
compounds, for it forms none. It is, indeed, in the highest degree
improbable that, had compounds existed, none should have been found
in Nature. There are, it is true, a few elements, such as platinum
and those resembling it, which always occur native, _i.e._ in the
elementary state; but even they yield to the attack of the agents tried
with argon. It cannot, of course, be stated with absolute certainty
that no element can combine with argon; but it appears at least
improbable that any compounds will be formed.

It was therefore necessary to adopt some other method in attempting to
determine the atomic weight of argon,--some method dependent on its
physical rather than its chemical properties, for argon, unlike almost
all other elements, appears to be devoid of chemical properties.

In order better to follow the train of reasoning based on experiment,
it will be well to begin with an account of why the atomic weight
of mercury is accepted as 200. The amount of mercury which combines
with 16 parts by weight of oxygen is easily found by heating a
weighed quantity of oxide of mercury, as Priestley and Scheele did,
and weighing the residue of metal. The results of the most accurate
experiments show that 200·36 grams of mercury combine with 16 grams
of oxygen, and if the compound consists of one atom of each element,
200·36 must be the atomic weight of mercury. The first idea which
naturally occurs is to find out the relative weight of mercury gas.
This has been done, and it is found to have the ratio to that of oxygen
of 100 to 16. Doubling these numbers will give the molecular weights,
since a molecule of oxygen consists of two atoms, and must therefore
possess twice the weight of one atom. We thus obtain the numbers 200
and 32 as the molecular weights of mercury and oxygen respectively. It
might therefore be concluded that 200 is not the true atomic weight of
mercury, but 100, and that the compound of mercury with oxygen contains
not one but two atoms of mercury, and should therefore be represented
by the formula Hg_{2}O, not HgO. But on surveying all known compounds
of mercury, there is not one which contains less than 200 parts by
weight of mercury in a molecule of the compound, or in which the
mercury cannot be conceived to replace 2 parts by weight of hydrogen.
And on weighing as gases the compounds of mercury with other elements,
where such compounds do not decompose on heating like the oxide, the
amount of mercury present must be always taken as 200, in order to add
up to the molecular weight found. For example, a compound of mercury
with carbon and hydrogen, named mercury methide, has a density of 120
compared with oxygen taken as 16, hence the comparative weight of its
molecule must be 240. Now it is known to contain two atoms of carbon
and six atoms of hydrogen, the atomic weights of which are 24 + 6 = 30.
And deducting 30 from 240, 210 remains as an approximation to the
atomic weight of mercury. It might, it is true, be the weight of two
atoms of mercury, but if so it is singular that no compound contains
a smaller proportion; and there is another reason, which follows
immediately, that leads us to believe that 200 is correctly taken as
the true weight of an atom.

It was discovered by Dulong and Petit, early in the century, that the
higher the atomic weight of an element the less heat is required to
raise its temperature through a given number of degrees. This heat can
be measured by dropping a fragment of the element, carefully weighed
and heated to a known temperature, into a known weight of cold water,
and ascertaining what rise of temperature the water undergoes, owing to
the heat communicated to it by the element. These comparative amounts
of heat, if water is chosen as the standard, are termed specific heats.
And as the specific heats of elements have been found by experiment
to be inversely as their atomic weights, the product of the specific
heat of any element and its atomic weight will give a constant number.
If the quantity of element weighed is one gram, and its rise of
temperature one degree, the numerical value of this product is
about 6·4.

Now the specific heat of mercury has been found to equal 0·032; that
is to say, it requires only a fraction of the value of 0·032 of heat
to raise the temperature of say 1 gram of mercury through one degree,
whereas the amount of heat necessary to raise 1 gram of water through
one degree is represented by the number 1. Hence this number, 0·032,
multiplied by the atomic weight of mercury, should yield the product
6·4; and it is seen at once that that number must be 200, for 200 ×
0·032 = 6·4. This is an additional reason for believing that the atomic
weight of mercury must be represented by the number 200.

We come next to a confirmatory piece of evidence which greatly
strengthens the view that the atomic weight of mercury must be 200;
but before entering into detail let us see what an atomic weight of
200 involves. The density of mercury gas is 100, and its molecular
weight must be 200. But if its atomic weight is also 200, it follows of
necessity that its molecule and its atom must be identical; that unlike
oxygen and hydrogen, its molecule consists, not of two atoms, but of
one single atom. There is nothing strange in this conclusion; there is
no evident reason why single atoms should not act as molecules, or
independent particles, able to exist in a free state, uncombined with
each other or with any other molecules.

The specific heat of a gas is measured in much the same manner as
that of a solid. A known volume of the gas is caused to pass through
a spiral tube, heated to a certain definite high temperature; it then
enters a vessel containing a known weight of water, traverses a spiral
tube immersed in the water, and parts with its heat to the water.
Knowing, therefore, the weight of the gas and its initial temperature,
and also the rise of temperature of the water, the specific heat of
the gas can be compared with that required to raise an equal weight of
water through one degree. But gases are found to possess two specific
heats. If the volume of the gas is kept constant, so that the gas does
not contract during its loss of heat, one number for its specific heat
is obtained; while if it is allowed to alter its volume a higher figure
represents its specific heat. It will be necessary to consider the
cause of this difference, in order to understand what conclusions can
be drawn respecting the molecular nature of argon from a determination
of the ratio between its two specific heats--that at constant pressure
and that at constant volume.

If a gas is allowed to expand into a vertical cylinder so as to drive
up a piston loaded with a weight, it is said to “do work.” The work
is measured by the weight on the piston, and also by the height to
which it is raised. Thus, if the weight is one pound, and the height
one foot, one foot-pound of work is done; if the mass is one gram and
the height one centimetre, one gram-centimetre of work is done. During
this process the gas must expand; and if it were enclosed in some
form of casing through which heat could not pass--we know of no such
casing, but we can contrive casings through which heat passes very
slowly--the temperature of the gas would fall during its expansion, and
it would lose heat. For each loss of one heat-unit or calory--_i.e._
the amount of heat given off by 1 gram of water in cooling through 1°
Centigrade--the gas would perform 42,380 gram-centimetres of work; it
would raise a weight of nearly 4¼ kilograms, or about 9⅓ lbs.,
through 1 centimetre, or nearly half an inch.

