THE ATMOSPHERE, 



37 



produce in a similar manner the gas known as car- 

 bonic acid, or carbon dioxide. This gas is poisonous 

 to animals when it exists in any considerable quan- 

 tity in the air, but is absolutely essential to the 

 growth of plants. 



75. Carbon dioxide consists of carbon and oxy- 

 gen, and is generated in all cases of ordinary com- 

 bustion, in putrefaction, fermentation, and decay. 

 It is also a product of respiration, and would soon 

 accumulate in sufficient quantity to destroy animal 

 life, were it not for the fact that it is absorbed by the 

 leaves of trees and plants, there deprived of its car- 

 bon, and pure oxygen restored to the air whence it 

 came (28). 



76. Every one has observed the soft, porous tex- 

 ture of the under surface of the leaves of plants; 

 through these pores the carbon dioxide is inhaled as 

 it floats in the atmosphere, and under the influence 

 of sunlight the process of digestion takes place with- 

 in the plant whereby the carbon is retained as food, 

 and the oxygen exhaled, thus preserving the purity 

 of the air. 



77. The relative amount of carbon dioxide in the 

 atmosphere is only about one twenty-fifth of one per 

 cent., and yet it is enough for the purposes of vegeta- 

 tion. A distinguished chemist says, " The gigantic 

 trees which adorn the forests of tropical regions with 

 the dense pine woods of more northern zones, and 

 the abundant though less conspicuous vegetation 

 of temperate climes, all derive their stock of carbon 



