CnAPTER III. 



THE A'niOSPHERE. 



THE atmosphere inclu(k'S the air and other gases 

 and vapors which surrijund the earth. Its com- 

 position is as follows : — 



1. Air. — Air forms the hulk of the atmosphere. It 

 is composed of oxygen and nitrogen, in the ])ro])ortion 

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

 not chemically united into molecules, but the molecules 

 of each are thoroughly mixed togethei'. It is the oxygen 

 of the atmosphere that is essential to plant and animal 

 life. The purpose of the nitrogen seems to he 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 ])asses into the air l)y evapoi-ation from the 

 surface of l)odies 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 ui)on the outside of a 

 pitcher of cold water, in the moisture that accunudates 

 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. 



(41) 



