Popular Science Monthly/Volume 47/August 1895/Argon
THE NEW CONSTITUENT OF THE AIR.
By Dr. JOHN TAPPAN STODDARD,
PROFESSOR OF CHEMISTRY IN SMITH COLLEGE.
ON the 31st of January last the Royal Society of England held a special meeting in Burlington Gardens. Formal invitation to this meeting had been extended to the members of two other scientific bodies, and an audience of at least eight hundred, which included the most distinguished scientific men of England, assembled to listen to the account of the discovery of a new substance in our atmosphere. This discovery, made by Lord Rayleigh and Prof. Ramsay, had been announced at the Oxford meeting of the British Association last August; but five months of patient and strenuous work proved necessary before the investigators felt prepared to publish the detailed results of their research.
Our atmosphere consists essentially of a mixture of oxygen and nitrogen. To the oxygen it owes its power of supporting respiration and combustion; while the nitrogen, inert and incapable of chemical union under ordinary conditions, acts as a diluent, tempering the fierceness of the chemical activity which un-mixed oxygen possesses. Both of these gases were discovered more than one hundred and twenty years ago; they have long been recognized as elementary substances, and innumerable analyses have established the proportion in which they occur in air.
When a measured quantity of air, carefully freed from the moisture and carbon dioxide which it always contains, is passed through a tube filled with red-hot copper, the oxygen is fixed by the copper, and the residual gas, amounting to four fifths of the original volume, is found to be incapable of supporting combustion. It is, in fact, what all chemists have considered, up to the time of this brilliant discovery, pure nitrogen.
It is now proved beyond all possible doubt or question that this atmospheric nitrogen is not a single substance, but contains, mixed with it to the amount of about one per cent, another heavier gas, whose existence was previously unknown and unsuspected. To this new substance, which out-nitrogens nitrogen in its chemical inertness, its discoverers give the name of argon
Besides its occurrence in the free state in air, nitrogen is found in combination in animal and vegetable substances, in saltpeter or niter (from which its name is derived), and is a constituent of many chemical compounds, from some of which it can readily be prepared. The identification of atmospheric nitrogen with that contained in niter and nitric acid is due to Henry Cavendish, whose exact and skillful work not only established this fact, but led to an observation of great interest in connection with the discovery of argon. In a paper which appeared in 1785 Cavendish says: "As far as experiments hitherto published extend, we scarcely know more of the phlogisticated part of our atmosphere [nitrogen] 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 animals; and that its specific gravity is not much less than that of common air"; and raises the question "whether there are not in reality many different substances compounded by us under the name of phlogisticated air." He then describes an experiment for the purpose of deciding this point. By passage of electric sparks through a mixture of air and oxygen, the nitrogen was converted into a compound absorbed by the dilute alkali over which the gases were confined. The sparking was continued until no further diminution of volume took place, when, on removing the excess of oxygen by absorption in "liver of sulphur" "only a small bubble of air remained unabsorbed." From this he concludes that "if there is any part of the phlogisticated air of our atmosphere which differs from the rest, and can not be reduced to nitrous acid, we may safely conclude that it is not more than a hundred and twentieth part of the whole." Cavendish was apparently satisfied with this as a proof of the simple character of atmospheric nitrogen, and his work has been accepted as conclusive for more than a century; but we now know that this "small bubble of air" which survived his experiment must have been argon.
It seems strange that a substance present in the air all about us, and whose actual quantity is enormous, should have defied detection through so many years of exact and searching chemical work; but the explanation lies largely in the fact that argon forms no compounds, so far as is known, and thus fails to assert itself in the presence of the almost equally indifferent nitrogen with which it is mixed.
Indeed, the hint which led to its discovery was obtained in the course of a purely physical investigation. For some years Lord Rayleigh has been engaged in the exact determination of the densities of some of the more permanent gases. In dealing with nitrogen, it was found that this gas, when prepared from chemical compounds, was about one half per cent lighter than the nitrogen obtained from air. This discrepancy at once suggested contamination with some known impurities. A careful search proved, however, that this was not the case. The possible explanation then occurred, that the lightness of the "chemical" nitrogen was due to a partial dissociation or breaking up of the molecules of the gas into single atoms under the conditions of its preparation. This, too, was negatived by experiment. One or the other of the gases must be a mixture, containing an ingredient much heavier or much lighter than ordinary nitrogen. To suppose a lighter ingredient mixed with the chemical nitrogen required the existence of two kinds of nitric acid, which was out of the question. "The simplest explanation in many respects was to admit the existence of a second ingredient in air from which oxygen, moisture, and carbon dioxide had already been removed."
