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The gases of the atmosphere, the history of their discovery online

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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

' ' O

of fish is richer in argon than atmospheric air.
But these experiments lead to a further



result. They show that nitrogen, procured from
its compounds, when treated in the same way as
atmospheric nitrogen, yields no trace of argon.
And it must therefore be taken as proved without
doubt that argon is actually present in the 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 an
accurately measured sample of air, for small gains
and losses are apt to occur. It is necessary to
keep the air out of contact with water as much as
possible, because argon, being more soluble than
nitrogen, dissolves in larger proportional amount
in the water, and is thereby partially removed.
The air was therefore entirely manipulated over
mercury. The processes were like those previously
employed : most of the nitrogen was removed with
magnesium, and the residue was freed from all
nitrogen by sparking with oxygen. Experiments
directed to this end were carried out by Mr. Kellas
in Professor Kamsay's laboratory, and independently


by M. H. Schloesing in Paris. The results were
identical. "Atmospheric nitrogen" consists of
pure nitrogen mixed with 1*186 per cent of its
volume of argon.

It is now possible, knowing the percentage of
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 Kayleigh's determinations of the
density of atmospheric nitrogen were correct. The
weight of one litre of pure nitrogen is 1*25092
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*25718 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 nitrogen and a little argon ;


the water from " Allhusen's Well," Middlesborough,
which evolved gas of an inflammable nature con-
sisting mainly of nitrogen, but also containing
marsh-gas, and argon to the extent of 0*4 per cent;
water from boiling springs in Iceland evolved gas
containing somewhat more argon than air does,
viz. 1*14 per cent; and lastly, water from the
Harrogate sulphur springs yielded a gas largely
consisting of a mixture of sulphuretted hydrogen,
carbon dioxide and nitrogen, but giving also
an appreciable amount of argon. Such 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
Eayleigh, Professor Eamsay, Mr. Travers, and Mr.
Kellas. 1

Similar experiments have been made by Dr.
Bouchard in Paris 2 on effervescing waters from
Cauterets in the Pyrenees. One of those springs
yielded a mixture of nitrogen with a small amount
of argon and helium ; another yielded only nitro-
gen and argon ; while a third gave nitrogen and

1 Proc. JRoy. Soc. vol. lix. p. 68. Phil. Trans, vol. clxxxvi. p. 227.
2 Compt. rend. vol. cxxi. p. 394.


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



THE density of a gas is one of its most character-
istic and important properties. Avogadro's law,
which postulates that equal volumes of gases, at
equal temperature and pressure, contain equal
numbers of molecules, renders it possible to 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 nitrogg^-the smallest
portion which can exist in frJHBn^ uncombined
with other elements is -Jths 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 . . . .11*11 per cent
Oxygen . . .< . 88'88

If the smallest portion of water capable of free
existence contains one atom of hydrogen and one
of oxygen, then, placing the weight of an atom of
hydrogen as unity, the weight of an atom of
oxygen is eight times as great. And although we
do not know the absolute weight of any single
atom, we are justified in supposing that an atom
of oxygen is eight times as heavy as an atom of
hydrogen. But have we any right to make the
assumption that a molecule of water contains one
atom of each element? Dalton came to the conclu-
sion that this supposition was a justifiable one ; but
there are strong reasons against it. We have already


seen that Cavendish discovered approximately, and
that Gay-Lussac and Humboldt determined 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 consideration (1) Boyle's dis-
covery that if the pressure on a gas be increased, the
volume of the gas, whatever it may be, diminishes
in like proportion ; and (2) Gay-Lussac's and
Dalton's discovery, that all gases, when equally
raised in temperature, expand equally. It would
be very remarkable if one gas, containing twice as
many particles in unit volume as another, should
show exactly similar behaviour towards pressure
and temperature. Hence it appeared not 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 " is
here used in the meaning of " small portion " ; such
particles may be molecules or they may be atoms.)
When steam is formed by the union of hydrogen
with oxygen, it has a volume equal not to the sum


of the volumes of the hydrogen and the oxygen,
but to two-thirds of the sum, or equal to that of
the hydrogen alone, or twice that of the oxygen.
And as steam, like hydrogen and oxygen, follows
Boyle's and Gay-Lussac's laws it must be supposed
that in the steam there are as many particles
as in the hydrogen from which it was formed.
But the particles of steam must necessarily be more
complex than those of the hydrogen, inasmuch as
the steam contains oxygen as well as hydrogen.

