1826.] Mr. Powell 07i Terrestrial Light and Heat. 37 J 



part in a solid body ; but, according to all analogy, we should 

 here expect that in doing so, it would give out a considerable 

 quantity of latent heat, which would be rendered sensible in 

 raising the temperature of the body. The above theory, how- 

 ever, will not allow of such a view of the matter; for the heat 

 produced is here the substance itself which is united to the body, 

 and not the lesult of that union. Moreover that a substance 

 should at once be in combination with another body, and at the 

 same time act upon other bodies as if it were free and uncom- 

 bined, is still further contrary to analogy. 



If then we assume the materiality of light, and wish to take 

 such a view of the mode in which its heating effects are produced 

 as shall at once make the smallest assumption, and be most 

 analogous with other phenomena, we must seek for some other 

 hypothesis than that just alluded to. 



(21.) It may be considered as established that the portion of 

 hght which is not reflected from a surface undergoes an absorp- 

 tion, and changes its state; whether it form a true chemical 

 combination with the body is a point which is probably beyond 

 our means of investigation. It is, however, certain, that imme- 

 diately on the absorption taking place, heat is produced in the 

 body ; but since we are in ignorance of the nature of the combi- 

 nation formed by light with the body, it is surely a most unwar- 

 rantable assumption to say, that the combined substance is heat. 



On the other hand, seeing an extremely subtle substance enter 

 into combination with a solid body, and finding heat produced 

 in that body, what idea can we more naturally and indeed una- 

 voidably form, than that the increase of temperature is here, as 

 in all other cases, occasioned by the giving outof latent from the 

 absorbed substance. 



(22.) In conformity with the phenomena of the changes of 

 state in all other sorts of matter, we here readily perceive that, 

 first, when light is absorbed and enters into combination with 

 common matter, heat is given out, and different degrees of heat 

 by different species of light ; secondly, light is not generated 

 without a certain degree of heat. All bodies at some temperature 

 become luminous, and after arriving at a certain temperature, an 

 excess of heat, which continues to be generated, is employed in 

 giving the form of light to some particles of the body by becom- 

 ing latent in the elastic matter into which they form. 



(23.) The view which 1 have taken of the subject appears tO" 

 me to be one to which we are directly led by tiie phenomena of 

 the experiments. It has long appeared to me a very vague 

 opinion, as well as one very much at variance with all analogy, 

 to say, that " light and heat nuitualiy evolve each other," or 

 that they are modihcations'of the same substance. The view I 

 have adopted exhibits the circumstances of the union and sepa- 

 ration of these agents in a way perfectly conlbrmable to other 



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