CONSTITUTION OF THE ATMOSPHERE 225 



land than over the sea, and in the lower atmosphere than in the 

 upper. 



The dust particles consist of inorganic materials, such as (1) tiny 

 particles of mineral matter blown up from dry roads and fields or 

 shot out of volcanoes, (2) particles of smoke from chimneys, and 

 organic particles. Among the last are bacteria of various sorts, 

 and the spores of many plants. The number of bacteria found in 

 a cubic meter of air at Montsouris (France) Observatory was 345, 

 while in the same amount of air in the heart of Paris the number 

 was 4,790. These figures give some idea of the relative purity of 

 country and city air. 



The dust particles in the atmosphere are important in several 

 other ways. They scatter the light of the sun, so as to illuminate 

 the whole atmosphere. Without the dust in the air, all shady 

 places would be in darkness. The sun would probably appear in 

 dazzling brilliance, shining from a black sky, in which the stars 

 would be visible even in the daytime. The blue color of the sky, 

 and the sunset and sunrise tints, are influenced by the dust in the 

 atmosphere. Dust particles also serve as centers about which 

 water vapor condenses. 



