CHAPTEE VIII. 



CLOUD-FORMATION EVAPORATION. 



Water exists in different forms without, 

 however, undergoing any chemical change. It 

 is when condensed into the fluid state that we 

 call it "water," and then it is heavier than 

 the atmospheric air and therefore seeks the 

 low places upon the earth's surface, the lowest 

 of which is the bed of the ocean. Wherever 

 there is water or moisture on the face of the 

 globe there is a process going on at the surface 

 called evaporation. This process is much 

 more rapid under the action of heat than when 

 it is colder. In other words, as the heat in- 

 creases evaporation increases within certain 

 limits and bears some sort of a ratio to it. 

 Evaporation is not confined to water, but as 

 our subject has to deal with atmospheric phe- 

 nomena we will speak of it only in its rela- 

 tion to aqueous moisture. 



The heat that is imparted to the earth's 

 surface by the rays of the sun is able to sepa- 

 rate water into minute particles, which, when 

 so separated, form what is called vapor, which 



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