108 FOOD OF PLANTS, AND HOW THEY TAKE IT. 
the ground, through the grasses upon which they feed. Man 
*nakes no compensation for this. Whence can it come ? un¬ 
doubtedly from the atmosphere, which is the great medium 
through which it is silently conveyed from the great craters 
of volcanoes that pour out with their molten lavas invisible 
streams also of carbonate of ammonia. 
BoussingaulCs experiments, which were conducted on a 
large scale, confirm these views: he had them continued on 
one spot during twenty-one years; he carefully weighed all 
the manure applied, and all the crops harvested, and the 
quantity of carbon, hydrogen, oxygen, nitrogen, and ashes, of 
both manure and crop, were carefully estimated in the whole 
of both, by means of chemical analysis. Now for the result: 
the annual harvest, on an average, gained twice as much nitro¬ 
gen, three times as much carbon and hydrogen, and four times 
as much oxygen, as had been applied to the soil in manure. 
Here we find the value and the beautv of the science of 
•/ 
chemistry and its application to practice; this depends too 
upon what chemists call the law of proportions, which may 
je thus stated:—Each element or different kind of simple 
natter will combine with others in certain definite proportions; 
thus, carbonic acid, a compound, is always composed of 
oxygen and carbon, in the proportions of 16 by weight of 
the former united with 6 of the latter substance or element. 
Ammonia always contains 3 of hydrogen with 14 nitrogen; 
but, referring to the experiments of Boussingault, and 
believing that carbonic acid, ammonia, and water are the food 
of plants, we find that they contain more oxygen than is 
needed for the plants—more than their tissues contain; 
hence we infer that oxygen must be set free—a fact long ago 
discovered by absolute experiment, and verified by every tyro 
in chemistry: and here we have another beautiful proof of 
the wisdom of the arrangement. This oxygen, necessary for 
animal life, as it is the great supporter of respiration, and 
equally necessary to combustion, is eliminated by plant life, and 
sent to renew and purify the atmosphere by the very plants 
that had first removed the noxious compounds from the air: 
Carbonic acid - 
Oxygen . 
. Carbon . 
. 16 
. 6 
. 2 
. 1 
Water . . * 
'Oxygen . 
Hydrogen 
. 8 
. 1 
. 1 
. 1 
Ammonia . . - 
' Hydrogen 
. Nitrogen 
. 3 
. 14 
o 
. 3 
. 1 
Here we find a grand exchange or interchange of elements 
of matter between the three great kingdoms of nature. (( De¬ 
composition and the process of respiration set free all vegetable 
and animal substances (diminishing the amount of oxygen 
