CARBONIC ACID OF THE ATMOSPHERE. 87 



quantity is so great that a distinguished chemist (Liebig) has 

 advocated the opinion, that plants derive their carbon wholly 

 from the carbonic acid of the atmosphere.* 



The quantity of acid thus absorbed and decomposed varies 

 greatly in different plants, even when placed under the same 

 circumstances. Saussure has proved that the portion depends 

 upon the surface ; hence, those plants, which have thin leaves, 

 absorb more than the Jleshif-leaved plants. The same is true, 

 as we have seen, with regard to their power of absorbing oxy- 

 gen gas. 



Plants require different quantities of carbonic acid at dif- 

 ferent periods of their growth. The young plant requires but 

 little, because its leaves are not sufficiently vigorous to absorb, 

 and decompose it. The quantity required increases with the 

 size of the leaves ; hence, the greatest quantity is required 

 when the leaves have obtained a mature growth, which peri- 

 od is near the middle of summer in most plants, and it is at 

 this period that the greatest quantity is furnished by the fer- 

 mentation of manures, and vegetable substances in the soil. 



Carbonic acid has been considered prejudicial to the ri" 

 pening of grain, because its presence, in large quantities, 

 stimulates the leaves, and increases their bulk at the expense 

 of the grain. 



As plants during the summer season absorb the carbonic 

 acid of the atmosphere, it would seem that during the winter 

 season a much larger quantity would be found in the air, than 

 in the summer ; particularly as larger quantities are produ- 

 ced by combustion, and smaller quantities brought to the earth 

 by rain. This would be the case were it not for the fact that 

 in the tropics there is always a vigorous vegetation ; the air 

 is constantly circulating from the tropics towards the poles, 

 and the reverse. By this constant motion the equilibrium 

 is maintained, and there actually is a larger quantity of acid 

 in the air in the summer than in the winter, because the 



* See third chapter. 



