THE DISCOVERY OF ARGON
With the advent of Lavoisier’s system of representing the phenomena
of combustion, and the expression in his terms of the various changes
resulting in air when metals are oxidised, and when carbonaceous
substances burn, the investigation of air was abandoned. It was no
longer regarded as a mysterious element, possessed of “chaotic”
properties, but was held to be a mixture of oxygen, nitrogen, and small
quantities of carbon dioxide and water vapour, together with a trace
of ammonia. More exact determinations of the proportion between its
oxygen and so-called nitrogen than Cavendish had made by the nitric
oxide method were carried out in 1804 by Gay-Lussac and Humboldt, by
explosion with measured quantities of hydrogen, according to the method
suggested by Volta; and they concluded, from a large number of analyses
made on specimens collected in all weathers and from various
localities, that 100 volumes of air contained 21 volumes of oxygen and
79 volumes of nitrogen. These experiments, too, led Gay-Lussac to the
conviction that oxygen and hydrogen unite to form water in the exact
proportion of one volume of the former to two volumes of the latter;
and he published, some years later, accounts of numerous experiments
of the same kind, as the result of which he found that, when two gases
combine or react with each other, they do so in some simple number of
volumes; for example, one to one, one to two, or one to three.
The almost constant relation between the volumes of oxygen and nitrogen
in air made it appear not unlikely, in the opinion of some, that air
was a compound, and not a mixture; for the law of combination in
definite proportions had by this time been enunciated by Professor
Thomas Thomson, Dalton’s intimate friend. But between the numbers
21 and 79 there exists no such simple ratio; and, moreover, on
artificially producing air by mixing oxygen and nitrogen, there are
none of the usual phenomena which characterise the formation of a
compound: there is no rise or fall of temperature, nor does the product
differ in any way in properties from the constituents. And in 1846
Bunsen showed that the proportion between oxygen and nitrogen is not a
constant one, but that the oxygen varies between 20·97 and 20·84; the
experimental error did not exceed 0·03 volume, while the difference
found amounted to 0·13 volume. Regnault, Angus Smith, A. R. Leeds, and
von Jolly confirmed these results at later dates, from analyses of air
collected from all parts of the world.
That air contains ammonia was first observed by Scheele. He found that
the stopper of a bottle containing muriatic acid, when exposed to air
became covered with a film or deposit which he recognised to be sal
ammoniac, or ammonium chloride.
The amount of ammonia in atmospheric air is, however, exceedingly
small, and it is best detected in rain-water, which dissolves it; thus
the air is considerably poorer in ammonia after a shower. The ammonia,
small though its proportion is, plays a great part, although not an
exclusive one, in yielding to plants their supply of nitrogen. The
rain, percolating through the soil, leaves the ammonia behind, in some
form of combination; and it is then attacked by the nitrifying ferments
and converted into nitrates, from which the plants derive the nitrogen
which forms part of their substance, in combination with carbon,
oxygen, and hydrogen.
There are also traces of nitric and nitrous acids in air, which are
apparently in combination with ammonia. While the ammonia has been
found to vary between 0·1 and 100 volumes per million volumes of
air--the latter number refers to Manchester streets--nitrous and
nitric acids are present in still smaller amounts; and in spite of
the widespread opinion that ozone is contained in air, its occurrence
is still a matter of dispute. That some powerful oxidising agent
such as ozone or hydrogen peroxide is present appears certain; but
the characteristic test for ozone--the formation of peroxide of
silver on exposure of metallic silver to its influence--has never
been successful. On the other hand, a small quantity of hydrogen
dioxide--also, like water, a compound of oxygen and hydrogen, but one
containing more oxygen than water--appears to be almost constantly
present in air. Its amount is also extremely minute: it does not exceed
one part per million. Its presence in air was discovered by Schönbein.
The atmosphere further contains dust, some of which appears to
consist largely of metallic iron, which is conjectured to be of
extraterrestrial origin--minute meteorites in fact--and also the
spores of micro-organisms; but these spores, however important from
a biological or a sanitary point of view, hardly come within the
scope of the chemical composition of air. They serve to emphasise the
conjectures of Boyle and of Scheele that air may contain “corpuscles”
of all sorts, some in the form of dry exhalations, while other
innumerable particles may be sent out from the celestial luminaries.
