1814.] Dr, Hulls' Essay on Dew. 503 



colder than the air ; and the most Teadily dewed metals are toe 

 ones which become coldest soonest. 



The quantity of dew deposited de]>enrls not merely on the differ- 

 ence between the temperature of the air and of those bodied on 

 which the dew condenses : it depends likewise, and in a very con- 

 siderable degree, upon the quantity of moisture which exists in the 

 atmosphere. 



When wool is equally exposed it becomes colder than gross; 

 while silk, cotton, and flax, become colder than wood. Swandf v-i 

 becomes still colder than any of these bodies. Fresh unbroken 

 straw, and shreds of white paper, were found, likewise, to become 

 colder than wool. Of powders, fine river sand became the least 

 cold; glass, and chalk, in powder, became more so; and charcoal, 

 lamp-black, and brown calx of iron, became coldest of all. Glass, 

 brick, cork, and oak-wood were inferior to filamentous substances. 

 Snow likewise became a good deal colder than the air ; but in this 

 respect it is inferior to swandown. 



.Such are the phenomena observed by Dr. Wells to accompany 

 the formation of dew. In tiie second part he treats of the theory of 

 the formation of dew. Aristotle supposed that dew was a s[>ecies of 

 rain, formed by the moisture of the atmosphere being condensed by 

 the cold of the night; an opinion still adopted by Mr. Leslie. The 

 discovery by Muschenbroek, that metals will be fiea from dew 

 while other bodies attract it copiously, led to the conclusion that the 

 formation of dew is an electrical phenomenon ; that it is deposited 

 on non-conductors, but not on conductors. Mr. Wilson and Mr. 

 Six conceived that its formation was accompanied by the evolution 

 of cold ; an opinion at first embraced by Dr. Wells : but sul>se- 

 quent observations led him to doubt its accuracy, and he afterwards 

 ascertained, by direct experiment, that the temperature of bodies 

 sinks before any dew is deposited on them ; that the subsequent de- 

 position of dew is the consequence of this coldness; and therefore 

 that the deposition of dew lias precisely the same cause as the ap- 

 pejeance of moisture on the outside of a glass or metallic vessel, 

 when a liquid considerably colder than the air has been poured into 

 it shortly before. 



Hut why, it will be asked, do bodies become colder than the air 



with which they are in contact ; and why do some bodies acquire ■ 



greater degree of cold than others in their vicinity ? These questions. 



could not have been answered in a satisfactory manner previous to 



the discoveries of Mr. Leslie and Conni Rurnford respecting heat. 



But iii consequence of these discoveries we are able to answer them 



in a satisfactory manner. All bodies have the property of radiating 



heat. During the day, the heat lost by radiation is more than rap* 



plied by the sun ; so that the temperature ol bodies dp ring the day 



■ increased instead of being diminished. But the contrary is the 



during the night. The heal radiated by the bodies on the rat> 



of the earth penetrates into the sky, and does not again return 



tq them. Hence their temperature must be constantly diminishing 



6 



