The Gases of the Atmosphere: The History of Their Discovery

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THE GASES OF THE ATMOSPHERE

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THE GASES

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THE ATMOSPHEEE

THE

HISTORY OF THEIR DISCOVERY

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BY

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ot

Sm WILLIAM BAMaAy; :]fj:jCr.B., F.R.S.

OrnCIEB X>B LA LXOION. D-UOH.f K'JR
PBOFBHOB or CHKMiSTKt IK U:riVER8ITT COLLEO^,.. LGMJi/k *"

- ■• ■

m^ »< '

.THIRD EDITION

•->•

'• .'

WITH PORTkAITO

ILonHon

MACMILLAN AND CO., Limitsj)

NEW YORK: TUB MACMILLAN COMPANY

1905

A/I rig^Ats tesetind

PREFACE TO THE THIRD EDITION

Tee radioactive gasee, products of the disintegra-
tion of the remarkable elements radium and radio-
thorium, have been recently added to the list of
the constituents of the atmosphere. An attempt
has been made in an additional chapter to give an
account of these ; but the reader who is interested
in the subject is advised to consult the much
more complete works by Professor Rutherford, Mr.
Frederick Soddy, and the Honourable R. J. Strutt,
if he wishes to learn more about the nature of the
disintegration of the parents of these gases, radium
and thorium, aud the astonishing changes which
they spontaneously undergo. The beginning of
the twentieth century has been characterised by a
revolution in chemical thought more wonderful
than any which has ever been seen, and this has

1

had its iDfluence on our conception of the nature
of the atmosphere. But the ground has only been
surveyed ; future years will certainly bring fuller
knowledge.

May 1905,

PREFACE TO THE SECOND EDITION

SmcE the publication of the first edition, atmo-
spheric air has been found to contain other four
inactive gases, belonging to the same class of
elements aa argon. These are helium, discovered
by myself in 1895 in certain rare minerals, but
first separated from the atmosphere in 1900; and
neon, krypton, and xenon, discovered in conjunc-
tion with Dr. M. W. Travers in 1898, and separated
from argon and from each other during the years
1899 and 1900. An additional chapter has been
added, giving an account of these elements. I
have also to 'thank Mr, S. Lupton and Dr. Hartog
for some interesting details of the lives of Mayow
and of Priestley, which, along with some errata,
will be found at the end of the book. >

PREFACE TO THE FIRST EDITION

The discovery of a new" elementary gas iu the
I 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, necessarUy 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 under-
stood 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 b«
accompanied by a full account of the reasoning oi
which they are based. An endeavour to fulfil thi
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. J

January 1896,

t

CONTENTS

CHAPTER I

Thi Experiments and Speculations of Botle, Mayow,
AND Hales .....

)

CHAPTEE n

PAGE

" Fixed Air " and " Mephitic Air " — Their Discovert
BT Black and bt Rutherford

39

CHAPTER m

The Discovert of " Dephlogisticated Air" bt
Priestlet and bt Scheele — The Overthrow
OF the Phlogistic Theort bt Lavoisier

69

CHAPTER IV

" Phlogisticated Air" investigated bt Cavendish —

His Discovert of the Composition of Water

xi

121

/

CHAPTER V

Thb Dibcovibt oy Argon ....

CHAPTER VI

The pROPEEiiEa op Arook ....

CHAPTER VII

The PoBiTioN of Argon a«osg the Elements

CHAPTER VIII
Tek Other Ik&ctive Oases: Heliuu, Neon, Krypton,

CHAPTER IX
The RADiOAorrvE Qabkb : tbk " Euanations "

LIST OF PORTRAITS

Stkphen Hales

FrontUpiece

RORERT BOTLE

John Mayow .

Joseph Black
Daniel Rutherford .
Joseph Priestley
Carl Wilhelm Scheele
Antoine Augusts Lavoisier
Hon. Henrt Cavendish

. To face

page 8

»

16

>»

^

»»

62

»

72

>»

84

»

102

• «

121

Xlll

CHAPTER I

THE EXPERIMENTS AND SPECULATIONS (
MAYOW; *ANb' WAT-TM

To tell the story ';.£(?• the develcfpiHent of men's
ideas regarding tUe nature of atmospheEic air is in
great part 'to write a history of. chemistry and
physics. Thisfiistory is an attractive and varied
one : in its 'ea^y-. atages it was -eaprfisaed in the
quaint tenna of.'aiicieptniythoilogyv while in its later
developments it illu^atea the advantage of careful
experimental inquiry. He human mind is apt to
reason from insufficient premises ; 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 overlook

N

THE GASES OF THE ATMOSPHERE

Among the errors which misled the old^
speculators, three stand out conspicuously. ThesS.
are —

First, The confusion of one gas with another.
Since gases are for the most part colourless, and
always transparent, they make less impression O!
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 cf which had to be
inferred from the "testimony of ccbBreenses; 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 substan-
tiality was leas than that of other objects more
accessible to our senses. We ineet with a relic
of this view in words still in common use. Thus
the Greek words iriJeo), I blow, and -nveiifui, a spirit
or ghost, are closely connected ; in Latin we have
spiro, I breathe, and spiritus, the human spirit;
in English, the words gfiost and g^ist are cognate.
And the same connection can be traced in similar
words in many other languages.

Our sense of smell is affected by extremelj
minute traces of gases and vapours — traces bo am,

adA
mad

aa 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 gaaea only;
liquids and solids, if introduced into the nostrils,
irritate the olfactory nerves, but do not stimulate
tbem so as to incite the sense of smell ; yet the
admixture of a minute trace of some odorous vapour
with air appears entirely to cliange 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 tunes 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 ob-
servation that "air" obtained by distilling animal

THE GASES OF THE ATMOSPHERE

and vegetable matter, or by the action of acids
iron and zinc, differed from ordinary air by being
inflammable; but such "airs" were regarded
atmospheric air, modified in some manner, as
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 " fermenta-
tion" (/ervere, to boil).

Second, Erroneotis ideas regarding the phe-
nomena of combustion. — While it was recognised
that a burning candle was extinguished if placed
in a confined space, its extinction was attributed 3
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 J
nature as air. Together with earth and water, air
and flame or fire formed the four elementary prin-
ciples of the Ancients ; and all substances — stones,
metals, animals, and vegetables — were regarded aa
partaking of the properties of these elements, and
often as being constituted of the latter in varying
proportious, accordiug aa 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 Kopp's History
of Chemistry, Bodwell's Birth of Chemistry, E.
von Meyer's History of Chemistry, and in other
similar works. But we shall be obliged to consider
the later developments of such ideasin 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 regard-
ing the nature of combustion, and ignorance &3 to
the part played by the atmosphere in the phenomena
of burning, the true nature of air was uudiscoverable.
Third, TTte 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, notliing visible being produced from it. The
products, we now know, are gaseous and invisible,
and possessed of greater weight than the unbumt
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 cloud-
lesB sky ; the growth of vegetables in earth, from
which, apparently, they did not derive their sub-
stance ; and the reputed growth of metalliferous
lodes in mines — these were all adduced as proofe of
the creative power of Nature. With such ideaa,
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 tb
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 moat distinguished scientific men of his own, or
indeed of any, age, and in his spirit of calm philo-
sophical inquiry he was far in advance of his contem-
poraries. He waB 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, prin-
cipal Secretary of State for Ireland, died while he ■
was atill a child. Yet she must have lived in the
recollection of her son Robert, for he wrote ; " To
be Buch parents' son, and not their eldest, was a
happiness that our Fhitarethes (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 om-
nivorously, and could hardly be induced to join in
games. The next six years of his life he spent oa
the Continent with his elder brother ; and on hift

*

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 retire-
ment, 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 pubhc 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.

Boyle did not confine his attention exclusively i
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 ap-
peared 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

A

BOYLE, MAYOW, AND HALES

" The Schools teach the air to be a warm and
moist element, and consequeutly 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 bo
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 ex-
halations 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 fecu-
lency, but a confused aggregate of different effluvia,
One principal sort of these effluvia in the atmo-
sphere 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 GASES OF THE ATMOSPHERE

the air, in many places, seems to abound in cor-
puscles of a nitrous nature ; but I don't find it
proved by experiments to possess a volatile nitre.
In all my esperimenta 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." '

Boyle then proceeds to collect and comment on
the effluvia from volcanoes aud 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

• Mtmoin for a General History ef Uu Air ; Shtw'a Abridgment of
Ilojle'a works, edition I72B, vol. iii. p. 26.

J

r BOYLE, MAYOW, AND HALES

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 th«m, such as are gener-
ative, 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 :

" 1 conjecture that the atmospherical air con-
aistfi 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, miuerals, 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 magnetieal 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 " pro-
duce 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 infiammability. 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 Bpirit-of-salt" in a vacuous receiver.

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

" For tho', by reason of its great thinness and
of its being, in its usual state, devoid both of
tast and smell, air seems wholly unfit to be a
menstruum [or solvent] ; yet it may have a dis-
solving, 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." '

Boyle, shortly after, describes the production
of " an efflorescence of a vitriolic nature " on mar-

is about some bidilen qiis.litio8 of tlie Air," ibid, p. 77.

casite (or sulphide of irou) whicli has been esposed
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 esperieneed 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, rt'ndera
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
Bubaistanee 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
ita vertue ; for after the extinction of the flame the
air in the receiver was not visibly alter'd ; and for
ought 1 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 sub-
stance, if I may bo call it, diffused thro' the
air ; whether it be a volatile nitre, or rather
some anonymous substance, sidereal or subter-
raneal ; tho' not improbably of kin to that which
Beems so necessary to the maintenance of the
other flames."

The experimental part of Boyle's work in this
connection relates to the oxidation of cuprous to
cupric compounds, with the change of colour from
brown to blue or green, either in ammoniaeal 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

' t BOYLE, MAYOW, AND HALES 15

. " To separate air from liquore by boiling.

1 " To separate air from liquors by the air-pump.

^^^ " To produce air by corrosion, especially with

^^B spirit of vinegar.

^^B "To separate air by animal and sulphureous

^^H dissolvants.

^^H "To obtain air in an exhausted receiver by

^^H burning-glasses and red-hot irons. j

^^^P " To produce air ont of gunpowder and other

1 nitrous bodies. I
" 2. To examine the produced aerial substances

^^B by their preserving or reviving animals,

^^H flame, fire, the light of rotten wood, and

^H of

^^^1 "To examine it by its elasticity, and the

^^^1 duration thereof.

^^H "To do the same by its weight, and its

^^^P 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 j
before it did. Some of these experiments were j
indeed made by John Mayow, his contemporary, |
whose work and speculations we shall now proceed Jt
to consider. ^M

John Mayow was born in the parish of
St. Dunstan in the West, London, in 1643. His
family was originally Cornish, having come from
Bree, in Cornwall. He entered Wadham College,
Oxford, on April 3rd, 1658, at the early age of
fourteeli, and was shortly afterwards made a pro-
bationer-fellow of All Souls College. On May 30th,
1665, after nearly seven years of study, he took
the degree of B.A. ; in 1670, he graduated as
Doctor of Laws (LL.D.) ; 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 - five years of age,
he wrote two essays on Respiration, ascribing the
inflation of the lungs to the action of the inter-
costal muscles. These " Tractatus duo" were
published in 1668. Some years later he produced
the treatise on which his fame rests ; it is en-
titled " Tractatus quinque medico-physici, quorum
primus agit de sal-nitro et spiritu nitro-aereo;
seeunduH, de resptratione ; tertius, de respiratione
foetus in utero et ovo ; quartus, de motu muscu-
lari, et spiritibus animdibus ; ultimus, de rhachi-
tide ; studio Joh. Mayow, LL.D. & Medi(?i, nee

non Coll. Omn, Anim. in Univ. Oxon. Socii. Oionii
e Theatro Sheldoniano, An. Dom. mdclxxiv." The
work was dedicated to Mr. Henry Coventry. It was
inserted in an abridged fonn in the Philosophical
Transactions of the Royal Society some time after
its publication, but received only ecant 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 bis contemporaries, for he died in
October 1679, five years after the publication
of his tracts, in his thirty-seventh 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
farthered the knowledge of the true explanation
of the nature of air, and its function in sup-
porting combustion and respiration, and that his
views would have been accepted more than a
eentaiy before Lavoisier — with fuller knowledge,
and with the scientific position which at once
gained a hearing — forced precisely similar doc-
trines upon the attention of tbe scientific world.
[ftyow was a contemporary of Boyle, and fre-

^

quentlymadeuseof 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 ao
young a man. And shortly after Mayow's death,
the views of Becber, 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, first by Blumenbaeh, in his Jnstitutiones
Physiological ; later by T. Beddoes, in 1790, who
wrote a digest of Mayow's work under the title
" Chemical Experimetfts and Opinions extracted
from a Work published in the Last Century " ; and
later by Johann, Andreas Scherer, in a work pub-
lished 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 paragrapha^
with a paraphrase in German of the Latin text, which
he quotes in full. Yeats' treatise is more especially

coDcerned with the medical aspect of Mayow's work,
althougli it also deals with the purely chemical por-
tionat considerable length.^ In the following account
of Mayow's researches free use has been made of
both of these works, as well as of hia 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 gas at least : one of these, termed
" pitro-aerial particles," is necessary for the sup-
port, 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 - aeriai particles " are also
present in saltpetre or nitre, as can be shown by

I 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,

I immediately plunging the open end of the tube

I ' A Gormiui trnDsUtion a( Mftjav'a chemii^al meirchm bM been

publiibed by K. G. Dona&n iu Osticiild*s Clataiixr (EDgclmann, Lsipiig).

i

THE GASES OF THE ATMOSPHERE

L

N

under water. The sulpliur 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 con-
tain such "sulphur" can be shown by expos-
ing it alone to heat, when no change takes place,
except fusion. Besides, nitre is compounded oi
"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-ait i
particles, because, as will be shown later, they aie
not present in the alkali.

One difficulty occurs to Mayow. How is it
that so large a quantity of gas as is necessaig^
to support combustion can be contained in
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
leas than pure water does. He also prepares'

BOYLE, MAYOW, AND HALES

Bal^tre by mixing nitric acid and alkali in a
viouum ; a brisk effervescence occurs, and the
dried-up salt is ordinary saltpetre. Hence salt-
petee cannot contain elastic air. Mayow conse-
quently draws a distinction between " air " and
"MT-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 con-
fined 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 absorp-
tion 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 pEirticles ; and these are

evidently due to the nitric acid, because the
residue, — the alkali, — if mixed with sulphar and
infiamed, does not produce ignition.

2, 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 sub-
stance from a vitriol (or sulphate) is obtained by
melting together alkali and sulphur ; and no
eflfervescence 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 con-

Ltained in sulphur, it would hinder the union of the j
aulphur with the alkalL ■

It is to be noticed that the volatile sulphuric
acid, from the combuation 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 col-
cothar {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. " 1 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 extin-
gaished 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
Buch 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.

Wbile gunpowder bums owing to the fire-aii
particles which it contains, and requii-ea no susten-
ance from external air, the combustion of vege-
tables 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.c. 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, paitly 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 modem viewa, but Mayow's mind is some-
what 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, 30 a loss of heat should, in his opinion, pro-
duce 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
combnstible 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 ordi-
nary 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 fer
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 combina-
tion of the two gases is attended by rise of tem-
perature, and is in so far analogous to combustion.

It would lead ua too far to consider in detail
Mayow'a theories of fermentation and of respira-
tion. Suffice it to say that he ascribes the pro-
duction 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
in opposition to that held by his contem-
poraries, 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 dis-
covery 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 combus-
tion. But these discoveries were not made until a
century after his death. Had he lived, there cau
be little doubt that, unless discouraged by the
want of appreciation with which hia ideas were
received, he would have continued to labour in the
feuitfiil fields from which he had already reaped
80 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 Reyexhibited someleauingtowardstbemodem
methods of experimentation, he still lay fettered in
the bonds of mediasval scholasticism. In discuss-
ing the weight of air and fire, he finds oeeaBion to
consider the question whether a vacuum can exist.
Win 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 ? Tiius tlie universe could reach anni-
hilation 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, aa
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 deduc-
tions of Boyle and Mayow.

The next stage in the history of our subject ia
the consideration of the work of Stephen Hales
and of Joseph Priestley. Both of theae philoso-
phers 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 aa 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 hia
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 experi-
ments 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 Vege-
tation : 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 Ted-
dington, Middlesex,"

In hia " 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 rime were of a somewhat vague character.

I

and it often happened that tlie conclusions drawn
from them were incorrect. Hence it is with a feel-
ing 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 crea-
tion 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 mount-
ains 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 quan-
tity 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 ex-

perimenta ; because it were impossible for such
great quantities of it to be confined in the sub-
stances 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,
Halea proceeded to distil a great number of sub-
stances of animal and vegetable origin, such aa
hogs' blood, tallow, a fallow-deer's horn, oyster-
shell, oak, wheat, peas, amber, tobacco, camphor,
aniseed oil, honey, bees'-wax, sugar, Newcastle coal,
earth, chalk, pyrites, a mixture of salt and bone-
ash, of nitre and bone-ash, tartar, compound aqua-
fortis, and a number of other substances. He col-
lected 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 posi-
tive conclusion, because of the crudeness of his
appliances. The compressibility or " elasticity " of
the air from tartar, however, was found to be iden-
tical with that of common air.

^

Hales does not appear to have made any special
experiments on the properties of bis 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 fermenta-
tion 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 com-
pression 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

^^B^H

■^

M

PUBLIC UBRARY ^H

1

metals. Agua-regia and gold, aqua-regia and
antimony, aquafoi'tis and iron, dilute oil-of-vitriol
and iron, yielded gases whicli contracted on stand-
ing in contact with water. This, in the case of the
oxides of nitrogen, is to be ascribed to their reacting
the oxygen of the air accidentally present in the
Ter ; but in the last case Hales noticed that the
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 absorl)ed 1^ cubic inches of air, and
one with copper filings twice aa 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 sometimea theorises a little, aa 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. Homherg, Mr. Lemery, and

some others imagine, then such aulphureous 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 pro-
perties 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 in- ,
flammable 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 tha
parts of matter were only endued with a strongly
attracting power, whole nature would then imme-
diately become one unactive cohering lump ; where-
fore it was absolutely necessary, in order to the
actuating and enlivening this vast mass of attracting '

I BOYLE, MAYOW, AND HALES 35

mutter, that there should be everywhere iutennis'd
with it a due proportion of strongly repelling
elastick particles, which might enliven the whole
masa, 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 pro-
perty 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,

I salt, Bulphur, 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 ferment-

THE GASES OF THE ATMOSPHERE

ing mixture ; the action and re-aetion 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 culinarya
fires are produced and maintained. I

" 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 fore-
going Experiments, evident proof that its elasticity
is easily and in great abundance destroyed ; and is
thereby reduced to a fist state by the strong attrac-
tion 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 salin^
particles which abound in air. Whence it is reason^
able to conclude that our atmosphere is a Chao^
consisting not only of elastick, but also of unclastidc
air-partic!es, which in plenty float in it, as well
the sulphureous saline, watry, and earthy particleSj
which are no ways capable of being thrown ofl" intd

& permanently elastiek 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 ehymieal 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 fer-
mentation, its former elastiek 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 ehymieal principles,
be immutably fixt, — since then this is the ease,
may we not with good reason adopt this now fist,
now volatile Proteus among the ehymieal 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 entitled 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 doei
adopt any one of Mayow's conclusions. One would
have thought that, having prepared so many gasea
by means of apparatus well adapted to their pur-
pose, and having observed that certain substances
introduced into air produced contraction, he would
have drawn the conclusion that such "airs" wera
essentially different kiuds 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.

"FIXED air" and "MEPHITIC AIR "—THEIR DIS-
COVERT BT BLACK AND BY ROTHERFORD

Before relating the history of the discoveries of
Black, Rutherford, and Priestley, it wili be appro-
priate to give an account of a theory which pro-
fessed 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 develop-
ing. Ite genn is to be traced to the writings of
Johann Baptist van Helmont of Brabant, Seigneur
of Merode, Royenboch, Oorshot, and PeUines, who
wae bom 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. Be-
lieving that water was the true principle and origin
of everything (for he had succeeded in producing

a willow tree, weighing 164 Iba., 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 de-
veloped a vapour, to which van Helmont gave the
name of "gas." A "gas" was a substance inter-
mediate lietween 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 waa
Mas, 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 appeal's from his treatise " De
Flatibus." His gas sylvestre was evolved from
fermenting liquors, and he knew that it was formed

FIXED AIR AND MEPHITIC AIR

1

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 pmgue, gas siccum, or gas
fuliginosv/m.

