58 AIR AND FIRE 



Bodies that burn slowly in air burn vigorously in oxygen. Oxygen is 

 a part of all living things. 



Respiration consists of breathing and of oxidation. The heat pro- 

 duced in the oxidation keeps the body warm. Water animals use -the 

 oxygen that is dissolved in water. 



Nitrogen is obtained by removing the oxygen from air; also 

 by heating a solution containing ammonium chloride and sodium 

 nitrite. 



Nitrogen is colorless, tasteless, odorless, lighter than oxygen, and 

 inactive. 



Nitrogen is held loosely in many substances, and is set free from 

 most explosives. 



Nitrogen is necessary to life, since it is a part of proteids. Proteids 

 are made by plants. 



Clover, beans, etc., support colonies of bacteria that make proteids 

 out of nitrogen and other materials. 



Liquid air is a mixture containing liquid nitrogen and liquid oxygen. 

 Most substances change their properties when exposed to it, owing 

 to its low temperature. 



The atmosphere as a whole does not become unfit for breathing 

 because of the winds, rain, sunlight, oxidizing bacteria, and plants. 



60. Exercises. 



1. How can you tell a bottle of air from one of oxygen? One of air 

 from one of nitrogen? One of nitrogen from one of carbon dioxide? 



2. What gases do plants take from the air? Animals? What gas 

 does each restore to the air? 



3. If the cover of a fruit jar has an area of 5 square inches, and the 

 jar is empty (a vacuum), how great a weight must I lift to get the cover 

 off, not counting friction? 



4. Why does the air seem so refreshing after a rain or a snowfall? 

 Why should sunlight and air be admitted into the rooms in which we 

 live? 



