Dr Stark cwi the Injluence of Colour cm Heat 73 



light, white discharges it most copiously, and scarlet is next in 

 order, its emissions being very bright and dazzling *." 



«* In illustration of this,"' says Sir John, " I painted three sides 

 of a square tin cannister with lamp-black, whiting, and minium, 

 — each being worked up with as little size as would give con- 

 sistence to the pigment. Being presented to the reflector, the 

 eflPect of the black surface was 100% that of the white only 85% 

 and that of the red Wf." 



Sir John made other experiments on the rate of cooling of a 

 globe of planished tin filled with warm water, in which a ther- 

 mometer was inserted. The air of the room was at 35° centi- 

 grade, and perfectly steady, and the progress of the ball in cool- 

 ing was carefully remarked. " From the station of 35° till the 

 internal thermometer sunk to the middle point or 25% the time 

 elapsed was 156'."" Sir John now painted the surface of the 

 ball with a coating of lamp-black, and again filling it with warm 

 water, scrupulously repeated the experiment. " The same 

 effect,*' says he, " was now produced, or one-half of the heat 

 expended, in the space of only 81' X-"^ 



Sir John appears evidently to have been struck with this re- 

 sult, though in perfect accordance with all the experiments 

 which had been previously made, of which, however, he does 

 not seem to have been aware. " The application of a coat of 

 pigment to a metallic surface, instead of retarding the effect, 

 almost doubles its discharge of heat. This fact," says he, 

 " equally curious and important, is most contrary to the preva- 

 lent notions, and seems not to have been observed. Had the 

 reverse taken place, we should have readily satisfied ourselves 

 with attributing it to the slow conducting power of the super- 

 ficial crust §.*" 



Notwithstanding these results, Sir John rather inconsistently 

 concludes his remarks on the subject of colour, by saying, that, 

 *' on the whole, it appears exceedingly doubtful if any influence 

 of that sort can be justly ascribed to colour. But (says he) the 

 question is incapable of being positively resolved, since no sub- 

 stance can be made to assume different colours without at the 

 same time changing its internal structure ."|| 



• Essay on Heat, p. 93. t Ibid, p. 93, 94. t Ibid. p. 268. 



§ Ibid. p. 270. 11 Ibid. p. 95. 