* * * * *
Up to within the last few years it was supposed that the constituents
of air had all been discovered. But Lord Rayleigh and Professor William
Ramsay have recently found that the supposed nitrogen of the air is
in reality a mixture of nitrogen with a new gaseous element, to which
they have given the name “argon,” on account of its chemical inactivity
(=ἂργον=, idle, inactive).
In his presidential address to Section A of the British Association at
Southampton in 1882, Lord Rayleigh alluded to an investigation which he
had begun on the densities of hydrogen and oxygen, relatively to each
other. The object of the research was to discover whether the atomic
weights of these gases, determinable from their densities and from the
proportions by volume in which they combine, was actually as 1 to 16,
or whether some fractional number was necessary to express the weight
of an atom of oxygen relatively to that of hydrogen. In 1888 his first
account of the determination was published in the _Proceedings_ of the
Royal Society. In 1889 he published a continuation of his first paper,
and in 1892 he gave his final results; the number obtained was 15·882
for the atomic weight of oxygen, calculated from its density, hydrogen
being taken as 1. In 1893 further experiments on densities were
published,[25] those of oxygen and nitrogen being specially considered
with reference to the density of air. He found the weights of one litre
of oxygen, nitrogen, and air to be
Oxygen 1·42952 grams
“Nitrogen” 1·25718 "
Air 1·29327 "
A simple calculation leads to the composition of purified air. The
percentage of oxygen must be 20·941, and that of “nitrogen” 79·059, in
order to give a mixture of which the weight of a litre is 1·29327. Now,
this corresponds with the results of the best analyses, quoted above.
And the accuracy of these determinations of density is confirmed by
this means, as well as by results of other experiments made by Leduc,
von Jolly, and Morley.
But Lord Rayleigh was not content to prepare his gases by one process
only. The oxygen, of which the mean value of the weight of a litre is
given above, was prepared in three different ways: by the electrolysis
of water, by heating chlorates, and by heating potassium permanganate.
The results showed that the only difference which could be detected,
and that an extremely minute one, must be attributed to experimental
error. The actual weights of the contents of his globe were--
Electrolysis, May 1892 2·6272 grams
" " " 2·6271 "
Heating chlorates, May 1892 2·6269 "
" " June " 2·6269 "
Heating permanganate, January 1893 2·6271 "
These numbers are subject to a deduction of 0·00056, due to the fact
that when the globe was empty of air, its capacity was somewhat
reduced, owing to the external pressure of the atmosphere.
It was next deemed necessary to test whether nitrogen was homogeneous
by preparing it too by several different methods. In the same paper
Lord Rayleigh (p. 146) mentions that nitrogen, prepared from ammonia,
its compound with hydrogen, is somewhat lighter than “atmospheric
nitrogen,” the deficiency in weight amounting to about 1 part in 200.
Now it is evident from inspection of the numbers quoted above, that
the accuracy of the density determination may be trusted to within
1 part in 10,000, and that the balance would detect a discrepancy
one-fiftieth of that observed in the densities of “atmospheric” and
“chemical” nitrogen. In a letter to _Nature_, Lord Rayleigh asked for
suggestions from chemists as to the reason of this curious anomaly, but
his letter went without reply. He himself was inclined to believe that
the difference was due to the decomposition of some of the ordinary
molecules of nitrogen, usually believed to consist of two atoms in
union with each other, into molecules consisting of one atom; and as it
is held that equal numbers of molecules inhabit the same volume,
temperature and pressure being equal, if the total number of molecules
in his globe were increased by the splitting of some double-atom
molecules into single-atom molecules, the effect would be that, owing
to an admixture of some lighter molecules, the density would be
somewhat reduced.
But two other suppositions were entertained as possible. The oxygen
might have been imperfectly removed from the nitrogen derived from the
atmosphere; or, on the other hand, the nitrogen from ammonia might
conceivably have retained traces of hydrogen. In the former case, the
nitrogen would have an increased weight owing to admixture of some
heavier oxygen; in the latter, a diminished weight, due to the presence
of the lighter hydrogen. The first of these suppositions is out of
the question, inasmuch as it would have required that the nitrogen
should contain one-thirtieth of its volume of oxygen, or one-sixth of
that present in air, in order that its density should be raised by one
two-hundredth; for the densities of oxygen and nitrogen are not so very
different. The second supposition was negatived by introducing hydrogen
purposely, and removing it by passing the gas over red-hot copper
oxide, which oxidises the hydrogen to water. This yielded nitrogen of
the same density as that which had not undergone that treatment.
