CHEMISTRY. 177 



required to convert this carbon into carbonic acid. 

 The azotic gas of the atmosphere does not appear to 

 exert any positive influence on living bodies. 



Limestone is a combination of carbonic acid and 

 lime. 



The Nourishment of Plants. 



Carbonic acid gas is formed during fermentation, 

 combustion, respiration, and a variety of other ope- 

 rations, taking place at the surface of the earth, and 

 there has been no process discovered in nature by 

 which it can be destroyed, except vegetation. 



The principal consumption of the carbonic acid 

 gas of the atmosphere seems evidently to be in the 

 affording of nourishment to plants. 



Hard Water. 



Carbonate of lime (common lime) is held in solu- 

 tion by water containing a superabundance of car- 

 bonic acid. 



Soda Water. 



Soda water, prepared in the best manner, ought to 

 contain a very small portion of carbonate of soda, 

 which has a tendency to correct acidity in the stomach. 

 It should also contain about eight times its own bulk 

 of carbonic acid gas, which is generated in the gazo- 

 meter from chalk and diluted sulphuric acid. Much 

 that is sold under the name of soda water, contains 

 scarcely any soda, being merely water impregnated 

 with carbonic acid gas by means of a forcing pump, 

 and consequently liable to be contaminated by cop- 

 per, zinc, or lead, according to the vessels in which 

 the condensation is carried on. 



A pleasant saline draught is made by dissolving 

 thirty grains of cajbonate of soda or potash, and 



