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



v THE DISCOVERY OF ARGON 157

oxidises the hydrogen to water. This yielded
nitrogen of the same density as that which had not
undergone that treatment.

One other possibility was considered : the
atmospheric nitrogen might contain some mole-
cules of greater complexity than two-atom mole-
cules, say N 3 -molecules. Now it is known that
when oxygen is electrified by the passage of a rain
of small sparks through it, it acquires new 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 Kayleigh made such experiments,
but without changing the density in the least : the



158 THE GASES OF THE ATMOSPHERE CHAP.

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



v THE DISCOVERY OF ARGON 159

also combined with the metallic magnesium. As
absorption proceeded, more nitrogen was admitted
from a reservoir, and after a certain quantity had
been absorbed, the residual gas was extracted from
the tube by a mercury pump, and weighed.

The amount weighed was very small, smaller
perhaps than had up till then been thought possible,
if accurate results were to be obtained. But here
large differences were to be looked for. Only 40
cubic centimetres the twenty-fifth part of a litre
was weighed ; and its weight was only O'OSO
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



CHAP, v THE DISCOVERY OF ARGON 161

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 redness, to absorb nitrogen ;
over red-hot copper oxide (a) (b), 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

M



162 THE GASES OF THE ATMOSPHERE CHAP.

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

The gas was then weighed, this time in a larger
bulb, the weight being 0*2190 gram; and such is
the possibility of precision in weighing on a good
balance, that a difference of one two-thousandth of
the whole weight was detectable. The density of
the gas was now found to be 1 6'1. 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-atom molecules, so the action of the mag-
nesium on the nitrogen might be to withdraw one
atom of nitrogen from the two-atom molecule, leav-
ing a single uncombined atom, which might not
improbably find two partners, each of its own kind,
to form with them a three-atom molecule a sort of
nitrogen-ozone, in fact. Hence it was resolved to
continue the absorption with fresh magnesium for a



v THE DISCOVERY OF ARGON 163

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
further purified gas was again determined, when it
was found that a litre now weighed 1*7054 gram,
corresponding to a density of 19*086.

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

A tube such as is usually employed in examin-



164 THE GASES OF THE ATMOSPHERE CHAP.

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



FIG. 2.

terminals of a RuhmkorfFs coil, and the tube
is partially exhausted, a brilliant glow appears
in the capillary portion. If viewed through a
glass prism, different gases show different sets of
coloured lines crossing the usual gradation of
colours of the spectrum. Thus hydrogen exhibits
three striking lines, one bright red, one peacock-
blue, and one violet ; nitrogen shows a large
number of somewhat hazy bands, red, orange,
yellow, and yellow - green in colour, besides a
number of bands of a violet colour ; but the new
gas, while exhibiting the bands characteristic of
nitrogen, showed in addition certain groups of red



v THE DISCOVERY OF ARGON 165

and green lines which did not appear to belong to
the spectrum of any known gas.

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

These small amounts are not proportional to



166 THE GASES OF THE ATMOSPHERE CHAP.

the quantities of air taken ; but, as will afterwards
be seen, the discrepancy is owing to the solubility
of the new gas in water. Still they served to
show that from a comparatively large amount of
air, more of the new gas could be obtained than
from a smaller amount.

At this stage the two discoverers joined forces,
and letters passed almost daily between them,
describing the results of experiments which one
or other had made. And just prior to the meeting
of the British Association at Oxford in August 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 discovery. 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



v THE DISCOVERY OF ARGON 167

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

An experiment was therefore devised, in which
a large quantity of air was made to stream slowly
through a long train of stems of churchwarden



168 THE GASES OF THE ATMOSPHERE CHAP.

tobacco-pipes, placed inside a glass tube, the latter
being closed at each end, except for the entrance
and exit tubes of the tobacco-pipes ; in the 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 air as such.

There is another method of proof, however,
which was not left untried. Experiment showed



v THE DISCOVERY OF ARGON 169

that the solubility of the new gas in water is
considerably greater than that of nitrogen, although
less than that of oxygen. In 100 volumes of water
at the ordinary temperature, about 1'5 volumes of
nitrogen will dissolve, about 4*5 volumes of oxygen,
and about 4 volumes of the new gas, to which the
name finally chosen for it, " argon," may now be
applied. Now the proportion in which the 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 4*5 = 4*5 volumes
Nitrogen . . . 4 x 1 *5 = 6 '0

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

The gases can be almost entirely extracted by



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 is a powerful flame, so that
the water in its passage becomes heated to the
boiling-point, and gives off its gas before it escapes.
Of course the gas collected contained oxygen, but
this was easily removed by the usual method of
passing it over red-hot copper. The density of the
residual gas was determined, and it was found to be
at least as much greater than that of "atmospheric"
nitrogen as the density of " atmospheric " nitrogen
exceeded that from chemical sources. Hence it was
to be concluded that the new constituent of air,
argon, was being concentrated by dissolving air
in water, and extracting the dissolved mixture of
gases. A third proof that argon exists in air will
be given farther on.