One other possibility was considered: the atmospheric nitrogen might
contain some molecules of greater complexity than two-atom molecules,
say N_{3}-molecules. Now it is known that when oxygen is electrified
by the passage of a rain of small sparks through it, it acquires
new properties: it possesses an odour, and attacks metallic mercury
and silver. And this product, ozone, has been shown to consist of
three-atom molecules of oxygen, by various experiments of which an
account cannot be given here.
It was not inconceivable that if such a “silent electric discharge”
were to be passed through “atmospheric” nitrogen, it might increase the
number of such three-atom molecules, and might render the gas still
denser; or if passed through “chemical” nitrogen, it might increase its
density so as to make it equal to that of “atmospheric” nitrogen. Lord
Rayleigh made such experiments, but without changing the density in the
least: the nitrogen from ammonia or from oxides of nitrogen, which has
been termed “chemical” nitrogen, still remained too light by about one
two-hundredth, and the atmospheric nitrogen still remained too heavy by
the same amount.
At this stage Professor Ramsay asked and received permission to make
some experiments on the nitrogen of the atmosphere, with the view of
explaining its anomalous behaviour. He had several years before made
experiments on the possibility of causing nitrogen and hydrogen to
combine directly, by passing the mixture over heated metals; among
these was magnesium, and although no direct combination to any great
extent was observed, still it was noticed that magnesium was a good
absorbent for nitrogen, when that gas was passed over the red-hot
filings of the metal. This process was therefore applied to the
absorption of “atmospheric” nitrogen, in order to find out whether any
portion of it was different from the rest. The plan adopted was to
heat turnings of magnesium, which can be made very thin and loose, to
redness in a tube of hard glass, in contact with the nitrogen of the
atmosphere, carefully purified from oxygen, which would otherwise have
also combined with the metallic magnesium. As absorption proceeded,
more nitrogen was admitted from a reservoir, and after a certain
quantity had been absorbed, the residual gas was extracted from the
tube by a mercury pump, and weighed.
The amount weighed was very small,--smaller perhaps than had up till
then been thought possible, if accurate results were to be obtained.
But here large differences were to be looked for. Only 40 cubic
centimetres--the twenty-fifth part of a litre--was weighed; and its
weight was only 0·050 gram. But with careful weighing the error should
not exceed one five-hundredth of the amount weighed; and if there were
to be any increase in density, that increase should be expected greatly
to exceed this small fraction.
The first weighing--in May 1894--showed that the nitrogen had increased
in density by reason of the operations, and instead of being fourteen
times as heavy as hydrogen, it was nearly fifteen times as heavy.
[Illustration: FIG. 1.]
The result was encouraging, and led to the probability of the nitrogen
being altered in some way, or of the presence of some new component of
the atmosphere. An experiment was therefore begun on a larger scale,
the atmospheric nitrogen being passed backwards and forwards from
one large glass gasholder A to another B, through a tube filled with
magnesium heated to redness G, to absorb nitrogen; over red-hot copper
oxide (_a_) (_b_), so that any carbonaceous matter such as dust should
be oxidised to carbon dioxide and water; and these, if produced, were
absorbed by placing in the train of tubes, one filled with a mixture
of soda and lime F and I, to absorb any carbon dioxide which might
possibly be formed, and two filled with pentoxide of phosphorus D and
H, to dry the gas, so that water-vapour, carried along with the gas
from the gasholders (which contained water) might be removed before
the gas passed over the red-hot magnesium; for water acts on hot
magnesium, forming oxide of magnesium and hydrogen, and the gas would
have become contaminated with the latter had this precaution not been
taken.