Theae 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 horn at Anspach in
1660; he studied and graduated in medicine at
Halle, and in 1694 he was appointed second pro-
fessor of medicine at that University, where he
continued to teach for twenty-two years. His
most important work was his Fundamenta chymiae
dogmaticae et experimejitale. His theoretical
views are contained in the last part of this work.
He there treats of zymotechnia, or fermentation ;
kalotechnia, or the production of salts ; and j)yrd-
technia, or the doctrine of combustion. It is the
last of these sections which gives an account of the
doctrine oi phlogiston.

THE GASES OF THE ATMOSPHERE chap.

The fundamental couception of this doctrine is
that all combuBtible 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 phos-
phorus, when burnt, give acids; and the metals form
ccUcea. Non-combustible substances, such as Ume,
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 com-
bustible bodies whatsoever ; it corresponds in kind
with the " sulphur " of more ancient writers, but
di£fers 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 conveision 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 bis contemporaries. For Stabl 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
auch substances as phosphorus, sulphur, or metals
are heated, they burn, and are changed into phos-
phoric 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, pit-
coal, sugar, flour, etc., phlogiston is restored to the
burnt substance, and the original material, phos-
phorus, 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 appli-
cations of the theory and to confirm its truth.

Substances were not supposed always to be
completely deprived of phlogiston by combus-
tion. Indeed, if the phlogiston were removed
wholly, or neai-ly so, it was by no means easy to

i

THE GASES OF THE ATMOSPHERE

restore it. Thus the calx of ziuc, or of iron, which
was regarded as nearly devoid of phlogiston, is
diiiicult 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 aa regards colour or
metallic lustre, and these vary accordiug 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 tevj-a pinguis, and it was
represented by Becker and by Stahl as a dry
substauce 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
dame, 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 Eey 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 antimoDy by the aid of a burning-
glftss, and rightly conjectured that the substance
produced was the same as that formed by treat-
ing it with nitric acid, and subsequent ignition.
Boyle's view waa 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 he offered. It was that phlo-
giston 18 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
i undergo in changing into calces.

It is indeed astonisbiiig 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 facta to which they wished
to call attention in this manner, partly because
they were not in a position to replace tlie theory
by a more rational one. It is not easy to revo-
lutionise 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 diflScult to under-
stand Priestley's attitude, which we shall have to
consider later ; for Priestley was first of all an
experimentalist, ami 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

FIXED AIR AND MEPHITIC AIR

knowledge of tlie nature of air was of such a kind as
to retard its progress. For how could that know-
ledge be furthered, when the moat 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, which furnished hut a lame explanation of
phenomena, and which had been anticipated by
another theory, subsequently proved to be coiTect.
Its sole support was the authority of its inventors
or adapters, and the deeply-ingrained notions of
eentaries. 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 associ-
ated with famous names it may be. This was
happily expressed by Boyle wht-n he wrote : ' " 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 yeara or ages since, or came

■ A Fttt Ittquiry ifUo Iht V'^Ugar NatiiM ^ Naiurt ; PrefaUry reiokrki.

48 THE GASES OF THE ATMOSPHERE chap.

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 nr no
it be genuine, and does or does not deserve to
have been curreut. 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
recseive 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

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PUBLIC LIBRARY

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carbon dioxide, that conBtituent 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 80 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 bom 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. CuUen, then
Lecturer on Chemistry at the College tliere, who
is mentioned by Professor Thomas Thomson in his
History of Cliemistry as an excellent and instruc-
tive 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.

' so THE GASES OF THE ATMOSPHERE CBAftl

He began hia medical studies in Edinburgh in the
yearl751,andin 1755 he published, as bis thesis for
the degree of M.D., the work which has rendered his '
name famous. It appears that as early as 1752 be
had been occupied with investigations on quicklime,
which was then attracting attention as a remedy
for urinary caleuU. Opinion was divided regard-
ing its virtue. In a manuscript copy of notes of
Black's lectures, which the author is so fortunate
as to possess, he mentions that hia 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 success-
fully 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 under-
gone by limestone on being calcined with its loss
V on being dissolved in muriatic acid. These experi-

ments appear from hia journal to have been made
before November 1752.

His thesis was not published, however, until
1755. Immediately after, in 1V56, he succeeded
Dr. Cullen aa Professor in Glasgow, where he re-
mained until 1766. During these ten years he

I 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 be termed them, the
" latent beats " 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 hia last students ; he writes :

' — "I never listened to any lectures with so much
pleasure as to his ; and it was the elegant simplicity
of bis mnnner, the perfect clearness of his state-

I ments, 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."

THE GASES OF THE ATMOSPHERE cha».

Black's original thesis for bis degree was entitled
Experiments upon Magnesia Alba, Quicklime,
amd other Alkaline Substances. It was published
in 1755, and several times reprinted. It is now to
be had in 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 aa 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 stdt '
from sea-water, and also from Epsom salts, " which
is evidently composed of magnesia and the vitriolic
acid." The magnesia is thrown down from tho
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 sub-

J

FIXED AIR AND MEPHITIC AIR

ntlj heated this magnesia, and found that it
' lost " a remarkable proportion of its weight in the
fire," and hia " attempts were directed to the in-
vestigation 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
{tojn 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 magTiesia 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
'eii that alkalme salts contain a large quantity

lundance I

THE GASES OF THE ATMOSPHERE

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 ia
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 ordinar}' condi-
tion, 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 suffi-
cient to expel it entirely from fixed alkalis, or
take away their power of efiervescing with acid
salts.

" These considerations led me to conclude that

the relation between fixed air and alkaline sub-
Btances 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 nii:i 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 acri-
mony 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
affeo with lime-water, the air leaves the magnesia

and joins itself to the lime ; and as both magnesia
and calcium carbouate are insoluble in water, the
water is left pare. Similarly quicklime deprives
alkalies of their air and renders them caustic. And
it foUowa that if caustic alkali be added to a salt of
magnesia or of lime, it will Hcparate 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 cap-
able 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 oi fixed air,
and perhaps very improperly ; but I thought it
better to use a word already famiHar in philosophy
than to invent a new name, before we be more fully
acquainted with the nature and properties of this

aubstanee, 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 ia required to
saturate the caustic alkali as to saturate the mild
alkali from which the caustic alkali had been pre-
pared. 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 in-
gredieut. The volatile alkali (ammonium carbon-
ate) 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 at-
tempts to render mild alkalies caustic by heat alone
(t.e. to expel carbon dioxide from potassium car-
bonate), Black examines the action of the "seda-

^ Bait" or boracie acid on mild alkalies, by rubbing

them together in presence of some water. At first
there is no eflfervescence, but on adding successive
quantities of boracic acid, brisk effervescence finally
takes place, borax being formed. "Tiiia pheno-
menon 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 potash) to the
air, in a broad and shallow vessel, for the apace 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 potassium car-
bonate). It therefore follows that such alkaline
sabstances 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 mdd, like a solution of salt of

tartar. Tliese 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
extevipore, 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 dis-
covered 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

6o THE GASES OF THE ATMOSPHERE

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, show-
ing 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 £xed
air from water, which then deposits its dissolved
calcium carbonate, present in solution as bicarbon-
ate. 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. 1 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 measure-
ment. It was not long since Hales had pro-
nounced 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 difi'ering in many of its
properties from common air, especially in its being
absorbed by quicklime and by alkalies. It must

1

be remembered that at that time carbon was not
recogniaed aa an element ; and hence, though Black
knew that fixed air was a product of the combus-
tion of charcoal, he did not attribute it to the
union of carbon with oxygen, although the sen-
tence 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 ita
description formed a thesis for hia 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 1 2th September. He then travelled
for three years in England, France, and Italy, and
in 1773 he returned to his native town, where he
practised hia 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

FIXED AIR AND MEPHITIC AIR

of Physicians of Edinburgh from 1796 to 1798.
During tiie greater part of his life lie suffered from
gout; be died in 1819, at the age of seventy.

Rutherford does not seem to have pursued
the etudy of chemistry further : hia duties led him
into other fields. Hia genial, pleasant face, seen in
the portrait by Raebum, shows him to have pos-
sessed 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 Zhssertatio Inauguralis de aere Jixo
dicto, aut mephitico. It was published at Edin-
burgh in 1772, seventeen years after Black's
memorable dissertation on Fixed Air. As will be
seen shortly, Priestley had nearly anticipated
Rutherford ; and, indeed, he speculated on the
nature of the residual gas, left after combustion
and absorption of the fixed air produced. Evi-
dently Black had noticed that a residue waa 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

en J

THE GASES OF THE ATMOSPHERE c

a student of his, the advantage of further investi-
gating the naatter, and ascertaining the properties '
of the residual gas.

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

DeniquB res oranea debent in corpore habere
Aera, quaadoquidem rara sunt corpora et aer
Omnibus est rebus clrcumdatus apposltusque.

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

After acknowledging hia debt to his iUustrious
preceptor Black, be proceeds to quote from the
latter to the efl'ect that mephitic or fixed air is the
air which proves fatal to animals and extinguishea
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 ferment-

II FIXED AIR AND MEPHITIC AIR 65 ^^^

ation. Its density, compaj-ed with that of ordinary
air, is 83 15| or 16 to 9 ; hence it can be kept for
some time in an open gljtss, and if a jar of it
be inverted over a lighted candle, the t-andle
is extinguished. It has an agreeable taste and
smell; and it changes the colour of syrup of
violets from blue to purple. It prevents putre-
faction, but putrefied bodies are not made freah
, 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
1 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

1 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 srnoulder in

1 it for a short time. It is thus proved that after

1 the whole of the fixed air has been withdrawn by

F

THE GASES OF THE ATMOSPHERE chap.

alkalies, the residue is still incapable of supporting
life and combustion.

Some burning bodies deprive air of its " salu-
brity " 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 haa
burned until it is extinguished. Even after'
the absorption of all fixed air by alkalies, phoa-
phorus burns, emitting clouds of the dry acid
of phosphorus, which can be absorbed by lime-
water.

" It therefore appears that pure air is not con-
verted 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 cal-
cination 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 im-
pregnated with phlogiston ; and also frum that

from decaying flesh, which is a mixture of mephitic
air and combustible air.

He proceeds : — " I had intencled to add some-
thing regarding the composition of mephitic air,
and to seek for a reason for its unwholesome efl'ectB,
bat I have not been able to find out anything
with certainty. Certain experiments appear to
show, however, that it consists of atmospheric
air iti union with phlogistic material : for it is
never produced except from bodies which abound
iQ inflammable parts ; the phlogiston ever appears
to be taken up by other bodies, and is hence of
value in reducing the cakes 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]. ... 1 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
wholeaomeness ; and that mephitic air, added to
air from putrid flesh, partly mitigates its unwhole-
ftome character. But I have been unable to try
such experiments."

We see, then, that Rutherford's claims to the

THE GASES OF THE ATMOSPHERE chap, ir

dUcovery of nitrogen amount to this : He removed
the oxygeu from ordinary air by combustibles auch
aa cbarcoal, phoapliorua, or a candle ; and having got
rid of the carbon dioxide, iu those cases when it was
formed, by alkali or lime, he obtained a residue,
now known as nitrogen. Hia 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'
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.

CHAPTER III

DISCOVERY OF " DEPH LOGISTIC ATED AIE BY
JESTLEY AND BY SCHblELE — THE OVERTHROW
IF TEE 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 minium, or 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
&om 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
simultaueougly by Priestley and Scheele, though it
69

appears from the recent publication of Scheele'a
laboratory notes by Baron Nordenskjold 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 experi-
ments were undertaken in an almost haphazard
manner, probably as a relazatioiL " For my
own part," he says,^ " I will frankly acknowledge
that, at the commencement of the experlmentB
recited in thia 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 lieen 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:* — "I perceived the
necessity of a knowledge of fire, because without
this it is impossible to make any experiment ; and

' Bxperinunlt and Observationa an Different Kindt of A\
By Josepli PneBtley, LLD., F.K.S. Second eUition (1776), p. 20.
> CKemitaX TrtatiMmtAir a!idFiTt{\'m),li.

without fire or heat it is impoaaible to utilise
the action of any solvent. I began, therefore, to
dismiss from my mind all esplanations of fire, and
undertook a series of experiments in order to gain
as full knowledge as possible of these lovely phe-
Qomeoa. I ere long found, however, that it was not
possible to form any correct opinioa concerning the
appearances which fire exhibits, without a know-
ledge of the air. After a aeries of experiments, I
saw that air really is concerned in the mixture
termed fire, and that it is a constituent of Hame and
sparks. I learned, moreover, that such a treatise on
fire aa 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 advan-
tage of priority of publication, we shall commence
I an account of his researches.

iph Priestley was born in 1733 at Field-

THE GASES OF THE ATMOSPHERE

head, 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 know-
ledge 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 Nant-
wich in Cheshire, where he preached and also taught
a school. Here his income was improved, though
still miserably small ; yet be managed to buy some
books, a small air-pump, and an electrical machine.
He removed to Warrington in September 1761,
being employed there in teaching and in literary
work ; there he began to pay some attention to

XCV.K

"the «-'^ ^o r^v

»NOX

TW-PiH

►•:ToohoTv«-.

fOV)

OF DEPHLOGISTICATED AIR

chemistry by attending a course of lectures delivered
by Dr. Turner of Liverpool. WLUe at Warring-
ton he wrote a History of Electricity, which first
brought him into notice, and which procured for him
the degree of LL.D. of Edinburgh, thus giving bim
a right to the title of Doctor, by which he was always
afterwards known. At Warrington, too, he married
a daughter of Mr. Wilkinson, an ironmaster of
Wrexham. We next find him being asked in 1767
to take the pastorship of Mill Hill Chapel at Leeds,
a call which he accepted. The house in which he
took up his abode before the "minister's house"
had been completed was in Meadow Road, next
door to the brewery of Jakes and Nell, and this
circamstance first induced him to take up the
subject of the chemistry of gases, which has made
bis 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 Conti-
nent. While with Lord Shelburne he published
the first three volumes of Experiments on Air, nnd
carried out investigations which were recorded in a
fourth volume, published after his removal to Bir-

miughaiQ. After some years spent in this way he was
pensioned off, and settled as minister of a meeting-
bouse in Birmingham, where he employed his time
partly in theological controversy, and partly in
prosecuting researches in chemiBtry. He published
during this period other three volumes giving a
description of his experiments on air, and com-
municated several papers to the Philosophical
Transactions of the Royal Society, of which he bad
been made a Fellow. Towards the year 1 790 he was
80 unfortunate as to attack Burke's book on the
French Revolution ; and thia bad the eff'ett of
rousing popular opinion against him, more especi-
ally that of the local clergy, whose political viewB
he had frequently opposed. During the riots which
took place at Birmingham in 1791 his house was
burned, and be was obliged to escape to London
under an assumed name. After some years spent
in the charge of a meeting-house at Hackney, be
left England for America. His opinions, though by
no means uncommon at the present day, were so
antagonistic to those of bis English contemporaries
that be was cut by his Fellows of the Royal Society,
and be therefore resigned bis 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 waa well received, and settled
at Northumberland, not far from Philadelphia.
There he died in 1804.

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
osed now : the Hasks with bent delivery-tubes, the
Woulfe's bottles with two Decks, and the pneumatic
trough filled with water or mercury were his chief
uteosUs. By means of such apparatus, gases can
be collected in a state of comparative puiity : 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 leas 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 fiask
in order to expel air ; an empty bladder waa then
over the neck, and the gas entered the

J

bladder. When he wished to transfer the gas to
a bottle, the bladder was tied at some distance
£rom 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 thia eork 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, Thia 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 superi-
ority by their survival.

The earliest date at which Priestley began to
experiment with gases was the beginning of the
year 1766, led, no doubt, by the lectures be had
heard at Warrington. In 1767, at Leeds, he had
made some experiments on the conductivity of
" airs" for electricity, using for this purpose common,
inflammable, and fixed airs. He appears to have
abandoned the study of gases in 1768. and to have

resumed it in 1 772. The first gas whicli he then
investigated was " nitrous gas," or, as it is now
named, nitric oxide. It had previously been pre-
pared by Mayow (see p. 25) by the action of nitric
acid on iron ; aod 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 pro-
perties from ordinary air. This made his mind
familiar with the thought that different kinds of air
exist, not necessarily modifications of atmospheric
aix. He had previously from his experiments
come to the conclusion that " atmospheric air is

THE GASES OF THE ATMOSPHERE

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 aa
to render it altogether unfit for inflammation,
respiration, and other purposes to which it is sub-
servient; and I had discovered that agitation in
water, the process of vegetation, and probably
other natural processes, by taking out the super-
fluous phlogiston, restore it to its natural purity.
But I own I had no idea of the possibility of going
any further in this way, and thereby procuring air
purer than the best common air."

On the Ist of August 1774, Priestley heated by
means of a burning-glass red oxide of mercury.
This was produced by heating mercury until it oxi-
dised, 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 ciindle burned more
brightly, and a piece of red-hot wood sparkled in
it. This air he also produced from " red precipi-
tate," the product of heating nitrate of mercury ;
and at the same time from red-lead, or minium.

It diflfered from " modifiud nitrous air," in which a
candle also bums brightly, inasmucb ae 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 ob-
tained by distilling nitrates, which is essentially a
mixture of nitric peroxide with oxygen. A candle
bums in such a mixture, depriving the nitric per-
oxide of part of its oxygen, and converting it into
nitric oxide mixed with nitrogeu. Nitric oxide,
deprived of the excess of peroxide by shaking with
water, with which the peroxide reacts and Is ab-
sorbed, 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 inter-
vals from August 1774 till March 1779, and at
that date it occurred to him to mix with his de-
phlogisticated 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 com-
bustion 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 whit;h nitric oxide
had not been added — when it lived for another half-
hour, and was quite vigorous when withdrawn.

Subsequeut experiments with nitric oxide
showed that air from red precipitate or from
" mercuritis ccUcinattis " {red oxide of mercury ia
each case, although prepared in different ways) was
"between four and five times as good as common
He proceeds : ' — " Being now satisfied with
respect to the nature of this new species of air, viz,

' Loc. cit. p. 46.

air. ne j

I respect to tl

that being capable of taking more phlogiston from
nitroQa 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 operatious, air
which was " five times as good " as common air.
This process formed lead nitrate, which on distilla-
tion 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
citric 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: " Atmosphei'ical
air, or the thing we breathe, consists of the nitrous
add and earth, with so much phlogiston as is
necessary to its elasticity ; and likewise so much

r

more as is required to bring it from its state of
perfect purity to the mean condition in which we
find it" '

When such experiments were made by heating
nitrates in a gun-barrel, " phlogisticated air " was
obtained. Tliia was nitrogen, for the iron had re-
duced 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
earthi."

" Dephlogisticated air may be procured from any
kind of earth with which the spirit or 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

Ill DISCOVERY OF DEPHLOGISTICATED AIR 83

Phlogiaticatod air, and weighed 7 dwts. 15 grs.
Nitrous air „ „ "^ >< 1^ n

Common air „ „ 7 „ 1 7 „

Dephlogisticated sir,, „ T „ 1 9 „

He concludes (taking into consideration that in-
flammable air is very light) ' ' that the less phlogiston
any kind of air contains, the heavier it is ; and the
more phlogiston it containsj the lighter it is."'
Strange that this should not have led to the
rejection of the phlogistic hypothesis 1

Priestley had the curiosity to breathe his
"good" air. He says: "My reader will not
wonder that, after having ascertained the superior
goodness of dephlogistieated air by mice living in
it, and the other teats 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

THE GASES OF THE ATMOSPHERE

in luxury ? Hitherto only two mice and myself
have had the privilege of breathing it." '

It will be Been 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, hia chemi-
cal pursuits were to him a recreation, and were
undertaken during the intervals of hia necessary
work. Hia 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 delight-
fully familiar one : he exposes his inmost thoughts
with perfect frankness, and his writings are there-
fore very readable. — We have now to compare hia
work with that of his coutemporary, 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 bom 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. I

■ ' Xoc ffU. p. 103. I

CARL Wn.HELM SCHLELE

i

^n ^^^^^H

■

■

1

^

THK WEW Yf BK
PUBLIC LIBRARY

1

J

After receiving his education, partly in a private
school, partly in the public achool {gymnasium) at
Stralsand, he was apprenticed at the age of four-
teen to the apothecary Bauch in Gothenburg. In
those days an apothecary was in large measure a
maimfitcturer 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, veutured to pre-
scribe in mild cases. Scbeele's master taught him
sach methods, and in addition instructed him in the
Dse of the chemical symbols in vogue at that date ;
these he afterwards freely employed in his manu-
scripts, and this renders them exceedingly difficult
to decipher. There still exists a catalogue of the
drags 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 Ubrary, which included the
works of Boerhaave and Lemery, and his master

THE GASES OF THE ATMOSPHERE

devoted much pains to his insti-uction. In a letter
to Scheele's father, however, he expressed a fear
that too great devotion to study and experimental
work would uadermiae the health of a growing lad.
In 1765 the buaineas was sold, and Scheele
obtained a situation in Malmd with an apothecary-
named Kjellatrom. 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 be 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 MalmS
for Stockholm ; but here the exigencies of his duties
interfered with bis leisure for experimentation.
While there, in conjunction with his friend Retziua,
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 ProfesBor of ChemiBtry 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 hia
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
be 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 Kdping, where he passed the rest of hia
days, in spite of urgent appeals to engage in more
remunerative work ; indeed, he was strongly pressed
to go to Berlin, and also, it ia said, to London, for
his publications had led to his recognition as one of
the greatest chemists of the age. His book on Fire

88 THE GASES OF THE ATMOSPHERE

and Air was not published for some years after the
manuBcript had been in the printer's hands. We
learn from his letters that he was much a&aid 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, Seheele is depicted as an amiable
and honourable man, singularly free from vanity
and selfishness. 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 Ma 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, extra-
ordinary aptitude in experimenting, and high
intellectual powers place him among the foremost
men of science of his day.

