74 ELEMENTS OF GENERAL SCIENCE 



78. Nitrogen and argon. While oxygen is a very active 

 substance chemically, readily entering into combination with 

 a great variety of other substances, quite the opposite is true 

 of nitrogen and argon. These gases serve to dilute the oxygen 

 of the air, but they remain unchanged under most circum- 

 stances. Certain compounds of nitrogen are of great impor- 

 tance, however ; the living substance of plants and animals 

 contains nitrogen compounds, and these compounds are essen- 

 tial constituents of the food of all living things. 



