JMjO Professor Powell on tlie Influence of Colour on Heat. 



er excitation of heat when the substance is exposed to the action 

 ©flight. 



Some experiments of Sir E. Home are next referred to as con- 

 tradictory to those of Franklin and others, while the author 

 deems the explanation given by Sir H. Davy as insufficient. 

 Here again I may be allowed to observe, that if he had referred 

 to my report (p. 288), he would have found what appears to 

 me a satisfactory explanation of the point, in accordance with 

 Sir H. Davy's suggestion. It was originally given in a paper 

 in the Annals of Philosophy. Sir E. Home's experiment, I be- 

 lieve, shews only this much, that the scorching effect on the 

 skin which sometimes takes place by the sun's rays through 

 semi-transparent white cloth or linen, is prevented by the absorp- 

 tive power of black cloth ; or, again, that it strikes through the 

 transparent skin of a white, but is absorbed by that of a Negro; 

 and thus, in either case, by the absorption of the black substance, 

 the effect is gradually converted into that of heat of temperature. 



With regard to the highly interesting remarks of the author, 

 bearing on the flnal causes of the change to white, during win- 

 ter, of the hair and feathers of some animals, I would merely 

 observe, that what 1 before said with respect to the want of data 

 for discriminating between the " causa'''' and " non causa" to be 

 found in the circumstance of colour, or of some peculiarity in 

 substance and texture, will, I think, fully apply here. If, in- 

 deed, the difference of colour in hair depend upon any peculiar 

 colouring matter secreted, more or less copiously, or not at all, 

 according to circumstances, surely such difference in the actual 

 component matter of the hair will afford a sufficient cause to ac- 

 count for a greater or less radiating or conducting power in 

 a way strictly analogous to a variety of well understood cases, 

 which it would be more philosophical to adopt as an explana- 

 tion, rather than have recourse to a principle so little capable of 

 distinct apprehension, and so little referrible to any classes of 

 facts of which we have a satisfactory analysis, as the influence of 

 a disposition to reflect certain rays of light upon the emission of 

 heat. 



It would be a mere repetition of what I have already urged, 

 to express my hesitation as to the same conclusion which the 

 author deduces respecting the influence of snow in preserving 