Near the beginning of his Treatise on Air and
Fire,^ Seheele defines air. It is that fluid invisible
substance which we continually breathe ; which

' The accursts truiHUtiou of SohseU's Trfaliae published bj ths
Alembic Club (William F. Cla;, 18fl4} hui beau made ueh: of hera.

DISCOVERY OF DEPHLOGISTI GATED AIR

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 exhala-
tions, which are so finely divided in it that they are
scarcely visible, even in the sun's rays." ' It also
contains another elastic substance resembling air,
termed aerial acid by Bergman (identical with
Black's fixed air). Siuce atmospheric air has not
been completely converted into fixed air by admix-
ture of foreign materials, " I hope I do not err if 1
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 other-
wise an innumerable multitude of varieties of air
would have to be assumed, since it is very probable
that ail substances can be converted by excessive
heat into a vapour resembling air."*
' 8 *■ " s 6.

THE GASES OF THE ATMOSPHERE chap.

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 pro-
duce 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 solu-
tion of iron in vinegar, and a solution of cuprous

chloride. " In noae 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 import-
ance." ' 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."'

THE GASES OF THE ATMOSPHERE

To decide this question, Scheele burned in air
substances such as phosphorus, which do not pro-
duce 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 loae
one-fifth of its volume. On burning a candle,
some spirits of wine, or some charcoal, in a con-
fined quantity of air, very little, if any, diminu-
tion of volume waa noticed ; but on shaking the
air with nulk 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 extin-
guished 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 sub-
stances. It is these two elastic fluids, separating
themselves from such a flame, which present no

DISCOVERY OF DEPHLOGISTICATED AIR 93

small hindrance to the fire which would otherwise
bum much longer, especially since there is here
no current of air hj 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."' 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.""
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

J

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. Oue 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 pre-
supposes some weight, I much doubt whether such
hypothesis has any foundation." ' He had formerly
conjectured that hydrogen, the "air" obtained by
the action of vitriol on zinc, might be phlogiston;
" stiU, 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."

DISCOVERY OF DEPHLOGISTICATED AIR

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 bum 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 bum ; 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." ^ How
history repeats itself! Here is Scheele, in 1772,
reproducing Mayow'a 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 produca
nitre with alkalies ; moreover, it can be prepared
from substances which have nothing in common.
with nitre, no compound of nitre having been usedi
during their preparation. Scheelc next describes
experiments proving that "fire-air" is produced.
hy the distillation of black oxide of manganese
with concentrated oU of vitriol, or with the
borus acid of urine " {phosphoric acid), by
distilling nitrate of magnesium, made by dissolv-
ing the "white magnesia employed in medicine"
(magnesium carWnate) 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 limo.
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,"

■II DISCOVERY OF DEPHLOGISTICATED AIR 97

The brown-red precipitate obtained by adding
" alkali of tartar " to " corrosive sublimate " {po-
tassium 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
modem language-, by heating mercuric nitrate, a
pure " fire-air,'.', untnised with;- '■ aerial acid," was
the product. ■ And lastly, arsenjc acid, when heated
gave ordinary white arsenic together with " fire-air."

This', fire^air was completely ^Jjsorbed by " liver
' of sulphur ■'.■(a polysulphide of-potassium, formed
by heating together potassiums, -carbonate and sul-
phur) ; and a i^ixture bf-.foar 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 com-
pletely established.

As " vitiated air " is lighter than ordinary air,

THE GASES OF THE ATMOSPHERE chap.

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 metaliie imerciiry and " fire-air."
" This 18 a remark^le , 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 ihe calx
is simply maile.\Fcd4iot,"' Is it not -remarkable
that the true explanation, should- no6. have forced
itself upon Scheele'?-. .mind, whigh was bo acute,
and BO capable of forming- trae 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 lungB
had the same effect. A few flies, bees, and cater-
pillars 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."
> ISO,

DISCOVERY OF DE

"These are accordingly strange circumstances, ttat
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."' And so he makes experiments
which prove that it is the fire-air which is con-
verted 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.
Seheele 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 GASES OF THE ATMOSPHERE

the oxygen falls below a certain not very small
amonut, while insects appear to be capable of ex-
hanating the oxygen to a great extent ; and it is
probable that the plants, under the unnatural cir-
cumstances in which they were placed, gave off a
considerable amount of carbon dioxide. Scheele'a
explanation in terms of phlogiston is not suceessfuL
He wrote : — " I am inclined to believe that 6re-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,'"
This is reversing what may be termed the true
explanation on the basis of the phlogistic theory.
For Scheele supposes that oxygen contains phlo-
giston, and by losing it, yields carbon dioxide. On
the other hand, the consistent explnnation would
be that carbon is carbonic acid plus phlogiston,
and that when it bums 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
MM.

to formulate views which are now universally
accepted.

In the concluding sections of his treatise
Scheele describes experiments which prove the
aolubility 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 althougii we
cannot but feel surprise that his discoveries did not
lead him to take the step of renouncing the hypo-
thesis 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 Caven-
dish, 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 uncer-

i

THE GASES OF THE ATMOSPHERE

CHAT. 1

i doubt I

tainty concerning the nature of heat, and the doubt
whether it was not a form of imponderable matter,
may have led both Scheele and Cavendiah to retain
the older viewa. It was Lavoiaier 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 boru in Paris on
the 26th of August 1743. His father was wealthy,
and spared no expense on his education. In hia
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

THS NEW YORK

PUBLIC LIBRARY

A8T«m. L*NOfK
TILD£N FOUNOATIONf

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-g^n^ral," or collector of Government
revenue ; and after his death she became the wife
of Count Rumford, another distinguished acientific
man. Made a " fermier g^ni^ral" himself, it waa
during his tenure of this office that Lavoisier waa
accused — along with others holding aimilar posi-
tions — of misappropriating revenue moneys, with
the result that, under the dictatorahip of the in-
famous 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'a last
plea, presented by Hall^ — for permission to finish a
research — was refused by Coffinhal, with the brutal
phrase, " La Repubtique u'a pas besoin de savants ;
il faut que la justice suive son coura." Within
twenty-four hours the execution took place.

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

The first account which we possess of Lavoiaier'
revolutionary ideas, for revolutionary they were

I

ere J

t04 THE GASES OF THE ATMOSPHERE

then deemed, was in a sealed note, placed in the
hands of the Secretary of the Academy on the Ist
of November 1772. It is to the following effect: —

" About eight days ago I discovered that sul-
phur, 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 com-
bustion 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 lith-
arge 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

^

m OVERTHROW OF THE PHLOGISTIC THEORY 105

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.
Aa 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.

"Paeis, 11(4 Noaimber 1772."

There is no account in Hales' work of his re-
ducing litharge in closed vessels. It is to be pre-
sumed 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.'

' The foUoving pusage in a Utter from Klagellan to Madsmo Laroieier
nulcw it probable that Lavoiiivr bad beeu Told of Unyon'a work : —
"Quand, apr^s lea d^uvvrtei de I.aToisier, on lui opposa Mayor, dont
il n'avoit jamaia entendu pailer, i1 char);ea sod ami Magellan, qui habitait
Loodrei, de lui procurer lea ceuvres ilu savant anglais. KlagelUn a'adreaM

It was not until Priestley, wben dining with him
in the autumn of 1774 (being in Paris with Lord
Shelburne at the time), had informed Lavoisier of his
discovery of "dephlogiaticated" air, that the ideas
of the latttT upon the subject became precise. Priest-
ley's own words are: — "Having miide the discoveiy
some time before I was in Paris, in the year 1774,
I mentioned it at the table of Mr. Lavoisier, when
moat 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-had. 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

en Tkin a tous Ua Ubrairea ile Lonitret ; il lai fut impauible lie Imiifsr
nns «itnipl(iir8 de» tEUvrea de M«yow." Yet this work was in ths
osUlogiie of ths Rojsl Society's libi'aiy ai thtc date.

which an account is given in the Memoirs of the
French Academy for 1774, and which were funda-
mental 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 capil-
lary, was then sealed, the air expelled haviug 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 cal-
cination 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 waa 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 equ:il to the loss of
weight of the air in the retort, measured by the
quantity of air which entered on breaking the

THE GASES OF THE ATMOSPHERE cha».

beak of the retort, less the Jiir 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 describea
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
fijrst time he points out that fixed air is a codu
pound of carbon with the elastic Jtuid contained
in the calx. He then describes the preparation
of oxygen by Priestley's process of heating red
oxide of mercury (mercurius precipilatus jyer
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
MSmoires of the Academy, deals with the combustion
of phosphorus ; and here he recapitulates Rutlier-

Ill OVERTHROW OF THE PHLOGISTIC THEORY 109

ford's experiments, and shows that one-fifth of the
air disappears, and that the residue, to which he
gave the name " mouffette atmosph^rique," is in-
capable 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, bad made a similar ex-
periment with a similar result. From these obser-
vations, 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 an-
other, also published in 1777. Its title is, "On
the Combustion of Candles in Atmospheric Air,
and in Air eminently reapirable." In this memoir
he distinguishes between four kinds of air : —
I. Atmospheric air, in which we live and which
we breathe. 2. Pure air, alone fit for breath-
ing, 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 un-
known. 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 liypo-
thesis : 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 hia 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 con-
verted into calces in combination with the respective
acids. Thia fact wiis 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 phlo-
giston 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 sucli a theory was that
the calx was iieavier than the metal, and that

Ill OVERTHROW OF THE PHLOGISTIC THEORV in

hydrogen, though light, still possessed weight.'
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 (ofijs, an
acid).

In 1777 he read another memoir, "On the Solu-
tion of Mercury in Vitriolic Acid, and on the
Resolution of that Acid into Aeriform Sulphurous
Acid, and into Air eminently respirable." Priestley
ha<l already shown that this process yielded sulphur
dioxide; Lavoisier carried the temperature higher,
and, decomposing the sulphate of mercury, produced

* Tliia, u prsrioml}' retnarkac!, bn-d Blnady been □oticed. In
Ibqncr*! £l4meni dt Chj/mit-pratiquc, published in 1762, a work wbicti
ran threngh man; edition*, we read (p. 307); "Thsre bippetis dnriog
b1] thsM ctlDinatioDa, and especicllj ia tliat of Ivad, ■ Terj atraiigs
pheDomanan for which it ii y»Tj difficult to aisign a reaBoii. It ia that
tbuM bodiei, wbich loae nn amall proportion of tbeir sul>stance, vbcther
by the iliacipation of phlogiaton, or bicau^a part of the mfltd is exhaled
■■ TapoOT, yield eaUa iooreased in veigbt after calcination ; and tbli
increaw ii bj no means inconsiderable. . . . PliyBiciats and cbemista
bave derinnl many iDgenious ayatems to accouut for thii phenomenon,
but no on* of them is ahsaluUly aatiifsclorj. As no well-establisbed
th*orr bos bwo daviied, we aball not undertake to attempt aa explana-
tion of thii lingular fact."

metallic mercury, sulphur dioxide, and oxygen.
It appeared therefore that sulphurous differed fi-om,
sulphuric acid in coutaiuiug a smaller proportion
of oxygen.

He also experimented with iron pyrites, and
his experiments recall those of Boyle. Boyle found,
that "marcasite," a disulpLide of iron, on exposure
to air, gained in weight, while vitriol of iron w»b
formed. Lavoiaier 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 hs
found that the air was rendered incapable of sup-
porting 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 burn-
ing 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

1

HEORY 113 I

m OVERTHROW OF THE PHLOGISTIC THEORY

fiom 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 exist-
ence of phlogiston ia purely hypothetical, and quite
unnecessary for the explanation of the phenomena.
Bat 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 combina-
tion of hydrogen and oxygen to form water was
first proved by Cavendish. And as soon as Sir
Charles Blagden, in 1781, had communicated Caven-
dish's results to Lavoisier, the latter at once saw
their bearing on the new theory which he was en-
deavouring to uphold, and perceived how they

iey I

r

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 redaction
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, Sit
Charles Blagden also being present. A consider-
able quantity of water was produced, and the"
volumes of the combining gases were found to be
I 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 wa*
collected in the gasholder.

The explanation of the solution of metals in
acids was now easy : it depended on the decompo-
sition 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

m OVERTHROW OF THE PHLOGISTIC THEORY 115

by the action of steam on red-hot iroo met with an
equally simple explanation : the oxygen and iron
united to form an oxide, — the ancient etkiops
maHial, — while the hydrogen escaped. The con-
veree took place during the reduction of a calx
to the metallic state by hydrogen. Here the
hydrogen Bei2ed 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, BerthoUet,
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 acta was to publish a defence of the
phlogistic theory. We shall see later how Caven-
dish carefully considered the rival theories, and what
reasons induced him to east his vote for the older

one. m

ii6 THE GASES OF THE ATMOSPHERE craf.

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 ozygen. The azote ox
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 aa 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
S^guin. He had nearly finished his experimental
work, but had drawn up no account of it. Hit^

request that his life might be prolonged until
he had compiled a statement of his results was
refused ; but S^guin, who was fortunately spared,
undertook the task. The facta 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, Traite elementaire de Chimie, in
which his views are stated in order, and with
great clearness. The nomeijelature 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 nomen-
clature 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 embarrassmentthat Lavoisier wrote his Treaiise.

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 neceasarilj
contain all those gases capable of existence at
the ordinary temperature ; and he explains how
water-vapour must be one of them, seeing tha^
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, stand-
ing 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 Tuercurius calcinatus per se,
which, when collected, weighed 45 grains. The
residual air in the retort and in the jar was incap-
able of supporting life or combustion ; but the red
precipitate, when heated, lost 3^ grains of ita
weight, yielding 41j grains of metallic mercury,
while it evolved 7 or 8 cubic inches of oxygen,
capable of supporting the combustion of a candle

TiTidly, and of causing cbarcoEil to burn with a
crackling noise, throwing out sparks. Oxygen waa
thus successfully separated from air, and obtained
from it in a pure condition for the first time, in a
UDgle series of operations.

In Lavoisier we see a maBter mind, not only
capable of devising and executiug beautiful ex-
periments, 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 con-
viction. 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
hie 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

i

of the constituents of air, and to note bis argu-
ments in favour of the phlogistic theory. Although
Cavendish never publicly acknowledged its in-
sufficiency, 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.

While Lavoisier was engaged in experiments
on oxygen, Cavendish, too, was devoting his
attention to the constituents of air, but in a some-
what 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 com-
bustion. His second paper deals with the inactive
constituent of air — the mephitic portion, now known
as nitrogen or a^ote. But before considering these,
a sketch of his life will prove of interest.

The Honourable Henry Cavendish was a very
6ingular man, retiring and uncommunicative to
bj^cgree ; hence little is known of his early life.

ife. A

He was the elder son of Lord Charles Cavendieh,
who waa the third sod 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 waa born at Nice in October 1731.
His mother died when he was two years old.
Nothing is known of hia 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 ia
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. Tke writer of his obituary
notice, M. Biot, epigraramatically said : — " II ^tait
le plus riche de tous les aavans, et le plus savant
de tous les riches."

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
t&lk to his neighbour at table so long as others
did not join in the conversation ; but if the con-
versation took a general turn, he was silent.

His death took place in February 1810, and
was as solitary as iiia life. It is related by his
servant that Cavendish, on feeling his end ap-
proaching, dismissed him from the room, telling
him to come back in half an hour. He disobeyed
instructions, and, being anxious, found some pre-
teit 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

>id A

THE GASES OF THE ATMOSPHERE

I of any sucli sensation. Indeed, his ezperimente

I were in many cases not published until long after

I 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 laboura
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 ita
two oxides. He held that arsenic acid was more
thoroughly " deprived of phlogiston " than arsenious
I 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. Hia
next experiments related to heat ; and had be pub-
lished them, he would doubtless have anticipated
Black in his discovery of latent heat. His paper on
"Factitious Airs," pubHshed in the Philosophical
Transactions for 1766, deals with the properties
of hydrogen, carbon dioxide, and the gases pro-
duced 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 un-
known 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 ren-
dered 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 ^ths of fixed air, burned
about 26 seconds in common air mixed with the
same proportion of this burnt air."' He named it,
as usual, "mephitic air," and it is certain that,
although Cavendish did not publish his results, his

' BrU. Akoc. Scporf, 1839. p. 64.

THE GASES OF THE ATMOSPHERE chak

discovery was not later in date than Rutherford's.
Dealing next with the phenomena observed wheo
that curioua 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 atmo-
sphere Cavendish made use of nitric oxide in
presence of water, as a means of removing oxygen.
This process, originally devised by Mayow, waa
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, pub-
lished in the Philosophical Tra7isactions for 1783,
is " Of a new Eudiometer." The term "eudio-
meter" signifiying "measurer of goodness," was

devised when it was supposed that ordinaiy air
presented conaiderable variations in its power of
Bapporting respiration and combustion, according
to the seasons, and according to the place from
which it was collected. Dr. Ingenhousz had found
a greater abaorption when aJr 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 Abb6 Fontana,
too, had made similar experiments, and had come
to similar conclusions. Cavendish modified Fon-
tana'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, some-
times at Marlborough Street, sometimes at Ken-
sington, which was then a country village. The
results of hia analyses establish as the com-
position of air, freed from carbon dioxide by
potash :

79-16 per cent of phlogiaticated air (nitrogen).

20'84 per cent of de phlogiaticated air (oxygen).
This result does not differ materially from those

THE GASES OF THE ATMOSPHERE

obtaiaed by the best modern analyees, which give,
within very small variations :

79'04 per cent of nitrogen, argon, etc.,

2096 per cent of oxygen,

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

In the following year, 1784, Cavendish pub-
lished the first of his great memoirs, entitled .Ec-
periTnents 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 suflFer by all the various ways in which it ia
phlogisticated, and to discover what becomes of
the air thus lost or condensed."

Cavendish chose processes for "phlogisticating"
ail' in the course of which no fixed air should be pro-
duced. He therefore avoided the use of animal and
vegetable materials, and confined himself to com-
bustibles, such as sulphur or phosphorus, to the
calcination of metals, the explosion of inflammable
air, and the admixture 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 pblogistication
of the air, and production of fixed air, in this

PHLOGISTI GATED AIR

1

process is owing to the burning of some inflam-
mable matter in the apparatus." We shall see
later what magnificent results arose from this last
mode of "phlogistieating" air.

He begins with an account of a repetition of an
experiment of Mr. Warltire's, related by Priestley,
in which a mixture of hydrogen and air was ex-
ploded in a copper vessel, with the result that they
observed a loss of a few grains in weight ; it is also
stated by Warltire 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 Warltire's, could remark no change
of weight; and be concluded that 423 measures of
hydrogen, or " inflammable air " as be named it, are
"nearly sufficient to completely phlogisticate 1000
of common air, and that the bulk of the air remain-
ing after the explosion is then very little more than
^ths 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 in-

flammable air, and about ^tb part of the common
air, lose their elasticity, and are condensed into thi
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 2^ times that quantity of
common air, and the burnt air made to pass through
a glass cylinder 8 feet long and about f 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 conse-
quently that almost all the inflammable and about
Jth of the common air arei turned into pure
water."

But on firing little by little a mixture of " de-
phlogisticated air" or oxygen, obtained from red
precipitate (that is, mercuric oxide prepared by
heating the nitrate), with twice its volume of " in-:
flammable air" or hydrogen, the resulting water
was acid to the taste, and on evaporation with

PHLOGISTI GATED AIR

alkali gave a Kmall quantity — about 2 grains — of
nitre. Cavendish Buspected that the acid came
from the nitrate of mercury in his red precipitate,
and, to test this, procured his oxygen from other
aources — from red-lead and sulphuric acid, and from
the leaves of plants— but still with the same result :
nitric acid waa 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 dephlo-
gisticated air is exploded in such proportion that
the burnt air is not much phlogisticated, the con-
densed liquor contains a little acid, which is always
of the nitrous kind, whatever substance the de-
phlogisticated 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 dephlogisticatt'd air is converted into pure
water." The quantity of uncombiued 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
aud iuflammable air, and consequently that if those
airs could be obtained perfectly piire, the whole
would be condensed."

