234 ESSAY 



noted in the foregoing part of this Essay. Gold, 

 silver, copper and tin, are there said to resist 

 the formation of dew more strongly, than other 

 substances of the same class ; but these metals, 

 according to Mr. Leslie, radiate heat the most 

 sparingly. -On the other hand, lead, iron and 

 steel, which, according to the same author, 

 radiate heat more copiously than the former 

 metals, were found by me to acquire dew more 

 readily. I do not know, if the radiating power 

 of platina has been ascertained by direct ex- 

 periments ; but, as its conducting power is 

 small, its radiation must be great, since these 

 qualities exist always in opposite degrees in the 

 same substance ; and I have accordingly ob- 

 served it to be dewed, while the four first-men- 

 tioned metals were dry. I am ignorant both of 

 the radiating and the conducting power of zinc, 

 as determined by ordinary experiments ; but I 

 infer, from its being more easily dewed than 

 gold 'or silver, that it radiates heat more co- 

 piously than they do ; unless indeed, the pieces 

 which I used, from having had their surfaces 

 roughened by friction with sand, which was 

 employed to brighten them, had acquired a 

 radiating power, greater than that possessed by 

 polished pieces, agreeably to the results of some 

 of Mr. Leslie's experiments *. 



* I once intended to subjoin here an explanation of some 

 very curious observations by Mr, Benedict Prevost on 



