34.5^ Professor Powell on tlie hifluence of Colour on Heat. 



other the radiating, powers of different substances. To the ex- 

 periments of Sir E. Home, (here more fully stated), I have al- 

 ready referred ; and the same remark will apply to the sugges- 

 tion of a black dress as most suitable, not for a warm climate, 

 as such, but for exposure to the sun^ and the avoidance of scorch- 

 ing. Whether the accumulation of the same heat, in a more 

 equable manner, be more desirable, would be another question. 



With reference to the often recited experiments of Sir John 

 Leslie, it appears to me that since his blackened balls were used 

 for the effects of the solar rays, there is none of that occasion 

 for surprise which the author seems to evince at his expressing 

 himself doubtful as to the influence of colour in simple heat. 

 Nor when he had tried the experiments referred to, does his 

 cautious inference from them seem to me more than is fully jus- 

 tified by the considerations I have already referred to. So far 

 from perceiving any of that " inconsistency"" with which Dr Stark 

 taxes him, I confess I am disposed to think his hesitation was 

 grounded on the most philosophical views. But even if Dr 

 Stark (as I presume) is an opponent of Leslie''s peculiar theory 

 of the radiation (or rather, as he would have termed it, pulsation) 

 of simple heat, yet surely he must allow the philosopher to be 

 consistent with himself, in doubting the influence of colour in 

 the latter case, which, according to his view, was not connected 

 even by any sort of analogy with the radiation of luminous heat. 



The researches of Sir W. Herschel, I would suggest, were 

 not directed to the " modification of heat by colour,'' but to the 

 heating property of the sun's rays, as analyzed either by pris- 

 matic refraction, or absorption by coloured glasses. Again, 

 when Sir H. Davy speaks of " the temperature of bodies being 

 affected by rays producing heat," it is evident, from the whole 

 tenor of the passage, and of the experiments referred to, that he 

 means rays from luminous sources. The remark of Dr E. Tur- 

 il^r, quoted at length (p. 75), I regard as the most perspicuous 

 statement of the matter which can be given in a few words. In 

 short, upon the whole of this historical review, I venture to re- 

 mark, that the deficiency pointed out as existing in all previous 

 researches, seems to me rather inherent in the nature of the sub- 

 ject, whe» distinctly considered, than one which could have been 