The process was continued for ten days, by which time most of the
nitrogen had become absorbed. The apparatus was then somewhat altered,
so as to make it possible to work with a smaller quantity of gas; but
the tubes destined to absorb nitrogen, hydrogen, etc., were filled with
the same materials as before. In a few days more the volume was reduced
to one-seventh of what it had been when the transference to the smaller
apparatus was made, or about one-eightieth of the original volume of
the atmospheric nitrogen taken.
The gas was then weighed, this time in a larger bulb, the weight being
0·2190 gram; and such is the possibility of precision in weighing on
a good balance, that a difference of one two-thousandth of the whole
weight was detectable. The density of the gas was now found to be 16·1.
At this stage it was still believed that the new gas was an ozone-like
modification of nitrogen, difficult to attack by magnesium. It was
supposed that just as oxygen, when exposed to an electric discharge,
undergoes a cleavage of its molecules, two-atom molecules becoming
one-atom molecules for an instant, which then unite to form three-atom
molecules, so the action of the magnesium on the nitrogen might be to
withdraw one atom of nitrogen from the two-atom molecule, leaving a
single uncombined atom, which might not improbably find two partners,
each of its own kind, to form with them a three-atom molecule--a sort
of nitrogen-ozone, in fact. Hence it was resolved to continue the
absorption with fresh magnesium for a still longer time, in the hope of
its being possible to isolate the three-atom nitrogen molecules. But
it became apparent that the bright metallic magnesium was now not much
attacked; and on estimating the total amount of nitrogen absorbed,
by treating the compound of nitrogen and magnesium with water, and
liberating the nitrogen as ammonia, it appeared that only a small
quantity of magnesium nitride had been formed. The density of this
further purified gas was again determined, when it was found that a
litre now weighed 1·7054 gram, corresponding to a density of 19·086.
A portion of this gas was mixed with oxygen and exposed to a rain of
electric sparks in presence of caustic soda; in fact, Cavendish’s old
plan of causing nitrogen to combine was now resorted to. Contraction
occurred, and on removing the excess of oxygen, the diminution of
volume was found to amount to 15·4 per cent of the original volume
taken. Making the supposition that the gas of density 19 still
contained nitrogen, and allowing for its influencing the density, it
followed that the pure gas should be twenty times as heavy as hydrogen.
A tube such as is usually employed in examining the spectra of gases
at low pressures was next filled with the gas of density 19. Such a
tube, called a Pflücker’s tube, after its inventor, contains wires of
platinum sealed through at each end, where it is about half an inch
in width; the middle portion of the tube is about 3 inches long, and
its bore is a fine capillary. When the platinum wires are connected
with the secondary terminals of a Ruhmkorff’s coil, and the tube is
partially exhausted, a brilliant glow appears in the capillary portion.
If viewed through a glass prism, different gases show different sets
of coloured lines crossing the usual gradation of colours of the
spectrum. Thus hydrogen exhibits three striking lines, one bright red,
one peacock blue, and one violet; nitrogen shows a large number of
somewhat hazy bands, red, orange, yellow, and yellow-green in colour,
besides a number of bands of a violet colour; but the new gas, while
exhibiting the bands characteristic of nitrogen, showed in addition
certain groups of red and green lines which did not appear to belong to
the spectrum of any known gas.
[Illustration: FIG. 2.]
While these experiments were in progress, Lord Rayleigh was occupied in
preparing nitrogen from other sources, and in determining its density;
and in every case it was evident that nitrogen from all sources except
the atmosphere weighed somewhat less than atmospheric nitrogen.
He therefore proceeded to repeat Cavendish’s experiment, and like
Cavendish, he obtained a small residue of gas which would not disappear
on sparking with oxygen, in presence of caustic soda. The sparks, as
they passed, could be observed through a spectroscope (which consists
of an arrangement of prisms and lenses so designed as to examine the
components of the light emitted by the sparks), and he, too, was struck
with the unusual character of the spectrum. His experiments proved,
besides, that the amount of residue was roughly proportional to the
amount of air taken; thus, beginning with 50 cubic centimetres of air,
the residue was 0·32 cubic centimetre; and from 5 cubic centimetres of
air, only 0·06 cubic centimetre of gas was obtained.
These small amounts are not proportional to the quantities of air
taken; but, as will afterwards be seen, the discrepancy is owing to
the solubility of the new gas in water. Still they served to show that
from a comparatively large amount of air, more of the new gas could be
obtained than from a smaller amount.