When a gas expands into the atmosphere it may be regarded as “raising
the atmosphere” through a certain height, for the atmosphere possesses
weight, equal on the average to 1033 grams on each square centimetre
of the earth’s surface, or between 15 and 16 lbs. on each square
inch. Suppose a quantity of air, weighing 1 gram, to be enclosed in
a long cylindrical tube of one square centimetre in section. At the
usual pressure of the atmosphere on the earth’s surface, and at 0°
Centigrade, the volume of the air would be 773·3 cubic centimetres;
and, as the sectional area of the tube is 1 square centimetre, the air
would occupy 773·3 centimetres’ length of the tube. If heat be given
to this air, so that its temperature is raised from 0° to 1°, it will
expand, as Gay-Lussac showed, by 1/273rd of its volume. Now the product
of 773·3 and 1/273 is 2·83 centimetres; the level of the surface of
the air will rise in the tube through that amount. In doing so it will
perform the work of raising 1033 grams through 2·83 centimetres, or
2927 gram-centimetres. Careful measurements have shown that, in order
to do this work, heat to the amount of 0·0692 calory must be given to
the gas. But it has been found that to heat the air through one degree,
without allowing it to expand, requires 0·1683 calory; that is, the
same amount of heat which would raise a gram of air through one degree,
its volume being kept constant, will raise a gram of water through
0·1683°; or, in other words, the specific heat of air is 0·1683. But if
allowed to expand, more heat is required--an additional 0·0692 calory
must be given it; consequently its specific heat at constant pressure
is greater; it is actually the sum of these two numbers,
0·1683 + 0·0692 = 0·2375.

We have thus--

     Specific heat at constant pressure  0·2375
         "        "       "      volume  0·1683

           Ratio between these numbers:  0·2375/0·1683 = 1·41

This ratio is termed the ratio between the specific heats of air, and
such a ratio is represented usually by the letter =γ=.

But it is not necessary to determine both kinds of specific heat in
order to arrive at a knowledge of the value of this ratio. One plan,
adopted by Gay-Lussac and Désormes at the suggestion of Laplace,[28] is
to actually measure the fall of temperature by allowing a known volume
of gas, of which the weight can of course be deduced, to expand from
a pressure somewhat higher than that of the atmosphere to atmospheric
pressure. It is true that heat will rapidly flow in through the walls
of the vessel; but by choosing a sufficiently large vessel, and
surrounding its walls with badly-conducting material, the entry of heat
will be so slow that it may, for practical purposes, be neglected. The
number for this ratio, actually found by Gay-Lussac and Welters for
air, was 1·376; but subsequent and more accurate experiments have given
as a result 1·405, which is almost identical with that calculated above.

This method, however, can be employed only when an unlimited supply of
gas is at disposal, for it entails the use of large vessels, and the
compressed gas must be allowed to escape into the atmosphere, and is
lost. There is, fortunately, another method by which the same results
can be obtained, and which requires only a small amount of gas.

Sir Isaac Newton calculated that the velocity of sound in a gas was
dependent on its pressure and on its density, in such a manner that

            __________
     _c_ = √ (_p_/_d_),

where _c_ stands for velocity (celerity), _p_ for pressure, and _d_
for density. When waves of sound are transmitted through air, the
air is compressed in parts and rarefied in parts, in such a manner
that compression follows rarefaction very rapidly, that part which is
compressed at one instant being rarefied at the next, compressed again
at a third, and rarefied at a fourth, and so on. Laplace was the first
to point out that during such rapid changes of pressure as occur while
a sound-wave is passing, the pressure will not rise proportionally to
the density, as would be the case if Boyle’s law were followed; for
on sudden rise of pressure the temperature of the compressed portion
of the gas will be increased; and, correspondingly, on sudden fall
of pressure, the wave of compression having passed, the temperature
will fall. He showed that instead of two pressures being inversely
proportional to their two volumes, under such circumstances, as they
are according to Boyle’s law, or

     _p_/_p__{1} = _v__{1}/_v_,

they must be inversely proportional to the volumes raised to a power,
the numerical expression of which is the ratio of the specific heats of
the two gases, =γ=, thus:

     _p_/_p__{1} = (_v__{1}/_v_)^γ;

or as

     _v__{1} : _v_ :: _d_ : _d__{1},

        ___________
     _c_ = √( γ_p_/_d_), and γ = _c_^2_d_/_p_.

The ratio of the two specific heats can therefore be determined by
finding the velocity of sound in the gas, and by noting at the same
time its density and its pressure.

To determine the velocity of sound in a gas, it is not necessary to
adopt the plan which has been successfully carried out with air; that
is, to make a sudden sound at one spot and to measure the interval
of time which the sound takes to travel to another spot some miles
distant. There is a simpler method, depending on the fact that the
lengths of the waves of compression and rarefaction are proportional
to the velocity of the sound. So that, knowing the velocity of sound
in air, the velocity in any other gas may be found by determining the
relative length of the sound-waves in air and in that gas.

[Illustration: FIG. 4.]

The simple apparatus with which such determinations are made is due to
the physicist Kundt. It consists of a glass tube, through one end of
which a glass rod passes, so that half the rod is enclosed in the tube,
while the other half projects outside it. In the experiments on argon,
the rod was sealed into the tube; in other cases, it is better to
attach it with indiarubber, or to cause the rod to pass through a cork.
The open end of the tube is connected with a supply of the gas, so
that, after the tube has been pumped empty of air, the gas, in a pure
and dry condition, can be admitted. Some light powder (and for this
purpose lycopodium dust--the dried spores of a species of fungus--is
best) is placed in the tube, and distributed uniformly throughout it,
so that when the latter is in a horizontal position, a streak of the
powder lies along it from end to end. The portion of rod outside the
tube is rubbed with a rag wetted with alcohol, when it emits a shrill
tone or squeak, due to longitudinal vibrations; the pitch of the tone
depends, naturally, on the length of the rod, a long rod giving a
deeper tone than a short one. The vibrations of the rod set the gas in
the tube in motion, and the sound-waves are conveyed from end to end of
the tube through the gas. As the tube is closed at the end through
which the gas was admitted, these waves echo back through it; and a
great deal of care must be taken to make the echo strengthen the waves,
so that the compressions produced by the back waves are coincident in
position with the compressions produced by the forward waves travelling
onwards from the rod. The gas, could we see it, would represent
portions compressed and portions rarefied at regular intervals along
the tube. Where the gas is compressed, it gathers the lycopodium dust
together in small heaps, the position of each heap signifying a node
of compression. Hence, comparing the distances between the nodes of
compression for any gas and for air, we find the relative wave-lengths
of sound in the two gases; and, as the velocity of sound in air has
been accurately measured, we thus determine the velocity of sound-waves
in the gas under experiment.

