146 ESSAY 



when it does form upon metals, commonly 

 sullies only the lustre of their surface ; and 

 even when it is sufficiently abundant to gather 

 into drops, these are almost always small and 

 distinct. Two other facts of the same kind 

 are ; fast, that the dew, which has formed 

 upon a metal, will often disappear, while other 

 substances in their neighbourhood remain wet ; 

 and secondly, that a metal, which has been pur- 

 posely moistened, will often become dry, though 

 similarly exposed with bodies which are attract- 

 ing dew. This inaptitude to attract dew, in 

 metals, is communicated to bodies of a very 

 different nature, which touch or are near to 

 them. For I have found, that wool laid upon 

 a metal will acquire much less dew, than an 

 equal quantity laid upon grass in the immediate 

 vicinity. 



A large metallic plate, lying on grass, resists 

 the formation of dew more powerfully than a 

 very small one similarly situated. I conclude 

 from various collateral facts, that a considerable 

 difference in the thickness of two pieces of 

 metal, exposing equal surfaces to the sky, will 

 be attended with a similar consequence, where- 

 ever they be placed, though I have no observa- 

 tion, which proves this directly. If, however, 

 a large and a very small plate be suspended 

 horizontally, at the same height, in the air, the 