The next paragraph ia interesting, "During
the last summer also [of 1781] a friend of mine
[Sir Charles Blagden; see p. 112] 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 aira
could be converted into water."

And next cornea an important deduction.
" Phlogiaticated 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

n "^PHLOGISTICATED AIR

1

matter by the expression, "Nitrogen ia 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 phlo-
gistication 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, forma the
volatile sulphuroua acid and vitriolic acid air, both i
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
1 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 dephlogiatieated air was still un-
explained. To settle this point Cavendish added
to an explosive mixture of oxygen and hydroguu a

tenth of ita volume of nitrogen, and found that the
water was much more strongly acid ; and if hydro-
gen 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 dephlogistieated air"
is only water deprived of its phlogiston, and that
inflammable air, as was before said, is either phlo-
gisticated water or else pure phlogiston, but in all
probability the former." In a footnote he givea
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 dephlogistieated 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

Btatement : " Instead of saying air is phlogisticated
or dephlogiaticated by any means, it would be more
strictly just to say, it is deprived of, or receives, an
addition of dephlogiaticated air ; but as the other
expression is convenient, and can scarcely be con-
sidered 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 ia 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
dephlogiaticated air; that nitrous air, vitriolic acid
air (aolphur dioxide), and the phosphoric acid are
also combinations of phlogisticated air, sulphur, and
phoaphorus 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 sub-
stance, — commonly, however, with a mixture of

GASES 6?" THE A'fMbgPHERE ch^^

fixed air ; that on exposing the calces of the
perfect metala to sufficient heat, all the dephlo-
gisticated air is driven off, and the calces are
restored to their metallic form ; but aa 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 cal-
cination, 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 differ-
ence, 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 substanceB

J » PH LOGISTIC ATED AIR

1

*i Consist chiefly of three different bases, one of

J ffiiieh [hydrogen], when united to dephlogisticated

■ air, forms water ; another [carbon] fixed air ; and

1 the third phlogisticated air [nitrogen] ; and that,

[ hy 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 reatoretl to their

pristine form.

, " It seema, 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 de-

, phlogisticated air to a body comes to the same

tiling 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 dilBcuIt 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,

J have adhered to that."

" Another thing which Mr. Lavoisier endeavours
to prove 13 that dephlogisticated air is the acidify-
ing 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 nitrouSj vitriolic, phosphoricj
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 loosing 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 : ' — " Lastly, the same
effect [i.e. the diminution of the volume of
common air], I find, is produced by the etectrio

PHLOGISTICATED AIR

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
L.^tt taking the electric spark in given quantities
ptfseveral 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 procea3 ; from whence I con-
cluded that the electric matter either ia 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 phlogisti-
cation 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
ia very different from what I suspected, and
depends upon the conversion of phlogistieated 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 con-

THE GASES OF THE ATMOSPHERE

i

nectiug the mercury in oae of the glasses with »
ball placed near the prime conductor of an
electric machine, and the other with 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-thirda of ita original volume; nor did the lime-
water give a precipitate on introducing some fixed
air, thus showing that it had been saturated by an
£icid. 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 triala
proved that " when five parts of pure dephlogisti-
cated 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 con-:
tained no suphuric 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

1

ji » PHLOGISTICATED AIR 141 ^^^

^f jKopitate which this solution gave with silver
m nitrate with that ohtained from his "soap-lees," he
■ fonnd them identical. There was therefore no
1 "muriatic acid " present, which would have yielded
1 chloride of silver, of appearance somewhat similar
1 to the nitrite.

1 As it had previously been shown to be probable
that phlogisticated air is nitrous air united with
phlogistoL, and that nitrous air is nitric acid united
with phlogiston, " we may safely conclude that in
the present experiments the phlogisti' ated air was
enabled, by means of the electric sparii, to unite to,
or form a chemical combination with, the dephlo-
giaticated air, and was thereby reduced to nitrous
acid, which united to the aoap-lees and formed a
solution of nitre ; for in theee experiments the two
airs actually disappeared, and nitrous acid was
actually formed in their room." " A further con-
firmation of the above-mentioned opinion is that,
as far as I can perceive, no diminution of air la
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
waa the same that the soap-leea would have pro-
duced 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 thi'se last quotations. No
loophole is left unstopped ; every precaution is
taken to make the proof aa 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 ascer-
tain it experimeutally, 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 phlogisti-
cated part of our atmosphere than that it is not
diminished by lime-water, caustic alkalies,
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, tliongh it waa reason-

able to suppose that part at least of the phlogisti-
cated 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 diflFerent substauces con-
founded together by us under the name of phlo-
gistieated air. I therefore made an experiment to
determine whether the whole of a given portion of
the phlogiaticated 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 fore-
going 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. 1 then, in order to
decompound as much as I could of the phlogisti-
cated air which remained in the tube, added some
dephlogisticated air to it, and continued the spark
imtil no further diminution took place. Having

by these means condensed as much aa I could
the phlogisticated air, I let up some solution of liver'
of sulphur to absorb the dephlogisticated air ; aft«
which only a small bubble of air remained unab-
sorbed, which certainly was not more than x^th of
the bulk of the phlogisticated air let up into the
tube ; 90 that, if there is any part of the phlogisti-
cated 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 y^'^h part
of the whole." We shall afterwards see that this
is a marvellously close estimate. There is actually
gif th 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. Itwas still conceivable that the
potash might contain some " inflammable matter "
which would diminish the air on sparking, and there-
fore 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 sub-
stituted for potash with the same result ; but if
litmus was added to the water the colour was die-

IV PHLOGISTICATED AIR !«

charged, and lime-water introduced into the tube
gave a cloud, showing tbat " the litmua, if not
burnt, was at least decompounded, so as to lose
entirely its purple colour and to yield fixed air ; 80
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 com-
bustible substances."

Such are the chemical researches of Cavendiah.
Of all experimenters on the subject he was un-
doubtedly the greatest, though Mayow and Scheele
were near rivals. But his researches were so com-
plete that it is scarcely posaible 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 dis-
torted facts. Lavoisier tried to repeat Cavendish's
experiments, but without success ; but an account

146 THE GASES OF THE ATMOSPHERE

is to be found in the last part of his Experimenb f<
on Air, published in 1788, of the successful » i^
petition by a Committee of the Royal Society 4 \
the conversion of nitrogen into nitric acid by tlit
electric spark in presence of oxygen and potash.

His remaining papers deal with meteorolo]
and astronomical subjects. One, published
1790, refers to the beight of a remarkable ai
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 re-
marked in passing that the method consisted
measuring, by means of a torsion balance, 1
attraction of one leaden ball for another, and tl
recent experiments, made with the utmost refiai
ment, have barely altered the number which
obtained, 5-4, to 5-527.

His lost paper, on an improvement in a machil
for dividing astronomical instruments, was publislu
in 1809, the year before his death.

Nothing has been said as yet regarding therii
claims of Watt to the discovery of the compositi

COMPOSITION OF WATER

( water, and little need be said. The discovery
ras made by both in 1784, yet Cavendish visited
Tatt at Birmingham, in 1785, and was apparently
ta the best of terms with him ; and Watt, as proved
ij Cavendish's diary, showed him many of his
leviceB connected with the steam-engine. There
itn be no doubt that Watt had also discovered
bat when hydrogen and oxygen are exploded
logether water is the sole product, but he coupled
Ihe phenomenon with views involving the material
toture of heat, or caloric, as it was then called,
which Cavendish repudiated.

Cavendish's later work was carried out in a villa
St Clapham, which was fitted as a laboratory, work-
top, and observatory, but he had a town-house near
the British Museum, at the comer of Gower Street
Ind Montague Place. He had also a library in
pean Street, Soho, which was available for any
eientifie man who chose to present himself. So
lingnlar were Cavendish's habits that when he
iriflhed a book he went to this house and borrowed
b as from a public library, giving a receipt for it.

Of all men. Cavendish was probably the most
fingalar, but there can be no question of his extra-
genius.

With the advent of Lavoisier's system of repre-
senting the phenomena of combustion, and the
expression in hia terms of the various changes
resulting in air when metak 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 bfr
tween its oxygen and so-called nitrogen than Caven-
dish had made by the nitric-oxide method wew,
carried out in 1804 by Gay-Lusaac and HumboIdt»
by explosion with measured quantities of hydrogen,
according to the method suggested by Volta ; and
they concluded, from a large number of analyi

made on specimens collected in all weathers and
firom 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 com-
bine 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.
B The almost constant relation between the
^BBfapnes of oxygen and nitrogen in air made it
^H^nr 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 Thom-
son, Dalton's intimate friend. But between the
numl)ers21 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 eonatituenta. And in 1 846 Bunsen showed
that the proportion between oxygen and nitrogen is
not a constant one. but that the oxygen varies be-
tween 2097 and 2084 ; the experimental error did
not exceed 03 volume, while the difference found
amounted to ■ 1 3 volume. Regnault, Angus Smith,
A. R. Leeds, and von Jolly confirmed these re-
sults 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 ia,
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 i
rain, percolating through the aoil, leaves the am-
monia behind, in some form of combination; and

V THE DISCOVERY OF ARGON

it is then attacked by the nitrifying ferments and
converted into nitrates, from which the plants
derive the nitrogen which forms part of their sub-
stance, 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 Man-
chester streets — nitrous and nitric acids are present
in still smaller amounts ; and in spite of the wide-
spread 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 char-
acteristic 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 It^t-
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 dis-

covered by Schonbein. Tbe atmosphere farther
contains dust, some of which appears to consiat
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 compoaition 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. But it has been found
that air contains corpuscles of another nature, the
consideration of which wiU come later.

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

In his presidental address to Section A of the
British Association at Southampton in 1882, Lord
Rayleigh alluded to an investigation which he had

THE DISCOVERY OF ARGON

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 gtises, detenninable from their densities and
from the proportions by volume in which they com-
bine, 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
detennination 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 esperiraents on densities
were published,' 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, at normal temperature
and pressure to be —

Oxygen

" Nilrogpn "

1-42952 grama

1-25718 „
1-29327 „

A simple calculation leads to the com]
of purified air. The percentage of osygen must be
20-941, and that of "nitrogen" 79-059, in order to
give a mixture of which the weight of a litre ifl
1 '29327. Now this corresponds with the results of
thebcst analyses, quoted on p. 128. 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 difi'erent ways: by
the electrolysis of water, by heating chlorates, and
by heating potassium permanganate. The reeulta
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 „

Heatingpermanganate, 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
aeveral 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 eYident from inspection
of the numbers quoted above that the accuracy
of the density determination may be trusted to
within I 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 Kayleigh
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, in molecules consisting of one atom ;
and as it is held that equal numbers of molecules

THE GASES OF THE ATMOSPHERE

CRAP. ■

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 iccreased 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 intro-
ducing hydrogen purposely, and removing it by
passing the gas over red-hot copper oxide, which

oxidisea 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 mole-
cules of greater complexity than two-atom mole-
cules, say Ng -molecules. Now it is known that
when oxygen is electrified by the passage of a rain
of small sparks through it, it acquires new pro-
perties : it possesses an odour, it attacks metallic
mercury and silver, and its density is increased.
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 ; their presence accounts for the increased
density of oxygen thus treated.

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

THE GASES OF THE ATMOSPHERE

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 Profesasr 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
possibUity 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 6nd 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

V THE DISCOVERY OF ARGON 159

also combined with the metallic magnesium. As
absorption proceeded, more nitrogen was admitted
from a reaervotr, 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.

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

THE DISCOVERY OF ARGON

„on a larger scale, the atmospheric nitrogen being
passed backwards and forwards from one large glass
gasholder A to another B, through a tube G filled with
magnesium heated to reduesa, to absorb nitrogen ;
over red-hot copper oxide (a) (6), so that any car-
bonaceous 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 gas-
holders (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 pre-
caution not been taken.

The process was continued for ten days, by
■which time most of the nitrogen had been
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

THE GASES OF THE ATMOSPHERE

i

same materials as before. In a few days more the
volume was reduced to one-seventh of what it
been when the transference to the smaller apparai
was made, or about one-eightieth of the origi
volume of the atmospheric nitrogen taken.

The gas was then weighed, this time in a largi
bulb, the weight being 0'2190 gram; and such is
the posaibihty 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 mag-
nesium. 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-atxim molecules, so the action of the mag-
nesium on the nitrogen might be to withdraw oi
atom of nitrogen from the two-atom molecule, lea'
ing a single uncombiued atom, which might m
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 fi^sh magnesium fore

■6

3

V THE DISCOVERY OF ARGON

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 mag-
nesium was now not much attacked ; and on estimat-
ing the total amount of nitrogen absorbed, by treat-
ing 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
farther purified gas was again determined, when it
was found that a litre now weighed 1'7054 gram,
corresponding to a density of 19'08G.

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
I 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 examin-

ing the spectra of gases at low pressures was
next filled with the gaa of density 19. Such a
tube, called a Plucker's tube, after its inventor,
containa wires of platinum sealed through at eacli
end, where it is about half an inch in width ; the
middle portion of the tube is about 3 inches
long, and ita bore is a fine capillary. When the
platinum wires are connected with the seuoniJar)'

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 seta 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 largft'
number of somewhat hazy bands, red, orange^,
yellow, and yellow - green in colour, besidee %
number of bands of a violet colour ; but the new
gas, while exhibiting the bands characteristic ol
nitrogen, shtiwed in addition certain groups of red

THE DISCOVERY OF ARGON

.65

and green lines which did not appeal- t-o belong to
the Bpectrum of any known gas.

While these experiments were in progress, Lord
Rayleigh was occupied in preparing nitrogen &om
other sources, and in determining its density ; and
in every case it was evident that nitrogen from
all sources except the atmosphere weighed some-
what less than atmospheric nitrogen. He therefore
proceeded to repeat Cavendish's experiment, and
like Cavendish, he obtained a small residue of gaa
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"0G cubic centimetre of gas was
obtained.

small amounts are not proportional to

THE GASES OF THE ATMOSPHERE

the quantities of air taken ; but, as will afterwards
be seen, the discrepancy is owing to the solubilitj
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
&om 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 1894,
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 discoveiy. The state-
ment 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

t

procesaes designed for its separation, and attention
was first turned to devising s complete proof of
its actaal 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 aa 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 1 6 and I 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

iam Slowly i

irchwarden J

THE GASES OF THE ATMOSPHERE c

tobacco-pipeB, 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 encas-
ing 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 aJr 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'fi 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 con-
stituents of a mixture of gases will dissolve in a
solvent is conditioned first by their relative solu-
bilities, 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 45 = -i'S volumefl

Nitrogen . . 4 x 15 = 60 „

So that the proportion of oxygen to nitrogen in
Buch 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.
^^^!he gases can be almost entirely extracted by

F

170 THE GASES OF THE ATMOSPHERE chap.

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 ia a powerful flame, so that
the water in its passage becomes heated to the
boQing-poiot, and gives off its gas before it escapes.
Of course the gas collected contained oxygen, but
this was easily removed by tbe 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.

In order that tbe properties of the newly-dis-
covered gas, argon, might be thoroughly investi-
gated, 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 re-
moving the oxygen and nitrogen which have been

already described. Supposing the new gas to have
the density 20 compared with oxygen aa 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,
aB 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, con-
fined in a large glass balloon of about 6 litres
capacity, over a weak solution of caustic soda.
For this purpose a very powerful rapidly alter-
nating current is necessary. The latest, and ap-
parently 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 foantain
in the balloon, and flows in a uniform stream down
its sides, covering its inner HUrface 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 playa a double part. It 1

prevents the undue heating of the gas balloon, |

which otherwise must be sunk in running water in ||

order to keep it cool ; and it exposes a very large '1

THE DISCOVERY OF ARGON

and constantly renewed surface of soda to tbe
nitrous fumes which are produced by the combi-
uatioD 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 per-
oxide, NOy This is absorbed by the soda, giving a
mixture of nitrite and nitrate of sodium, NaNO,
and NaNOj. 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
obt^ned 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,
aud the atmospheric nitrogen collected in a gas-
holder. Long tubes of eorobuation-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
aa are used in organic analyses ; and through these

I

the " atmospheric nitrogen," dried by passage
over Boda-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 com-
bustion ia in progress. The gas issuing from the
tube ia collected in a small gasholder. When one
tube of magnesium is exhausted, another is substi-
tuted 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, found 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 tempera-
ture. The process involves the preparation of pure
lime by heating artificial carbonate, A mixture of
equal amounts of magnesium powder and this pure
lime may be readily heated in a glass tube, without
danger of fusing the glass. All the nitrogen may

be removed by a single passage through a tube thus
charged and heated ; but hydrogen and carbonic
oxide are introduced in small amount, and must
subsequently be got rid of by passage over copper
oxide and soda lime. Porcelain tubes are attacked
by the magnesium, and crack on cooling ; and iron
tubea are difficult to clean out.

This preliminary operation, if magnesium alone be
used, 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) througli
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 efiectually dries the gas,
previous to its passage over the red-hot magnesium,

THE GASES OF THE ATMOSPHERE c\M.

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. If, however, the lime-magnesium mixture
be employed, argon free from nitrogen may be at
once obtained.

It is difficult to chooBe between these two
methods : both are troublesome, and require a con-
siderable 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 one rare mineral, malacone. No animal or
vegetable substance appears to contain it. Ex-
periments were made in the summer of 1895 by
Mr. George MacDoiiald and Mr. Alexander Kellaa,
in order to decide whether argon was a constituent
of any living matter. Some peaa 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 statfl

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 moat
part by meana of magnesium, and the small un-
sbsorbed 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 of 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 interest-
ing 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. It has
been found, however, that the gas in the air-bladders
of fish is richer in argon than atmospheric air.
But these experiments lead to a further

178

THE GASES OF THE ATMOSPHERE

1 directed

L iu Profes

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 atmo-
sphere 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 per-
centage of argon in atmospheric air next invited
inquiry. It is by no means very easy to absorb
quantitatively the whole of the nitrogen from aa
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
iu Professor Kamsay's laboratory, and independently

by M. H. Sehloesing ui 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
crude argon in atmospheric nitrogen (for it will be
seen later that other gases allied to argon are
also present) and its density (19"94), to calculate
whether Lord Rayleigh'a determinations of the
density of atmospheric nitrogen were correct. The
weight of one litre of pure nitrogen is r25092
gram, and of argon, 1"7815 gram ; hence a litre of
a mixture of 98"814 volumes of pure nitrogen with
1*186 volume of argon must possess the weight
1"25711 gram. The actual number found by Lord
Rayleigh was 1 '257 18 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, helium, and neon—gases of which more
hereafter. The waters actually examined were the
Bath waters, which contain much nitrogen, a little
argon, and traces of helium and neon ; the Buxton
waters, containing uitrogen and a little argon ;

the water from "Allhusen'a Well," Middleaborough,
which evolved gaa of an inflammable nature con-
sisting mainly of nitrogen, but alao containing
marsh-gas, and argon to the extent of 0'4 per cent;
water from boiling springs in Iceland evolved gaa
containing somewhat more argon than air does,
viz. 1'14 per cent; and lastly, water from the
Harrogate sulphur springs yielded a gas largely
eonsistiag of a mixture of sulphuretted hydrogen,
carbon dioxide and nitrogen, but giving also
an appreciable amount of argon. Such deter-
minations show that argon is not merely confined
to the atmosphere above the earth, but that it pene-
trates the earth and is contained in subterraneous
water. These results have been obtained by Lord
Rayleigh, Professor Ramsay, Mr. Travers, and Mr.
Eellas.'

Similar experiments have been made by Dr.
Bouchard in Paris* on efl'ervescing 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 nitro-
gen and argon ; while a third gave nitrogen and

' /Vdc Boh. Soc Tol. lii. p. 88. i*iV, Trana. vol. olinTi p. 227.
* Compt. rend. toI. cizi. p, Sfll.

THE DISCOVERY OF ARGON

i8i

helium. Such are, up to the present, the sources
of argon.

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

The density of a gas is one of its moat character-
istic and important properties. Avogadro's law,
which postalates that equal volumes of gaaea, at
equal temperature and pressure, contain equal
numbers of molecules, renders it possible to com-
pare 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, uneombined
with other elements — is ^ths of the weight of a
single molecule of oxygen. Hence a determination
of the density of argon leads directly to a know-
ledge 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 neces-
sary 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 approximately —

Hydrogen
Oxygen

11 "11 per cent

If the smallest portion of water capable of free
existence contains one aforn 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 conclu-
sion that this supposition was a justifiable one; but
there are strong reasons against it We have already

l84 THE CASES OF THE ATMOSPHERE C

seen that Cavendish discovered approximately, and
that Gay-Lussac and Humboldt detennined accur-
ately, 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 improb-
able, when we take into conaideration (I) Boyle's dis-
covery that if the pressure on a gas be increased, the
volume of the gas, whatever it may be, diminishea
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 unreason-
able to suppose that the composition of water was
expressed by one particle of oxygen in union with
two particles of hydrogen. (The word " particle " ifl
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

VI THE PROPERTIES OF ARGON 185

of the volumeB 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 over-
come 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 otherj 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 on»
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 mole-
cules 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

n THE PROPERTIES OF ARGON

themaelves, so that groups of atoms, or moIeculeB,
each containing two atoms of hydrogen and one of
oxygen, are fonned. To such an arrangement the
formula K,0 is applied, while ordinary hydrogen
molecules may be represented as H,, and molecules
of oxygen as 0,.

