CHAPTER III. 



THE ATMOSPHERE. 



r 1HHE atmosphere includes the air and other gases 

 JL and vapors which surround the earth. Its com- 

 position is as follows : 



1. Air. Air forms the bulk of the atmosphere. It 

 is composed of oxygen and nitrogen, in the proportion 

 of one part oxygen to four parts nitrogen. The two are 

 not chemically united into molecules, but the molecules 

 of each are thoroughly mixed together. It is the oxygen 

 of the atmosphere that is essential to plant and animal 

 life. The purpose of the nitrogen seems to be to dilute 

 the oxygen and reduce its force. 



2. Water Vapor. There is always present in the air, 

 and distributed through it, more or less moisture, or 

 vapor of water. The quantity varies from one fiftieth to 

 one two-hundredth part of the bulk of the air. This 

 moisture passes into the air by evaporation from the 

 surface of bodies of water, from the surface of the land, 

 and from all moist substances. 



We are reminded in various ways that the air contains 

 moisture, as in the drops that form upon the outside of a 

 pitcher of cold water, in the moisture that accumulates 

 upon the window-pane and forms a thick covering of frost 

 in winter and in the moisture that appears upon the 

 grass in the morning. 



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