GEOLOGY OF AGRICULTURE. ^9 



106. Nitric acid (NO'). — This, like carbonic acid, 

 can hardly be said to be a permanent ingredient of 

 the soil, except as it exists in combination with bases 

 forming nitrates. Eain-water is, however, sometimes 

 impregnated with it, particularly in thunder storms. 

 In highly manured soils, it is formed on the surface, 

 by a direct union of its elements, oxygen and nitro- 

 gen. It then combines with bases in the soil, forming 

 nitrates, which may often be seen on the surface, as a 

 kind of white mould. Such an appearance always 

 indicates well for the crops, for the nitrates are easily 

 soluble, and act as stimulants to the growth of plants. 



107. Watei' (HO). — The office of this compound, to 

 furnish the moisture required by plants, is too well 

 known to require to be spoken of here. There is an- 

 other, and most important office of water, which is 

 not so well understood ; it is that of dissolving the 

 foods of plants, and carrying them into the plant in 

 a state of limpid solutions. All the foods of plants 

 enter them, either as invisible gases through the 

 leaves, or in a state of perfectly limpid solutions, 

 through the roots. Now water will dissolve in itself 

 and hold in solution 3^ times its bulk of oxygen, 

 once and a half its bulk of nitrogen, once and a half 

 its bulk of hydrogen, once its bulk of carbonic acid, 

 and many times jts bulk of ammonia. In this way it 

 conveys these and other nutritious gases as food into 

 the plant. Water also dissolves solid substances, 

 some more anS others less, and thus carries them in 

 the form of transparent solutions into the plant, as 



