INFLUENCE OF VEGETATION ON THE ATMOSPHERE. 205 



multiplying this vast amount of carbon by sixteen, and dividing it 

 by six, we obtain an expression of the number of pounds of oxy- 

 gen gas that have in this process been supplied to the atmosphere. 

 360. Rightly to understand the object and consequences of this 

 immense operation, which has been going on ever since vegetation 

 began, it should be noted, that, so far as we know, vegetation is 

 the only operation in nature which gives to the air free oxygen gas, 

 that indispensable requisite to animal life. There is no other pro- 

 vision for maintaining the supply. The prevailing chemical ten- 

 dencies, on the contrary, take oxygen from the air. Few of the 

 materials of the earth's crust are saturated with it ; some of them 

 still absorb a portion from the air in the changes they undergo ; 

 and none of them give it back in the free state in which they took 

 it, in a state to support animal life, by any known natural 

 process, at least upon any considerable scale. Animals all con- 

 sume oxygen at every moment of their life, giving to the air car- 

 bonic acid in its room ; and when dead, their decaying bodies con- 

 sume still more. Decomposing vegetable matter produces the 

 same result. Its carbon, taking oxygen from the air, is likewise 

 restored in the form of carbonic acid. Combustion, as in burning 

 our fuel, amounts to precisely the same thing ; it is merely rapid 

 decay. The carbon which the trees of the forest have gathered 

 from the air in the course of centuries, their prostrate decaying 

 trunks may almost as slowly restore to the air, in the original form 

 of carbonic acid. But if set on fire, the same result may be accom- 

 plished in a day. All these causes conspire to rob the air of its 

 life-sustaining oxygen. The original supply is indeed so vast, that, 

 were there no natural compensation, centuries upon centuries 

 would elapse before the amount of oxygen could be so much re- 

 duced, or that of carbonic acid increased, as to affect the existence 

 of the present races of animals. But such a period would eventu- 

 ally arrive, were there no natural provision for the decomposition 

 of the carbonic acid constantly poured into the air from these va- 

 rious sources, and for the restoration of its oxygen. We have 

 seen that vegetation accomplishes this very result. The needful 

 compensation is therefore found in the vegetable kingdom. While 

 animals consume the oxygen of the air, and give back carbonic 

 acid which is injurious to their life, this carbonic acid is the prin- 

 cipal element of the food of vegetables, is consumed and decom- 

 posed by them, and its oxygen restored for the use of animals. 

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