£4 Mr. Powell on Light and Heat 



ratio of the heating power of light, to the simple heat, consider- 

 ably beyond what is found in the most intense flames. This ex- 

 periment I have not had an opportunity of trying : it is to be hoped 

 this notice may lead to its being tried. And, perhaps, the further 

 investigation of this point may promote some advance towards a 

 knowledge of the nature of the chemical influence accompanying 

 light under these circumstances. A power, of which, at present, 

 we can only say that it is exerted by the solar rays where no radi- 

 ation of simple heat is present ; and in the greatest degree by those 

 rays which have the least heating power. 



10. It forms one of the most interesting topics of inquiry to ex- 

 amine the nature of the solar heat. A variety of experiments have 

 long convinced me, that this heat consists solely of that kind which 

 belongs to light. Among other modes of trial, I have often applied 

 that here employed : which though not of itself sufficient to es- 

 tablish the two radiations, will yet infallibly shew whether there be 

 present the smallest radiation of non-transmissible heat With an 

 instrument graduated according to Professor Leslie's scale, a va- 

 riation of the 20th of a centigrade degree may be distinguished. 

 I have repeatedly tried the experiment with the screened bulb, 

 both plain and coated with whitewash, or with white silk. The 

 screen could here be placed at a sufficient distance to preclude all 

 interference. And, in these cases, after waiting till the instrument 

 placed in the sun had become perfectly stationary, I never per- 

 ceived the slightest increase of effect on interposing the screen. 



11. These experiments afford us a point of comparison between 

 solar and terrestrial heat. The former resembles, in all its pro- 

 perties, one species of the latter. But this is always accompanied 

 by another species totally distinct. 



With respect to the nature of the former, or the heating power 

 of light, various opinions have been held. And it is to an ex- 

 amination of these opinions, and of the conclusions which we may 

 deduce on the subject, from the experiments here described, that 

 the remaining portion of my remarks will be devoted. 



With respect to the heating power of the solar light, we have not 

 at present many data from which we can deduce a view ef its 



