COMPOSITION OF THE AIR 133 



dry regions of western United States, the days are very 

 hot. The sun's heat rays pass through the air unchecked. 

 At night, on the other hand, it is very cool, because the heat 

 has radiated away again. Water vapor and particles of 

 dust in the air absorb the heat. A cloud blanket docs not 

 permit of radiation from the earth, nor does it permit much 

 heat to pass through to the earth. On a cloudy night there 

 is not so much danger of frost as on a clear night, since the 

 heat is not radiated off into space so rapidly. 



Air is warmed by contact with its own water vapor, or 

 with the earth. When a surface of water is evaporating 

 heat is being absorbed; when water vapor condenses heat 

 is liberated. The specific heat of water is 1, whereas air 

 is only 0.240, and the weight of a given volume of air is -$%-$ 

 of the weight of an equal volume of water. Thus when a 

 large body of water warms up one degree and evaporation 

 takes place, a volume of surrounding air equal to 3200 times 

 the volume of the body of water is cooled down one degree. 

 And, conversely, when a body of water cools down one degree, 

 the surrounding air to the extent of 3200 times the volume 

 of water is warmed up one degree. This accounts for the 

 modifying effect of large bodies of water on the climate of 

 nearby land. This accounts, also, for the mild climate of 

 western Europe which is washed by the warm Gulf Stream. 



113. Composition of the Air. — The constituents of the air 

 other than water vapor, which is exceedingly variable, are 

 arranged in Table VII in the order of their amounts: 



Table VII. — Atmospheric Constituents 



Per cent. Per cent. 



Constituents. by volume. by weight. 



Nitrogen 78.03 7.""). 51 



Oxygen 20.99 23.14 



Argon 0.94 1.29 



Carbon dioxide 0.03 0.05 



Hydrogen 0.01 0.001 



Compounds of nitrogen .... Trace Trace 



Bacteria 



Dust, etc 



To give a striking illustration of the different amounts of 

 the constituents of the air, a slight modification of Grahams 



