REVIEW OF THE PRINCIPLES OF NUTRITION. 107 



312. Besides these four universal elements, many other sub- 

 stances, earthy and mineral, are found in quantities greater or 

 less in different species: thus forest trees and most other inland 

 plants contain potassa ; marine plants, soda, iodine, &c. ; the 

 grasses, silex and phosphate of lime ; rhubarb and sorrel, oxalate 

 of lime ; the Leguminosae, carbonate of lime. Now all these 

 ingredients, being found in plants, are inferred to be essential 

 elements in the food which they require for healthy vegetation ; 

 and an inquiry into the sources from which they may be supplied, 

 constitutes the chief object of Agricultural Chemistry, 



313. It is evident that plants do not create a particle of matter, 

 and therefore do not originate in themselves any of the ingre- 

 dients which compose them; consequently they must obtain 

 them from sources without. These sources are obviously air, 

 earth, and loater. Carbon is derived from the carbonic acid 

 which the atmosphere contains, and from the decaying vegetable 

 matter of the soil. Oxygen is derived from the water, and from 

 the carbonic acid of the atmosphere; hydrogen, from water and 

 ammonia ; and nitrogen, from ammonia alone, either drawn from 

 the air or the soil. 



314. The ATMOSPHERE contains about ^^^jVo part of carbonic 

 acid, diffused throughout the whole extent ; and, as this gas con- 

 tains 27 per cent, of carbon, it may be demonstrated, that the 

 whole atmosphere contains at least fourteen hundred billions of 

 tons of solid carbon, derived from the sources mentioned in 

 § 282, — an amount fully adequate to the vast and ceaseless drain 

 made upon it by the vegetable kingdom. 



315. Soil consists of two classes of materials ; viz. mineral 

 and organic. The former, called earths, consists of disintegrated 

 and decomposed rocks, — all the various mineral substances 

 which are found to enter into the composition of plants, as 

 potassa, soda, silica, lime, &c., all of which are more or less 

 soluble in water. The organic materials consist of the remains 

 of former tribes of plants and animals, mingled with the earths, 

 which, having access to air, are decomposed, evolving carbonic 

 acid and ammonia both to the air and the water. 



31 G. Water is composed of oxygen and hydrogen, in the pro- 

 portion of 8 to 1 by weight. Having pervaded the atmosjOn-re 

 10 



