SOURCE OF THEIR ORGANIC CONSTITUENTS. 185 



ingredients, in such a form that they may be detained when the 

 superfluous water is exhaled, this might furnish the whole organic 

 food of the vegetable ; since the plant may condense and accumu- 

 late the carbon and nitrogen, just as the extremely minute quantity 

 of earthy matter which the water contains is in time largely ac- 

 cumulated in the leaves and wood. 



326. As respects the nitrogen, nearly seventy-nine per cent, of 

 the atmosphere consists of this gas in an uncombined or free state, 

 that is, merely mingled with oxygen. And, being soluble to some 

 extent in water, every rain-drop that falls through the air absorbs 

 and brings to the ground a minute quantity of it, which is therefore 

 necessarily introduced into the plant with the water which the roots 

 imbibe. This accounts for the free nitrogen which is always pres- 

 ent in plants. 



327. The plant also receives, probably, a larger portion of its 

 nitrogen in the form of ammonia (or hartshorn), a "compound of 

 hydrogen and nitrogen, which is always produced when any ani- 

 mal and almost any vegetable substance decays, and which, being 

 very volatile, must continually rise into the air from these and 

 other sources. Besides, it appears to be formed in the atmosphere, 

 through electrical action in thunderstorms (in the form of nitrate of 

 ammonia). The extreme solubility of ammonia and all its com- 

 pounds prevents its accumulation in the atmosphere, from which it 

 is greedily absorbed by aqueous vapor, and brought down to the 

 ground by rain. That the roots actually absorb it may be inferred 

 from the familiar facts, that plants grow most luxuriantly when the 

 soil is supplied with substances which yield much ammonia, such 

 as animal manures ; and that ammonia may be detected in the 

 juices of almost all plants. Rain-water, therefore, contains the 

 third element of vegetation, namely, nitrogen, both in a separate 

 form and in that of ammonia. 



328. The source of the remaining constituent, carbon, is still to 

 be sought. Of this element plants must require a copious supply, 

 since it forms the largest portion of their bulk. If the carbon of a 

 leaf or of a piece of wood be obtained separate from the other or- 

 ganic elements, which may be done by charring, that is, by heat- 

 ing it out of contact with the air, so as to drive off the oxygen, hy- 

 drogen, and carbon, although a small part of the carbon is ne- 

 cessarily lost in the operation, yet what remains perfectly preserves 

 the shape and bulk of the original body, even to that of its most 



16* 



