ARGON, a gaseous constituent of atmospheric air. For more than zoo years before 1894 it had been supposed that the com position of the atmosphere was thoroughly known. Beyond vari able quantities of moisture and traces of carbonic acid, hydrogen, ammonia, etc., the only constituents recognized were nitrogen and oxygen. The analysis of air was conducted by determining the amount of oxygen present and assuming the remainder to be nitrogen. Since the time of Henry Cavendish no one seemed even to have asked the question whether the residue was, in truth, all capable of conversion into nitric acid.
Discovery of Inert Gas in Air.—The manner in which this condition of complacent ignorance came to be disturbed is in structive. Observations undertaken mainly in the interest of Prout's law, and extending over many years, had been conducted to determine afresh the densities of the principal gases—hydrogen, oxygen and nitrogen. In the latter case, the first preparations were according to the convenient method devised by Vernon Harcourt, in which air charged with ammonia is passed over red-hot copper. Under the influence of the heat the atmospheric oxygen unites with the hydrogen of the ammonia, and when the excess of the latter is removed with sulphuric acid, the gas properly desiccated should be pure nitrogen, derived in part from the ammonia, but principally from the air. A few concordant determinations of density having been effected, the question was at first regarded as disposed of, until the thought occurred that it might be desir able to try also the more usual method of preparation in which the oxygen is removed by actual oxidation of copper without the aid of ammonia. Determinations made thus were equally con cordant among themselves, but the resulting density was about 1 o 0o part greater than that found by Harcourt's method (Ray leigh, 1892). Subsequently when oxygen was substituted for air in the first method, so that all (instead of about ? part) of the nitrogen was derived from ammonia, the difference rose to 1%. Further experiment only brought out more clearly the diversity of the gases hitherto assumed to be identical. Whatever were the means employed to rid air of accompanying oxygen, a uniform value of the density was arrived at, and this value was 1% greater than that appertaining to nitrogen extracted from compounds such as nitrous oxide, ammonia and ammonium nitrate. No impurity consisting of any known substance could be discovered capable of explaining an excessive weight in the one case or a deficiency in the other. Storage for eight months did not disturb the density of the chemically extracted gas, nor had the silent electric discharge any influence upon either quality. ("On an Anomaly encountered in determining the Density of Nitrogen Gas," Proc. Roy. Soc., 1894.) At this stage it became clear that the complication depended upon some hitherto unknown body, and probability inclined to the existence of a gas in the atmosphere heavier than nitrogen, and remaining unacted upon during the removal of the oxygen —a conclusion afterwards fully established by Lord Rayleigh and Sir William Ramsay. The question which now pressed was as to the character of the evidence for the universally accepted view that the so-called nitrogen of the atmosphere was all of one kind, that the nitrogen of the air was the same as the nitrogen of nitre. Reference to Cavendish showed that he had already raised this question in the most distinct manner, and indeed, to a certain ex tent, resolved it. In his memoir of 1785 he writes:— "As far as the experiments hitherto published extend, we scarcely know more of the phlogisticated part of our atmosphere than that it is not diminished by lime-water, caustic alkalies or nitrous air; that it is unfit to support fire or maintain life in ani mals; and that its specific gravity is not much less than that of common air ; so that, though the nitrous acid, by being united to phlogiston, is converted into air possessed of these properties, and consequently, though it was reasonable to suppose, that part at least of the phlogisticated air of the atmosphere consists of this acid united to phlogiston, yet it may fairly be doubted whether the whole is of this kind, or whether there are not in reality many different substances confounded together by us under the name of phlogisticated air. I therefore made an experiment to determine whether the whole of a given portion of the phlogisticated air of the atmosphere could be reduced to nitrous acid, or whether there was not a part of a different nature to the rest which would refuse to undergo that change. The foregoing experiments indeed, in some measure, decided this point, as much the greatest part of air let up into the tube lost its elasticity ; yet, as some remained unab sorbed, 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 [oxygen] and common air, in the same manner as before [by sparks over alkali], till it was reduced to a small part of its original bulk. I then, in order to decompound as much as I could of the phlogisticated air [nitrogen] which re mained in the tube, added some dephlogisticated air to it and con tinued the spark until no further diminution took place. Having by these means condensed as much as I could of the phlogisticated air, I let up some solution of liver of sulphur to absorb the de phlogisticated air; after which only a small bubble of air remained unabsorbed, which certainly was not more than -2 , of the bulk of the dephlogisticated air let up into the tube ; so that, if there be any part of the dephlogisticated air of our atmosphere which dif fers from the rest, and cannot be reduced to nitrous acid, we may safely conclude that it is not more than h part of the whole." Although, as was natural, Cavendish was satisfied with his re sult, and does not decide whether the small residue was genuine, it is probable that his residue was really of a different kind from the main bulk of the "phlogisticated air," and contained the gas afterwards named argon. The announcement to the British Asso ciation in 1894 by Rayleigh and Ramsay of a new gas in the at mosphere was received with a good deal of scepticism. Some doubted the discovery of a new gas altogether, while others denied that it was present in the atmosphere. Yet there was nothing in consistent with any previously ascertained fact in the asserted presence of 1% of a non-oxidizable gas about half as heavy again as nitrogen. The nearest approach to a difficulty lay in the be haviour of liquid air, from which it was supposed, as the event proved erroneously, that such a constituent would separate itself in the solid form. The evidence of the existence of a new gas (named Argon on account of its chemical inertness, Gr. a- priva tive and ip'yov, work), and a statement of many of its properties, were communicated to the Royal Society by the discoverers in January 1895.
