98 THE ATMOSPHERE AND ITS SERVICE TO MAN 



Although the air appears to be a simple gas and was so 

 considered until the end of the eighteenth century, it has 

 been shown to be a* mixture of several different colorless gases. 

 One of these, oxygen, supports combustion, as we have 

 already learned; another, nitrogen, neither burns nor sup- 

 ports combustion. These two gases make up by far the 

 greater part of the air about us, and occur in the proportion 

 of about one part of oxygen to four parts of nitrogen. Car- 

 bon dioxide is also found in the air in the proportion of about 

 3 parts to 10,000. There are in addition very small quan- 

 tities of several other gases, but these are not of suffi- 

 cient importance to be studied here. Besides the gases, 

 the air contains other matter, such as water vapor, dust 

 particles, and microbes. 



Almost all of us have had occasion to observe that if there 

 is a slight leak of gas from the gas stove in the kitchen, the 

 " smell of gas " will permeate the whole house. It makes 

 no difference whether there are currents of air to carry the 

 gas or not. Gases, whether heavy or light, mix readily 

 with each other, or diffuse. As a rule, therefore, the propor- 

 tion of oxygen, nitrogen, carbon dioxide, and other gases 

 is the same for all places on the surface of the earth. 



Oxygen is the most important part of the air to animals, 

 for without it they could not live. They breathe in oxygen, 

 and breathe out carbon dioxide. All the heat and energy 

 animals have is due to their power of combining oxygen with 

 carbon. Plants also have need of oxygen, but to a smaller 

 degree than animals. 



The nitrogen is needed to dilute the oxygen. If oxygen 

 were undiluted, animals could not live; and a fire once 

 started would burn up iron as readily as it now does wood. 

 Plants and animals need nitrogen too, but it is of no use to 



