248 MOISTURE OF THE AIK; ITS FALL. 



below 0° C. (32° P.), water poured upon it freezes. 

 So if a sudden south wind brings, at such time, a 

 fall of rain, it freezes on the surface, forming a thin 

 film or varnish of very slippery ice. But even 

 when it does not rain, the cold ground withdraws 

 from the warm moist air of the south a part of its 

 charge of vapour, and covers itself with a similar 

 thin film of ice. This ice is even formed on the 

 floors of houses, if the cold has already found its 

 way into them. You see that the origin of this 

 surface ice is the same as that of the frost on the 

 windows. 



When the nights are cool, but not cold enough 

 to freeze, most bodies in the open air, plants 

 especially, become covered with those drops of 

 water which we call Dew. It was formerly believed 

 that dew came out of the earth. 



However, Wells, a Scotch naturalist, proved 

 most clearly that dew is set down from the air. 

 The temperature of many, perhaps all bodies, placed 

 in the open air after sunset, soon falls below that 

 of the air, and, under favourable circumstances, 

 even below the dew-point. Now from this 

 moment they behave like any cold body plunged 

 in moist air ; they become studded with drops of 

 water. The degree, to which this cooling is carried, 

 depends upon the power which the substances 

 have of radiating heat. Those bodies, therefore, 

 which are the best radiators, such as plants, leaves 



