FOOTNOTES:
[63] The total amount of nitrogen in the air has been estimated approximately at four million billion tons.
[64] See Introductory Chapter, pp. 40 to 45.
[65] Although ammonia is more abundant than nitrates and nitrites, it only amounts to a few parts per million of air. According to Müntz, the air at great heights contains more ammonia than in its lower strata. The opposite, however, is the case with regard to nitrates, which are only found in air near the surface of the earth. See p. 49.
[66] Nitric acid may also be formed by the oxidation of ammonia by ozone, or peroxide of hydrogen.
[67] According to Schloesing, the chief source of the ammonia present in the air is the tropical ocean, which yields gradually to the atmosphere, under the action of the powerful evaporation constantly going on, a large amount of nitrogen in this form. The sources of the nitrogen of the ocean are the nitrates which it receives from the drainage of land, animal and vegetable matter, sewage, &c.
[68] See Appendix, Note I., p. 155.
[69] To illustrate this point, it may be mentioned that on the least windy of days, when the wind is only moving at the rate of two miles an hour—and this, it may he added, is so slow as to be scarcely noticeable—the air in a space of 20 feet is changed over five hundred times in an hour. The combined nitrogen thus absorbed is probably entirely in the form of ammonia. It would seem so at any rate, from some experiments by Schloesing. See p. 132.
[70] No vegetable or animal cell exists which does not contain nitrogen.
[71] This is less on the whole than what has been found in subsoils by Continental investigators. Thus, for example, A. Müller found the average of a number of analyses of subsoils to be .15 per cent., and the late Dr Anderson found the nitrogen in the subsoil of different Scottish wheat-soils to run from .15 per cent to .97 per cent.