1824.] Mr. Powell on Solar Light and Heat. 327 



show that simple radiant heat exists distinctly in the solar beam; 

 or in other words, that the sun's heat is produced by an assent 

 identical with that which emanates from non-luminous hot bo- 

 dies ; it merely shows that the refractive and dispersive power 

 which the lens exercises on light is capable of eliciting from it a 

 certain agent or set of rays which produces the sensation of heat, 

 but not that of illumination ; and by no means shows that that 

 agent exists in a separate form when the rays of light are in 

 their direct natural condition. 



If the focus of heat be the same as that of light, and the expe- 

 riment of Sir W. Herschel (Phil. Trans. 1800, No. 15, Exper. 

 19, 20) be admitted, where the concentration of simple radiant 

 heat by a lens appears to be proved, the same remark applies as 

 that just made with respect to concentration by a mirror. 



(16.) The principal modification which the sun's rays are 

 made to undergo, and from which conclusions relative to the 

 nature of their heating power have been drawn, is the analysis 

 to which they may be subjected by the prism. Into any discus- 

 sion upon the controverted points respecting these experiments, 

 I shall abstain from entering. We will suppose it granted that 

 a set of invisible heating rays are separated beyond the visible 

 red rays. The existence of such rays in a distinct state in the 

 spectrum cannot be considered as any proof that they have that 

 distinct existence in the natural state of the rays. It by no 

 means proves that any such simple heating rays must have come 

 directly from the sun, and have been transmitted through the 

 prism, with merely a change in their direction. 



From the experiments above given, we may, in reference to 

 this point learn thus much : the direct rays are not accompanied 

 by any separate heating rays which are either stopped by glass, 

 or bear a relation to texture more than colour. It therefore 

 becomes an important object to try whether in the prismatic 

 beam these heating rays possess those characteristics or not. 

 With respect to one of the characteristics, viz. transmissibility 

 through glass, we have no ground to assume that the heating 

 prismatic rays possess it from the circumstance of their passing 

 through the prism, because, when the light impinges upon the 

 prism, we know that it has not any such separate rays accompa- 

 nying it ; and of the nature or properties of the rays during their 

 passage through the prism, we are altogether ignorant. 



(17.) My object in making these remarks is merely to attain 

 if possible some clear ideas on the subject in question ; and to 

 point out those parts of it which appear to me to want further 

 elucidation ; and to several of which I have attempted to direct 

 experimental research. The subject must always remain 

 perplexed and obscure so long as we dispute about such terms 

 as " calorific rays," " luminous," or " non-luminous heat," &c. 

 The only way of arriving at clearness of ideas, and thence being 

 able to pursue the inquiry in a satisfactory manner, is to fix upon 



