ON DEW, &c. 



211 



this conclusion by the test of observation, since 

 it occurred to me. 



IV. When bodies become cold from radia- 

 tion, the degree of effect observed must de- 

 pend, not only on their radiating power, but in 

 part also on the greater or less ease, with which 

 they can derive heat, by conduction, from 

 warmer substances in contact with them. Thus 

 grass, on a clear and still night, was constantly 

 colder, sometimes very much colder, than the 

 gravel walk, though a small quantity of sand, 

 placed upon grass, was always nearly as cold as 

 this substance. In this case, the difference in 

 temperature, between the gravel walk and sand, 

 evidently depended on the different quantities 

 of heat, which they received from the parts 

 beneath. A like reason is to be given for dew 

 appearing in greater quantity on shavings of 

 wood, than on the same substance in a more 

 dense and compact form ; and for filamentous 

 and downy substances becoming colder than all 

 others, even than lampblack, which is placed 

 by Mr., Leslie, at the head of the best solid 

 radiators of heat. For the lampblack exposed 

 by me, being about 2 lines in depth, possessed, 

 in consequence, a fund of internal heat, which 

 would more readily pass to its cold surface, 

 than the heat of the lower parts of the downy 

 substances would to their upper surface. 