It has been shown already (p. 153) how Lord
Rayleigh obtained the number 15'882 for the
density of oxygen compared with that of hydro-
gen. To determine the atomic weights of elements,
the usual process has been to analyse their oxides,
for only a few elements form compounds with hydro-
gen. Thus the analysis of copper oxide yields the
nnmbers —

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 reason*

THE GASES OF THE.

akHe to nippoee thateadi
of oxygen coDtAina two fttoma, Ac mmlrTr 15*883
reprc«euta the weight ofanatomotfmjgtMCOMpMad
with that of an atom of faydrogea takoi as 1. B«t
this niunber baa oat been regarded as snffiaoilijr
establiabed bj experiment. Oiha obserrefs (tor
the importance of this rado has been
tedged since the b^inning of the cennuy) have
obtained resolte 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, woold be
I '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
clement baa 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 repre-
sented by whole numbers instead of by fractions ;
thus, sulphur has the atomic weight 32, if oxygen
be made 16, whereas, if it were 1588^, 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 1994; 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

THE GASES OF THE ATMOSPHERE chai-.

this important quantity two methods may be chosen.
One is to make compounds of the element, and
thia 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 ex-
ample, reverting to the former instance of water, the
relative proportions by weight of oxygen and hydro-
gen are, in round numbers, 16 to 2. Reasons have
already been given showing why its formula should
be HgO and not HO ; its molecule must contain
two atoms of hydrogen. But another compound of
oxygen and hydrogen is known in which the propor-
tions are 16 parts by weight of oxygen to 1 part bj
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^Oj. No
other compound of oxygen and hydrogen is known ;
and it may be stated briefly that no compound of

THE PROPERTIES OF ARGON

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 aa the atomic weight of oxygen.

If now it were possible to prepare compoundB
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 con-
sistent 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 ex-
posing 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 aloDg with the vapours of phosphorus, sulphur,

r^^^

^^" 191 THE GASES OF THE ATMOSPHERE chai'.

tellurinm, 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 iufluence
of oxygen and an alkali in the shape of fused
1 potassium nitrate or red-hot peroxide of sodium
1 was also without effect. Gold would, however,
^ have resisted such action, but 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 eiperimente 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 thia 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. Ita
volume certainly decreases, but whether this de-
crease is to be attributed to true combination or
not LB 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

THE GASES OF THE ATMOSPHERE

Professor RamBay has also made experimente
OD the action of a silent electric discharge upon
a mixture of argon with the vapour of carbon
tetrachloride ; the latter deeomposea, giving, not
a resin, but crystals of he xachloro benzene and free
chlorine ; but the volume of the argon was un-
changed. It was all recovered without loss. Next,
the rare elements titanium and uranium have been
heated to redness in a current of argon with no
alteration or absorption of the gas. And more
recently, attempts have been made to cause argon
to combine with the very electro-positive elements,
rubidium and caesium, by volatilising them in an
atmosphere of argon. Numerous experiments, in
which electric sparks have been passed through
argon cooled with liquid air between poles of
every attainable element, have also been made, but
without result. It -was hoped that possibly at
a very low temperature, - 185° C, a compound of
argon might be caught before it had time to
decompose, and retained in the solid state as an
incrustation on the walls of the tube. Just aa
nitrogen and oxygen can be made to combine in
quantity, when electric sparks are passed through
the mixture, provided the product, nitrogen per-

oxide, is withdrawn by caustic alkali as it is made,
as in Cavendish and Lord Rayleigh's experi-
ments, so it may be withdrawn by freezing, if the
vessel be immersed in liquid air. But with argon,
all results were negative. In short, all likely agents
have been tried as absorbents for argon, but in
no ease 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

196 THE GASES OF THE ATMOSPHERE

pb3'sical 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 conaiets 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 aa the mole-
cular 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^O, not HgO.
But on surveying all known compounds of mer-
cury, 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 com-
pounds 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 mercun

N

198 THE CASES OF THE ATMOSPHERE cn»R

DO compound contains a smaller proportioD ; 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 aud 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 com-
municated 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

'I THE PROPERTIES OF ARGON 199

only a fraction of the value of 0"032 of a unit of
Aeat 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
thifl number, 0032, multiplied by the atomic
weight of mercury, should yield the product 64 ;
and it is seen at once that that number must be
200, for 200x0032 = 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 1
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 aud its atom
must be identical ; that unlike oxygen and hydro-
gen, 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, imcombined
with each other or with any other molecules.

The specific heat of a gas is meastired in much
the same manner as that of a solid. A known
volume of the gas is caused to pass throngh s
spiral tube, heated to a certain definite high tem-
perature ; it then enters a vessel containing a
known weight of water, traverses a spiral tube
immersed in the water, and parts with its beat to
the water. Knowing, therefore, the weight of
the gas and its initial temperature, and also the
rifle 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 diS'erence, in order to un-
derstand 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.

THE PROPERTIES OF ARGON

If a gas is allowed to expand into a Tertical
cylinder bo 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 ia 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 easing through which heat could not
pass — we know of no such casing, but we can con-
trive 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 l'
Centigrade — the gas would perform 42,380 gram-
centimetres of work ; it would raise a weight of
nearly 4^ kilograms, or about 9^ 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 ou the average to 1033

rsoa
grams
surface
inch.

THE GASES OF THE ATMOSPHERE

grams on each square centimetre of the earth'ft.
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 sm-face, 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 raJBed
from 0° to 1°, it will expand, as Gay-Lussac
showed, by ^^^rd of its volume. Now the product
of 773'3 and ^^r ^ ^'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 woric
of raising 1033 grams through 283 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 gaa.
Bat 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 beiug kept constant, will raise a

n THE PROPERTIES OF ARGON 303

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 prosBure . 0'2375

volume . 0'1683

This ratio is termed the ratio betwiieu the specific
heats of air, and such a ratio is represented usually
by the letter 7.

But it is not necessary to determine both kinds
of specific heat in order to arrive at a ItDowledge
of the value of this ratio. One plan, adopted by
Gay-Lussac and D^sormes at the suggestion of
Laplace,' is to actually measure the fall of tem-
perature by allowing a kuown 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

' Miehanique cilaU, rol. t. p. 12!<.

true that heat will rapidly flow in through the
walls of the vessel ; but by choosing a sufficiently
large vessel, and surrounding its walla with badly-
conducting material, the entry of heat will he
BO 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 experi-
ments 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 gaa 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

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 = !i

Pi "'

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, 7, thus ;

Pi ^e/ '

THE CASES OF THE ATMOSPHERE

■.d:dj. e

'J^r

The iiitio of the two specific heats can therefore be
determiiied 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
Buecessfully 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 deter-
mining the relative length of the sound-waves in
air and in that gas.

The simple apparatus with which such deter*
minations 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

VI THE PROPERTIES OF ARGON 307

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 clubmoss — is best)
is placed in the tube, and distributed uniformly
throughout it, so that when the latter is in a hori-
zontal 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. Tlie 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. Aa the tube is closed at the end
through which the gas was admitted, these waves

k

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 com-
pressions produced by the forward waves travelling
onwards from the rod. The gaa, could we see it,
would represent portions compressed and portiona
rarefied at regular intervals along the tube. Where
the gas is compressed, it gathers the lycopodium
duBt 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 gaa 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 7 was r67;
that is, in the equation

the value of r67 had to be ascribed to 7, in order to

THE PROPERTIES OF ARGON

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 —

V^o *"->"■

-V'J'^

where n is the number of vibrations per second,
X the wave-length of sound, and a the coefficient
of the expansion of a gas for a rise of 1° in tem-
perature, t, viz. 000367. Now if the expression p
(1 +at) can be shown to be identical for argon and
for air, the value of y for argon cau be calculated
by the very simple proportion —

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

\L^

no THE CASES OF THE ATMOSPHERE cha».

of Boyle's and Gaj-Lnssac's laws for argon ; and

this research was succesBfully carried out by Dn
Randall of the Johns Hopkins University of
Baltimore, U.S.A., and Dr. Kuenen, of Leydcn,
working in Professor Ramsay's laboratory.' They
made use of a constant volume thennometer, 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 in air, heated from 0° to 100°, raises
pressure in the proportion of 1 to 1-3663

Argon 1-3668

Helimn 1-3665

It may therefore be taken for certain that, within
the limits of experimental error, the value of thc'
expression p (H-a() is identical for all three;
gases.

We see, then, that for argon, as for mercury gas,
the value of 7, 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.

> Pne. Sey. Sec toL Uk. p. 83.

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

On the usually accepted theory of the constitu-
tion of matter, it is held that for simplicity's sake
atoms may be regarded aa 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 con-
sidering 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 arrangements depending on their relative size

i_

312 THE GASES OF THE ATMOSPHERE chat.

and attraction for each other. It most be clearly
understood, however, that such mental pictures are
not to be taken as actually representing the true
coDstitution of matter, but merely as attempts to
picture auch forms as will allow of our drawing
conclusions regarding their behaviour &om known
configurations of large masses.

The molecules of gases are imagined to be in a
atate 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 can-
not be described by any of these expressions, but
such other directions may be conceived as par-
taking 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 trans-
ported 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 thia
motion. The other three consist in rotations iu.
three planes at right angles to each other.

Now, it can be shown that the product of pres-

aura and volume of a gaa, pv, is equal to frds of
the energy of translation of all molecules of the
gas, or

where N stands for the number uf 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 0° C and 76 cms. pressure (normal temperature
and pressure), the pressure {p), measured in grams
per square centimetre, is 1033, and the volume (u) is
773 '3 cubic centimetres; and the raising of the tem-
perature through 1°, iis was shown before, requires
2927 gram-centimetres of work. Further, since the
product of pressure into volume is equal to frds of
the energy due to motion, or the translational
energy of the gaa,

NR = Jpp = ^ X 2927 = 4391 gram-centime trea.

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 specihc

at constant volume ; but it has t
found equal to 0'1683, as already shown.

We have seen that to convert specific heai
constant volume into specific heat at constant pres-
sure 0'0692 must be added. Hence at constant pres-
sure 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 1|.
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 tempera-
ture. What this work may possibly be we shall
consider later.

But Kundt and Warburg found, from their ei-
periments on the ratio between the specific heats of
mercury gas, this ideal ratio, 1 to 1^ ; and Pro-
fessor 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 also for three other gasea
of the same group, and it must therefore be con-

eluded 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
dumb-bell -like structures ; and here we enter on a
theoretical conception put forward by Professor
Boltzmann, but which has not been accepted
universally by physicists.

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, free-
dom 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 dia-
tomic 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 si
perfectly smooth. But if the molecules are tria-
toniic — as, for example, COj or N^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

Fia. e.

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 gaaea it ia —

Oxygen 1402

Nitrogen 1'411

Hydrogen 1-412

Carbon monoxide . . . . 1-418

In all cases the numbers are too large, and this ia
a serious dilEculty, because any tendency to rotate
round the central line would cause the values to
be less, not greater than 1*4, For triatomic mole-
cules the calculated value of 7 is 1^, but in actual

fiict the ratio in the case of triatomic molecules,
such as HgO, CO2, N,0, etc., ia always less than 1^.
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, neon,
krypton, xenon, and helium, and it appears that
the atoms of these six 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 its congeners must also be mona-
tomic elements.

CHAPTER VII

THB 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 If, it follows that energy imparted to it
is employed solely in communicating tranalational
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 com-
pounds, from the density of its compounds in the
gaseous state, and from its own vapour-density,
as well as &om 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, othei-wise 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

220

THE GASES OF THE ATMOSPHERE

CHAP.

not lightly to be met. These difficulties arise from
a consideration of the position of argon when it is
classified with other elements.

After a preliminary attempt by de Chancourtois,
which met with no attention, Mr. John Newlands
pointed out in 1863, in a letter to the ChenUoal
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 further in 1869
by Professor Mendel^eff of St. Petersburg and by
the late Professor Lothar Meyer, and the table
may be made to assume the subjoined form (the

ThB ElBMENTS ARRA5C

Lithium

7-0

Sodium . .23*0
Potassiam . 89*1

Rubidium . 85*4
Caesium 133*0
? . . . . 170*0

221*0

Beryllium . 9*1

Magneaium 24 *4

Calcium 40*0

strontium . 87*6

Barium . . 137*4

« . . . . 172*0

Radium . 225

Boron .

11*0

Aluminium 27 *1
Scandium . 44*1

Yttrium . 89*0
Lanthanum 188*0
Ytterbium 173*0

? .

. . 230*0

Carbon . .
Silicon . .
Titanium

Zirconium
Cerium .
? . . .
Thorium

I

U

\]

•1

-^

atomic weights are given with only approximate
accuracy) : —

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 with-
out 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

?BBiODio Ststeu.

220

THE GASES OF THE ATMOSPHERE

CHAP.

not lightly to be met. These difficulties arise bom
a consideration of the position of argon when it is
classified with other elements.

After a preliminary attempt by de Chancourtois,
which met with no attention, Mr. John Newlands
pointed out in 1863, 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 further in 1869
by Professor Mendel^eff of St. Petersburg and by
the late Professor Lothar Meyer, and the table
may be made to assume the subjoined form (the

Thb Elements abbaw

Lithium

. 7-0

Beryllium

. 9-1

BoroD

11
27 1

Carbon . .

•

~^^ '-4 - junminium

Silicon . .

Copper .

. 63-4

Zioo. .

. 66-8

Gallium

. 69-9

0«nnaiua

Silver .

. 107-9

Cadmium

. 112-1

Indium

. 118-7

Tin . .

T. . .

. 156

?. . .

. 158-0

? . . .

. 159-0

Terbium

Gold .

. 197-2

Mercury

. 200-2

Thallium

. 204-2

LmuI .

atomic weights are given with only approxiniate
accuracy) : —

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 with-
out 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

pEBiODio Stbtem.

Hjdrogen

1-01

Helium

40

««

14-0

0iyg.li

19-0

FluoTina

19-0

Meon .

- 20-0

ph^

31-0

Salphnr

32 ■!

ChJorine

35-5

Argon .

. 39 9

Oc

76-0

Ssleniam

79-1

BromiD^

800

Krypton

. 81-8

wmj

lSO-0

TflUnrium

127 '6

Iodine .

128-9

.Xenon .

. 12B-0

»

186 ^)

t . . .

lfl7-0

! . . .

169-0

T

la

208 -6

! . . .

214-0

! . . -

219-0

!

80 on with the other columns. All elements in
vertical columns exhibit chemical similarity, aud,
indeecl, are often strikingly like in properties.

The subdivision, produced by folding the loose
slip, is intended to show that the elements repre-
sented 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 i-eplacea
one atom of hydrogen in some of its compoxmda,
and is thus a monad, forma 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 bi-
valent ; thallium, univalent and tervalent ; tin and
lead, bivalent and quadrivalent, and so on. It is aa
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 ia approximately 16, or some
multiple of 16, Thus we have hthium, sodium,

ARGON AND OTHER ELEMENTS

223

and potassium ; beryllium, magnesium, and cal-
cium ; 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 broak : 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 differ-
ence of about 16x3; 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 x 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

j numbers. But one thing has been remarked :

^im is the same seeming regularity between

THE GASES OF THE ATMOSPHERE our.

certain physical properties of elements and their
componnds : their specific volumes, their melting-
points, their refractive indices, and other properties
vary from member to member of the same column
in a maoDcr 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 grada-
tion of properties, bear a striking resemblance to
the elements thus arranged. Thus we have the

CH, Methane
CjHg Ethane
C,Hj Propane
C^H,(, Butane
Cj,H^j Pentane

4

and a host of others up to a compound of the
formula C^Hgj ; in each case there is a constant
difiference 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 con-
stant difference should exist in the relations of thft
atomic weights of the elements, and that they too

are undecomposable compounds of two unknown
elements, U 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 ia, that
although the differences are irregular {e.g. between
B= 11 aiid C= 12 the difference is 1, while between
= 16 and F = 19 the difference is 3), yet there are
two marked displacements in the order of arrange-
ment of the elements, iuasmuch as two elements
have atomic weights lower than those preceding
them in the horizontal line. It is tellurium and
iodine, and nickel and cobalt which are thus mis-
placed ; and the same is evidently the case with
potassium and argon. With an atomic weight of
39'88, the natural position of argon would lie
between potassium and calcium ; but there is no
room for it. And for this reason considerable
doubt has been thrown on the validity of the
conclusion to be drawn from the found ratio of
its specific heats, 1%, 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 2'0 would for
the same reasons then attach, would be 17'94, one
not incomparable with 16. But, as before re-
marked, 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 monatomio
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 tbat elements with high
atomic weights have, aa a rule, higher boiling-
points than those with low atomic weights in the
same columns. Perhaps the most striking cose is
that of the elements fluorine, chlorine, bromine,
and iodine. Whereas fluorine has only lately been
liquefied {chiefly owing to difficulties of manipula-
tion, due to its extraordinarily energetic action
on almost every element and compound), and boils
at about — 185^ chlorine boils at —34°, 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
(analagous to a mixture of the two supposed
constituents of the argon mixture) the pressure
of the bromine gas in the mixture would be
only yjoths of the normal pressure, or 53'2
millimetres. At this pressure the boUing-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
30° above the boiling-point of chlorine ; and there

»

would therefore be no difficulty whatever in detect-
ing sDcb a percentage of bromine in a mixture of
chlorine and bromine gases on cooling the mixtnte
to a moderately tow temperatnre.*

Aigon was first Uqaefied in 1895. A sample
of pare ai^n was sent by Professor Ramsay to
Professor Olszewski of Cracow, well known for his
accurate researches at low temperatores ; and he
found the boiling-point of argon at atmospheric
preasure to be — 186"9', and its melting-point to
be — 189'6°.' There was no appearance of Hqaid
before the boiling-point was reached, nor was there
any alteration of temperatore as the argon boiled
away, and these are signs of a single sabstance,
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 elements.

Other evidence may be sought for in the
spectrum of argon, which has been carefully ex-

' Theu coiuidgntioDB would hold on th< uiuinptioD that no cam*
bi'iiiCion take* placa between chlorine uid bromine.

' I>»t»^r determinitions bj Eamuy tnd Tn'en b»»e altered tbcae
numbento -186-l°*ad -W9'{Fha. TranM. 1901, A, i^. 41},

AND OTHER ELEMENTS

amined by Sir William Crookes and others. 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
ultra-violet light, although invisible to the eye,
impressea 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
containiug argon at a low pressure. By interposing
a Leydeu jar between the secondary terminals of
the induction-coil &om 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 diflference, 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

k

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,
adduced reasons for concluding that helium, the
gas from cleveite, is a mixture ; it appears to show
lines belonging to two spectra, each series of lines
exhibiting certain regularities. But this is also
the case with oxygen, which is not considered to
be a mixture of gases.

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 more sparingly soluble, — only 1'4 volume
per 100, — it ia not unreasonable to suppose that,
if argon consisted of a mixture of elements in
argon, one should be more soluble than another;
but Lord Rayleigh has made experiments which
render it very improbable that any separation of
its constituents 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 smfill 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, did not effect
any separation.

The evidence was therefore at first distinctly
against the supposition that argon ia 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 dia-
tomic, 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
_frQm diatomic to monatomic molecules, and the

13J THE GASES OF THE ATMOSPHERE

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, I
which is 7 1 ; but it corresponds to that of a mix-
ture of monatomic and diatomic molecules. Here
fall of temperature causes combination of monatomic ,
molecules with each other to form diatomic mole-
cules ; and rise of temperature increases the number
of monatomic molecides, 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. Randal 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 r667, the theoretical

VII ARGON AND OTHER ELKMEXTS 233

figure. A mixture of 5 per cent of diatomic mole-
cules 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.

