EVAPORATION. 243 



after the sun has begun to shine. All this water 

 is dissipated into the air ; it evaporates, that is, it 

 passes into the state of vapour. 



Evaporation does not always take place with 

 the same quickness. You cannot fail to have 

 remarked that it is closely connected with heat 

 and with the draft of air. The property of air, of 

 taking up vapour into the space which it occupies 

 itself — its capacity for vapour— increases with the 

 temperature; for this reason evaporation is fa- 

 voured more by warm than by cold air. But in 

 all cases the amount of vapour, which a given bulk 

 of air can take up at a given temperature, is 

 limited. If the limits of this capacity are reached, 

 evaporation ceases, until the air thus charged with 

 moisture is removed, and other drier air takes its 

 place. The free air of the atmosphere is probably 

 never quite dry, but it seldom holds as much 

 moisture as it could take up at the temperature at 

 which it is. For this reason a draft of air favours 

 evaporation, and that the more effectually, the 

 drier and the warmer it is. Moist air cannot aid 

 the formation of vapour, however swiftly it may 

 move. The air is commonly moister and stiller 

 over low lying spots than on heights. You will 

 now quite understand why it is necessary to seek 

 high places, if the object is to obtain the quickest 

 evaporation in the open air. 



Air, which at any time contains as much vapour 



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