60 Mr. Powell on Light and Heat 



account for effects which we suppose to be sui generis, only because 

 we have not examined whether they are in accordance with any 

 other class of phenomena, 



22. What then are the most proper terms in which to describe 

 the facts ? 



The phenomena which light exhibits in its relations to heat, 

 agree in the closest conformity with those presented by the changes 

 of the ordinary forms of matter. When light is absorbed and 

 enters into combination with other matter, heat is given out ; on 

 the other hand, light is not generated or evolved without the ap- 

 plication of a certain degree of heat. All bodies at some tem- 

 perature become luminous ; and when they arrive at that point, a 

 portion of the heat is employed in giving the form of light to some 

 matter belonging to, or in combination with, the body by becoming 

 latent in it. 



23. To this view of the subject,! conceive we are directly, led by 

 the foregoing experiments : the different results all afford a strong 

 corroboration of each other ; and from them it appears, that in- 

 stead of the vague opinion that " heat and light mutually evolve 

 each other," we have it in our power to explain the phenomena 

 which their union and separation present, in a way equally simple 

 and satisfactory, and agreeing in the closest analogy with other 

 physical phenomena ; and when the terms " absorption of latent 

 heat," ^-c, are carefully used in their strict experimental sense, it 

 is obvious that we do not, in applying them, necessarily assume 

 the materiality of light, or of heat ; though, perhaps, the facts 

 here brought forward may be considered as new arguments for it. 



It would lead into details of too great length here, to proceed 

 to the application of the principle above inferred, and to point 

 out the ready explanation of many phenomena which it affords, 

 and which, upon the received theories, are allowed to involve 

 great difficulties. But it may be proper briefly to mention one 

 or two instances. 



24. The light from phosphorescent bodies is too feeble to allow us 

 to ascertain whether it possesses any heating power when absorbed. 

 In the absence of proof, analogy would lead us to suppose that 