For this purpose a small Leyden jar is connected as usual to the secondary terminals, and if neces sary the force of the discharge is moderated by the insertion of resistance in the primary circuit.
When with a fairly wide slit the yellow line is no longer visible, the residual nitrogen may be con sidered to have fallen below 2 or 3%. During this stage the oxygen should be in considerable ex cess. When the yellow line of nitrogen has disappeared, and no further contraction seems to be in progress, the oxygen may be removed by cautious introduction of hydrogen.
The development of Cavendish's method upon a large scale involves arrangements different from what would at first be ex pected. The transformer working from a public supply should give about 6,000 volts on open circuit, although when the electric flame is established the voltage on the platinums is only from 1,60o to 2,000. No sufficient advantage is attained by raising the pressure of the gases above atmosphere, but a capacious vessel is necessary. This may consist of a glass sphere of so litres' capac ity, into the neck of which, presented downwards, the necessary tubes are fitted. The whole of the interior surface is washed with a fountain of alkali, kept in circulation by means of a small cen trifugal pump. In this apparatus, and with about one horse-power utilized at the transformer, the absorption of gas is 21 litres per hour. ("The Oxidation of Nitrogen Gas," Trans. Chem. Soc., 1897.) In one experiment, ,specially undertaken for the sake of measurement, the total air employed was 9,25oc.c., and the oxy gen consumed, manipulated with the aid of partially deaerated water, amounted to io,82oc.c. The oxygen contained in the air would be 1,942c.c.; so that the quantities of atmospheric nitro gen and of total oxygen which enter into combination would be 7,3o8c.c. and r 2,762c.c. respectively. This corresponds to N+1•750, the oxygen being decidedly in excess of the proportion required to form nitrous acid. The argon ultimately found was 75•oc.c., or a little more than 1% of the atmospheric nitrogen used.
The other method by which nitrogen may be absorbed on a considerable scale is by the aid of magnesium. The metal in the form of thin turnings is charged into hard glass or iron tubes heated to a full red in a combustion furnace. Into this air, pre viously deprived of oxygen by red-hot copper and thoroughly dried, is led in a continuous stream. At this temperature the nitrogen combines with the magnesium, and thus the argon is concentrated.
It is not, in general, practicable to get rid of the last traces of nitrogen by these methods. For some purposes, however, e.g., the use in incandescent lamps, this is not necessary, and a few units % of nitrogen may, without serious detriment, be allowed to re main. On the other hand, for experiments on the physical proper ties of argon, and on its behaviour under the electric discharge, high purity is necessary. On a small scale this is conveniently attained by purification with heated turnings of metallic calcium, which absorb nitrogen far more readily than the magnesium originally used, and have the advantage of absorbing a moderate quantity of other likely impurities as well. For larger-scale puri fication, heated calcium carbide may be used as an absorbent of the residual nitrogen, but, though cheaper, it is not so convenient in use as calcium.
A demonstration experiment may here be described by which the presence of argon in atmospheric air can be shown very quickly and simply. A vacuum discharge tube (fig. 2) is used consisting of two elongated electrode bulbs united by a capillary tube of, say, 1.5mm. internal diameter. The bulbs each contain a pool of the liquid alloy of sodium and potassium, introduced with a pipette before the glass work is sealed together. This tube is excited by an induction coil, the current being passed during continued exhaus tion by a high-va cuum purfip until the alloy ceases to give off hy drogen. A small dose of air, say o•rc.c., is then introduced by means of the small space included between the stop-cocks. The alloy is, of course, avid of oxygen, but it also rapidly removes atmospheric nitrogen under the influence of the discharge, and the well-known band spectrum of the latter disappears, giving place to a line spectrum characteristic of argon. A further dose of air is then introduced, and similarly treated. If the tube is initially in the proper condition, enough argon to show the spectrum strongly can be separated in this way in a minute or two.
The spectrum of argon, when isolated, is somewhat complicated, and consists of numerous lines extending over the whole visual spectrum. None of these is of outstanding intensity, and the inte grated effect in a vacuum discharge is of a rather pale red colour, easily distinguished by an accustomed eye from the red of nitro gen, hydrogen or neon. For recognizing the presence of argon the group of green lines: 5,650•7, 5,607.8, 5,495.9, is conveniently made use of. Argon shows very conspicuously the abrupt change from an arc spectrum to a spark spectrum, the light changing from a red to a steely-blue colour when a condensed discharge is used. The blue spectrum is now known to be due to the ionized atom.
