Dr Stark on the Influence of Colour on Heat. 71 



the flame of a wax candle. In one instrument in which the sur- 

 face was naked, or of its natural colour, the time of cooling 

 through the standard interval of ten degrees was 55^ minutes ; 

 while in another with the surface blackened in the manner men- 

 tioned it was only 36j minutes. The black matter carefully 

 wiped off with a piece of linen rag, which was accurately weigh- 

 ed before and after the experiment, was found to be ^^ of a grain 

 Troy weight. It had covered a surface of polished brass equal 

 to fifty superficial inches. " How this very thin covering (says 

 the Count) which, if the specific gravity of the black matter 

 were only equal to that of water, would amount to no more than 

 17 ug ol' an inch in thickness, could expedite the cooling of the 

 instrument in the manner it was found to do, is what still remains 

 to be shown.""* 



Count Rumford afterwards varied this experiment by cover- 

 ing the cylinders with an animal substance — gold-beaters' skin. 

 " Having covered," says he, " the two large cylindrical vessels 

 No. 3. and 4. with gold-beaters' skin, I painted one of them 

 black with Indian ink, and filling them both with boiling hot 

 water, I exposed them to cool, in the manner already described 

 in the air of a quiet room. No 4, which was blackened, cooled 

 through the standard interval of ten degrees in 23^ minutes ; 

 while the other (No. 3.) which was not blackened, took up 28 

 minutes in the same interval -f-." 



The result of these and numerous other experiments was, 

 " that those substances which part with heat with the greatest 

 facility or celerity, are those which acquire it most readily or 

 with the greatest celerity J." 



Count Rumford seems to have rested in this conclusion, and 

 does not appear to have extended his inquiries as to the effects 

 of heat or cold on other colours. This appears the more re- 

 markable, as he asserts, after relating the unexpected results of 

 these experiments, " that nothing should prevent him from mak- 

 ing the experiment of blackening his skin, or at least of wear- 

 ing a black shirt," if he were called upon to live in a very hot 

 country. 



In connexion with this idea of Count Rumford, and leading 



• Philosophical Transactions, 1804, p. 95, 96. t Ibid. p. 59. 

 + Ibid. 1804, p. 128. 