These difficulties may, however, be easily 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 other, or with atoms of some other element.
Two volumes of hydrogen, therefore, contain twice
as many particles as one volume of oxygen ;
to such particles the name " molecules " is now
universally applied. And as these molecules
are themselves each made up of two smaller
particles, now termed atoms," there exist in two
volumes of hydrogen twice as many atoms as in
one volume of oxygen. On combination, the atoms


in the molecules of hydrogen and oxygen rearrange
themselves, so that two atoms of hydrogen and one
atom of oxygen combine to form a molecule of
water- vapour, containing three atoms. The steam
now contains as many molecules as did the hydrogen
before combination ; but whereas the molecules of
hydrogen originally consisted of two atoms each,
the molecules of steam contain three atoms. It
is this which causes the contraction from three
volumes to two when hydrogen and oxygen 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


themselves, so that groups of atoms, or molecules,
each containing two atoms of hydrogen and one of
oxygen, are formed. To such an arrangement the
formula H 2 is applied, while ordinary hydrogen
molecules may be represented as H 2 , and molecules
of oxygen as 2 .

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

Copper . . . 7 9 '9 6 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-


able to suppose that each molecule of hydrogen and
of oxygen contains two atoms, the number 15*882
represents the weight of an atom of oxygen compared
with that of an atom of hydrogen taken as 1. But
this number has not been regarded as sufficiently
established by experiment. Other observers (for
the importance of this ratio has been acknow-
ledged since the beginning of the century) have
obtained results differing from that given above,
although not to any great extent. And as it is a
matter of indifference what basis or standard be
taken for atomic weights, which represent only
relative numbers, it is common to accept the
atomic weight of oxygen as 16, in which case that
of hydrogen, if Lord Kayleigh's determination of
its density be regarded as accurate, would be
1*0074. Hence if we place the atomic weight of
oxygen as 16, that of copper would be 63*34.
And as with copper, so with most other elements.
It is very seldom that the atomic weight of an
element has been directly compared with that of
hydrogen ; it is, in fact, almost always ascertained
by analysis of its chloride, bromide, or oxide ;
and the atomic weights of chlorine and bromine
have been very carefully compared with that of


oxygen. There is, besides, another convenience
in accepting 16 as the atomic weight of oxygen :
it is that many atomic weights are then 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 15*882, the atomic
weight of sulphur would be 31*764, a number much
more difficult to remember.

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

But this gives no information regarding the
relative weight of an atom of argon. To ascertain


this important quantity two methods may be chosen.
One is to make compounds of the element, and
this will be first considered. Since an atom of
an element is defined as the smallest amount
which can exist in combination, then, if numerous
compounds of an element be examined, that one
which contains proportionally the least amount of
the element may be regarded as containing an atom,
unless there are reasons to the contrary. For 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. Eeasons have
already been given showing why its formula should
be H 2 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 by
weight of hydrogen. Here also there are reasons
for believing that this compound, hydrogen peroxide,
contains two atoms of hydrogen ; whence it follows
that it must contain two atoms of oxygen, or 32
parts by weight to 2 parts by weight of hydrogen,
and must therefore have the formula H 2 2 . No
other compound of oxygen and hydrogen is known ;
and it may be stated briefly that no compound of


oxygen with any element whatever is known in which
less than 16 parts by weight enters compared, of
course, with the atomic weight of the other element
or elements in the compound. Hence 16 is accepted
on this ground as the atomic weight of oxygen.

