CHEMISTRY IN AGRICULTURE AND PHYSIOLOGY. 287 
made up of four substances, viz. silica (sand), alumina (clay) 
lime, and magnesia. 
Plants require for their nutrition or support but fourteen 
elements; these are, carbon, oxygen, hydrogen, nitrogen, 
potash, soda, lime, magnesia, phosphorus, sulphur, silica, iron, 
chlorine, and iodine ; these go to make up all the vegetables 
and all the animals found upon the face of the earth. 
The carbon, oxygen, nitrogen, and hydrogen are supplied 
to plants by the atmosphere, the other elements are obtained 
from the soil. These four elements, by some extraordinary 
power of vital alchemy, form no less than ninety-four per 
cent, of the whole vegetable kingdom; for example, 100 
pounds of wheat, if burnt, yield only two pounds of ashes or 
inorganic matter, the ninety-eight pounds which disappear 
into the atmosphere are called organic matter. 
The chemist, by proximate analysis, ascertains that the 
grain of wheat contains, in 100 pounds, fifty-five of starch, 
fifteen of albuminous matters, fifteen of woody fibre, thirteen of 
water, and two pounds of ashes. By ultimate analysis, that 
starch and woody fibre are composed of carbon, oxygen, and 
hydrogen ; and albumen, of the same elements with nitrogen. 
When wheat or any other grain is burnt, the carbon unites 
with oxygen from the atmosphere and form carbonic acid, a 
portion of the hydrogen combines with nitrogen, and forms 
ammonia, and the remaining hydrogen with oxygen forms 
water, and all pass into the atmosphere. 
If the vegetables are consumed as food by an animal, the 
carbon and hydrogen, by a process of slow combustion (oxi¬ 
dation),support the heat of the body and are eventually expired 
by the lungs and skin as carbonic acid and watery vapour; 
the nitrogen, after entering into the tissues of the body, is 
excreted as urea by the kidneys. Urea, exposed to atmos¬ 
pheric air, is rapidly converted into ammonia. 
Lastly, if vegetables are collected into a heap, they ferment, 
and carbonic acid, water, and ammonia are found rapidly, 
escaping into the atmosphere. 
Hence it is obvious, that it matters not w r hat intermediate 
changes vegetable substances may undergo, their entire 
organic matters eventually reach the atmosphere as carbonic 
acid, water, and ammonia. 
Now carbonic acid is a poison, and ammoniacal gas is also 
highly destructive to animal life. We naturally ask, if 
immense quantities of these poisonous gases are being con¬ 
stantly thrown into the atmosphere, how does it happen that 
the proportion found there of each is constant ? 
One man consumes by respiration twenty-five cubic feet of 
