from Terrestrial Sources, 61 



such a power must exist, though probably of very small intensity ; 

 and the phenomena which these substances present, on the prin- 

 ciples here advanced, agree in the closest analogy with other 

 phenomena of nature. Some bodies, as water, mercury, ^•c., change 

 into elastic fluids at common temperatures ; in the same way the 

 common temperature of phosphorescent bodies may be sufficient to 

 afford to some of their peculiarly- constituted particles, the requisite 

 degree of latent heat, to evolve them in the form of light. 



25. Phosphori, when exposed to light, absorb it, by which some 

 degree of heat is necessarily communicated to them. This is in 

 part occupied in causing the light to be again evolved, by becom- 

 ing latent in it, as soon as the body is removed from exposure to 

 light. An increase of temperature accelerates this process, and 

 too great an increase soon exhausts the supply of luminifiable 

 particles. 



26. In general, I may further observe, it is on these principles easy 

 to conceive, that there may be cases in which, for the evolution 

 of light, no increase of sensible temperature should be necessary. 

 The whole of such increase, though really generated, becoming 

 latent in the light. 



In general, also, the heating power of light evolved from dif- 

 ferent bodies, need not follow any proportion to its illuminating 

 effect. It is conceivable that a body may produce light contain- 

 ing any given degree of latent heat, but of such tenuity, or so deep 

 in colour, as to have very little illuminating intensity : of this we 

 have seen an instance in incandescent metal. 



I might proceed to the application of the principle here adverted 

 to, in a variety of other cases ; but upon these topics it will not be 

 necessary to go any farther. I will merely observe, in conclusion, 

 that as Newton has put the query, whether light and common 

 matter be not convertible into each other, we here perceive phe- 

 nomena which render it highly probable that such a change ac- 

 tually does take place ; since, whatever the change be, it is 

 accompanied by precisely the same phenomena in regard to latent 

 heat, as those by which the changes of state, in ordinary matter, 

 are accompanied. 