This explanation was put to the test by an attempt to isolate the suspected gas, with the result that by two entirely distinct methods a new substance was obtained from air.
One of these methods was that of Cavendish, already described. Air confined over dilute alkali is subjected to the action of electric sparks, while oxygen is added from time to time until, with an excess of oxygen present, no further absorption occurs. The oxygen is then removed by alkaline pyrogallate, and argon is left.
The second method for the separation of argon is based on the fact that red-hot magnesium unites with nitrogen, forming a nonvolatile compound. Air from which moisture and carbon dioxide have been removed is freed from oxygen by passing it over heated copper, and then from nitrogen by means of magnesium turnings at a red heat. The removal of the last portions of nitrogen is a tedious operation, requiring some two days. The residual gas is pure argon.
The gas obtained by both of these methods is the same, and its behavior proves conclusively that it is a new substance. Prof. Crookes finds that it gives two spectra, according to the strength of the induction current, one characterized by red and the other by blue lines; and testifies that he has "found no other spectrum-giving gas or vapor which yields spectra at all like those of argon"; and that "as far, therefore, as spectrum work can decide, the verdict must, I think, be that Lord Rayleigh and Prof. Ramsay have added one, if not two, members to the family of elementary bodies."
The behavior of argon at low temperatures and under high pressures has been examined by Prof. Olszewski, of Cracow, who is well known for his researches on the liquefaction of air and other gases, its critical temperature—that is, the temperature at which its liquefaction under pressure first becomes possible—is —121º C., and at that point it is condensed to a liquid by a pressure of 50·6 atmospheres. Liquid argon becomes an icelike solid at a still lower temperature, melts at —189·6º, and boils under ordinary pressure at —187º. Its critical and boiling points lie between those of oxygen and nitrogen, nitrogen having the lowest of the three.
Argon is four tenths heavier than nitrogen, and much more soluble in water. As already stated, and as is evident from the methods employed for its preparation, argon is more inert than nitrogen; so great is its chemical indifference, that all attempts to bring about reactions with even the most active substances at high temperatures have thus far proved abortive. It is unaffected by phosphorus or sulphur at red heat; sodium and potassium may be distilled in it without loss of their metallic luster; it is unaltered by fused and red-hot caustic soda or niter; aqua regia and other wet oxidizing and chlorinating agents are entirely without action; and it resists the attack of nascent silicon and boron.
Though thus unique in its chemical inactivity, it would be premature to conclude that argon may not form compounds under conditions yet untried, and that it is an absolutely "idle" and useless thing. Prof. Roberts-Austen suggests that it may possibly play a part in certain metallurgical operations in which air is largely employed. In making Bessemer steel, for instance, not less than one hundred thousand cubic feet of air are blown through each charge of metal for the purpose of removing the carbon, silicon, phosphorus, and other impurities. In this air there must be over one thousand cubic feet of argon. Now, Prof. Roberts-Austen has found by experiment that the nitrogen which can be pumped out of Bessemer-blown metal, and which is twice the volume of the metal, contains little or no argon; and the question arises, whether the argon may not have united with the iron, as nitrogen undoubtedly does, and confer upon Bessemer steel some of the peculiarities which distinguish it from other steel. It is, of course, possible and perhaps more likely that the argon passes through the molten metal without combining with it; but the suggestion is an interesting one, and well worth experimental examination.
Further, it may prove that argon is in some way taken up by plants, and contributes in an important manner to their nourishment and growth; although the attempts to extract argon from vegetable and animal substances have thus far yielded only negative results. As is well known, plants are unable to take nitrogen directly from the air, but obtain it from nitrogenous compounds which are absorbed in solution by their roots. The air is, however, the original source of these compounds, as well as of all other naturally occurring nitrogenous substances, most of which are produced by the life-activity of micro-organisms; and from the natural substances all chemical compounds containing nitrogen are prepared. Considering, therefore, the identity of the source, it seems improbable that the nitrogen of plants or animals should contain argon, while that of inorganic chemical compounds is without it. It is, however, possible that argon may enter the plant in a manner quite different from nitrogen; for it does not follow that, because it is associated with nitrogen in the air, argon must always play the part of an inseparable companion.
