88 BIOLOGY OF PLANTS. 



causes of its production are more abundant, and some of 

 them more active. 



There are other causes which tend to abstract the carbon- 

 ic acid from the atmosphere. 



1. As we approach the centre of lakes or sail out upon the 

 sea, ihe carbonic acid of the air gradually diminishes. This 

 we know is due to the fact that the water absorbs it in large 

 quantities, but as it is capable of absorbing, under the ordina- 

 ry pressure, only a quantity equal to its own bulk, we should 

 suppose that it would soon become saturated ; this, however, 

 is not the case, and the water does not, so far as is known, 

 return it again to the atmosphere, but disposes of it in some 

 other way. 



2. The water of rivers is constantly carrying down sub- 

 stances which have derived carbonic acid from the atmos- 

 phere, and it thus becomes fixed in the bottoms of lakes and 

 seas. 



3. In temperate climates, vegetable matter accumulates in 

 the form of peat which permanently fixes large quantities of 

 this acid. 



The presence of carbonic acid is absolutely essential to veg- 

 etation. This fact has been shown by Saussure, who found 

 that when lime was put into a glass vessel containing plants, 

 so that the carbonic acid, both of the air and of the soil, was 

 absorbed, they no longer continued to grow, and the leaves 

 in a few days fell off. On examining the air, it was found 

 to be deprived of carbonic acid ; if, however, the plant is 

 placed in the shade, the presence of lime to absorb the car- 

 bonic acid promotes vegetation ; that is, plants grow better 

 without the acid in the shade than with it. This process, 

 however, cannot be continued long, as the presence of light 

 and acid are required to continue vigorous growth. 



By adding a small quantity of carbonic acid to that which 

 already exists in the atmosphere, vegetation is promoted. 

 When the atmosphere contains ^L- part of carbonic acid. 



