AIR 71 



cles separate and settle very much as fine clay separates 

 from water which is not disturbed. There are many 

 different kinds of organic impurities present in the air. 

 The most objectionable of these are decayed refuse mat- 

 ters, particularly of animal origin. The air which passes 

 over swampy, undrained land, is contaminated with im- 

 purities. 



74. Air as a Source of Food. In both animal and 

 plant life, air plays an important role. It contains oxy- 

 gen necessary for the existence of animals and carbon 

 dioxid essential as food for plants. Over 90 per cent, of 

 the total food of our agricultural and useful plants is ob- 

 tained from the air as carbon dioxid, or from rain which 

 finds its way into the soil. Food which is oxidized in 

 the body requires oxygen for combustion. Hence it will 

 be seen that the air is the source of the larger portion of 

 the total food of both plants and animals. 



