TIIK PLANT. 15 



bodies (those which have possessed animal or vegetable 

 life, that is, organic matter), and others, also, in 

 the breathing of animals. Experiments have proved 

 that if the atmosphere consisted of pure oxygen every 

 thing would be speedily destroyed, us the processes of 

 combustion and decay would be greatly quickened, 

 and animals would be so stimulated that they would 

 soon die. One use of the nitrogen in the air is to 

 ilihite the oxygen, and thus reduce the intensity of 

 its effect. 



Besides these two great elements, the atmosphere 

 contains certain impurities which are of great impor- 

 tance to vegetable growth ; these are, ca.i'bu ,<;,. <i<-'nl, 

 water, amnwnia, etc. 



CARBONIC ACID. 



Carbonic acid is, in all probability, the only source 

 of the carbon of plants, and consequently supplies 

 more material to vegetation than any other single 

 sort of food. It is a gas, and is not, under natural 

 circumstances, perceptible to our senses. It consti- 

 tutes about O/QO of the atmosphere, and is found in 

 combination with many substances in nature. Marble, 

 limestone and chalk, are carbonate of lime, or car- 

 bonic acid and lime in combination ; and carbonate 

 of magnesia is a compound of carbonic acid and mag- 

 nesia. This gas exists in combination with many 

 other mineral substances, and it is contained in all 

 water not recently boiled. Its supply, though small, 

 is sufficient for the purposes of vegetation. It enters 