Properties.—Argon is soluble in water at 12° C. to about 4.0%, that is, it is about 21 times more soluble than nitrogen. The den sity of argon, prepared and purified by magnesium, was found by Sir William Ramsay to be 19•941 on the 0 = 16 scale. The vol ume actually weighed was 163c.c. Subsequently large-scale oper ations with the same apparatus as had been used for the principal gases gave an almost identical result for argon prepared with oxygen. We should thus expect to find it in increased propor tion in the dissolved gases of rain-water. Experiment has con firmed this anticipation. The weight of a mixture of argon and nitrogen prepared from the dissolved gases showed an excess of 24mg. over the weight of true nitrogen, the corresponding excess for the atmospheric mixture being only I img. Argon is contained in the gases liberated by many thermal springs, but not in special quantity. The gas collected from the King's Spring at Bath gave only 1%, i.e., half the atmospheric proportion. The refractivity of argon is 0•961 times that of air. The viscosity is 1•21 referred to air, somewhat higher than that of oxygen, which stands at the head of the list for the principal gases. The behaviour of argon at low temperatures was investigated by K. S. Olszewski. The fol lowing results are extracted from the table given by him:— Critical Critical Boiling Freezing Name temp. press. point point Nitrogen . • C 35.9 atmos. C C Argon . . —121.0 50•6 —187 —189.6° Oxygen . . —118•o 5o•8 —182.7 The smallness of the interval between the boiling points and freezing points is noteworthy.
From the manner of preparation, it was clear from the first that argon would not combine with magnesium or calcium at a red heat, nor with oxygen, hydrogen or nitrogen under the influence of the electric discharge. Numerous other attempts to induce combination also failed; nor have observations on positive rays given evidence of even temporary association of argon atoms with one another, or with atoms of another element.
The positive-ray investigations of F. W. Aston indicate that argon consists in the main of one isotope, of exact atomic weight 39.971. A small quantity of amounting perhaps to I% of the whole, is present in addition.
The most remarkable physical property of argon relates to the constant known as the ratio of specific heats. When a gas is warmed one degree, the heat which must be supplied depends upon whether the operation is conducted at a constant volume or at a constant pressure, being greater in the latter case. The ratio of specific heats of the principal gases is 1.4, which, according to the kinetic theory, is an indication that an important fraction of the energy absorbed is devoted to rotation or vibration. If the whole energy is translatory, the ratio of specific heats must be 1.67. This is precisely the number found from the velocity of sound in argon as determined by Kundt's method, and it leaves no room for any sensible energy of rotatory or vibrational motion. The same value had previously been found for mercury vapour by Kundt and War burg, and had been regarded as confirmatory of the monatomic character attributed on chemical grounds to the mercury molecule. In the case of argon we have no chemical evidence to go upon to determine the atomicity, but the discoverers regarded it as mon atofnic on the grounds above stated.
Atomicity of Argon.—It was difficult to give any reason on the classical theory of atom mechanics why a single atom should be devoid of rotational energy, so at that time the evidence for the monatomicity of argon did not rest on a very secure founda tion. The view now taken is as follows :—A very small moment of inertia results from the estimated minute dimensions of the atomic nucleus, in which the atomic mass is chiefly concentrated. If the energy of rotation is to be made large enough to be sensible in comparison with the known energy of translation at, e.g., the or dinary temperature, a very rapid rotation of so small a fly-wheel will be needed. But Bohr's frequency condition, which has a weight of varied evidence behind it, only allows a certain limited series of rotational energies at a given frequency ; and, with the rapid rotation indicated, even the lowest of the admissible ener gies is considerably too large to meet the requirements ; so that the alternative of no rotational energy remains. This "explanation" is, of course, incomplete in the same measure as all explanations in terms of the quantum theory.
The conclusion then holds that argon is monatomic. This can be confirmed from other lines of evidence. Thus, the wave-length of the K-absorption edge in the X-ray spectrum of argon fixes the atomic number as 18, since it falls between chlorine (17) and po tassium (I 9) . The atomic weight must, therefore, be about double 18, and this leaves us no doubt that 4o must be taken, which is equal to the molecular weight, rather than 20, which would be half the molecular weight. Thus the atom and the molecule are identi cal. If we consider the periodic law (q.v.), and take the known X-ray spectra of the allied gases krypton and xenon into account, no room for doubt remains, and it is not necessary to enter on yet further evidence which might be cited.
Importance of Argon.—The discovery of argon was the start ing point of many of the recent developments of physics. It led directly to the discovery of the other inert gases, helium (q.v.), neon, krypton and xenon (see ATMOSPHERE), and thus filled up a large lacuna in our systematic knowledge of the fundamental forms of matter. Through the discovery of helium, it led to fur ther far-reaching discoveries. Argon has commercial importance as constituting the atmosphere in "gas filled" incandescent lamps. Its chemical inertness makes it much preferable to nitrogen for this purpose.