At this stage the two discoverers joined forces, and letters passed
almost daily between them, describing the results of experiments which
one or other had made. And just prior to the meeting of the British
Association at Oxford in August 1895, it was decided that the proof of
the existence of a new constituent gas in air was sufficiently clear to
render it advisable to make to the Association a short announcement of
the discovery. The statement was received with surprise and interest;
chemists were naturally somewhat incredulous that air, a substance of
which the composition had been so long and so carefully studied, should
yield anything new. One of the audience inquired whether the name of
this new substance had been discovered; as a matter of fact it was then
under consideration.
But it was still conceivable, although improbable, that the new gas was
being produced by the very processes designed for its separation, and
attention was first turned to devising a complete proof of its actual
presence in air. Now it is known that the rates of diffusion of gases
through a narrow opening, or through a number of minute holes, such
as exist in a pipe of porous clay, _e.g._ a tobacco-pipe stem, are in
inverse proportion to the square roots of the densities of the gases.
Oxygen is, in round numbers, sixteen times as dense as hydrogen; the
square roots of 16 and 1 being 4 and 1, it was found by Graham, who
first carefully investigated this subject, that four times as much
hydrogen would pass through a porous diaphragm, in a given time, as
oxygen. The _compound_ of hydrogen and oxygen, however, in the state
of gas, _viz._ steam, is not separated by such a process into its
constituents; it diffuses as such, and since it is nine times as dense
as hydrogen, the relative rates of diffusion of steam and hydrogen are
as 1: √9, or as 1 to 3; that is, for every 3 parts of hydrogen
passing through such a septum, 1 part of steam would pass in the same
time.
An experiment was therefore devised, in which a large quantity of air
was made to stream slowly through a long train of stems of churchwarden
tobacco-pipes, placed inside a glass tube, the latter being closed at
each end, except for the entrance and exit tubes of the tobacco-pipes;
in the encasing glass tube a vacuum was maintained, and the gases,
passing through the walls of the pipe-stems, were pumped off and
discharged. According to what has just been said, these should be the
lighter gases, nitrogen and oxygen, which ought to pass through the
porous stems more quickly than the supposed heavier constituent of air;
while the air issuing from the end of the train of pipes should contain
relatively more of the heavier constituent, and should in consequence
have a greater weight than an equal volume of air. But it was obviously
convenient to remove the oxygen before weighing this sample of altered
air, and this was done in the usual way by passing the mixed gases
over red-hot copper. It was found that such nitrogen was even heavier
than ordinary atmospheric nitrogen; not much, it is true, but still
consistently heavier. The denser constituent could, in fact, be
concentrated by this means. The proof was therefore indubitable that
the new gas existed in air as such.
There is another method of proof, however, which was not left untried.
Experiment showed that the solubility of the new gas in water is
considerably greater than that of nitrogen, although less than that of
oxygen. In 100 volumes of water at the ordinary temperature, about 1·5
volumes of nitrogen will dissolve, about 4·5 volumes of oxygen, and
about 4 volumes of the new gas, to which the name finally chosen for
it, “argon,” may now be applied. Now the proportion in which the
constituents of a mixture of gases will dissolve in a solvent is
conditioned first by their relative solubilities, and second, by their
relative proportion. Thus, if air be considered to be simply a mixture
of 1 volume of oxygen and 4 volumes of nitrogen, the gas extracted from
water which has been shaken with air will have the composition--
Oxygen 1 x 4·5 = 4·5 volumes
Nitrogen 4 x 1·5 = 6·0 "
So that the proportion of oxygen to nitrogen in such a mixture of gases
is considerably greater than in air: instead of being approximately 1
to 4, it is nearly 4·5 to 6. The discovery of this law concerning the
composition of the gases dissolved in liquids was due to Dr. Henry, one
of the biographers of Dalton.