THE OTHER INACTIVE GASES : HELIUM, NEON,
KRYPTON, AND XENON

In 1868 aa eclipse of the sun was visible in India.
The spectroscope waa tliea for the first time em-
ployed to examine the chromosphere, or coloured
atmosphere round the son ; and a brilliant yellow
line was observed, and supposed to be the " D "
lines of sodium. The well-known French astronomer,
M. Janssen, however, noted its non- coincidence
with those lines ; and it was supposed to be due to
hydrogen, or to water- vapour ; but all attempts to
reproduce the line in the laboratory failed. Messrs.
Lockyer and Frankland, who investigated the
chromosphere spectrum, found that the line which
waa distinguished as " D^" could not be ascribed
to the spectrum of any known element ; and as a
matter of conveuient reference, Frankland suggested

, GASES 235 I

," a name derived I

aupposed element I

liiipd n.It.liniinli n

THE OTHER INACTIVE GASES

the provisional name of "helium,
from the Greek word for the sun, ^Xtoi.

No certain proof that this
existed on the earth was obtaiued, although
specimen of gas from a mud-vo!cano near Vesuvius
was said to have exhibited the line.

In seeking for compounds of argon, Professor
Ramsay was reminded by Professor Miers that
Dr. Hillebrand, of the U.S. Geological Survey, had
obtained relatively considerable volumes of gas,
which was supposed to be ''nitrogen^ by beating a
rare mineral named cleveite, after Cleve, the Pro-
fessor of Chemistry at Upsala. Of course, if a
substance were found, from which argon could be
obtained on heating, it would give a clue to the
elements with which an attempt to cause argon to
combine might be successful. A sample of cleveite
was procured, and heated with sulphuric acid ; and
a gas was collected, which, after purification by
sparking with oxygen in presence of caustic soda,
was examined with the spectroscope. The chief
characteristic of the spectrum was a brilliant yellow
line, much overpowering the others in intensity ;
and the first idea was that it must have been due
to the discharge making the soda in the glass of

the vacuum-tube iDcandescent. The position of
the line was not coincident, however, with that of
the sodium lines thrown into the field of vision for
the purpose of comparison ; the preconceived idea
that the line was due to sodium was hard to
eradicate ; and the spectroscope was dismantled,
the prisms readjusted, and the spectra again com-
pared. This time there could be no doubt ; the
lines were not coincident. Reference to a table of
the solar spectrum soon made the matter clear,
and terrestrial helium was discovered.' Like argon,
it ia a gas, with no pronounced tendency towards
combination ; it is, like argon, near!)' insoluble in
water ; while 100 volumes of water at atmospheric
temperature (15° C.) dissolve 4'1 volumes of argon,
they dissolve only 1 "4 volumes of helium ; for the
solubility of helium ia nearly the same as that of
nitrogen, the least soluble of gases. Attempts
made to induce helium to enter into combination
failed, like those made with argon ; and it is there-
fore reasonable to place it in the same class of
elements as argon, especially as the ratio between
its specific heats shows it to resemble argon in

■ It wu aamewh&t later, bat iodepenilentlj, r«di»00Tered \ij Langlvt,

THE OTHER INACTIVE GASES

being a monatomic gas. Its density is nearly 2'0
— that of oxygen .being taken at 16'0 ; next to
hydrogen, the density of which is 1'007, it is the
lightest gas known.

It was subsequently found that many minerals,
chiefly those which contain the rare element
uranium (the element of highest atomic weight
known), contain helium, and give it off when
heated ; among these are broggerite, fergusonite
(which turas white-hot during the evolution of
helium), and monazite, a mineral now mined in
large quantity in the United States, and used as a
source of the thoria of which incandescent gas-
mantles are made. The curious behaviour of
fergusonite deserves explanation.

It is familiar to all that a burning object, such
as coals, or a candle, gives out heat. This is
generally the case when chemical combination
takes place. Even when chlorine combines with
elements such as iron or phosphorus, heat is
evolved. Now, there are compounds which are
formed not with evolution of heat, but with absorp-
tion. And such substances give off heat when
they decompose ; iudeed they usually decompose
with explosion. Gun-cotton and the new

;wer forms M

of smokeless powders are instances in point;
another is acetylene gas, which owes its high
luminosity to the great heat given oflF when it
decomposes ; this heat adds itself to that evolved
by the burning of the carbon and hydrogen of
which it consists, and the particles of carbon which
separate in the flame are raised to brighter incan-
descence than if they owed their temperature to
the heat due to the burning of the carbon and
hydrogen alone, as is the case with ordinary coal-
gas. Compounds which behave thus are termed
" endothermic," and fergusouite is an eiidothermic
compound. Now it is found that endothermic
compounds are not readily produced from their
elements ; for chemical combination takes place
generally only when heat is evolved during com-
bination — when the reaction is " exothermic." But
it is possible to produce endothermic compounds
directly ; that can be achieved if energy is given
them during combination. For example, nitrogen
doea not burn in oxygen ; if it did, our atmosphere
would be inflammable. But if an electric dis-
charge be passed through a mixture of nitrogen
and oxygen, combination does occur : that is one
of the methods employed for removing nitrogen

from a mixture of tbat gas with argon or helium.
Now, the fact that fergusonite decomposes with
evolution of heat implies that the helium which it
evolves when heated must have entered into com-
bination with the constituents of the mineral with
absorption of heat. How thia combination was
originally induced will appear later.

That helium is a pretty abundant constituent
of the earth is proved by its being contained in
many mineral waters. The springs at Cauterets,
in the Pyrenees, evolve it in fair quantity ; and,
nearer home, the mineral waters of Bath are rich
in it. It doubtless escapes from the eoil in many
places ; and, as will hereafter appear, it is a con-
stituent of our atmosphere. Like argon, helium
has neither taste nor smell ; indeed, its inactive
character would have rendered this probable, even
without direct evidence. It also resembles argon
in being a monatomic gas ; for the ratio of its
specific heats is 1 to If, as explained on p. 203;
and with such a ratio, the atom and the molecule
are identical.

The spectrum of helium is a very brilliant one.
Besides the particularly brilliant yellow line, by
sanB of which it was originally recognised da the

sun's chromosphere and in many of the fixed stars,
it exhibits two red lines, of which one is fairly
brilliant ; also, besides other fainter ones, a green
line, a peacock-blue line, and a violet line. It was
at first conjectured that what was named helium
was a mixture of two gases, and, indeed, the name
" parhelium " or " asterium " was given to the
supposed partner ; but this supposition was found
to be erroneous, for the arguments in its favour
(namely, that the various lines in its spectrum
could be arranged in two series, the lines in each
series exhibiting numerical relations to each
other, if arranged according to the frequency
of vibration of the light-waves) were shown to
apply equally to oxygen ; and it is not believed
that oxygen is a compound gas. Professors Kayser
and Ruuge, who made this suggestion, afterwards
disproved it ; and another suggested argument,
which was that diffusing the gas through porous
pipe-clay separated a light from a heavy portion,
the one giving a more brilliant green, and the
other a more brilliant yellow line, also turned out
to be inaccurate ; as a matter of fact, if the pressure
in a vacuum-tube containing helium is reduced,
the yellow line is relatively weakened in intensity,

while the green line grows stronger and more
luminoua. Careful experiments, by which helium
was fractionally diffused many hundred times, also
proved the homogeneity of helium. The spectrum
of helium, too, like that of argon, is altered by
the interposition of a jar and a spark-gap ; but the
change ia by no means so striking as with argon.

After the discovery of helium, it appeared cer-
tain that other gaaes remained to be discovered,
similar to those which had already been isolated.
The reasons for this belief were stated by Professor
Eamsay in an address given to the Chemical
Section of the British Association, at its meeting
at Toronto in 1897. As it may appear wonderful
that the existence of new and undiscovered
elements can be thus prophesied, an attempt will
be made to make clear the arguments in favour of
the forecast.

Not long after John Dalton, in 1803, had re-
introduced the old Greek hypothesis of the atomic
constitution of matter, and had made hia somewhat
unsuccessful attempt to determine the relative
weights of the atoms of the elements, speculation
began as to some possible relationship between the
weights of these atoms. These speculations finally.

24^ THE GASES OF THE ATMOSPHERE

aa has already been remarked, culminated in the
periodic table, reproduced on p. 220. Tlie last
column of that table contains the elements helium
and argon. The elements of preceding groups
show approximately regular differences between
their atomic weiglits ; thus, for example, the differ-
ence between the atomic weights of nitrogen, 14,
and phosphorus, 31, ia 17; that between oxygen,
16, and sulpliur, 32, ia 16; hydrogen and fluorine
show a difference of 18, and fluorine and chlorine
of 16'5 ; and hthium, sodium, and potassium have
differences of 16 and 16'1 respectively. It was
highly probable, therefore, that an element should
exist, having an atomic weight about 16 units
A;i„„**' higher than that of tmtu, and about 17 or 18
units lower than that of argon. It should have a
brilliant spectrum ; it should be a gas with a
boiling-point when liquefied higher than that of
helium, yet lower than that of argon ; like them
it should be monatomic, and it should display
inactivity in resisting combination with other
elements. Similar arguments would lead to the
conclusion that other two elements of higher
atomic weights should also be found — one with
an atomic weight somewhat higher than that of

THE OTHER INACTIVE GASES

bromine, 80, but somewhat lower than that of
rubidium, 85'4 ; and that a third should succeed
iodine, with atomic weight greater than 127, but
less than 133. As no elements are known in
the chlorine or sodium column with stiil higher
atomic weights, it was imagined that it would be
unlikely that any element with a higher atomic
weight than, say, 130 would be discovered belong-
ing to the helium column.

But where were these elements to be sought?
A very large number of minerals were heated in
a vacuum, and the gases they gave off extracted
by pumping ; some few yielded no gas whatever ;
but the majority evolved carbon monoxide and
dioxide, and hydrogen, in small quantity, while a
considerable number evolved helium, and one, a
mineral named malacone, containing zirconium,
evolved both helium and argon. The spectra of
the inactive gases were carefully examined, but
showed no unknown lines. The helium from
mineral waters, too, was introduced into vacuum-
tubes, but its spectrum likewise failed to show the
presence of any new constituent. The diffusion
of helium, which might have been expected to
separate a light from a heavy constituent of

the i

THE GASES OF THE ATMOSPHERE ca«.

mixture, was also unsuccessful in revealing any
impurity, except a trace of argon ; the only clue,
and that not a very hopeful one, was that argon,
when systematically ditfuscd, gave two portions —
one slightly heavier, the other slightly lighter,
than the original gas. But the difference was
extremely minute, and was probably to have been
accounted for by experimental error.

However, as all other possible sources had been
examined, it appeared to be the only one left
untried ; and after an examination of sea-water,
which proved fruitleaa, a large quantity of argon
was separated from the atmosphere, with the view
of its liquefaction and distillation — a process which
would separate small quantities of light and heavy
constituents more perfectly than any other method.

The boiling - point of argon, at atmospheric
pressure, is 86'9° absolute, or — ISe"!" centigrade ;
hence, in order to liquefy it, a plentiful supply of
liquid air was necessary. Dr. William Hampson,
who had devised an apparatus which yields liquid
air easily and in large quantity, kindly placed a
litre at the disposal of Professor Ramsay and his
coadjutor Dr. Travcrs. This liquid air was not
used, however, for the liquefaction of argon, but

experiments were made with it, to obtain practice
in manipulation, before risking the fifteen litres
of argon which were ready for liquefaction. On
the chance that the "dregs" or last residues of
this air might contain some one of the supposed
higher-boUing constituents of the atmosphere, the
final portions, after almost all had boiled away,
were collected ; the sample consisted largely of
oxygen, because, as nitrogen has a lower boiling-
point than oxygen, and is more volatile, the
spontaneous evaporation of the air had deprived
it of most of this constituent. On removal of the
oxygen by means of red-hot metallic copper, and
of nitrogen by magnesium, the inert residue waa
examined spectroscopically. While it showed the
well-known argon spectrum, two brilliant lines
were also visible — one in the yellow and the
other in the green part of the spectrum. The
density of the sample was 22'47; hence it was
considerably higher than that of argon, which,
it will be remembered, is approximately 20. The
ratio of the specific heats of the sample was found
to be normal, viz. 1 "66 ; hence this gas, hke helium
and argon, is also raonatomic. This gas was
discovered on May 30, 1898, and was named

THE GASES OF THE ATMOSPHERE

^^^K " krypton," or hidden — a name which had previously
^^^H been considered as a possible one for argon.
^^^B The fiftoen litres of argon were next Uquefied,
I by causing it to enttT a bulb, surrounded by liquid

air, boiling under reduced pressure. The liquid
1 argon, which occupied about 11 cubic centimetres,

I was seen to be a colourless, mobile liquid ; it could

^^H easily be frozen, by a slight reduction of tempera-
^^H ture ; and it then formed a white, ice-like solid.
^^H^ The first portions of the gas which boiled off
this liquid were collected separately, and examined
with a spectroscope ; a complicated and extremely
beautiful spectrum was observed, consisting of a
great number of red, orange, and yeUow lines.
The density of this sample was 14*67. These
densities, of course, must not be taken as dnal
numbers, but merely as indicating that the argon
obtained by the evaporation of air contained a
heavier companion, and that the gas distilled
from liquid argon contained a lighter constituent,
than argon itself. To this constituent the name
" neon " or netv was given.

The recognition of the presence of new gases in
the atmosphere, and their separation on a scale
sufficiently large for their study, are two very

vui THE OTHER INACTIVE GASES

different things. While the gases were discovered
in June 1898, it was not until October 1900 that
the investigation of their properties was completed.

It was first necessary to prepare them in
considerable quantity. And two quite distinct
operations were required to separate the lighter
constituent, neon, from air, and the heavier con-
stituent, krypton. Although both processes were
immediately commenced, as soon as a suitable
machine for producing liquid air had been pro-
cured from the Brin Oxygen Company, through
Dr. Harapson, it will conduce to clearness to
describe the operations separately.

The preparation of neon was carried out by
liquefying air, compressed to about 1-1 50th of
its volume ; this is achieved by allowing the com-
pressed air to pass downwards through a tightly-
wound copper coil, enclosed in a thin metal box,
which box is itself surrounded by a packing of
wool, contained in an exterior ease. The air
escapes at the bottom of the coil. Air, thus
compressed, approximates to a liquid, in so far
as its molecules or ultimate particles are so close
together that they attract one another. Now, in
order to separate particles of water from each

THE CASES OF THE ATMOSPHERE

Other, two methods are possible : either heat ma7
be applied, when the water changes to steam, a
body occupying a much larger volume than the
same weight of water; or if pressure is removed
from the surface of the water by means of a pump,
the particles at the surface fly off, whUe the tem-
perature of the water is lowered. With compressed
air, allowed to expand through a valve, a somewhat
similar phenomenon takes place. The air-particles,
separating suddenly from each other, absorb heat,
for the attraction between the particles is over-
come, and to effect this, heat is necessary ; as,
however, little external heat is allowed to enter,
the heat is derived from the air itself, during the
act of expansion ; and the air is cooled. The cold
air passes upwards over the copper coils through
which the compressed air is passing downwards ;
and the copper pipe is cooled. The effect naturally
is that the air passing downwards is progressively
cooled to a lower and lower temperature ; and
finally, its temperature is so greatly reduced that
it issues in the state of liquid. The process is a
wonderfully rapid one ; in less than ten minutes
after the compression-pump starts, liquid air begins
through the valve.

THE OTHER INACTIVE CASES

Of the two main eonstituenta of air, nitrogen
has the lower boiUng-point^ — 194'4'', for oxygen
boils at — 183°. Hence, the liquefied portion of
air, which probably does not exceed one-twentieth
of what passes through the coils, contains a re-
latively larger proportion of oxygen than of
nitrogen, while the escaping portion consists more
largely of nitrogen. The couDecting pipes were so
arranged that the issuing gas returned to the com-
pressor, to be again forced into the coUs, and partly
liquefied. In this manner, the heavier constituents
of the air were condensed out, and the lighter
constituents, on compressing the remainder at a
pressure of about two atmospberes into a bulb
immersed in liquid air made to boil at a low
temperature (about — 205°), by being connected
with a vacuum-pump, were liquefied. This liquefied
portion, of course, contained the lower - boiling
constituents. Air was then blown through the
liquefied portion, causing it to evaporate ; and
about one-third was collected in a large gas-holder.
The operation was repeated until a considerable
quantity had been obtained. It was then freed
from nitrogen by help of magnesium dust and
lime ; and the residue, consisting chiefiy of argon,

THE GASES OF THE ATMOSPHERE

nd frac- I

bat containing also neon, was liquefied and frac-
tionated by distillation.

And next began a tedious and troublesome
process of repeated fractionation ; tbe gas was
liquefied, distilled, and collected in successive
frBctions in a methodical manner, until the
heavier and higher boiling argon had been sepa-
rated from lighter and lower boiling constituenta.
Finally, a quantity of gas free from argon was
obtained, which would not liquefy, however much
the temperature of the liquid air which was used
Eis a cooling agent was lowered. This gas showed
the spectrum previously described as that of neon ;
but in addition, it showed the well-known lines
of helium. The spectrum of helium had previously
been recognised in that of atmospheric argon by
Drs. Kayser and Friedlauder, but it was not
believed to contain helium in such considerable
amount. The helium appeared to form about
one-third of the total volume of gas ; the remain-
ing two-thirds were neon. As neither of these
gases can be liquefied at the temperature of
liquid air, even when its temperature is lowered
as far as possible by boiling it in a vacuum, it
was impracticable to separate them by distillation.

Many months were spent in attempting to effect
a separation by diffusion ; partial success was
attained, but perfect separation was impossible.
Dissolving the gases in liquid oxygen, and frac-
tionation from the solvent were also attempted, but
without success. Finally, an apparatus somewhat
similar in design to Dr. Hampson's air-liquefying
apparatus was constructed by Dr. Travers, in
which hydrogen was liquefied, and at the very low
temperature of boiling hydrogen, 20'5'' absolute,
the neon froze solid, while the helium remained
gaseous ; the helium was removed by help of a
pump, and the solid neon was allowed to warm
up, gasified, and collected separately. It may
be mentioned incidentally that, on removing the
liquid hydrogen from the bulb which contained
solid neon, the atmospheric air froze on it, and
encrusted it with a snowball of solid air, which
melted, dropped, and evaporated.

The krypton was prepared in a different
manner. It was left in the residue from about
thirty litres of liquid air, which had been used
for various operations. Instead of allowing the
last fractions of this air to boil away and mix
with the atmosphere, they were collected sepa-

and the lighter gaaea, helium and neon, had
abready evaporated, Thia argon, which amounted
in bulk to several litres, was liquefied, and frac-
tionated in an apparatus of which an idea may
be gained from the annexed figure. The bulb (b)

THE OTHER INACTIVE GASES

1

was cooled by being immersed in liquid air ;
the argon waa introduced from a reservoir (a)
through the stopcock, under some pressure, and
liquefied ; and the first portions were allowed to
boil back into the gas-holder. The remainder
was again liquefied, and separated into six
fractions.

It was found, on inspecting the spectrum of
the lowest boiling of these fractions, that little
krypton was present, and that the gas consisted
mainly of argon. It is fortunate that the brilliant
yellow and green lines of krypton render its
recognition especially easy ; hence we had no
hesitation in rejecting gas in the spectrum of
which these lines were not visible ; but the
intensity of the spectrum misled us at first in
estimating the relative quantity of krypton in the
gaseous mixture.

The remaining fractions were re-fractionated
again and again ; but we found that anomalous
results were being obtained, for it appeared that
while krypton could be pumped away slowly,
even while the bulb was surrounded by liquid
air, a residue remained which, after removal of
the air-jacket, gasified. It turned out to be still

AQother gas, to which the oame "xenon," or the
stranger, was given.

To describe in detail the numerous fractiona-
tions by which a separation of argon, krypton,
and xenon were effected would be tedious. The
final result was that two gases were obtained
which were not further altered by repeated frac-
tionation — one of density 408, and the other of
density 64'0. These numbers imply the atomic
weights 81'6 and 128, for reaaona already given
on p. 197. The total amount of these gases was
disappointingly small ; for the krypton was only
about half a fluid ounce (about 15 cubic centi-
metres) in volume, while the xenon had barely
a quarter of that volume.

An attempt has been made to estimate ap-
proximately the amounts of these gases in atmo-
spheric air. The process for krypton and xenon
was to compress the ordinary atmospheric air,
and to run it through the liquefier ; the air which
had escaped condensation was passed through a
large gas-meter. During the experiment no less
than 1797 kilograms, or nearly 400 lbs. of air,
passed the meter, or about 4500 cubic feet, while
114 kilograms or 25 lbs, were liquefied. The total

THE OTHER INACTIVE GASES

quaatity was therefore 191 kilograms of air. It
may be remarked in passing that the liquid air
contained twice the normal quantity of argon, for
during liquefaction more argon liquefies proportion-
ally than nitrogen.

