66 ELEMENTS OF GENERAL SCIENCE 



67. More about oxygen. The gas oxygen is the most abun- 

 dant simple substance upon the earth. In an uncombined state 

 it makes up about one fifth of the air, as noted above. It is 

 found in combination with almost every known simple sub- 

 stance, forming eight ninths (by weight) of water and about 

 one half of the rocks of the earth's crust. It is estimated that 

 about 50 per cent of the earth is oxygen. 



68. 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. 



69. Summary The air is composed of molecules of several 

 kinds of gases. These molecules are all moving about and tend 

 to get away as far as possible from each other. This is what 

 explains the expansion of gases. 



Each molecule is ordinarily composed of two or more atoms. 

 The molecules of simple substances contain atoms of the same 

 kind. In a compound, such as carbon dioxide, a molecule 

 contains atoms of more than one kind. 



When a candle is burning in the air, atoms of oxygen from 

 the air unite with atoms of hydrogen from the candle wax to 

 form water, and with the atoms of carbon to form carbon 

 dioxide. 