Such experiments were made by Kundt and by his co-worker Warburg on
mercury gas, and they found that in this case the value of γ was
1·67; that is, in the equation

     γ = _c_^{2}_d_/_p_
the value 1·67 had to be ascribed to γ, in order to render it
equal to the product of the square of the velocity into the density,
divided by the pressure.

Similar experiments with argon led to the same result as Kundt and
Warburg found for mercury gas; but the calculation becomes more
simple if it is allowable to take for granted that the elasticity,
or alteration of pressure produced by unit alteration of volume, is
identical in the case of argon and air. The full equations are--

                               __________________________
     _n_λ_{air} = _c__{air} = √γ(_p_/_d_)(1 + _at_)_{air},

and

                                   ____________________________
     _n_λ_{argon} = _c__{argon} = √γ(_p_/_d_)(1 + _at_)_{argon},

where _n_ is the number of vibrations per second, =λ= the
wave-length of sound, and _a_ the coefficient of the expansion of a
gas for a rise of 1° in temperature, _t_, viz. 0·00367. Now if the
expression _p_ (1 + _at_) can be shown to be identical for argon and
for air, the value of =γ= for argon can be calculated by the very
simple proportion--

     λ^{2}_d__{air} : λ^{2}_d__{argon} :: 1·408 : γ_{argon}.

This involved a measurement of the rate of rise of pressure of argon,
_p_, per degree of rise of temperature, _t_; or, in other words,
the verification of Boyle’s and Gay-Lussac’s laws for argon; and
this research was successfully carried out by Dr. Randall of the
Johns Hopkins University of Baltimore, U.S.A., and Dr. Kuenen, of
Leyden, working in Professor Ramsay’s laboratory.[29] They made use
of a constant volume thermometer, and measured the rise of pressure
corresponding to a definite rise of temperature, comparing the gases
argon and helium in this respect with air. The values found between 0°
and 100° for air, argon, and helium were--

    One volume air, heated from 0° to 100°, raises
          pressure in the proportion of 1 to         1·3663
    Argon                                            1·3668
    Helium                                           1·3665

It may therefore be taken for certain that, within the limits of
experimental error, the value of the expression _p_(1 + _at_)
is identical for all three gases.

We see, then, that for argon, as for mercury gas, the value of =γ=,
the ratio between the specific heats at constant volume and at constant
pressure, is 1 to 1·66, whereas for air, hydrogen, oxygen, nitrogen,
carbon monoxide, and nitric oxide, it is 1 to 1·4.

We have now to consider what conclusion can be drawn from this
difference.

On the usually accepted theory of the constitution of matter, it is
held that atoms may be regarded as spheres, hard, elastic, smooth, and
practically incompressible. True, we really know little or nothing
regarding the properties of such particles, if particles there be;
but in considering their behaviour it is necessary to make certain
suppositions, and to see whether observed facts can be pictured to our
minds in accordance with such postulates. If, from the known behaviour
of large masses, conclusions can be drawn regarding small masses, and
if these conclusions harmonise with what is found to be the behaviour
of large numbers of small masses acting at once, the justice of the
supposition is, although not proved, at least rendered defensible as
one mode of regarding natural phenomena.

Molecules, on this supposition, may consist of single atoms, or they
may consist of pairs of such atoms, joined in some fashion like the
bulged ends of a dumb-bell; or lastly, they may consist of greater
numbers of atoms arranged in some different manner, the arrangement
depending on their relative size and attraction for each other. It must
be clearly understood, however, that such mental pictures are not to
be taken as actually representing the true constitution of matter, but
merely as attempts to picture such forms as will allow of our drawing
conclusions regarding their behaviour from known configurations of
large masses.

The molecules of gases are imagined to be in a state of continual
motion, up and down, backwards and forwards, and from side to side.
It is true that they must also move in directions which cannot be
described by any of these expressions, but such other directions may be
conceived as partaking more or less of motions in the three directions
specified; _i.e._ in being resolvable into these. To these motions
have been applied the term “degrees of freedom.” Such motions through
space, in which the molecule is transported from one position in space
to another, form three of the possible six degrees of freedom which a
molecule may possess, and the molecules are said to possess “energy
of translation” in virtue of this motion. The other three consist in
rotations in three planes at right angles to each other.

Now, it can be shown that the product of pressure and volume of a gas,
_pv_, is equal to ⅔rds of the energy of translation of all molecules
of the gas, or

     _pv_ = ⅔(NR),

where N stands for the number of molecules in unit volume, and R for
their energy of translation; inasmuch as a pressure diminishing a
volume is of the nature of work, or energy. For one gram of air at O°
C. and 76 cms. pressure (normal temperature and pressure), the pressure
(_p_), measured in grams per square centimetre, is 1033, and the volume
(_v_) is 773·3 cubic centimetres; and the raising of the temperature
through 1°, as was shown before, requires 2927 gram-centimetres of
work. Further, since the product of pressure into volume is equal to
⅔rds of the energy due to motion, or the translational energy of the
gas,

     NR = 3/2_pv_ = 3/2 × 2927 = 4391 gram-centimetres.