The liquid air was boiled down in a large glass
flask under a partial vacuum ; its boiling-point was
— 195° C. ; the residue measured about 200 cubic
centimetres. This liquid was then allowed to boil,
and the resulting gas was passed over red-hot
copper contained in a large tube; much oxygen
was absorbed, and the final volume of gas was
50 litres ; after all nitrogeu had been removed by
magnesium-lime mixture, the argon left measured
12'5 litres. It was then fractionally distilled, so as
to separate the argon, boiling at a low temperature,
from the krypton and xenon, of which the boiling-
points are much higher. Then the gases were
sepai'ated from each other by a long series of
operations of the same kind.

On comparing the volumes of krypton and
xenon with that of the air from which they had
been obtained, surprisingly small quantities were
obtained.

The estimation of the helium and neon was

156

THE GASES OF THE ATMOSPHERE

made in qaite a ditferent manner. Sir James
Dewsr made the ingenious discovery that if air
be admitted into a vessel containing charcoal cooled
to a low temperature by liquid air surrounding
it, the oxygen and nitrogen are absorbed, while the
helium remains uncoiidensed, and not much neon
condenses. This property of gases to be absorbed
by charcoal has been long known ; but Dewar was
the first to apply it at low temperatures,

A measured quautity of air, about 17 litres,
waa admitted to a bulb containing 100 grams of
cocoa-nut charcoal, cooled to — 100°C. A large
proportion of the oxygen and nitrogen waa
absorbed, but the neon and helium remained un-
absorbed. The unabaorbed gas was pumped off,
and again treated iu the same manner with a
smaller quantity of charcoal, so as to remove
moat of the oxygen and nitrogen. The remaining
mixture, which still contained a little nitrogen and
oxygen, was mixed with excess of oxygen, and
sparked over caustic soda. The residue of inert
gases was measured, after withdrawal of excess of
oxygen by phosphorus. The mixture of neon and
helium was then admitted to a bulb containing
charcoal cooled to —185° C, when the neon was

viii THE OTHER INACTIVE GASES

almost completely absorbed, the helium being left.
It was pumped off and measured, and when the
charcoal warmed up, the neon was also collected
and measured.

The amounts of these rare gases in crude argon
were found to be —

Heliiim . 1 part in 2300 of argon by volume.
Neon 1 ,> „ 757 „ „

Krypioii . I „ „ 200,000

Xenon

„ 1,700,000

In air, the gases are present in the following
proportions, approximately —

»

Helium

1 part

in 2*6,300 by vol

Neon
Argon

80,800
106-8

Krypton .

1 •.

20 millione „

Xenon

1

ITO millionB „

It is surprising to think that there is much more
gold in an average sample of sea-water than there
is xenon in the air.

The density of pure argon, freed from these
gases, was determined by Sir William Ramsay and
Dr. Travers ; the two most reliable determinations
gave the figures 19'952 and 19'9G1. But knowing
the relative volume of neon and helium in crude

iS8 THE GASES OF THE ATMOSPHERE CHAt.

argon and their density, the density of pure argon
can be calculated ; it is 19'953 — a number in close
agreement with the result of direct experiment

To determine the properties of these elements,
apparatus had to be constructed on a very minute
scale. The boiling-points of argon, krypton, and
xenon were determinud at all presBures between a
few millimetres and forty metres of mercury ; that
of argon at atmospheric pressure is 186'1° below
zero centigrade ; that of krypton, ISl'?"; and that
of xenon, 1091°. On compressing these gaaes, the
highest temperature at which liquefaction occurs,
or the critical temperature, is for argon, — 117"4°,
for krypton, — 62'5°, and for xeuon,+ 147" ; hence
on compressing xenon to about 50 atmospheres on
a cold day, it liquefies. They all form transparent,
colourless liquids, indistinguishable in appearance
from water ; and they likewise all freeze to white,
ice-like solids.

The position of the atmospheric elements in the
periodic table may be seen on p. 221. But it will
conduce to clearness if the elements are placed in
juxtaposition to those of neighbouring atomic
weights ; and an excerpt from the periodic table is
therefore introduced.

THE OTHER INACTIVE GASES

: Position of the Inactivk Elements in thk
Periodic Table.

Hydrogen. Helium.

Beryllium,

Fluorina

Neon.

19

20

Chlorine.

Argon.

355

40

Bromine.

Krypton.

80

82

Iodine.

Xenon.

127

128

Calcium.

40
Strontium.

Professors Lothar Meyer and MendeUeff, after
constructing the periodic table of the elements,
drew attention to the fact that the pliysical
properties of the elements are periodic functions
of, or vary regularly with, the atomic weights of
the elements in each column : thus, for example,
we have in the first column of the elements, in
the table above, the elements hydrogen, fluorine,
chlorine, bromine, and iodine. For some unknown
reason, the first element of such a column diverges
considerably in properties from the rest ; thus there
is no great analogy between the behaviour of
hydrogen and that of the halogens, as the remain-

THE GASES OF THE ATMOSPHERE

ing elementa of the column are termed. But con-
trasting the latter among themselves, we see that
while the colour of fluorine is pale yellow, that
of chlorine ia a darker greenieh-yeUow ; that of
bromine -gas red, and that of iodine-gas violet
Again, fluorine has the lowest boiling-point, and
iodine the highest ; fluorine is the most active, or
in this column the most highly electro-negative
element of the group, and iodine the least ; for
fluorine attacks almost every other element, and
indeed compound, forming fluorides; whUe iodine
is comparatively inactive, and is somewhat diflicult
to induce to combine with electro-negative elements.
On the other hand, while iodine forms relatively
stable compounds with that other highly electro-
negative element, oxygen, no compound of fluorine
and oxygen has been isolated.

Now in certain cases such properties admit of
a numerical value being attached to them. For
example, the volume of liquid or solid occupied by
a weight of the element taken in grams, at some
suitable temperature, implying the same condition
for each, is seen to vary progressively, with increase
in atomic weight. Thus, in the instance chosen
the volumes in the liquid state at the boiling-point

of 1 gram of hydrogen, of 19 grams of fluorine, of
35 '5 grams of chlorine, of 80 grama of bromine,

and of 127 grams of iodine are —

Hydrogen. Fluorine. Chlorine. Bromine. Iodine.
C.cs. . H-3 17-16 23-5 27-1 34-2

In the annexed figure it will be seen that with
ordinates as atomic weights, and with abacissae as

..

j

1

JVn

«—

—"•

^

ta

K

•Kr

p

\i

.,

\

Bt

2

"

n'*

^

i

"

'

^cJ-

"

n

J

U J

4

u

1

1

1}

1*i

Atomic Weights.
F[0. 8.

atomic volumea, broken curves are obtained both
in the fluorine and in the argon column of elements
which show such periodic variation. The same
kind of irregular variation of properties with atomic
weights is to be found if other properties be con-
sidered. Thus, when light passes through
transparent material, it suflers more or less

i

ough any J

33 retai-da- I

36a THE GASES OF THE ATMOSPHERE ch

tion, depending on the nature of the material. If
the retardation due to passage tbrough a known
length of air at a certain tcmjierature and pressure
is taken as unity, that due to passage through the
same length of the new gases under similar con-
ditions of temperature and pressure is shown
below —

Ktypton. XenoD.
1-450 2-364

Here, again, the increase with rise in atomic weight
is well defined. These numbers display among
themselves a very simple relationship, as has been
pointed out by Mr. Clive Cuthbertson. If the
value of the refractivity of heUum be taken as
ij, theu the aeries becomes

Helium. Neon. Argon, Krypton. Xenon.

Mr. Cuthbertson lias also shown that a similar
relationship holds for other elements which can be
obtained in the form of gas. Thus, if certain
columns of the periodic table (see p. 221) be
considered, we have, if we take neon as 139 —

THE OTHER INACTIVE GASES

Helium.

139 « h

Nitrogen.

Oxygen.

Fluorine.

Neon."

297^ 1

270. 1

192 X 1

139 X 1

Phosphorus.

SuJphur.

Chlorine.

Argon.

297x4

270 "4

192 x4

139 x4

Arsenic.

Selenium.

Bromine.

Krypton.

I

1

192 .; 6

139 x6

Antimony.

Tellurium.

Iodine.

Xenon.

f

?

192x10

139- 10

These numbers are values of the expression
(^ — l)xlO', and are proportioual to the retarda-
tion of light in passing through equal uumbers
of molecules of the gases, or of the elements in
the gaseous state. Their significance has not yet
met with any explanation, but it is evident that
the exceedingly simple relation must be connected
with some fundamental facts relating to the con-
stitution of matter.

The same pertodii^ity is manifest with other
properties, such as the melting and boiling-points
of argon, krypton, and xenon, the critical tem-
peratures, and, indeed, the whole curve of vapour-
pressures. Moreover, the properties of these
elements are functions, nut merely of their atomic
weights considered in reference to each other, but

also in reference to those of other elements : thus,
for example, while the atomic volume of sulphur
(atomic weight, 32) is 21'6, that of chlorine (355)
is 23-5, that of argon (39'9) ia 32'9, and that of
potassium {391) 45'4,

But it is the electric behaviour of these new
elements which has moat interest. For while
fluorine, chlorine, bromine, and iodine are the most
electro-negative of the elements, being separated
from their compounds witli metals at the positive
pole, — and while elements of the sodium group,
namely, lithium, sodium, potassium, rubidium,
and caesium, are the most electro-positive of the
elements, separating at the negative pole when a
solution of one of their compounds, or the fused
compound, provided it is a conductor of electricity,
is electrolysed, — these elements occurred in con-
tiguous columns in the periodic table of the
elements. Now it is difficult to see how elements
of such opposite properties should be next each
other, without some transition. " Natura nihil
Jit per saltum " ; and it would be reasonable to
expect a bridge to unite columns containing
elements of such opposite properties. This bridge
has now been discovered : it consists of the neutral

elements of the argon group, which have no electric
polarity, seeing that tliey form no compounds. It
is owing to this neutrality and to their low boiling-
points that they occur in the atmosphere. The
boiling-point of any substance appears to be influ-
enced greatly by its molecular weight, as well as
by the nature of the elements forming the com-
pound ; and these gases, being raolecularly simple
(for their molecules are identical with their atoms),
have especially low boiling-points, and therefore
occur only as permanent gases.

It is curious that although the presence of
helium is revealed in the sun and in many of the
fixed stars by its spectrum, that of argon has not
been detected. This leads to the suspicion that a
hypothesis put forward by Dr. Johnstone Stoney
contains a considerable element of truth. It is
that gases are continually leaving our atmosphere,
owing to the intrinsic rate of motion of their mole-
cules. A molecule of hydrogen, for example, when
it arrives at the confines of our atmosphere, may
escape, provided its rate of motion is sufficiently
rapid. And it may be proved that some molecules
of hydrogen possess sufficient velocity to carry
them beyond the sphere of the earth's attraction ;

266 THE GASES OF THE ATMOSPHERE out.

it would follow that, given sufficient time, all mole-
cules of hydrogen would ultimately fly off and
would find a home when they reached a body of
sufficient mass, and therefore of sufficient attractive
force to retain them permanently. Such a body is
the sun ; and it has been abundantly proved that
free hydrogen exists in quantity in the solar
atmosphere. But M. Gautier and Lord Kayleigh
have shown that our terrestrial atmosphere con-
tains a detectible quantity of free hydrogen ; and
Sir William Ramsay and Dr. Travers have proved
it to contain helium. Why do these gases remain
in our atmosphere ? Why, in the course of ages, do
they not leave it, each molecule pursuing its way
as an independent wanderer, until it comes under
the sway of the sun, or of some planet of sufficient
mass to retain it in its atmosphere ?

The answer to this question must be that
hydrogen and helium are continually being evolved
from the earth in such quantity as to replenish the
drainage of these gases into space. The existence
of helium in the gases from mineral waters leads
to the very probable guess that that gas must be
escaping in appreciable quantity from the soil ; and
it is well known that hydrogen is produced by

VIII THE OTHER INACTIVE GASES 267

imperfect combustion, and tbua finds its way into

our 3

Tlie absence of the spectrum of argon from the
sun's atmosphere is more puzzling. The explana-
tion may perhaps lie in the fact that the spectrum
of argon ia easily masked by that of other gases.
It is impossible to see argon lines in a mixture
containing a small amount of nitrogen ; and the
spectrum is much enfeebled, too, if oxygen be
present in the vacuum-tube. If this is not the
explanation, it must be concluded that the relative
quantity of argon in the sun's chromosphere is
small compared with that in the atmosphere of the
earth ; or possibly that the compounds of argon
are stable at the enormously high temperature of
the sun, — a suggestion which has something in its
favour; for compounds which arc formed with
absorption of heat acquire greater stability the
higher the temperature ; and it is not inconceivable
that although argon, at atmospheric temperature,
and under atmospheric conditions, refuses to com-
bine, it may yet form compounds under the much
greater extremes of electric disturbances and high
temperature which obtain in the sun and in the
fixed stars. It is only recently that the spectrum

THE GASES OF THE ATMOSPHERE

of oxygen has beeu recognised in the sua, and a
possible reason of its feebleness may be the stability
of some of its compounds endothermic under
normal conditions.

Shortly after the wave-lengths of the lines in
the spectrum of krypton were published, Sir William
Huggina, in a private letter, suggested to Sir Wil-
liam Ramsay that its brilliant green line appeared
to be identical with that seen in the spectrum of
the aurora borealis. The same remark was made
somewhat later by ProfesBor Schuster, in a letter
to Nature.

The aurora borealis or Northern Lights gene-
rally appears in the north, on frosty evenings, as a
luminous arch, from which streamers descend, and
emit light, sometimes white, sometimes green, and
sometimes crimson. The height of this arch appears
to be from 50 to 125 miles. The spectrum contains
numerous lines, all of which have been shown by
Mr. Baly to be identical with strong lines in the
spectrum of krypton, but the strongest is one of
wave-length 5570 Angstrom units.

Now this krypton line persists at great rare-
factions. Even when the amount of krypton is
reduced to one twenty-three-miUionth part of its

normal pressure, the Hoe still is visible. It can
be calculated that the pressure of the atmosphere
would be equal to that amouut at a height of 80
miles, a number which falls within the limits given
above.

Sir William Ramsay has succeeded in producing
an artiBnial aurora by causing a riug-ahaped dis-
charge to take place through krypton in the
interior of a flask, and by a powerful electro-
magnet, suitably placed, the "streamers" can also
be reproduced. Such an aurora shows all the
peculiarities of the natural aurora, including the
spectrum characteristic of krypton.

The progress of events has resulted in the dis-
covery of new atmospheric gases, the peculiarity
of which is their short life ; and the next chapter
will be devoted to a description of their sources
aud properties.

CHAPTER IX

THB RADIOACTIVE GASES : THE " EMANATIONS

The year 1896 was remarkable for the discovery
by M. Henri Becquerel that the metal uranium
and its salts were capable of impressing a photo-
graphic plate, even after they had been kept for
years in the dark ; they appeared to be able to
emit a constant and unceasing flow of something
analogous to light. Moreover, the rays emitted
were found to discharge an electrified body, so that
no charged object could retain its charge in their
immediate neighbourhood. But the effect of such
" uranium rays " was feeble. Madame Curie, two
years later, announced that she had succeeded in
extracting from pitchblende, the natural ore of
uranium, a metal resembling bismuth, to which
she gave the name "polonium" — a word derived
from Poland, of which she is a native. This sup-

THE RADIOACTIVE GASES

posed metal possessed these properties of uranium,
but in a much higher degree. Shortly afterwards,
she announced the discovery of another element
still more active, and happily named it " radium."
Compounds of the element thorium were also
found to exhibit similar properties ; and later,
another substance was separated from pitchblende
by M. Debierne and by Professor Giesel, named
by the former " actinium," and by the latter
" emanium." The property of emitting " rays,"
which were at first imagined to resemble those of
light, has been termed " radioactivity."

Perhaps the simplest way to test for radio-
activity is to place the substance undtr examination
on the top of an ordinary photogi-aphic plate,
wrapped up in black paper^ so that no light can
reach it. But the photographic method is not well
fitted for quantitative experiments. A much more
satisfactory instrument is the gold-leaf electroscope.
It consists of a metal chamber with two windows
of glass or mica opposite each other, through which
the gold-leaf can be observed. A tin oil-can forma
an efficient chamber, It is closed liy an india-
rubber cork, perforated with two holes. Through
one hole passes a thin brass rod, to the lower end

H£ GASES OF THE ATMOSPHERE cha».

of which % skort rod of fdsed silica is attached. A
slip of braiSB. ablaut ^ inch thick, ^ inch wide, and
ti inches long, is cemented on to the lower end of
che alioa rod : and attached bj a dash of gum to
the upper end of this hnss slip is a strip of gold
leaf of the same length and width, which hangs
down parallel to the brass slip, so long as it is not
electrified, bat when charged, the gold leaf stands
oat more or less &om the brass slip like an A
with one leg vertical (the strip), and the thin leg
representing the gold leaf. To impart a charge to
the go'id leaf and scrip, a stiff brass wire passes
through the seo^nd hole in the india-rubber cork ;
this wire is t-ent at the lower end, so that on
twisting it round, the lower end makes contact
wirh the brass slip. By rubbing a piece of ebonite
or sealiiicr-wax, it is charsretl, and on touehino: the
wire with it. the wire being in contact with the
brass slip, the latter is charged, and the gold leaf
diverges. The wire is then twisted, so as to break
contact with the slip : and it is then advisable to
connect the charging wire to earth, by means of
a piece of thin wire attached to a gas-pipe. The
bottom of the oil can should be removable ; and it
is well to pierce the india-rubber cork with a third

ra THE RADIOACTIVE GASES 273

hole through which a glasa tube passes. In order
that it may be possible to introduce a gas into the
metal chamber. If a solid is to be tested for radio-
activity, the electroscope is charged, and the solid
is laid on the bottom, which is then replaced. Rays
from the radioactive substance have an eifect on
the air contained in the can, termed " ionisation " ;
ionised air has the property of discharging an
electrified body ; and the amount of ionisation, and
therefore the rate of discharge, is proportional to
the intensity of the radiation of the radioactive
substance. If a radioactive gas is to be tested, a
measured quantity is blown through the glass tube
into the can ; it wilt discharge the electroscope
more or less quickly according to the extent of its
radioactivity. By observing the rate of fall of the
charged gold leaf through a telescope fitted with
a scale in its eye-piece, comparative experimente
may be made, and the relative radioactivity of two
substances compared.

More accurate measurements may be made with
an electrometer ; but enough has been said to give
a fair idea of a practicable method of testing for
and estimating radioactivity.

Compounds of radium, thonum, and uranium

diBer from those of uranium and polonium in that
they continuously evolve gases ; but these gases
are unlike others with which we are acquainted,
for they decompose or disintegrate in a short time.
Only one of the products of such decomposition has
been indeutified with any known chemical element ;
it is helium, which is produced from the gas evolved
from compounds of radium. To these gases the name
" emanation " has been given by Professor Ruther-
ford, tlie discoverer of the first of these to be ob-
served — namely, the emanation from thorium. The
thorium emanation, like other gases, mixes with air,
and air, thus mixed, acquires and retains the property
of diachiirging an electroscope, so loug as the eraaua-
tion remains undecomposed. In conjuction with
Mr. Frederick Soddy, Rutherford showed that the
emanation can be condensed by passing it through
a tube cooled below - 154" C. by means of liquid
air; this, as the reader has observed, is now a
familiar method of separating two gases from each
other.

Monsieur and Madame Curie observed that radium
compounds, too, had the power of imparting radio-
activity, lasting for a considerable time, to air with
which they were in contact ; but they did not divine

the true cause of the radioactivity — namely, the
evolution of a gas. The radium-gas was also
investigated by Rutherford and Soddy, and found
to be condensable, like the thorium gas.

In 1900, Profesaor Geitel and Mr. C. T. R.
Wilson independeotiy discovered that a positively
or negatively charged body, placed in a closed
vessel, gradually lost its charge. And Elster and
Geitel in 1901 tried to extract the radioactive
substance from the air, believing that the loss of
charge in the closed vessel was due to some radio-
active constituent of the atmosphere. Their method
of extraction depended on an observation made by
Rutherford, that the gas from thorium was attracted
by a negatively charged object, and could be made
to deposit its decomposition -product on it ; and as
this decomposition-product is also radioactive, its
presence could be detected, and its comparative
quantity measured. The same method would
attract the radium emanation, and lead to its ,
detection.

