THE ATMOSPHERE AND ITS COMPOSITION 109 



80. THE ATMOSPHERE AND ITS COMPOSITION 



It is necessary for every living being to breathe, and the 

 oxygen required is supplied from the atmosphere which sur- 

 rounds the earth. This atmosphere obtains its name from 

 two Greek words meaning " breath-sphere." It extends up 

 from the earth between two and three hundred miles, but, 

 since it is very compressible, fully half of all the atmosphere 

 is within three miles of the surface of the earth. 



The composition of the atmosphere is not uniform. This 

 fact proves that the atmosphere is not a chemical compound, 

 but merely a mixture of various gases. It can therefore be 

 separated mechanically into its component parts, which con- 

 sist roughly, by volume, of nitrogen, 78 per cent, oxygen, 21 

 per cent, and argon, 1 per cent. Besides these, very small 

 quantities of carbon dioxide, ozone, dust, bacteria, and other 

 rarer substances are present. The amount of carbon dioxide, 

 bacteria, and dust depends upon the locality. These are 

 called impurities, and pure air necessarily contains a very 

 small amount of them. 



References : 



1. 1002 : 135-143. The Earth's Atmosphere and its Effects. 



2. 1304:229-231. Composition and . Weight of the Atmos- 



phere. 



3. 1702 : 66-71. Air a Mechanical Mixture. 



4. 1803 : 68-70. Extent and Character of Earth's Atmos- 



phere. 



a. 1102 : 2-8. Origin, Evolution, and Future of the At- 



mosphere Its Composition. 



6. 1301 : 33-41. Composition and Height of the Atmos- 

 phere. 



c. 1302 : 273-275. Composition of the Atmosphere. 



d. 1303 : 23-26. The Atmosphere. 