Dividing this number by 42,380, the mechanical equivalent of heat,
or the number of gram-centimetres corresponding to one calory, the
quotient is 0·1040 calory. If the energy of the air were due to the
translational motion of its molecules, we should expect this number,
0·1040, to stand for the specific heat of air at constant volume; but
it has been found equal to 0·1683, as already shown.

We have seen that to convert specific heat at constant volume into
specific heat at constant pressure 0·0692 must be added. Hence at
constant pressure the specific heat of such an ideal gas should be
0·1732. And the relation between specific heat at constant volume and
that at constant pressure should be 0·1040 to 0·1732, or 1 to 1⅔.
The conclusion to be drawn from these numbers for air, 0·1683 and
0·2375, which bear to each other the ratio of 1: 1·41, is that air
cannot be such an ideal gas; that in communicating heat to it some of
that heat must be employed in performing some kind of work other than
that of raising its temperature. What this work may possibly be we
shall consider later.

But Kundt and Warburg found, from their experiments on the ratio
between the specific heats of mercury gas, this ideal ratio, 1 to
1⅔; and Professor Ramsay obtained the same ideal ratio, or one very
close to it indeed, 1 to 1·659, for argon. He subsequently found this
ideal ratio also to hold for helium (1 to 1·652), and it must therefore
be concluded that such gases possess only three degrees of freedom; or,
in other words, their molecules, when heated, expend all the energy
imparted to them in translational motion through space.

This is the consequence which we should infer from the supposition
that such molecules are hard, smooth, elastic spheres. Were they each
composed of two atoms, we should have to picture them as dumbbell-like
structures; and here we enter on a theoretical conception put forward
by Professor Boltzmann, but which has not been accepted universally by
physicists.

[Illustration: FIG. 5.]

[Illustration: FIG. 6.]

Boltzmann imagines that to the three “degrees of freedom” of a single
atom molecule there may be added, provided the molecule consists of two
atoms, two other degrees of freedom, namely, freedom to rotate about
two planes at right angles to each other. The knobs at the end of each
imaginary dumb-bell may revolve round a central point in the handle
joining them, and it is clear that they may revolve in one horizontal
and in one vertical plane, as shown in Fig. 5. Such diatomic molecules
are said to possess five “degrees of freedom.” They will not, it is
supposed, rotate round the line joining the centres of the spheres,
because, as before said, the atoms are pictured as perfectly smooth.
But if the molecules are triatomic, as, for example, CO_{2} or N_{2}O,
they will have six degrees of freedom, for with the addition of an
additional atom they have an additional plane of rotation (see Fig.
6). Boltzmann has attempted to show that the ratio of the specific
heats of diatomic molecules should be as 1 to 1·4. In actual fact it
approximates to that number. For the commoner gases it is--

     Oxygen            1·402
     Nitrogen          1·411
     Hydrogen          1·412
     Carbon monoxide   1·418

In all cases the numbers are too large, and this is a serious
difficulty, because any tendency to rotate round the central line
would cause the values to be less, not greater than 1·4. For triatomic
molecules the calculated value of γ is 1⅓, but in actual fact the
ratio in the case of triatomic molecules, such as H_{2}O, CO_{2},
N_{2}O, etc., is always less than 1⅓. These speculations stand on a
basis very different from the first conception, namely, that all heat
must be employed in communicating translational motion to molecules of
mercury gas, argon, and helium, and it appears that the atoms of these
three elements must necessarily be regarded as having the properties
of smooth elastic spheres. The atoms and the molecules must in their
several cases be identical. And, inasmuch as the chemical evidence
regarding mercury leads to the same conclusion, it appears legitimate
to infer that argon and helium must also be monatomic elements.

[28] _Méchanique céleste_, vol. v. p. 123.

[29] _Proc. Roy. Soc._ vol. lix. p. 63.




CHAPTER VII

THE POSITION OF ARGON AMONG THE ELEMENTS


From what has been said in the preceding chapter there can be no doubt
that the molecular weight of argon is 39·88. We have now to consider
what this conclusion involves. Taken in conjunction with the fact that
the ratio between its specific heat at constant volume and that at
constant pressure is 1⅔, it follows that energy imparted to it is
employed solely in communicating translational motion to its molecules.
In the case of mercury gas such behaviour is taken as evidence that
the conclusion following from the formulae of its compounds, from
the density of its compounds in the gaseous state, and from its own
vapour-density, as well as from its specific heat in the liquid state,
namely, that its molecules are monatomic, is correct. Is it legitimate
to conclude that because argon in the gaseous state has the same ratio
of specific heats, therefore it also is a monatomic gas?

The conclusion will depend on our conception of an atom and a molecule,
and in the present state of our ignorance regarding these abstract
entities no positive answer can be given. It appears certain that,
on raising the temperature of argon, very little, if any, energy is
absorbed in imparting vibrational motion to its molecules; and our
choice lies between our ability or inability to conceive of a molecule
so constituted as to be incapable of internal motion. If there be any
truth underlying Professor Boltzmann’s conception, a molecule of argon
cannot consist of any complex structure of atoms, otherwise it would
possess more than three degrees of freedom, and heat would be utilised
in causing rotational motions. As we know for a fact that the ratio
between the specific heats of gases diminishes with the increasing
complexity of their molecules, perhaps the safest conclusion is the one
adopted by the discoverers of argon, that the balance of evidence drawn
from _this_ source is in favour of its monatomic nature.

But this hypothesis raises difficulties which are not lightly to be
met. These difficulties arise from a consideration of the position of
argon when it is classified with other elements.

In 1863 Mr. John Newlands pointed out in a letter to the _Chemical
News_ that if the elements be arranged in the order of their atomic
weights in a tabular form, they fall naturally into such groups that
elements similar to each other in chemical behaviour occur in the
same columns. This idea was elaborated farther in 1869 by Professor
Mendeléeff of St. Petersburg and by the late Professor Lothar Meyer,
and the table may be made to assume the subjoined form (the atomic
weights are given with only approximate accuracy):--

THE ELEMENTS ARRANGED IN THE PERIODIC SYSTEM.

