Professor Powell on the Influence of Colour on Heat 233 



tinctions in the most prominent light possible. If Dr Stark had 

 honoured that report with a perusal, I think either he would 

 not have fallen into one or two such misconceptions (as I esteem 

 them), or, if he considered my positions faulty, could not have 

 done less than have noticed and criticised them. The neglect 

 of such distinctions appears in several instances in the introduc- 

 tory part of the paper, and I shall notice some others in the 

 sequel. Thus, for example, (p. 286), he speaks of the experi- 

 ments of Franklin and Davy as referring to heat in general, 

 whereas they refer simply to the light of the sun, which, in the 

 act of absorption (whatever that be), in some way or other, of 

 which we are wholly ignorant, produces, excites, or sets free, a 

 certain quantity of heat. 



This subject, especially when the rays are subjected to pris- 

 matic analysis, is involved in much uncertainty ; but the re- 

 searches of Sir D. Brewster, and of MM. Nobili and Melloni, 

 are probably those which will tend most to elucidate it. (See my 

 Report, p. 293). Before entering upon the detail of Dr Stark's 

 experiments, I may here take the opportunity of referring 

 briefly to the distinction between the two kinds of terrestrial 

 heat, because I am aware that my view of the matter, though I 

 conceived I had explained it, both in my several papers and in 

 the report alluded to, with sufficient perspicuity, has not even yet 

 been correctly understood. My fundamental experiment there 

 referred to, (see p. 279), if the numerical results are considered 

 entitled to any confidence, and if no adventitious cause of error 

 can be pointed out, involve, as a mathematical consequence, the 

 conclusion that two distinct heating causes or agents emanate, 

 at one and the same time, from a luminous hot body, which are 

 marked by possessing different properties. It is certainly re- 

 markable that this result should have been entirely overlooked, 

 or palpably misunderstood, in the various works embracing the 

 subject (which I have happened to see), which have appeared 

 since 1825 ; and yet no experimenter, as far as I have been able 

 to learn, has refuted the investigation, or even suggested any 

 objection which might invalidate the result. Yet it is certainly 

 of importance, that if that result be worth nothing, it should be 

 proved to be so. In a word, l)oth De la Roche's experiments 

 and his theory have been maintained by nearly all writers since 



