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 Leguminose, 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 water. 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 y455 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. Som 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, &e., 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. 
316. WATER is composed of oxygen and hydrogen, in the pro- 
portion of 8 to 1 by weight. Having pervaded the atmosphere 
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