ON DEW, &c. '235 



XI. Thinking it probable, that black bodies 

 might radiate more heat to the sky, at night, 

 than white, I placed upon grass, on five different 

 evenings, equal parcels of black and white wool. 

 On four of the succeeding mornings, the black 

 wool was found to have acquired a little more 

 dew than the white ; whence I inferred that it 

 had, in consequence of its colour, radiated a 

 little more heat. But I afterwards remarked, 

 that the white wool was somewhat coarser 

 than the black ; which circumstance alone was 



which were published, first in the 44th volume of the French 

 Annals of Chemistry, and afterwards by Mr. Peter Prevost 

 of Geneva, in his Essay on Radiant Heat ; but fearing to be 

 very tedious, I have since given up the design. I will say, 

 however, that, if to what is now generally known on the 

 different modes, in which heat is communicated from one 

 body to another, be added the two following circumstances 5 

 that substances become colder, by radiation, than the air, 

 before they attract dew ; and that bright metals, when ex- 

 posed to a clear sky at night, become colder than the air 

 much less readily than other bodies j the whole of the ap- 

 pearances observed by Mr. Prevost may be easily accounted 

 for. 



Note to second edition."] I found, shortly after the publica- 

 tion of the former edition of this Essay, that the learned Dr. 

 Young had, several years before, in his great work on Na- 

 tural Philosophy, employed the principle of the radiation of 

 heat to account for several of the facts observed by Mr. B. 

 Prevost. On the subject of Dr. Young's explanation, I have 

 spoken somewhat fully in the 28th number of Dr. Thomson's 

 Annals of Philosophy. 



