204 ■ Mr. Powell on Solar Light and Heat. [March, 



no medium between identifying light with heat, and maintaining 

 a totally separate set of rays ? It appears to me that if we reject 

 either one of these opinions, we are not by any means obliged 

 to adopt the other. Without identifying the two agents, or 

 without supposing them inseparably united, without conceiving 

 the heating power absolutely inherent in every particle of hght, 

 and invariable in intensity, except as the intensity of light 

 varies, on the one liand ; or on the other hand, that the heat 

 consists of a distinct set of rays analogous to the rays of light ; 

 we may admit it to be in some very close state of union, combi- 

 nation, or dependence, yet so as to be susceptible of variation 

 without a corresponding variation in the other effects of light. 

 And such indeed, antecedently to the inquiry here adverted to, 

 would seem the most natural and obvious way of considering the 

 matter ; because we are ignorant whether light be matter, or 

 whether heat be motion, does it follow that there is any neces- 

 sity for explaining the phenomena in which both agents seem 

 concerned, bv assumino- them to be one and the same thin'g, on 

 the one hand; or by denying that there is any sort of union 

 between them, on the other .' 



(64.) To adopt a view of the subject which shall be a medium 

 between the two extreme theories hitherto adopted appears to 

 me not only to be what is most natural and most analogous to 

 the views we take of other natural phenomena, but what is 

 required by many strong facts. 



To suppose that rays of heat exist distinct from those of hght, 

 either in the direct solar rays, or in the prismatic beam, requnes 

 the supposition of a new and peculiar sort of radiant heat, as 

 different from common radiant heat as it is from light; by which 

 means I do not see that we obtain any more satisfactory expla- 

 nation of the phenomena than we did before. 



(65.) It is certain that whatever we suppose to be the state in 

 which the heat exists when it so inseparably accompanies the 

 sun's light, there must be some peculiar circumstance in the 

 mode of its union which makes its effects sensible only under 

 some particular circumstances ; and under others endows it with 

 properties which heat in its simple radiant state does not possess. 



In ordinary cases there is a direct communication of heat to 

 substances with which light comes in contact. This effect is 

 produced on all substances in some degree, but on seme much 

 more than others ; and these are of a character widely different 

 from those on which simple radiant heat is known to produce its 

 greatest effects. 



Heat accompanying light passes through the densest sub- 

 stances which are completely impervious to simple radiant heat 

 (unless first thoroughly heated), and yet produces less heating 

 effect on these than on any class of substances which are heated 

 at all by the impact of light. 