The gases can be almost entirely extracted by boiling the water. But to
boil large quantities of water at one operation in a vessel suitable
for collecting the escaping gas is not easy. It is much simpler to
cause the water to pass slowly through a can below which there is a
powerful flame, so that the water in its passage becomes heated to
the boiling-point, and gives off its gas before it escapes. Of course
the gas collected contained oxygen, but this was easily removed by
the usual method of passing it over red-hot copper. The density of
the residual gas was determined, and it was found to be at least as
much greater than that of “atmospheric” nitrogen as the density of
“atmospheric” nitrogen exceeded that from chemical sources. Hence it
was to be concluded that the new constituent of air, argon, was being
concentrated by dissolving air in water, and extracting the dissolved
mixture of gases. A third proof that argon exists in air will be given
farther on.
[Illustration: FIG. 3.]
In order that the properties of the newly-discovered gas, argon, might
be thoroughly investigated, it was necessary to prepare it on a much
larger scale than had hitherto been attempted, and this was carried out
by the two processes for removing the oxygen and nitrogen which have
been already described. Supposing the new gas to have the density 20
compared with oxygen as 16, the density of the atmospheric mixture of
nitrogen and argon compared with that of nitrogen alone shows that air
should, roughly speaking, contain less than one part of argon in one
hundred. Hence, to obtain a litre of argon, it was necessary to work up
a large quantity of atmospheric nitrogen. Now, as has just been said,
there are two ways of doing this. (1) One is to produce an electric
flame between two pieces of stout platinum in air, confined in a large
glass balloon of about 6 litres capacity, over a weak solution of
caustic soda. For this purpose a very powerful rapidly alternating
current is necessary. The latest, and apparently the best, method
of carrying this out, was described by Lord Rayleigh in his Royal
Institution lecture in January 1896. The neck of the balloon is placed
downwards, and connected by means of a glass tube, passing through
a cork which closes the neck, with a rotating fan or paddle-wheel
with curved blades, which forces through the tube a weak solution of
caustic soda; another tube, also entering through the cork, conveys
away the excess of soda to the fan, whence it is again forced into
the balloon. The soda solution makes a fountain in the balloon, and
flows in a uniform stream down its sides, covering its inner surface
with a thin layer of liquid. Through the cork the two electrodes,
with their thick platinum terminals, enter; and there is another tube
besides, which conveys into the balloon a mixture of air and oxygen
in such proportions that they combine completely on exposure to the
flame. The layer of soda solution plays a double part. It prevents the
undue heating of the glass balloon, which otherwise must be sunk in
running water in order to keep it cool; and it exposes a very large
and constantly renewed surface of soda to the nitrous fumes which are
produced by the combination of the nitrogen and the oxygen, and so
removes them as quickly as they are formed. It appears probable that
the union results initially in the formation of nitric oxide, NO, which
then unites partially with oxygen to form some nitrogen peroxide,
NO_{2}. This is absorbed by the soda, giving a mixture of nitrite and
nitrate of sodium, NaNO_{2} and NaNO_{3}. Working in this way, from 7
to 8 litres of mixed gases can be made to combine per hour. The rapidly
alternating current is best obtained by the use of a transformer; and
as the heating effect on the platinum terminals is very great, they
must be made of stout rods.
(2) To prepare a large quantity of argon by the absorption of
atmospheric nitrogen by magnesium is a somewhat tedious process.
The air must be first freed from oxygen by means of red-hot copper,
and the atmospheric nitrogen collected in a gasholder. Long tubes
of combustion-glass tubing, which stands a bright red heat without
becoming deformed, are packed with magnesium turnings and heated to
redness in long gas furnaces, such as are used in organic analyses;
and through these the “atmospheric nitrogen,” dried by passage over
soda-lime and phosphorus pentoxide, is then passed. The magnesium
begins to glow at that end of the tube nearest the entrance, owing to
its combination with nitrogen, and a hot ring is seen to travel
slowly down the tube to the other end, marking the place where such
combustion is in progress. The gas issuing from the tube is collected
in a small gasholder. When one tube of magnesium is exhausted, another
is substituted for it. Each tube is capable of absorbing about seven
litres of nitrogen, so that to obtain a litre of argon about one
hundred litres of “atmospheric nitrogen” must be employed, and about
fourteen tubes of magnesium are required. M. Maquenne, who has prepared
the nitrides of several metals, has stated that a mixture of lime and
magnesium, yielding metallic calcium, is more easily manipulated than
pure magnesium, owing to the absorption of the nitrogen at a lower
temperature. The process has not been tried on a large scale, but if
the temperature of combination of magnesium and nitrogen could be thus
reduced, it would much facilitate the operation, for the greatest care
has to be taken not to overheat the combustion tube, else it softens,
and blows into holes. Porcelain tubes are attacked by the magnesium,
and crack on cooling; and iron tubes are difficult to clean out.
