Professor Powell on the Ivjluence of Colour on Heat. 231 



exactly corroborate what all the previous results of Franklin, 

 Davy, and other philosophers lead us to expect, viz. that the 

 effects of the radiation from luminous sources, are in a totally 

 different ratio from those of simple radiant heat ; the former 

 always following the order of darkness of colour, even in the 

 midst of considerable diversity of otJier properties ; the latter 

 certainly not exhibiting any such constant or general relation : 

 but, as far as we can at present conjecture, the relation which 

 they do follow being one of an entirely different kind ; though, 

 of course, instances may occur when it presents an apparent, 

 and possibly in some cases a real, coincidence with the order of 

 colour. 



Indeed, the reason assigned by Sir J. Leslie appears to me 

 perfectly satisfactory against the prospect of success in such ex- 

 periments. I would even go further, and maintain that if we 

 have any substance, in the first instance of its natural colour, 

 and then proceed to dye or tinge it in any way with another, 

 one of two things rnust take place : either, 1st, Some heteroge- 

 neous matter is now mixed up with it, and consequently the sur- 

 faces which it is the object of experiment to compare are not of 

 the same material ; or, 2dly, The mechanical disposition of the 

 particles of the original body is altered, and it has no longer 

 precisely the same texture. Either circumstance would to- 

 tally invalidate a conclusion as to any change in its radiating 

 power being due to its colour as such.* 



Upon these grounds, I feel most forcibly the philosophical cau- 

 tion of the remarks of Dr T. Thomson and Dr Turner, which the 

 author has cited ; and, as the former has observed, the investiga- 

 tion in question " has hitherto been impossible,*" so, I am inclined 

 to think, that its impossibility is rendered nearly certain on the 

 ground first stated ; or rather, that though we may trace approxi- 



• It should be observed, that Professor Forbes has pointed out the ne- 

 cessary connexion between absorbing and radiating power, as dependent on 

 the arrangement of the particles of the surface to be one of the mathematical 

 consequences from Fourier's researches on the subject. This circumstance, 

 perhaps, may be regarded as diminishing still further the probability of any. 

 relation to colour. 



See Translation of M. Maurice's Abstract, &c. Lond. and Edin. Journal 

 of Science, Feb. 1833. Note p. 108. 



