CHAPTER V. The Discovery of Argon

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, 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.

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.

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.

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.

Similar experiments have been made by Dr. Bouchard in Paris 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.

Proc. Roy. Soc. vol. liii. p. 134.

Proc. Roy. Soc. vol. lix. p. 68. Phil. Trans. vol. clxxxvi. p. 227.

Compt. rend. vol. cxxi. p. 394.