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CH. v.] EVAPORATION. 67 



ever small. It is clear, therefore, that the atmosphere must 

 always contain aqueous vapour. Nor is it necessary to seek 

 far for its source. 



The damp towel on which you have just wiped your wet 

 hands does not stand long on the towel-horse before it 

 becomes dry again ; the water left forgotten in the flower- 

 vase a week ago has completely dried away. In such cases 

 the water passes imperceptibly as vapour into the surround- 

 ing air by a process termed evaporation. It is a quiet 

 process, very different from the noisy production of vapour 

 during ebullition, or boiling ; yet the same in its ultimate 

 result. The general process of converting a liquid into a 

 vapour, by any means whatever, may be called vaJ>orisation ; 

 and two modifications of this general process may be dis- 

 tinguished — evaporation and ebullition. Whilst ebullition 

 takes place only when the liquid undergoing vaporisation 

 reaches a definite temperature, called its boiling point, eva- 

 poration is a permanent process going on at all times and 

 in all places. Every piece of open water, from the narrowest 

 stream to the broadest sea, is constantly giving off vapour in 

 greater or less volume. More vapour will be drawn into 

 the air on a hot than on a cold day ; yet, on the coldest da}-, 

 the process of evaporation is simply slackened, not stopped. 

 Even a piece of ice, exposed to air at the freezing-point, 

 gradually diminishes in size, showing that vapour is given off 

 from the frozen surface. A fall of snow may be evaporated, 

 just as a shower of rain is dried up, but the process is 

 immeasurably slower. It is therefore by no means diflicult 

 to account for the watery vapour in the atmosphere. And 

 it should be remembered that, in addition to that which 

 reaches the air by direct evaporation from river, lake, and 

 ocean, there is a good deal of water thrown into the atmo- 

 sphere as vapour by the agency of living beings, exhaled from 



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