In order that the 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



v THE DISCOVERY OF ARGON 171

already described. Supposing the new gas to have
the density 20 compared with oxygen as 16, the
density of the atmospheric mixture of nitrogen and
argon compared with that of nitrogen alone shows
that air should, roughly speaking, contain less than
one part of argon in one hundred. Hence, to
obtain a litre of argon it was necessary to work
up a large quantity of atmospheric nitrogen. Now,
as has just been said, there are two ways of doing
this. (1) One is to produce an electric flame
between two pieces of stout platinum in air, 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 Eayleigh 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



172



THE GASES OF THE ATMOSPHERE



CHAP.



the balloon. The soda solution makes a fountain
in the balloon, and flows in a uniform stream down
its sides, covering its inner surface with a thin layer
of liquid. Through the cork the two electrodes, with
their thick platinum terminals, enter ; and there is



To Transformer




Gases In



FIG. 3.



another tube besides, which conveys into the balloon
a mixture of air and oxygen in such proportions that
they combine completely on exposure to the flame.
The layer of soda solution plays a double part. It
prevents the undue heating of the gas balloon,
which otherwise must be sunk in running water in
order to keep it cool ; and it exposes a very large



v THE DISCOVERY OF ARGON 173

and constantly renewed surface of soda to the
nitrous fumes which are produced by the combi-
nation 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, N0 2 . This is absorbed by the soda, giving a
mixture of nitrite and nitrate of sodium, NaN0 2
and NaN0 3 . Working in this way, from 7 to
8 litres of mixed gases can be made to combine
per hour. The rapidly alternating current is best
obtained by the use of a transformer ; and as the
heating effect on the platinum terminals is very
great, they must be made of stout rods.

(2) To prepare a large quantity of argon by the
absorption of atmospheric nitrogen by magnesium
is a somewhat tedious process. The air must be
first freed from oxygen by means of red-hot copper,
and the atmospheric nitrogen collected in a gas-
holder. Long tubes of combustion-glass tubing,
which stands a bright-red heat without becoming
deformed, are packed with magnesium turnings
and heated to redness in long gas furnaces, such
as are used in organic analyses ; and through these



174 THE GASES OF THE ATMOSPHERE CHAP.

the " atmospheric nitrogen," dried by passage
over soda-lime and phosphorus pentoxide, is
then passed. The magnesium begins to glow
at that end of the tube nearest the entrance,
owing to its combination with nitrogen, and a
hot ring is seen to travel slowly down the tube to
the other end, marking the place where such com-
bustion is in progress. The gas issuing from the
' tube is 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



v THE DISCOVERY OF ARGON 175

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
tubes 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) through
tubes containing red-hot copper, red-hot copper
oxide, red-hot magnesium, and cold soda-lime and
phosphoric anhydride. The copper serves to remove
traces of oxygen ; the copper oxide yields up its
oxygen to any hydrogen or carbon compound
dust and the like which may happen to be present ;
the soda-lime absorbs any carbon dioxide produced
by the combustion of the carbon compounds, and
at the same time partially dries the gas; while
the phosphoric anhydride effectually dries the gas,
previous to its passage over the red-hot magnesium,



176 THE GASES OF THE ATMOSPHERE CHAP.

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 choose 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 MacDonald and Mr. Alexander Kellas,
in order to decide whether argon was a constituent
of any living matter. Some peas were reduced to
powder and dried ; the carbon and hydrogen of the
peas were burned to carbon dioxide and water by
heating with oxide of copper, and under these
circumstances the nitrogen is evolved in the state



v THE DISCOVERY OF ARGON 177

of gas. Had argon been contained in the vegetable,
it too would have accompanied the nitrogen. The
nitrogen was then, as usual, absorbed for the most
part by means of magnesium, and the small un-
absorbed 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


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Online LibraryWilliam RamsayThe gases of the atmosphere, the history of their discovery → online text (page 9 of 16)