THE FORMATION OF DEW. SI 



vapour as dew, which supplies to a considerable extent the 

 place of rain in supporting vegetation. 



It is well known that heat radiates from the surfaces of all 

 substances, but more freely from some than from others. 

 That which radiates from the surface of the earth, when the 

 atmosphere is clear, seems to pass into space, and is lost to 

 our planet; such radiation, therefore, cools the surface of the 

 earth on a clear night. But on a cloudy night, the clouds 

 may return nearly as much heat to the earth as the earth 

 radiates, leaving the temperature almost undisturbed from 

 that cause. Indeed, it is conceivable that at times the radia- 

 tion from the cloud may be even greater than that from the 

 earth. A newly formed cloud is warmer than dry air at the 

 same height, and the radiation from the cloud is proportioned 

 to the temperature; it is therefore conceivable that the earth 

 may be cooled down to a low degree, and then a warm 

 cloud be formed over it, when an excess of radiation from 

 the cloud would warm the earth. But, whatever may be the 

 temperature of the cloud, it will radiate heat downward in 

 proportion to that temperature, and counteract to a greater 

 or less extent the cooling effect of radiation from the surface 

 of the earth: whilst in a clear sky there is no such return of 

 heat, and therefore in such a sky radiation cools the earth's 

 surface. 



On the earth being thus cooled to a temperature below 

 the dew-point of the atmosphere, a part of the atmospheric 

 vapour that rests on the earth will be condensed and form 

 dew, as already described. But it is found that this dew 

 does not form equally on all substances, because different 

 substances radiate heat with various degrees of force, and a 

 scale may be formed of these substances, showing their 

 radiating powers — as, say, Wool, Cotton, Down, Grass, 

 Leaves, Sand, Glass, Porcelain, Varnish, Wood, and Metals, 



From this scale it appears that the best radiating bodies 

 are organic and silicious, and this is more particularly the 