This method of detecting radioactive substances
by means of their discharging power is incomparably
more delicate than the most delicate chemical or
spectroscopic teat. It must not be supposed that

276 THE GASES OF THE ATMOSPHERE chap.

these emanations can, as a rule, be measured and
weighed; their amount is almost inconceivably
small. Moreover, the products of their decomposition
are also quite invisible. Their presence or absence
can be detected only by their power of ionising air,
and thus affecting an electroscope : and they are
differentiated from each other only by the time
during which they retain that power. For example,
the radium emanation, mixed with air, is measured ;
a known fraction of the whole — that is, a certain
number of cubic centimetres of the radioactive gas —
is blown into an electroscope, and the rate at which
the leaf falls is measured. That quantity contains
a certain amount of emanation which we shall call
X ; its absolute amount is unknown. The stock of
air is now kept for 92*6 hours, and a fresh portion,
the same in volume as the former, is blown into
the electroscope. It is again discharged, but the
time required is twice as long as the first, for one
half of the emanation has been changed into pro-
ducts which are non-volatile but also radioactive.
If these deposit on the sides of the vessel con-
taining the electroscope, the vessel would become
radioactive ; hence it is necessary, after the first
measurement, to remove the bottom of the vessel, and

THE RADIOACTIVE GASES

take care to expel the residual emanation completely
by a current of air. After a second period of 92'6
houra, a third equal quantity of the emanation,
which has now been kept for 185'2 hours, is admitted :
the time required for discharge is now four times as
long as the original time — that is, only one quarter
of the original emanation has survived. An example
of actual measurements is giveu below.

Age of
EmaDation
in honr*.

Conductivity

of Air in
Elactnweopa.

Agaof
Enianatioii
in houra.

of Air in
Elattroscope.

. 345

241

. 67-6

123-5 .

. H3

316

. 31-4

168

96-2

363

. 19-5

The conductivity is inversely proportional to the
time of discharge of the electroscope ; it is evident
that it falls off as the emanation grows older
and until it reaches nearly a zero value.

The law of decrease is a well known one ; it
may be likened to the inverse of the law of com-
pound interest. If a sum of money is leut, it is
customary to pay interest on it at stated intervals ;
for example, a yearly interval is usual. Thus, at
four per cent per annum, a sum of £100 yields, at
the end of a year, £4 interest. Supposing it to be

278 THE GASES OF THE ATMOSPHERE ckap.

agreed that the intreat is to be payable at the rate
of four per cent per half-year, tbeu it U clear that
£100 in ail months will have increased to X102.
During the second six months, the interest at 4 per
cent accrues not on £100, but on £102 ; it is there-
fore £2, i.e. the interest on £100 for six months,
plus about 94d., the interest on £2 for six months
at 4 per cent If it be agreed to pay at the rate of
4 per ceut per quarter, then the interest works out
as follows : —

luterest on £100 for 3 months at 4 p.c. = £100

101

101 2-1^ „

101 H ..

By this method, therefore, the interest at the end
of the year will be increased by about Is. Ijd. If
the period of payment were monthly, instead of
quarterly, it is clear that the sum gained would be
still larger; if daily, hourly, once every minute,
once every second, the increase would be pro-
gressively greater. Stated generally, the rate of
increase of the principal at any momeot depends
on the amount of the principal at that moment.
Now the decay of the emanation is an inverse

J

eaae of this. Let us take a supposititious iDStauce.
Imagine, for simplicity's sake, that in each day one
tenth of the total quantity of emanation present
Then we should have —

Amount of Emanation
present.

1-000

(1-000- 0-100) -0'900
(0-900 -0-090) = 0-810
(0-810 -0-081) = 0-729
(0-729 -0-073) = 0-656
etc.

1 x 1-000 = 0-100
1 y 0-900 = 0-090
1 X 0-890 = 0-081
1 X 0-729 = 0073
1 X 0-656 = 0066
etc.

At each moment the amount decomposing, how-
ever, is proportional to the quantity present. Now
the mathematical expression for this is

where lo ie the amount present at the beginning of
the change, It, the amount present in time, t, e a.
number equal to 2-718 . . ., and \ a constant.
The value of \ may be defined as the reciprocal of
the average life of a particle of the emanation.

In the case of the radium emanation, X = ^tsa^OTTo!
it means that that proportion of the total amount
of emanation present decomposes in a second. The
average life of a particle of this emanation is there-

THE GASES OF THE ATMOSPHERE

fore 463,000 seconds, or 5 days, 9 hours. The
thorium emanation has a much shorter life : it is
87 seconds, and \ has the value ^. Still shorter is
the life of the emanation from actinium : it is only
5"8 seconds; nearly ^th of the whole emanation
decomposes each second.

The gases from a solution of a radium salt (the
bromide is commonly used) consist for the most
part of a mixture of oxygen and hydrogen ; about
10 cubic centimetres per gram of radium per day
are evolved. There is always a small excess of
hydrogen, which amounts to about 6 per cent of
the total volume of the mixed gases. The gases
are evidently derived from the decomposition of
the water of the solution ; but it is not easy to
account for the excess of hydrogen : it may be due
to the formation of bromate of radium, Rd{BrO,)„
although this has not been satisfactorily ascer-
tained. Mixed with these gases is the emanation,
in extremely minute amouut,

Rutherford and Soddy investigated to some
extent the action of chemical agents on the thorium
and radium emanations. In each case they resemble
the inert gases of the atmosphere. Copper oxide
at a red heat, red-hot zinc dust, and red-hot

platinum black in presence of oxygen are without
effect on them ; their discharging power, property
of condensation, etc., are unaffected. And Ramsay
and Soddy confirmed this evidence of their indif-
ference towards reagents : neither sparking with
oxygen in presence of caustic potash, nor passage
over red-hot magnesium-lime mixture in any way
altered the radium emanation. It must, therefore,
be concluded that they resemble moat the gases of
the argon group in this respect.

Now it is remarkable that those elements which
display radioactivity have all a very high atomic
weight. Thus radium belongs to the barium series,
with a probable atomic weight of 225 ; thorium ia
allied to silicon, but has the high atomic weight
232 ; and uranium is the element with the highest
atomic weight known — 240.

It has recently been shown, however, by Dr.
Hahn, working in Ramsay's laboratory, that the
impure thoria from thorite contains a substance
for which the name "radiothorium" has been
proposed, of intense radioactivity. Indeed, it
appears to bear to thoria the same sort of relation-
ship as radium to uranium. It was separated,
along with radium, from a sample of a mineral from

s£2 THE GASES OF THE ATMOSPHERE chap.

^ -

CeTlon, named thomnite. Thifi sabetaDce was left
behind along with barium and radium sulphates,
aiier die mineral had been fiued with hydrogen
«:Mii:2m sulphate. Its bromide is more soluble than
ihai of radium; moreover, unlike radium, it is
predpiiable by ammonia. The quantity obtained
is loo small to have made it possible to detemune
:i5 axomic weight ; its activity, however, is at least
f-a-million times that of the crude thoria from
ieh ii was sep>arated. The emanation which it
evolves is identical in every respect with that evolved
frMQ ssJis thorium ; and the natural conclusion is
:}Lsr :i is the substance to which thorium compounds
oTTf iheir radioactivity. It is still doubtful whether
liorlum has a radioactivitv of its own, like uranium;
ihe assertion has l»een made bv two different observers
ih^r thorium oxide fit)m certain minerals is non-
laiioaotive. Like thorium salts, too, salts of this
substance give on precipitation with ammonia a
dlirate containing thorium X, a body which is at
first stronelv radioactive, but which soon loses its
radioactivity ; while the precipitate from which the
thorium X has been separated is less radioactive,
and regains its activity at a rate precisely the
same as that at which the thorium X loses activity.

I* THE RADIOACTIVE GASES aBj

We do DOt know the atomic weights of the gases
evolved from radium, from radiothorium, or from
actinium. Experimenta made with the view of
determining thia quantity by comparing their rates
of diffusion with those of gasea of known density
can hardly be pronounced satisfactory. It is said
that the density of radium emanation is approxi-
mately 100 ; if it is a monatomic gas, that would
imply an atomic weight of 200, and it might there-
fore follow xenon in the periodic table {see p. 220).
An experiment made with actinium emanation,
however, appears to point to its being a light gas.
If a piece of paper containing an actinium salt (of
course in a very impure condition, for aalts of
actinium have never been obtained free from
impurities) be held about the middle of a card-
board screen, covered over with phosphorescent
zinc sulphide, at about half-an-inch from the screen,
it can be seen that the gas rises. Suppose the
screen to be represented by the line, and the paper
by the dot, then in the position • there is hardly
any luminosity produced on the screen. In the
position '~~ the screen becomes intensely luminous.
In the positions /, and \! the evidence is again
that the emanation rises, care of course being taken

zl^ THE GASES OF THE ATMOSPHERE chaf.

scp sTCbi ooDTecdoa cnzrentB of air. The actiniam
fT^jfArion is puxicokilT well adapted for such
expEfimenta, for its effect is veij transitoiy ; it can
be Uoth awav, and it takes an appreciable time to
reappear. The phen<«nen(Hi is a very striking one»
and caskTevs the conviction that an invisible
sobsia&ee is streaming on to the screen to excite
faxminosicj where it toaches the phosphorescent
ooTering. Now, if the gas from actiniam is lighter
ih&n air, it man Lave a density of considerably
less ihan 14 '5 (hydrogen being taken as anity).
Can it have an atomic weight less than 4 — that of
heliam ?

It has heen already mentioned that radiom
emanation andeigoes decomposition, and that one
of its prodacts is helium. Professor Ratherford
and Mr. Soddy, after finding that thorium emana-
tion was an indifferent gas, remarked : " The
speculation naturally arises whether the presence
of helium in minerals and its invariable association
with uranium and thorium may not be connected
with their radioactivity." The state in which
helium is retained in minerals, too, is analogous
to the state in which radium emanation is retained
in salts of radium. It cannot be pronounced to be

THE RADIOACTIVE CASES

combiued ; it is almost certainly molecularly en-
tangled in the interior of the solid, and escapes
only when heat ia applied. This state has been
imitated by decomposing a compound of nitrogen,
mixed with a solid, when the nitrogen is similai'ly
retained.

The discovery that radium emanation, during
its change, is converted partially into helium was
the result of an unsuccessful attempt to obtain
the spectrum of the emanation, It was at first
imagined that if the emanation were mixed with
a gas of simple spectrum, such as helium, its lines
might be visible. But it soon became evident
that the amount of emanation present was so small
that specially minute apparatus would have to be
constructed in order to deal with it. The Pliicker
tube, instead of being of dimensions measurable
in inches, was constructed in dimensions measurable
in millimetres (1 inch = 25 millimetres). And the
volume tube, in which the amount of emanation
obtainable from a given weight of radium in a
given time was measured, was of the finest capillary
tubing. It has been mentioned that the gases
evolved from a solution of radium bromide consist
of a mixture of hydrogen and oxygen in nearly the

proportion (

hydrogen, and that the emanation is mixed with
these gases. The gases evolved in eight days from
60 milligrams of radium bromide in aqueous solution
were exploded by a spark in a small explosion
burette ; the residue of hydrogen was left in con-
tact with moist caustic potash for some time, in
order that any carbon dioxide which might have
arisen from dust on the surface of the glass being
burnt, should be absorbed. The residual hydrogen,
containing the emanation, passed through a tube
containing phosphorus pentoxide, su as to remove
water- vapour ; and it was driven upwards, by
means of mercury, until it entered a small bulb,
to the top part of which a capillary tube of known
bore was sealed. The bulb was then cooled with
liquid air, so as to condense the emanation, and
the hydrogen was entirely removed by pumping.
The jacket of liquid air was then removed, and the
emanation evaporated ; by raising the mercury
column, it was made to enter the capillary tube,
where it was measured.

It was a brightly luminous gas ; its volume was
nearly the fortieth of a cubic millimetre — that is,
the forty- thousandth of a cubic centimetre. It

was found to follow Boyle's law — that is, the
volame decreased in proportion as the pressure
was increased. From day to day the volume de-
creased, until after four weeks leas than one two-
thousandth of a cubic millimetre was left. This
minute bubble, however, was as brightly luminous
as at first, although, of course, there was much
less of it.

The mercury was drawn down the capillary
tube into the bulb, and was there frozen. On
heating the glass so as to expel helium, and passing
a discharge, the helium spectrum was visible. It
appears then that the emanation contracts to
practically nothing in about four weeks ; that
helium is formed from it, which penetrates the
walls of the glass tube, probably because the mole-
cules are shot oflf with enormous velocity ; and
that, on heating, some at least of the helium is
expelled, so that it can be recognised by its
spectrum.

In a second experiment, in which a capillary
tube of a different kind of glass was used, there
was no contraction, but, on the contrary, an ex-
pansion. The initial volume of emanation was
practically identical with that which had previously

ion was
'eviously ^

been found ; but it expanded to about ten times
ita original volume in three weeks. The gas was
then removed by pumping, and it showed a brilliant
helium spectrum ; some gas had also been absorbed.

It is easy to calculate from these data the
amount of emanation produced from one gram of
radium. It appears to be about one cubic centi-
metre from one gram in a year. But as this goes
on, the radium will continually decrease in weight,
and hence the actual amount of emanation evolved
will continually diminish, again according to the
inverse law of compound interest. It can be cal-
culated that the average life of an atom of radium
is about 1100 years, supposing that the only pro-
duct of its initial decomposition (not of subsequent
changes, in which the emanation is concerned) is
emanation.

Now if radium is changing at this rate, its pro-
duction must keep pace with its waste ; else it
would all have disappeared during the enormous
period of time of existence of the minerals in
which it is found. The natural supposition is that
uranium, the main constituent of the pitchblende
in which radium invariably occurs, may be changing
continuously into radium. This conjecture ia

Btrengthened by the fact that the amount of radium
in such minerals always bears a constant ratio to
that of uranium. But very careful experiments
by Mr. Soddy, in which he freed a solution of a
uranium salt completely from radium by repeated
precipitation of the radium with sulphuric acid in
presence of a barium salt, showed that the rate of
formation is by no means bo rapid. That radium
is formed appeared to be proved ; but not at the
expected rate. At present this want of concord-
ance of fact with theory has not been cleared up.

The spectrum of the radium emanation has also
been observed. The process of obtaining the pure
gas was practically the same as the one already
described ; but a minute Pliicker's tube was sub-
stituted for the capillary measuring tube. The
character of the spectrum is analogous to that of
the inert gases ; it is characterised by a number
of clearly-cut lines, chiefly in the green region.
These lines have been observed to be present in
the spectra of many of the fixed stars.

The disruption of the radium molecule is
accompanied by a relatively enormous heat evolu-
tion. Rutherford has found that of this heat, 75
per cent is derived from the emanation

1 and its A

390 THE GASES OF THE ATMOSPHERE

subsequent products of chauge. The Curies found
that 1 gram of radium evolves no leas than 100
calories per hour ; hence I '3 cubic millimetres, the
amount of emanation yielded per hour by one
gram of radium, must be responsible for 75 calories.
Comparing this with the heat evolved by a violent
chemical change of the ordinary character, the
difference is enormous. One cubic centimetre of
emanation, were it possible to obtain it, would
evolve about seven and a half million calories
during its complete change ; while a cubic centi-
metre of mixed oxygen and hydrogen gases evolves
on explosion only 205 calories, or 3'4 million times
less than the heat of disruption of an equal volume
of emanation — that is, on the ordinary assumption
of an equal number of molecules. The process of
change of an atom, therefore, while the same in
kind as an ordinary chemical reaction, differs
entirely in the magnitude of the result : the
amount of energy parted with during the dis-
ruption of an atom is hardly commensurable with
that due to its combination with another atom to
form a compound.

That a charged electroscope always leaks on
standing, and is slowly discharged, has been a

subject of annoyance to physiciata ever since its
invention. It was for long supposed to be due to
damp. As a matter of fact, if the insulating rod,
from which the gold-leaf is suspended, be of glass,
the discharge of the electroscope is closely conne<'ted
with the hygrometric state of the atmosphere.
For glass is attacked by water ; especially if, as is
always the ease in natural air, carbon dioxide is
present ; the solution of carbonic acid, no doubt,
decomposes the glass, forming sodium carbonate ;
and in moist weather, a conducting film is formed
on the glass, consisting of a solution of that salt.
Hence leakage takes place along the supposed
insulator. But if material be employed which is
not acted on by water, such as sulphur, ebonite,
amber, or fused quartz, the electroscope still slowly
discharges ; and it has often been shown that
the moisture of the air has no connection with
the observed leakage. The earth appears to be
negatively electrified: for if a wire, in contact
with the earth, extends upwards into the air, a
negative charge continuously leaks away. And
the leakage, if the wire be insulated from the
earth, is less in damp than in dry weather; indeed
it is least during a fog. The ions of atmospheric

292 THE GASES OF THE ATMOSPHERE chap.

gases are trapped by the small water-partides of a
fog and lemoTed; and as the leakage depends
oa the pressure of such ions, anything which
diminishes the number of ions in the air produces
a corresponding diminution of leakage of electricity.
Now, n^ative ions induce the formation of fog. If
air containing moisture be cooled, the moisture will
condense. In order that it may condense, unless the
degree of cooling be very great, nuclei are necessary.
These nuclei may be minute solid particles, such
as those of smoke ; or they may be ions, either
positive or n^ative. The negative ions attract
the fog particles, and induce liquefaction at a
higher temperature than do the positive ions.
Hence the negative ions are removed, as the fog
precipitates, leaving the positive ions still free in
the atmosphere. And thus these positive ions
discharge a negatively electrified body.

Owing to the rarity of radium and thorium, it
was long ere it was suspected that the ionisation
of the atmosphere was due to their presence in the
soiL But as soon as it was discovered that radio-
active gases are products of their disintegration, it
became evident that it is owing to the presence of
such gases that the atmosphere acquires its dis^

charging power. It v/as observed by Elster
and Geitel, to whom moat of our knowledge of
atmospheric electricity is due, that the air in
underground cellars has a much greater discharging
power than that of the free atmosphere; and this
suggested a source for the ionisation of the air —
namely, the presence of a greater amount of radio-
active gas.

Now Rutherford showed that when the thorium-
emanation decomposes, Its products are attracted
by a negatively electrified body, such as a platinum
wire connected with the negative pole of a battery
of several hundred volts. Elater and Geitel, there-
fore, tested for such products by connecting an
insulated wire, suspended in the air, with such a
source of negative potential for several hours ; and
it was easy to prove, by coiling the wire and
transferring it to an electroscope, that the " induced
activity " due to the presence of the decomposition-
product of radium emanation, was present on it.
That this is really a material coating on the wire
is shown by the fact that it can be removed by
polishing the wire with aand-paper, or by treating
it with an acid, so as to dissolve off the superficial
layer. The coating is specially rich if the nega-

I -* -It :»

ZK0L 'Si "mfrmn -snaaosaax^ iTuwigh pnwf has been

taa, 'x iisBM: sy mu ninii "iic ife petiod of decay of
1312: ^zurstsKti se^iLii/ T^jck diScis m die two

G«QKi*j garafrard wzA tbeae plieDomeiia is the
x*ci:fl*?irrnT of ziiijaal watersu The waters of
Bftoh a^i of Hazrc^ie, as well as those of some
GeniiS£i siiDsal spcings hare been shown to contain
ratdicm emaxuu5on in sohition : and Stratt has
found that the deposit on the sides of the Bath
springs contains a minate trace of radium. J. J.
Thomson, too, has proved that water fix)m deep
wells near Cambridge is radioactive. Indeed,
ra^lium appears to be a widely spread element,
although it occurs in almost infinitesimal quantity.
Such waters, no doubt, give ofi* their emanation to
the air; and as the life of the radium emanation
niiiy be taken as about a month, much survives in
ttu) uir, and by ionising its gaseous constituents,
conforH on them the power of discharging an
olectroscope. The emanation from thorium, or

n

IX THE RADIOACTIVE GASES agS

rather from radio thorium, has a much shorter
period of life ; it may be reckoned as at the moat
ten miautea; hence its radioactive power ia aoon
exhausted, and it can be detected only near the
soil. It is a remarkable fact that thorium emana-
tion is produced from some soils in quantity much
greater than be accounted for by the thorium which
they contain, and it is to be presumed that thia ia
due to the presence of radiotliorium in quantity
far too small for detection by any analytical
process.

The investigation of these remarkable gasea is
still very far from complete ; we do not know
where they should be placed in the periodic table,
but in all probability they belong to the argon
group. They must now be reckoned with as normal
constituents of the atmosphere ; and although the
proportion in which they are found is almost incon-
ceivably small, it is still possible that the enormous
quantity of energy with which they part when
they undergo their inevitable change may make
them potent factors in relation to living plants and
animals. The words of Boyle, quoted on p. 9, are
almost prophetic when he stated that " Our atmo-
sphere, in my opinion . . . consista in great number

THE GASES OF THE ATMOSPHERE CHav. ix

of numberless exhalations of the terraqueous globe
. . . with perhaps some subatautial emanations
from the celestial bodies." It is indeed conjectured
that corpuscles, almost inconceivably minute, which
have been termed " electrons," and which are shot
off with enormous rapidity during the changes
which the radioactive elements undergo, are actu-
ally constituents of out atmosphere ; that they owe
their origin to the sun ; and that they contribute to
the electrification of the atmosphere, and are the
cause of the Northern Lights, or aurora boreaiis.

i|

"II ■ '^

M

- IV