[16] Under the name of atmospheric air the chemist and physicist understand ordinary air containing nitrogen and oxygen only, notwithstanding that the other component parts of air have a very important influence on the living matter of the earth's surface. That air is so represented in science is based on the fact that only the two components above-named are met with in air in a constant quantity, whilst the others are variable. The solid impurities may be separated from air required for chemical or physical research by simple filtration through a long layer of cotton-wool placed in a tube. Organic impurities are removed by passing the air through a solution of potassium permanganate. The carbonic anhydride contained in air is absorbed by alkalis—best of all, soda-lime, which in a dry state in porous lumps absorbs it with exceeding rapidity and completeness. Aqueous vapour is removed by passing the air over calcium chloride, strong sulphuric acid, or phosphoric anhydride. Air thus purified is accepted as containing only nitrogen and oxygen, although in reality it still contains a certain quantity of hydrogen and hydrocarbons, from which it may be purified by passing over copper oxide heated to redness. The copper oxide then oxidises the hydrogen and hydrocarbons—it burns them, forming water and carbonic anhydride, which may be removed as above described. When it is said that in the determination of the density of gases the weight of air is taken as unity, it is understood to be such air, containing only nitrogen and oxygen.
[16 bis] Thanks to the remarkable discovery made in the summer of 1894 by Lord Rayleigh and Prof. Ramsay, the well-known component elements of air must now he supplemented by 1 p.c. (by volume) of a heavy gas (density about 19, H = 1), inactive like nitrogen, which was discovered in the researches made by Lord Rayleigh on the density of nitrogen as mentioned in note [4 bis]. Up to the present time this gas has been always determined together with nitrogen, because it combines with neither the hydrogen in the eudiometer nor with the copper in the gravimetric method of determining the composition of air, and therefore has always remained with the nitrogen. It has been possible to separate it from nitrogen since magnesium absorbs nitrogen at a red heat, while this gas remains unabsorbed, and was found to have a density nearly one and a half time greater than that of nitrogen (is it not a polymer of nitrogen, N3?). It is now known also that this gas gives a luminous spectrum, which contains the bright blue line observed in the spectrum of nitrogen. Owing to the fact that it is an exceedingly inert substance, even more so than nitrogen, it has been termed Argon. Further reference will be made to it in the Appendix.
[17] As a further proof of the fact that certain circumstances may change the composition of air, it will be enough to point out that the air contained in the cavities of glaciers contains only up to 10 p.c. of oxygen. This depends on the fact that at low temperatures oxygen is much more soluble in snow-water and snow than nitrogen. When shaken up with water the composition of air should change, because the water dissolves unequal quantities of oxygen and nitrogen. We have already seen (Chapter [I].) that the air boiled off from water saturated at about 0° contains about thirty-five volumes of oxygen and sixty-five volumes of nitrogen, and we have considered the reason of this.
[18] The analysis of air by weight conducted by Dumas and Boussingault in Paris, which they repeated many times between April 27 and September 22, 1841, under various conditions of weather, showed that the amount by weight of oxygen only varies between 22·89 p.c. and 23·08 p.c., the average amount being 23·07 p.c. Brunner, at Bern in Switzerland, and Bravais, at Faulhorn in the Bernese Alps, at a height of two kilometres above the level of the sea, Marignac at Geneva, Lewy at Copenhagen, and Stas at Brussels, have analysed the air by the same methods, and found that its composition does not exceed the limits determined for Paris. The most recent determinations (with an accuracy of ±0·05 p.c.) confirm the conclusion that the composition of the atmosphere is constant.
As there are some grounds (which will be mentioned shortly) for considering that the composition of the air at great altitudes is slightly different from that at attainable heights—namely, that it is richer in the lighter nitrogen—several fragmentary observations made at Munich (Jolly, 1880) gave reason for thinking that in the upward currents (that is in the region of minimum barometric pressure or at the centres of meteorological cyclones) the air is richer in oxygen than in the descending currents of air (in the regions of anticyclones or of barometric maxima); but more carefully conducted observations showed this supposition to be incorrect. Improved methods for the analysis of air have shown that certain slight variations in its composition do actually occur, but in the first place they depend on incidental local influences (on the passage of the air over mountains and large surfaces of water, regions of forest and herbage, and the like), and in the second place are limited to quantities which are scarcely distinguishable from possible errors in the analyses. The researches made by Kreisler in Germany (1885) are particularly convincing.
The considerations which lead to the supposition that the atmosphere at great altitudes contains less oxygen than at the surface of the earth are based on the law of partial pressures (Chapter [I].) According to this law, the equilibrium of the oxygen in the strata of the atmosphere is not dependent on the equilibrium of the nitrogen, and the variation in the densities of both gases with the height is determined by the pressure of each gas separately. Details of the calculations and considerations here involved are contained in my work On Barometric Levellings, 1876, p. 48.
On the basis of the law of partial pressure and of hypsometrical formulæ, expressing the laws of the variation of pressures at different altitudes, the conclusion may be deduced that at the upper strata of the atmosphere the proportion of the nitrogen with respect to the oxygen increases, but the increase will not exceed a fraction per cent., even at altitudes of four and a half to six miles, the greatest height within the reach of men either by climbing mountains or by means of balloons. This conclusion is confirmed by the analyses of air collected by Welch in England during his aëronautic ascents.
[19] The complete absorption of the oxygen may be attained by introducing moist phosphorus into a definite volume of air; the occurrence of this is recognised by the fact of the phosphorus becoming non-luminous in the dark. The amount of oxygen may be determined by measuring the volume of nitrogen remaining. This method however cannot give accurate results, owing to a portion of the air being dissolved in the water, to the combination of some of the nitrogen with oxygen and to the necessity of introducing and withdrawing the phosphorus, which cannot be accomplished without introducing bubbles of air.
[20] For rapid and approximate analyses (technical and hygienic), such a mixture is very suitable for determining the amount of oxygen in mixtures of gases from which the substances absorbed by alkalis have first been removed. According to certain observers, this mixture evolves a certain (small) quantity of carbonic oxide after absorbing oxygen.
[21] Details of eudiometrical analysis must, as was pointed out in Chap. III., Note [32], be looked for in works on analytical chemistry. The same remark applies to the other analytical methods mentioned in this work. They are only described for the purpose of showing the diversity of the methods of chemical research.