206 Mr. Powell on Solar Light and Heat. [Marc^, 



Those effects which we call effects of caloric in ordinary mat- 

 ter, pervade it in different ways, and are exhibited in several 

 sorts of union or connexion. In order then to adopt with pro- 

 priety this mode of describing the calorific phenomena of light, 

 the chief point is to examine carefully whether the analogy does 

 hold good ; and to show to what part of the phenomena of heat 

 in its combination with ordinary matter, those of its union with 

 light are to be compared. 



The first and most obvious idea is, can the effects be ascribed 

 to what we might call the high temperaliire of light? 



Since light is known to pass through many veiy dense media, 

 and communicate very little if any heat to them, it might be 

 inferred that it possesses no sensible temperature of its own ; but 

 this inference is obviously of no force : for in passing through 

 transparent media, most of the luminous particles are never in 

 contact with those of the medium, but pass probably between 

 them and that with inconceivable velocity ; so that whatever 

 heat they may possess, they are incapable of communicating it. 

 Some few rays are stopped and absorbed by the medium and 

 more as it possesses a less perfect transparency ; and in propor- 

 tion as this is the case, w^e know that heat is always communi- 

 cated, and all transparent bodies, after being some time exposed 

 to the sun's rays, become heated. 



When we come to consider the different development of its 

 heating power on bodies of different colour, the effects are 

 totally unlike those of temperature. On this principle, the hea- 

 ting effect would depend upon the impact of hght rather than 

 its absorption, and it should not be greater on a black than on 

 a white surface. But perhaps the difference of calorific power 

 in the prismatic rays is the strongest evidence against attributing 

 the effect to temperature ; for in this case how could such 

 difference of temperature be maintained, supposing it could be 

 originally communicated, when the rays are g^ll in contact, and 

 moving with equal velocities ? 



From these considerations, it would follo.w that the heat must 

 exist in some state of combination with the light, moi'e intimate 

 and more connected with its changes and modifications than 

 that belonging to heat of temperature. 



In order to be the better prepared for following up this 

 inquiry, I propose shortly to bring forward some experiments 

 and conclusions, which are supplementary to some researches 

 on light and heat from terrestrial sources lately read before the 

 Royal Society. (See reports of the Royal Society, Amtals, p. 

 224.) 



(To be continued.) 