  +---------------+---------------+----------------+---------------
  |               |               |                |
  |               |               |                |
  |Lithium     7·0|Beryllium   9·1|Boron       11·0|Carbon     12·0
  |               |               |                |
  |Sodium     23·0|Magnesium  24·3|Aluminium   27·0|Silicon    28·3
  |               |               |                |
  |Potassium  39·1|Calcium    40·1|Scandium    44·1|Titanium   48·1
  |               |               |                |
  |Rubidium   85·5|Strontium  87·5|Yttrium     89·0|Zirconium  90·0
  |               |               |                |
  |Caesium   132·9|Barium    137·0|Lanthanum  142·3|Cerium    140·3
  |               |               |                |
  |?         170·0|?         172 0|Ytterbium  173·0|?         177·0
  |               |               |                |
  |?         221·0|?         225·0|?          230·0|Thorium   232·4
  +---------------+---------------+----------------+---------------

    ----------------+----------------+--------------+-----------+
                    |                |Hydrogen  1·01|Helium  4·2|
                    |                |              |           |
    Nitrogen    14·0|Oxygen      16·0|Fluorine  19·0|?          |
                    |                |              |           |
    Phosphorus  31·0|Sulphur     32·1|Chlorine  35·5|Argon  39·9|
                    |                |              |           |
    Arsenic     75·1|Selenium    79·0|Bromine   80·0|?          |
                    |                |              |           |
    Antimony   120·3|Tellurium  126·3|Iodine   126·9|?          |
                    |                |              |           |
    Erbium     166·0|?          167·0|?        169·0|?          |
                    |                |              |           |
    Bismuth    208·1|?          214·0|?        219·0|?          |
                    |                |              |           |
                    |                |              |           |
    ----------------+----------------+--------------+-----------+

  +-------------+---------------+---------------+---------------
  |             |               |               |
  |             |               |               |
  |Lithium   7·0|Beryllium   9·1|Boron      11·0|Carbon     12·0
  |             |               |               |
  |Sodium   23·0|Magnesium  24·3|Aluminium  27·0|Silicon    28·3
  |             |               |               |
  |             |               |               |
  |Copper   63·4|Zinc       65·3|Gallium    69·9|Germanium  72·3
  |             |               |               |
  |             |               |               |
  |             |               |               |
  |Silver  107·9|Cadmium   112·1|Indium    113·7|Tin       119·1
  |             |               |               |
  |             |               |               |
  |?       156·0|?         158·0|?         159·0|Terbium  162·0?
  |             |               |               |
  |             |               |               |
  |Gold    197·2|Mercury   200·2|Thallium  204·2|Lead      206·9
  |             |               |               |
  |             |               |               |
  |             |               |               |
  +-------------+---------------+---------------+---------------

  ---------------+------------------+--------------+----------------+
                 |                  |Hydrogen  1·01|Helium       4·2|
                 |                  |              |                |
  Nitrogen   14·0|Oxygen        16·0|Fluorine  19·0|?               |
                 |                  |              |                |
  Phosphorus 31·0|Sulphur       32·1|Chlorine  35·5|Argon       39·9|
                 |                  |              |                |
                 |                  |              |{Iron       56·0|
  Vanadium   51·4|Chromium      52·3|Manganese 55·0|{Cobalt     58·7|
                 |                  |              |{Nickel     58·6|
                 |                  |              |                |
                 |                  |              |{Ruthenium 101·6|
  Niobium    94·0|Molybdenum    95·7|?        100·0|{Rhodium   103·0|
                 |                  |              |{Palladium 106·3|
                 |                  |              |                |
  Neodymium 140·8|Praseodymium 143·6|Samarium 150·0|? 152, 153, 154 |
                 |                  |              |                |
                 |                  |              |{Osmium    191·3|
  Tantalum  182·5|Tungsten     184·0|?        190·0|{Iridium   193·0|
                 |                  |              |{Platinum  194·3|
                 |                  |              |                |
  ?              |Uranium      240·0|?        244·0|                |
  ---------------+------------------+--------------+----------------+

The elements in the first column all agree in that they are white soft
substances, with metallic lustre, but tarnish rapidly in air, owing to
the action of water-vapour; they are all violently attacked by water,
and they are without exception monads, that is, they replace hydrogen
in its compounds atom for atom. The elements in column two are also all
white metals, attacked by water with more or less ease; but in their
case one atom replaces two atoms of hydrogen, whence they are called
dyads, or bivalent elements (worth two). And so on with the other
columns. All elements in vertical columns exhibit chemical similarity,
and, indeed, are often strikingly like in properties.

The subdivision, produced by folding the loose page, is intended
to show that the elements represented on it have a double set of
resemblances. But there are various anomalous and inexplicable
phenomena still attached to this arrangement of elements. For example,
copper, although it replaces one atom of hydrogen in some of its
compounds, and is thus a monad, forms more numerous and more stable
compounds in acting as a dyad and replacing two atoms of hydrogen.
Gold, which belongs to the same column, is at once univalent and
tervalent; mercury, both univalent and bivalent; thallium, univalent
and tervalent; tin and lead, bivalent and quadrivalent, and so on. It
is as if some elements had a tendency to enter a column not their own.

Again, on comparing the atomic weights of the elements, it is seen that
the differences are far from being regular. As a rule, the difference
in the vertical columns between any single element and the one
following it is approximately 16, or some multiple of 16. Thus we have
lithium, sodium, and potassium; beryllium, magnesium, and calcium;
boron, aluminium, and scandium; carbon and silicon; oxygen and sulphur;
fluorine and chlorine--all with a difference of 16 approximately. But
here we come to a break: silicon and titanium, phosphorus and vanadium,
sulphur and chromium, chlorine and manganese, each show a difference of
about 20.

Passing on, between the atomic weights of potassium, rubidium, and
caesium there is a difference of about 16 × 3; a similar difference
between calcium, strontium, and barium; between scandium and yttrium;
between titanium, zirconium, and cerium, and so on; but with wider
and wider divergence from the supposed constant, 48 = 16 × 3. In
short, we have a seeming regularity, but only a very approximate
one--a regularity, in fact, in which a vivid imagination must play a
conspicuous part in order to detect it.

