[CHAPTER V]
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).