This preliminary operation does not yield pure argon; it merely
removes a large portion of the nitrogen. To free the argon from the
remainder, it is caused to circulate (by means of a specially contrived
mercury-pump, where each drop of mercury in falling down a narrow glass
tube carries before it a small bubble of gas) through tubes containing
red-hot copper, red-hot copper oxide, red-hot magnesium, and cold
soda-lime and phosphoric anhydride. The copper serves to remove traces
of oxygen; the copper oxide yields up its oxygen to any hydrogen or
carbon compound--dust and the like--which may happen to be present;
the soda-lime absorbs any carbon dioxide produced by the combustion of
the carbon compounds, and at the same time partially dries the gas;
while the phosphoric anhydride effectually dries the gas, previous to
its passage over the red-hot magnesium, which in its turn removes the
nitrogen. It is necessary to continue this circulation for several days
before the litre of gas is entirely freed from nitrogen.
It is difficult to choose between these two methods: both are
troublesome, and require a considerable time, but in an ordinary
laboratory the latter is probably the more easily set in operation, for
the former requires a suitable electric current, and power, so as to
rotate the water-fan. Up to the present date, the only sources which
have yielded argon are atmospheric air, gases extracted from mineral
waters or from springs, one meteorite, and a few rare minerals. No
animal or vegetable substance appears to contain it. Experiments were
made in the summer of 1895 by Mr. George MacDonald and Mr. Alexander
Kellas, in order to decide whether argon was a constituent of any
living matter. Some peas were reduced to powder and dried; the carbon
and hydrogen of the peas were burned to carbon dioxide and water
by heating with oxide of copper, and under these circumstances the
nitrogen is evolved in the state of gas. Had argon been contained in
the vegetable, it too would have accompanied the nitrogen. The nitrogen
was then, as usual, absorbed for the most part by means of magnesium,
and the small unabsorbed residue was mixed with oxygen and exposed to
electric sparks for many hours, in presence of caustic soda. There
was _no_ residue left after absorbing the excess oxygen: the gas was
completely removed. Similar experiments carried out on animal tissue
led to a similar conclusion. Two mice were chloroformed, and when dead
they were dried in an oven until all the moisture of their bodies was
completely driven off, and it was possible to reduce them to powder. It
is interesting to note that one of these mice contained 73 per cent of
water, and the other 70·5 per cent. The dried animals yielded about 11
per cent of their weight of nitrogen. Absolutely no residue of gas was
obtained on causing this nitrogen to combine; hence it appears to be a
legitimate conclusion that neither animal nor vegetable tissue contains
any appreciable amount of argon.
But these experiments lead to a further result. They show that
nitrogen, procured from its compounds, when treated in the same way as
atmospheric nitrogen, yields no trace of argon. And it must therefore
be taken as proved without doubt that argon is actually present in the
atmosphere as such, and is not produced by any process to which the
nitrogen has been submitted in order to extract it.
This point having been settled, the actual percentage of argon in
atmospheric air next invited inquiry. It is by no means very easy to
absorb quantitatively the whole of the nitrogen from an accurately
measured sample of air, for small gains and losses are apt to occur.
It is necessary to keep the air out of contact with water as much as
possible, because argon, being more soluble than nitrogen, dissolves
in larger proportional amount in the water, and is thereby partially
removed. The air was therefore entirely manipulated over mercury. The
processes were like those previously employed: most of the nitrogen was
removed with magnesium, and the residue was freed from all nitrogen by
sparking with oxygen. Experiments directed to this end were carried
out by Mr. Kellas in Professor Ramsay’s laboratory, and independently
by M. H. Schloesing in Paris. The results were identical. “Atmospheric
nitrogen” consists of pure nitrogen mixed with 1·186 per cent of its
volume of argon.
It is now possible, knowing the percentage of argon in atmospheric
nitrogen and its density (19·94), to calculate whether Lord Rayleigh’s
determinations of the density of atmospheric nitrogen were correct.
