THE FOEMATION OP DEW. 55 



sphere, and when, consequently, radiation from the surface 

 cools the atmosphere contiguous to it, and condenses a por- 

 tion of the vapour which the air contains into minute globules 

 of liquid, which are sustained by the elastic force of the air. 

 Whilst dew, properly so called — that which is found attached 

 to various substances in the form of drops — is a result of 

 the cooling of certain bodies below the dew-point of the 

 atmosphere by radiation of heat from those bodies, and a 

 consequent condensation and abstraction of some of the 

 vapour which the air resting on them contained. And the 

 more anybody is thus cooled, the greater will be the quantity 

 of dew deposited on it. In the two last-mentioned modes, 

 dew supplies to a certain extent the place of rain. Where 

 clouds are freely formed, rain falls on the earth to some 

 extent ; but when rain is absent, and the sky is cloudless, 

 radiation of heat, by cooling and condensing vapour, gives 

 some moisture to the earth. Thus the sands of Africa and 

 Asia, which are never visited by rain, have their scanty 

 vegetation supplied with a certain amount of that moisture 

 which is so essential to the life of organized beings. 



