200 THK rOOD AND NUTRITION OF PLANTS. 



forests and herbage of the world, and add to the estimate all that 

 exists in the soil, as vegetable mould, peat, and in other forms ; all 

 that is locked up in the vast deposits of coal (the product of the 

 vegetation of bygone ages) ; and, finally, all that pertains to the 

 whole existent animal kingdom ; — and we shall have the aggregate 

 amount of a single, though the largest, element which vegetation has 

 withdrawn from the atmosphere. By multiplying this vast amount 

 of carbon by sixteen, and dividing it by six, we obtain an expression 

 of the number of pounds of oxygen 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 carbonic 

 acid in its room ; and when dead, their bodies consume a further por- 

 tion in decomposition. 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 been for 

 centuries gathering from the air, 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 reduced, or 

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

 present races of animals. But such a period would eventually 

 arrive, were there no natural provision for the decomposition of the 

 carbonic acid constantly poured into the air from these various 