Is argon an element, a mixture of elements, or a compound? While the evidence that it is a new substance is indisputable, the facts thus far obtained do not warrant a final decision in regard to its simplicity. There is no reason, however, to believe that it is a compound, but, on the contrary, there is a piece of most conclusive evidence against this view. This evidence is the ratio of its specific heats at constant pressure and at constant volume. This has been carefully determined, and is found to be in exact agreement with the value required by the mechanical theory of heat for a monatomic gas—that is, a gas whose molecules consist of a single atom each. Such a state of things is obviously impossible for a compound, which must have two atoms, at least, in every molecule. It is also unusual in elementary gases, whose molecules are in most cases diatomic, or of two atoms each. Argon is therefore either an element or a mixture of elements having structureless molecules. This evidence throws out of court also the view, which has been repeatedly urged since the first announcement of the discovery, that argon is an allotropic form of nitrogen, consisting of triatomic nitrogen, and analogous to ozone, which is triatomic oxygen.
As to the question whether it is a single element or a mixture, the argument for the mixture is based on the fact that it gives two spectra. Though suggestive, this can not be looked on as conclusive, for certain well-known elements—hydrogen and nitrogen—show the same peculiarity. On the other hand, a definite melting point, a definite boiling point, a definite critical temperature and pressure, all of which argon possesses, are generally accepted criteria of a pure substance. The evidence, therefore, is largely in favor of the simple elementary character of argon.
If subsequent investigation confirms this view, and argon proves to be a single monatomic element, a question of great interest is raised. For many years an accepted law of chemistry has been expressed in the so-called periodic classification of the elements. When the elements are arranged in the order of their atomic weights, the series may be broken into a number of well-defined periods, whose members show marked analogies to the corresponding members of the other periods, and a regular gradation of properties among themselves; or, in other words, "the properties of the elements and of their compounds are a periodic function of their atomic weights." The exigencies of classification, so that the elements of different periods may fall into their proper places in the tabulated scheme, have left many gaps in the table, which may represent elements yet awaiting discovery. In fact, three such elements have been discovered since the first formulation of the periodic law by Mendeleeff, and found to agree very exactly with the prediction made several years previously by Mendeleeff for the properties of elements which might be expected to fill certain gaps.
Now argon, if it is a monatomic element, must have an atomic weight of about forty. There is, however, not only no vacant place in the table for an element of this atomic weight, but the properties of the elements occupying this region are wholly unlike those of argon. Thus for the first time in its history the periodic law would fail in its hitherto triumphant provision for the results of discovery.
A law which expresses so much undoubted truth, and which has proved of such great service in the past, is not, however, to be at once discredited because it seems not to provide for this case. So great is the confidence felt in it, that many chemists consider its apparent failure in this case a conclusive argument against the monatomic character of argon. It must be remembered, however, that the evidence for monatomicity is founded on a deduction from the thoroughly established mechanical theory of heat; while the periodic law is, after all, as Prof. Rücker says, "an empirical law, which rests on no dynamical foundation," for which no adequate theory has yet been found. More evidence is needed in the case, and more will probably soon be forthcoming. Meantime the present situation will strengthen the feeling, by no means new, that, while the periodic law is a grand generalization containing much that is true, it is certainly not a complete or final expression of the relations which exist between the properties of the elements and their atomic weights, but rather a first approximation to the law which may ultimately be formulated.
Whatever the outcome of these speculative issues, there can be but one opinion in regard to the discovery itself. From every point of view it is a masterly achievement. The elements of recent discovery have all been metals which occur in minute quantities in rare minerals. No nonmetallic element has been discovered for nearly seventy years, and the existence of another element belonging to this group did not seem probable; still less likely did it appear that such an element could be present in our atmosphere. The discovery has well been called "the triumph of the last place of decimals"—that is, of work so exact that the worker knew that the small differences in the figures he obtained must be due to the presence of an unknown substance rather than to an error in his results. The prediction based on this observation, the search for the disturbing substance, and its discovery, form an achievement which, in the history of science, has perhaps only been surpassed by the prediction of Neptune by Adams and Leverrier, and its subsequent discovery by Galle.