Now, up to the present, no reason has been suggested to account for the
divergence from this irregular regularity, which a little expenditure
of time will enable any one to trace through all these numbers. But one
thing has been remarked; there is the same seeming regularity between
certain physical properties of elements and their compounds: their
specific volumes, their melting-points, their refractive indices, and
other properties vary from member to member of the same column in a
manner bearing more or less similarity to the periodic variation of the
atomic weights.

It happens that among compounds of carbon we are acquainted with series
of compounds which, in variation of molecular weights and gradation of
properties, bear a striking resemblance to the elements thus arranged.
Thus we have the series:--

     CH_{4}        Methane    16
     C_{2}H_{6}    Ethane     30
     C_{3}H_{8}    Propane    44
     C_{4}H_{10}   Butane     58
     C_{5}H_{12}   Pentane    72

and a host of others up to a compound of the formula C_{30}H_{62}; in
each case there is a constant difference of 14 between the molecular
weight of any one hydrocarbon and that immediately preceding or
succeeding it in the column. Such a series is termed a homologous
series. The analogy is very tempting; to suppose that a similar
constant difference should exist in the relations of the atomic weights
of the elements, and that they too are undecomposable compounds of two
unknown elements, is an attractive hypothesis, but one for which
there exists no proof; indeed, it is rendered improbable by the
irregularities just pointed out.

But there is one noticeable feature in the periodic arrangement of the
elements. It is, that although the differences are irregular (_e.g._
between B = 11 and C = 12 the difference is 1, while between O = 16
and F = 19 the difference is 3), yet there is no marked displacement
in the _order_ of arrangements of the elements, inasmuch as no element
has an atomic weight _lower_ than that preceding it in the horizontal
line. It was for some time supposed that tellurium and iodine were thus
misplaced; and indeed it is even now not quite established that they
are not, but the balance of evidence is in favour of tellurium having a
lower atomic weight than iodine.

Argon, however, is a marked exception. With an atomic weight of 39·88,
its natural position would lie between those of potassium and calcium;
but there is no room for it. And for this reason considerable doubts
have been thrown on the validity of the conclusion to be drawn from the
found ratio of its specific heats, 1⅔, viz. that its molecule and
its atom are identical. If it were a diatomic gas, like chlorine or
hydrogen, its atomic weight would be 19·94, and it would find a fitting
position after fluorine and before sodium. And the difference between
its atomic weight and that of helium, to which the atomic weight
2·1 would for the same reasons then attach, would be 17·84, one not
incomparable with 16. But, as before remarked, it is difficult, if not
altogether impossible, to conceive of a diatomic structure to which
all energy imparted in the form of heat should result in translational
motion, and as a matter of fact none such is known.

There are two methods of escape from this dilemma. If the gases termed
argon and helium are not single elements, but mixtures of monatomic
elements, then what has been termed their atomic weights will represent
the mean of the atomic weights of two or more elements, taken in the
proportion in which they occur. For example, supposing that argon is a
mixture of an element of atomic weight 37 with one of atomic weight 82,
the found atomic weight, nearly 40, would imply a mixture of 93·3 per
cent of the lighter, with 6·7 per cent of the heavier element. We must
therefore carefully examine all evidence for or against the supposition
that argon is a mixture of elements.

It is well known that elements with high atomic weights have, as a
rule, higher boiling-points than those with low atomic weights in the
same columns. Perhaps the most striking case is that of the elements
fluorine, chlorine, bromine, and iodine. Whereas fluorine has never
been liquefied (chiefly owing to difficulties of manipulation, due
to its extraordinarily energetic action on almost every element and
compound), chlorine boils at -102°, bromine at 59°, and iodine at
184°. And if a mixture of chlorine and bromine gases be cooled, the
bromine, if present in sufficient amount, will condense first, in a
fairly pure state, little chlorine condensing with it. But in a mixture
containing only 7 per cent of bromine with 93 per cent of chlorine
(analogous to a mixture of the two supposed constituents of the argon
mixture) the pressure of the bromine gas in the mixture would be only
7/100ths of the normal pressure, or 53·2 millimetres. At this pressure
the boiling-point of bromine is about -5°, so that, on cooling to that
temperature, bromine would begin to show signs of liquefaction. This
is, however, still nearly 100° above the boiling-point of chlorine;
and there would therefore be no difficulty whatever in detecting such
a percentage of bromine in a mixture of chlorine and bromine gases on
cooling the mixture to a moderately low temperature.[30]

Argon has been liquefied. A sample of pure argon was sent by Professor
Ramsay to Professor Olszewski of Cracow, well known for his accurate
researches at low temperatures; and he found the boiling-point of argon
at atmospheric pressure to be -186·9°, and its melting-point to be
-189·6°. There was no appearance of liquid before the boiling-point
was reached, nor was there any alteration of temperature as the argon
boiled away, and these are signs of a single substance, not of a
mixture; moreover, the melting-point was a definite one; and here
again, mixtures never melt suddenly, but always show signs of softening
before melting. So far as this evidence goes, therefore, it points to
the conclusion that argon is not a mixture of two elements.

Other evidence may be sought for in the spectrum of argon, which was
carefully examined by Mr. Crookes. It consists of a great number of
lines, extending all through the spectrum, from far down in the red to
far beyond the visible violet; the invisible lines were examined by
the aid of photography, for ultraviolet light, although invisible to
the eye, impresses a photographic plate. The most striking feature of
this spectrum is the change which can be produced in it by altering the
intensity of the electric discharge which is passed through the tube
containing argon at a low pressure. By interposing a Leyden jar between
the secondary terminals of the induction-coil from which sparks are
taken through the gas, the colour of the light in the tube changes from
a brilliant red to an equally brilliant blue. A large number of lines
in the red spectrum disappear, on interposing the jar, while many lines
in the blue-green, blue, and violet part of the spectrum, invisible
before, shine out with great brilliancy. There is no other gas in which
a similar alteration of intensity of discharge produces such a marked
difference, although in many gases, supposed to be simple substances,
similar changes may be produced. So far as we know at present, however,
such a change cannot be definitely ascribed to the presence of a
mixture of two elements, although it is in itself a very remarkable
phenomenon.

