122 77/2? HUMAN BODY. 



3, Air is a material substance, though invisible. It is 

 in the gaseous condition. It is composed chiefly of three 

 gases mixed. 



COMPOSITION OF AIR. 



Nitrogen, about 79 parts in a hundred. 



Oxygen, about .... 21 .. 

 Carbonic-acid gas, about . . .04 " " " 



Besides, there is watery vapor, and a little of various other 

 substances. 



4, Nitrogen gas, which constitutes nearly four-fifths of 

 the air, is not used either by animals or plants. Neither 

 is it injurious. Its office is to dilute the oxygen. If we 

 put a lighted candle into a jar containing pure oxygen, it 

 will burn up very rapidly. So, if an animal is made to 

 breathe pure oxygen, it is greatly excited; and its life, 

 which is partly a burning, is quickly burned out. The 

 nitrogen in the air dilutes the oxygen to just the right 

 strength. 



5, Carbonic-acid gas is not used by animals. On the 

 other hand, they are constantly throwing it off as a waste 

 product. But plants live on it: it is a large part of their 

 food. As they take it in, they give off oxygen. 



Animals take in oxygen, and give out carbonic-acid gas. 



Plants take in carbonic-acid gas, and give out oxygen. 



Fires, and the decay of animal and vegetable matters, are 

 adding constantly to the carbonic-acid gas. But there is 

 only one process which is adding to the oxygen ; and that 

 is, the action of the leaves of plants. On the whole, the 

 addition and subtraction are equal. The amount of car- 

 bonic-acid gas in the whole air remains the same, though 

 in a smoky city it is greater than it is in the country. 



