REVIEW OF TUi: PRINCIPLES OF NUTRITION. 107 



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

 stanceSj/earthy and mineral, are found in quantities greater or 

 less in different species : tlius 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 Lesuminosa^, carbonate of lime. ;, jS^ow a,ll these 

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

 elements in the food Avhich they require for healthy vegetation ; 

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

 constitutes the chief object of Agricultianl CJtemistry. 



313. It is evident.'tliat 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/'mr, 

 earthy and wafer.') 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 conta!ns(about yJW P^i't of carbonic 

 acid, diffused throughout the whole extent"i\and, as this gas con- 

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

 whole atmosphere I 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 tl>e water.^ 



316. Water is composed-, of oxygen and hydrogcn,")in the pro- 

 portion of 8 to 1 by weight. Having pervaded the" atmosphere 



10 