If now it were possible to prepare compounds
of argon, similar reasoning might be applied to
them, and that compound containing least argon
would be regarded as indicating its atomic weight.
Many attempts were therefore made to induce
argon to enter into combination. And the 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 >t cannot be
induced to combine with one of the most electro-
negative of elements oxygen, and one of the most
electro -positive magnesium. It also refuses to
combine with hydrogen or with chlorine when
sparked with these gases ; nor is it absorbed or
altered in volume by passage through a red-hot
tube along with the vapours of phosphorus, sulphur,


tellurium, or sodium. Ked-hot caustic soda, or a
red-hot mixture of soda and lime, which attacks
the exceedingly refractory metal platinum, was
without action on argon. The combined influence
of oxygen and an alkali in the shape of fused
potassium nitrate or red-hot peroxide of sodium
was also without effect. Gold would, however,
have resisted such action, but would have been
attacked by the next agent tried, viz. persulphide
of sodium and calcium. This mixture was exposed
at a red-heat to a current of argon, again without
result. Nascent chlorine, or chlorine at the moment
of liberation, obtained from a mixture of nitric
and hydrochloric acids, and from permanganate of
potassium and hydrochloric acid, was without action.
A mixture of argon with fluorine, the most active
of all the elements, was exposed to a rain of
electric sparks by M. Moissan, the distinguished
chemist who first succeeded in preparing large
quantities of fluorine in a pure state, without his
observing any sign of chemical combination.

An attempt was also made to cause argon to
combine with carbon by making an electric arc
between two rods of carbon in an atmosphere of
argon. It was at first believed that combination


had taken place, for expansion occurred, the final
volume of gas being larger than the volume taken ;
but subsequent experiments have shown that the
expansion was due to the formation of some oxide
of carbon from the oxygen adhering to the carbon
rods. On absorption of this oxide by the usual
absorbent, a mixture of cuprous chloride and
ammonia, the argon was recovered unchanged.

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


Professor Eamsay has also made experiments
on the action of a silent electric discharge upon
a mixture of argon with the vapour of carbon
tetrachloride ; the latter decomposes, giving, not
a resin, but crystals of hexachlorobenzene and free
chlorine ; but the volume of the argon was 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 as
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 Kayleigh'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 case has any true chemical combination been

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

It was therefore necessary to adopt some other
method in attempting to determine the atomic
weight of argon, some method dependent on its


physical rather than its chemical properties, for
argon, unlike almost all other elements, appears
to be devoid of chemical properties.

In order better to follow the train of reasoning
based on experiment, it will be well to begin with
an account of why the atomic weight of mercury is
accepted as 200. The amount of mercury which
combines with 16 parts by weight of oxygen is
easily found by heating a weighed quantity of
oxide of mercury, as Priestley and Scheele did,
and weighing the residue of metal. The results of
the most accurate experiments show that 200*36
grams of mercury combine with 16 grams of
oxygen, and if the compound consists of one atom
of each element, 200*36 must be the atomic weight
of mercury. The first idea which naturally occurs
is to find out the relative weight of mercury gas.
This has been done, and it is found to have the ratio
to that of oxygen of 100 to 16. Doubling these
numbers will give the molecular weights, since a
molecule of oxygen consists of two atoms, and must
therefore possess twice the weight of one atom. We
thus obtain the numbers 200 and 32 as the 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 2 0, 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 mercury, but if so it is singular that


no compound contains a smaller proportion ; and
there is another reason, which follows immediately,
that leads us to believe that 200 is correctly taken
as the true weight of an atom.

It was discovered by Dulong and Petit, early in
the century, that the higher the atomic weight of
an element the less heat is required to raise its
temperature through a given number of degrees.
This heat can be measured by dropping a fragment
of the element, carefully weighed and heated to a
known temperature, into a known weight of cold
water, and ascertaining what rise of temperature
the water undergoes, owing to the heat 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

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Online LibraryWilliam RamsayThe gases of the atmosphere, the history of their discovery → online text (page 10 of 16)