On the other hand, Professors Runge and Paschen, in a paper
communicated to the Royal Academy of Science of Berlin in July 1895,
have adduced reasons for concluding that helium, the gas from clèveite,
is a mixture; it appears to show lines belonging to two spectra, each
series of lines exhibiting certain regularities. But this, although an
important conclusion in itself, has no direct bearing on the question
of the simplicity of argon.

One method of separating the constituents of a mixture is by taking
advantage of their different solubilities in water, or in some other
appropriate solvent. And as argon was found to have the solubility of 4
volumes in 100 of water, while helium is very sparingly soluble, only
0·7 volume per 100, it is not unreasonable to suppose that, if argon
consisted of a mixture of elements in argon, one should be more soluble
than another. Exhaustive experiments in this direction have still to
be carried out; but Lord Rayleigh has made experiments which render it
very improbable that any separation into its constituents, if it be a
mixture, can be thus effected. Wishing to ascertain if there were any
helium in the air, he shook up atmospheric argon with water, until a
very small fraction remained undissolved. The spectrum of this small
residue was identical with that of the original argon, from which it
would appear that this method, at least, is incapable of effecting any
separation.

A completely decisive proof that argon is not a mixture has just been
furnished by experiments carried out by Dr. Collie and Professor
Ramsay, in which a large quantity of argon was submitted to fractional
diffusion. From what was said on p. 162, it will be seen that if argon
consisted of a mixture of two gases of different densities, such a
process should separate the mixture more or less completely into its
two constituents. After a long series of diffusions, however, the
density of that portion of argon passing first through the porous plug,
which would have been less had any gas of lower density been present,
was found to be identical with that of the last portions of gas. On the
other hand, by aid of the same diffusion-apparatus, a fair separation
of oxygen (density 16) from carbon dioxide (density 22) was effected,
although, as the reader will observe, the densities of these two gases
do not differ greatly. Hence, if argon consists of two kinds of matter
they must have the same density, and hence the same molecular weights,
and the difficulty is not removed. But as the spectrum of the first and
last portions was the same and was identical with that of argon, this
supposition is improbable.

The evidence is therefore distinctly against the supposition that argon
is a mixture of two or more elements.

There is, however, another possible method of accounting for the high
atomic weight of argon, which, if it could be reduced by a few units,
would fall into its place after chlorine and before potassium. It is
that argon consists of a mixture of many monatomic, with comparatively
few diatomic, molecules. If there were only about 500 molecules of
diatomic argon in every 10,000 molecules of the gas, its density,
supposing it to consist entirely of monatomic molecules, would be 19,
and its atomic and molecular weights 38, a number which would fit
between the atomic weight of chlorine, 35·5, and that of potassium,
39·1. Several instances of this kind are known. Chlorine itself, when
heated to high temperatures, changes from diatomic to monatomic
molecules, and the density decreases with the change. For example,
at 1000° the found density of chlorine is 27, implying a molecular
weight of 54; now 54 is neither the weight of a monatomic molecule of
chlorine, viz. 35·5, nor of a diatomic molecule, which is 71; but it
corresponds to that of a mixture of monatomic and diatomic molecules.
Here fall of temperature causes combination of monatomic molecules
with each other to form diatomic molecules; and rise of temperature
increases the number of monatomic molecules, at the expense of the
diatomic molecules. Is there no sign of similar behaviour with argon?

It has already been mentioned that the rise of pressure of argon with
rise of temperature has been carefully measured by Drs. Randall and
Kuenen, and that it is quite normal; no sign of splitting has been
observed. But the range of temperature was not great (it was only from
0° to 280°), and it is quite possible that the change, if there was
one, was so minute as to have escaped detection. Again, a more delicate
method of detecting such a change is in the measurement of the ratio of
the specific heats. The most trustworthy number obtained was 1·659
for the ratio, instead of 1·667, the theoretical figure. A mixture of
5 per cent of diatomic molecules should have reduced this ratio to
1·648. Here the evidence is, however, inconclusive. But on the whole,
the presumption is against the hypothesis that argon is a mixture of
monatomic with diatomic molecules.

It still remains for us, therefore, to account for the fact that in the
periodic table there is no place for argon, provided it be insisted
on that the elements must follow each other in the numerical order
of their atomic weights. If the numbers in the table actually showed
regular intervals, or if there were any regularity to be detected in
their differences, argon might be regarded as of wholly exceptional
behaviour. But this is not so. Argon is an extreme instance of
divergence, but similar divergences, though not of equal magnitude, are
common.

In attempting to offer an explanation of such anomalies, it must be
remembered that the question is in itself a far-reaching one; and that
although argon has served to direct attention anew to the anomalies
of the periodic table, yet these anomalies existed before argon was
discovered. It is necessary above all things to be clear as to what is
under discussion. We speak of “atomic weights,” or “atomic masses.”
What is meant precisely by these expressions?

By mass, we understand that property of a body, in virtue of which,
when acted on by a certain force for a certain time, it acquires a
certain velocity. The product of the mass into half the velocity
squared, or ½MV^2 (where M and V stand respectively for mass and for
velocity), is what is termed kinetic energy. If the mass chosen be 1
gram, and the velocity 1 centimetre per second, the unit of energy
is the product; it is termed an erg. The same unit of energy, the
erg, is derived by the action of unit force, termed 1 dyne, through
unit length, 1 centimetre. We have thus two equations, where F and L
represent force and length,

        Kinetic Energy = ½(MV)^2 and Linear Energy = FL.

Now we choose mass as a unit of measure of the _quantity of material_;
and we are justified in doing so, because experiments have shown that
material, confined in a closed space, does not appreciably alter its
mass. The mass is proportional to the _weight_, generally measured as
the force exerted at some definite latitude on the earth’s surface,
tending to pull the body towards the centre of the earth. This force
is equal to about 981 dynes at London. There is no known reason
why mass and weight should be proportional to each other; for the
cause of the attraction of the earth has never been satisfactorily
elucidated. We therefore use weight, or the attraction of the earth, as
a convenient means of determining the relative masses of two objects.
Hence it appears rational to prefer the expression “atomic weight” to
“atomic mass,” seeing that the former represents the actual result of
experiment, and also because we are dealing in atomic weights with
relative numbers. But this is really a matter of choice.

