[25] See p. 22.
[26] See Chapter III., pp. 120 and 131.
[27] Further reference is made to this subject in Chapter III., p. 136.
[28] See p. 6.
[29] See Phil. Trans., Part II., 1861, pp. 444-446. Lawes & Gilbert. Schloesing has found in the air in the neighbourhood of Paris 1 lb. of ammonia in 26,000,000 cubic yards; while Müntz found only about half that amount in a similar quantity of air on the top of the Pic du Midi.
[30] See Chapter III., pp. 119, 120; Appendix, p. 155.
[31] Some recent experiments by Dyer and Smetham would seem to show that comparatively small quantities of ammonia in the air prove actually hurtful to plant-life. Thus they found that one volume of ammonia in 1000 volumes of air was fatal to hardy plants; while one volume in 3000 volumes killed tender ones.
[32] According to the experiments of Hellriegel and Wollny. The quantity, it may be added, varies with the leaf-surface and the length of the period of growth of the plant. It is greatest with clovers and grasses, and least in the potatoes and roots.