v.] EVAPORATION. 69 



containing absolutely a large quantity of vapour. On the 

 other hand, if the temperature be low, a small quantity of 

 vapour may render the air damp, since it approaches nearer 

 to the point of saturation. Hence the paradox that, in 

 summer, dry as the air may feel, it usually contains more 

 moisture than in winter, when it is popularly said to be 

 damper. 



Another condition affecting evaporation is the rapidity 

 with which the air is renewed in the neighbourhood of the 

 water to be evaporated. A windy day soon dries a wet 

 pavement, the currents of air promoting evaporation. The 

 air immediately around the damp object takes up vapour 

 and soon receives its full complement, so as to prevent 

 further evaporation ; but, when the air is disturbed, the por- 

 tions which have become charged with vapour are rapidly 

 removed and fresh ones brought into their place, which 

 in turn become laden with vapour and are carried away 

 to make room for others. It need hardly be said, too, 

 that the rapidity of evaporation depends on the extent of 

 the exposed surface of liquid. Ink dries up quickly 

 in a wide-mouthed inkstand, but the same quantity may 

 be preserved much longer in a narrow bottle. In fact, 

 the vapour is derived only from the exposed surface of the 

 liquid, and herein lies one of the great differences between 

 evaporation and ebullition : in the rapid process of boiHng, 

 bubbles of vapour are generated throughout the mass of 

 liquid, while, in the slow process of evaporation, the vapour 

 is derived from the surface only. 



Meteorologists occasionally measure the rapidity of evapo- 

 ration by means of instruments called atmomete^-s} It is 

 more useful however to determine the proportion of moisture 



1 Atmometer, from arixus, atmos, vapour ; whence also .itmosphere, 

 the sphere of vapour or air. 