The weight of one litre of pure nitrogen is 1·2511 gram, and of argon,
1·7818 gram; hence a litre of a mixture of 98·814 volumes of nitrogen
with 1·186 volume of argon must possess the weight 1·2574 gram. The
actual number found by Lord Rayleigh was 1·2572 gram, which is almost
exactly identical with the number calculated.
Mineral waters, as a rule, contain small quantities of argon mixed
with oxygen, nitrogen, carbon dioxide, and in some cases sulphuretted
hydrogen and helium, a gas of which more hereafter. The waters actually
examined were the Bath waters, which contain much nitrogen, a little
argon, and a trace of helium; the Buxton waters, containing nitrogen
and a little argon; the water from “Allhusen’s Well,” Middlesborough,
which evolved gas of an inflammable nature consisting mainly of
nitrogen, but also containing marsh-gas, and argon to the extent
of O·4 per cent; water from boiling springs in Iceland evolved gas
containing somewhat more argon than air does, viz. 1·14 per cent; and
lastly, water from the Harrogate sulphur springs yielded a gas largely
consisting of a mixture of sulphuretted hydrogen, carbon dioxide
and nitrogen, but giving also an appreciable amount of argon. Such
determinations show that argon is not merely confined to the atmosphere
above the earth, but that it penetrates the earth and is contained in
subterraneous water. These results have been obtained by Lord Rayleigh,
Professor Ramsay, Mr. Travers, and Mr. Kellas.[26]
Similar experiments have been made by Dr. Bouchard in Paris[27] on
effervescing waters from Cauterets in the Pyrenees. One of those
springs yielded a mixture of nitrogen with a small amount of argon
and helium; another yielded only nitrogen and argon; while a third
gave nitrogen and helium. Such are, up to the present, the sources
of argon. It has been several times stated that the element helium,
which is closely allied to argon in its physical properties, and in
its inertness, is a normal constituent of our atmosphere, although in
very small amount. This, however, is not the case. For the argon of the
atmosphere has been very carefully examined for helium in two ways.
First, Lord Rayleigh dissolved a large quantity of argon in water,
leaving only a minute bubble undissolved. Now while 1000 parts of water
dissolve 40 parts of argon, they dissolve only 7 parts of helium; and
if helium were present, it should be found in the residue. But careful
spectroscopic examination failed to reveal the characteristic lines
of helium. Next, Professor Ramsay and Dr. Collie diffused argon
fractionally; and as the densities of argon and helium are very
different (2 and 20), the helium should have been visible in the
first portion which diffused. There was no trace to be detected.
But, further, they submitted this portion of argon to a discharge of
electricity for several hours in a vacuum-tube provided with platinum
electrodes. This process, as they proved, carries out the helium
with the platinum which is splashed on to the sides of the tube. The
argon was then removed by pumping it off in the cold. On heating the
splashed-off platinum, no helium spectrum could be observed. In a
similar experiment, in which a very minute trace of helium had been
added to the argon, there was no difficulty in detecting the helium on
heating the tube. The conclusion therefore follows that no helium is
present in our atmosphere.
It is, besides, exceedingly improbable that helium should be present.
For, as shown by Dr. Johnstone Stoney, the rate of motion of a molecule
of hydrogen is so rapid that, on finding its way to the confines of the
atmosphere, it would escape, and travel through space until it found a
planet of sufficient attractive force to hold it. The same is true of
helium. Were helium present in the atmosphere, it would ultimately
leave us for the sun, or for some planet of much greater mass than the
earth. This conception of Dr. Stoney’s tallies with the observation
that the moon, a planet of small mass, is devoid of an atmosphere, and
that the sun, a body with a mass 300,000 times as great as the earth,
shows in its chromosphere the spectra both of hydrogen and of helium
with great brilliancy.
It is now of interest to inquire what are the properties of argon and
how it is related to other elements.
[25] _Proc. Roy. Soc._ vol. liii. p. 134.
[26] _Proc. Roy. Soc._ vol. lix. p. 68.
_Phil. Trans._ vol. clxxxvi. p. 227.
[27] _Compt. rend._ vol. cxxi. p. 394.