PRELIMINARY OUTLINE 5 



surface, together with (2) such transient vapors as the various sub- 

 stances of the earth throw off. The first class form the permanent 

 gases of the atmosphere, and consist of nitrogen about 79 parts, 

 oxygen- about 21 parts, carbon dioxide about .03 part, together 

 with small quantities of argon, and several other rare constituents. 

 Chief among the second class is water vapor, which varies greatly 

 in amount from time to time and from place to place. Here too, 

 belong the gases which issue from volcanoes, and a great variety of 

 volatile organic substances. Dust and other matter suspended in 

 the air are usually regarded as impurities rather than constituents 

 of the atmosphere; but they are of great importance because they 

 affect its temperature and luminosity, and they facilitate the con- 

 densation of moisture. 



Mass and extent. The total mass of the atmosphere is esti- 

 mated to be THnrtannr of tne mass of tne earth. It exerts a pres- 

 sure of about fifteen pounds per square inch at the sea-level. Its 

 density decreases upward, but its actual height is not known. 

 There is no direct evidence of its existence above a few hundred 

 miles, but there are theoretical grounds for believing that it 

 extends out very much farther. 



Geologic activity. The atmosphere is the most mobile and 

 active of the three great subdivisions of the earth. Its direct and 

 indirect effects on water and rocks are so great that it must be 

 regarded as one of the great agents of change. It acts chemically 

 upon the rock substance of the earth, causing hardening of the rock 

 in some cases, but more often causing disintegration, by means of 

 which rock is reduced to soil-like material, and prepared for removal 

 by winds and waters. When in motion, the atmosphere acts 

 mechanically on the surface of the land, transporting dust and 

 sand. Its greatest function, however, is in furnishing the con- 

 ditions for water action. Rains, streams, glaciers, and all the 

 various forms of moving water upon land, are dependent in one 

 way or another on the atmosphere. On the ocean, too, wave 

 action is dependent largely on the winds. Streams and waves, 

 which are the most familiar agents of geologic change, are there- 

 fore to be credited as much to the atmosphere as to the hydro- 

 sphere. 



