CHAPTER V 

 THE ATMOSPHERE 



THE atmosphere is a light, transparent mixture of 

 gases. It rests upon the land and the sea, forming the 

 outermost part of the earth. The atmosphere becomes 

 less dense as the distance from the earth's crust increases, 

 one half of it being within four miles of the solid earth. 

 The change in density, however, is very gradual. Just 

 how deep the atmosphere is we cannot tell, but there is 

 probably very little of it beyond a distance of fifty miles 

 from the crust of the earth, although it has been estimated 

 that there is some atmosphere at a distance of two hun- 

 dred miles. 



The principal gases of the atmosphere are nitrogen 

 and oxygen mixed with a small amount of argon and 

 carbon dioxide. Below is a table showing the approxi- 

 mate percentage of each gas in the atmosphere, although 

 the amounts vary slightly. 



Nitrogen 78 per cent 



Oxygen. . . . . ...,-. 21 per cent 



Argon . . v V 1 per cent 



Carbon dioxide 03 per cent 



A number of experiments may be devised to prove that 

 air is matter. We cannot see air, but we are concerned 

 with it every moment of our lives, and we have only to 

 stand out of doors on a windy day to be convinced of its 

 reality. 



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