Professor Powell on the Influence of Colour on Heat. 239 



generated. Results of this kind, I think, must be considered 

 quite distinct from those which bear upon the absorption of heat^ 

 as such. 



In the experiments (p. 291), the tube containing coloured 

 wools as before, being heated to a given point, the time of cool- 

 ing a certain number of degrees was noted. The difference was 

 but small, but the effect more rapid with the darker wool. This 

 would be a consequence of the better conducting power which 

 the former experiment establishes. 



Similar experiments, with a similar result, were tried with the 

 substitution of flour differently coloured. 



These results, considered as referring to the conducting power, 

 and described with reference to the peculiarity, whatever it may 

 be, in the physical character of the substance connected with its 

 darkness of colour, appear to me an interesting addition to the 

 very few instances on record in which the conducting power has 

 been shewn to have any precise connexion with any one deter- 

 minate physical characteristic of bodies. 



The results (p. 292), with differently coated bulbs, and 

 (p. 294) glass bulbs containing water, evincing a greater rapidity 

 of cooling with the darker coloured pigments, and greatest of all 

 with the smoke of a candle, are explicable in regard to the last 

 mentioned case by the consideration of its texture as well as its 

 blacJaiess. In regard to the different-coloured paints, though I 

 am far from denying that the general agreement of so many 

 different experiments is a strong circumstance, yet I conceive 

 the remark before made ought here to be borne in mind ; and 

 the conclusion can hardly be regarded as strictly verified, unless 

 we could be sure of all the circumstances attending the laying 

 on, as well as the nature of the paint, so as to ensure an absolute 

 and perfect equalitt/ of thickness, conducting power, &c. But 

 supposing the validity of the conclusion admitted, it appears to 

 me to amount to this : That certain paints, in proportion as they 

 c»ntain some colouring matter of a darker hue, sustain an in- 

 crease in their power of radiating heat. Is this substance car- 

 bon ? At any rate, I conceive the inquiry still remains. Whe- 

 ther such increase has ani/ real analogy with the increased ten- 

 dency to absorb light, which constitutes darkness of colour, and 

 which, as such, is really and constantly accompanied by a great- 



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