The atomic weights therefore represent the relative masses in which
the elements generally unite. They often, however, unite in multiples
of these weights, as formulated by Dalton’s second law. The weights,
arranged in numerical order in columns, give us the periodic table.

Now energy can be measured in other units besides those of force and
mass. Heat is one form of energy, and it is measured by an interval of
temperature, and by a property which we term specific heat. It happens
that the latter property varies, not with the mass of the substance
heated, but with its atom, so that all elements have approximately
the same atomic heat; that is, quantities of elements proportional in
mass to their atomic weights require approximately equal increments
of heat to raise their temperature through an equal interval, say 1°.
This is the formulation of Dulong and Petit’s law previously alluded
to. But here we meet with irregularities, which have up till now
defied classification. The heat imparted to an aggregation of atoms
is not expended solely in raising their temperature; other work is
done also, as is generally supposed, in the nature of expansion, or
separation of parts, as in overcoming the attraction between the atoms
in the molecule, or in imparting special motion to the atoms; such
work, however, involves an expenditure of energy which is either very
small in proportion to the total energy imparted as heat, or is nearly
the same for all elements. At present we cannot decide between these
alternatives, owing to the lack of knowledge of the nature of liquids
and solids. The main fact, however, is incontestable: that the heat
energy required to raise different elements through the same interval
of temperature is the same, not for equal masses or weights of the
substances, but for their _atomic_ masses.

Again, many compounds when dissolved in water conduct an electric
current, while they themselves are decomposed; and the different
ingredients of the compound are often deposited at the points where the
electric current enters or leaves the liquid. Where they are not so
deposited, it is usually because of their action on the solvent water.
Now Faraday found that when elements are deposited, equal quantities of
electricity are conveyed either by equal numbers of atoms or by some
simple fraction of these numbers. To this fraction we apply the term
_valency_. Thus we say that an atom of oxygen is bivalent, or a dyad,
because it conveys twice as much electricity through the liquid in
which it is present as an atom of hydrogen, which is termed univalent,
or a monad. Here we see a direct connection between the conveyance of
an electric charge and the atomic weight. The electric unit of quantity
is in fact defined as that which can be conveyed by a certain weight of
hydrogen, by 8 times that weight of oxygen, by 108 times that weight of
silver, and so on. Therefore the electrical unit is connected, not with
unit of mass, or with the gravitational unit, but with the atomic unit.

We have therefore a number of systems, each capable of being equated
to a unit of energy, but of which the terms are in some as yet unknown
way related to each other, and often more directly than they can be
related to mass and weight. This relation is only an approximate one
in the case of specific heats; it appears to be an absolute one in the
conveyance of electrical energy. The arrangement of the elements in
the periodic table must therefore be considered by the light of such
general views.

I would venture to suggest, as a tentative method of solving this
problem, that it be considered whether mass or weight are such
invariable properties of matter as have generally been taken for
granted. That the relative weights or masses in which elements combine
always retain their invariable proportion is true, so far as we can
determine; but it must be remembered that we cannot cause them to
combine except under a very limited series of conditions. For example,
the act of combination equalises the temperature of two combining
atoms; it also, in all probability, equalises their electrical charges.
It is a legitimate speculation whether, could we maintain a difference
in their temperature or in their electric potential, their atomic
weights might not also change. Indeed, we are ignorant whether mass
is changed by alteration of temperature. Experiments made by Sir
John Airy, and interpreted by Professor Hicks, appear to show that
variation of temperature is not without some influence on gravitational
attraction. Others by Professor Landolt point in the same direction.

It therefore appears to me not impossible that the mass of the atoms
may be affected by the various and different properties which they
possess, some to a greater, some to a lesser extent. It must be
admitted that atoms differ from each other in the readiness with
which they combine with those of the same kind to form molecules; and
that molecules of different elements differ from each other in their
capacity to form molecular aggregates. Take, for example, such cases
as caesium and fluorine, each intensely active, but towards different
objects: caesium the most electro-positive of the metals, and fluorine
the most electro-negative of elements. Surely their activity must be
due to some cause which cannot but exert influence on their other
properties, such as their mass and their gravitational attraction, as
it doubtless has influence on their specific heats, and on many of
their other physical properties. And contrast these instances with
helium and with argon, the most indifferent of substances, the atoms
of which are unwilling, and apparently unable, to pair even with
themselves; it is hardly conceivable that these peculiarities should
leave their other, and, as we are in the habit of thinking, invariable,
properties unaffected. I venture to suggest that these powers of
combination, due to some configuration or to some attractive force,
tend to lessen the gravitational attraction by which we measure their
atomic weights; that helium and argon, which possess little, if any,
of such power to combine, show what may be termed the normal atomic
weights, inasmuch as their gravitational attraction is subject to no
deduction attributable to their reacting powers.

I cannot but think that, when some numerical values are assigned to
this combinational power, it will be found that they will so increase
their atomic weights as to display that regularity which, so far as we
can see at present, is conspicuous by its absence.

I am aware that these suggestions are of a wholly speculative
character; and yet I venture to put them forward in the firm conviction
that no true progress in knowledge has ever been made without such
speculations. It was the speculative phlogistic theory which combined
phenomena apparently so distinct as the burning of a candle and the
rusting of iron. It is true that that theory is now a phantom of the
past, yet it served its purpose in directing attention to phenomena
of a similar character. It would be easy to multiply instances of
the kind; in almost every case some useful object has been served by
speculation preceding exact knowledge. The object of science, as indeed
of inquiry in all departments of human interest, is to reconcile the
world of man with the world of nature, and to endeavour to know in part
that of which we hope one day to attain to a perfect knowledge.

                           THE END

[30] These considerations would hold on the assumption that no
combination takes place between chlorine and bromine.

 _Printed by_ R. & R. CLARK, LIMITED, _Edinburgh_.





End of Project Gutenberg's The Gases of the Atmosphere, by William Ramsay