58 Mr. Powell on Light and Heat 



must follow, (allowing for tlie nature of the determinations,) the 

 same law as that of the cooling of the hot body, since those of (A) 

 follow ft less rapid law, we might infer that part of the heat was 

 constantly abstracted, or ceased to appear in the form of heat ; and 

 this in proportion to the increased heating power of the Ught ; the 

 greater evolution of which thus contributes to the cooling process. 



10. If it be admitted, as I conceive lias been before shewn, that 

 the whole heating energy of a luminous hot body is not displayed 

 by the simple heat radiating from it ; but though partly thus dis- 

 played, is also partly as it were communicated to another agent, 

 through the medium of which it ultimately acts ; this conclusion 

 wiil receive still further confirmation from the arguments just 

 adduced. From them we perceive tht^t a continued and increasing 

 disappearance of simple radiant heat always accompanies the in- 

 creasing deyelopement of heating power in the luminous rays. 



20. I have adverted to the difficulties attending the supposition 

 that the portion of heat which is not radiated in its simple form is 

 converted into light ; but -since it is evidently abstracted, and afr 

 terwards appears again through a diflferent channel, it will hardly 

 be questioned that it is in some way employed in the extrication of 

 the light. It, therefore, becomes important to inquire (so far as we 

 c^n ascertain,) what is the real modification which it undergoes, 



The fects teach us thus much :• — This portion of the heat acr 

 quires totally different properties from those which it possessed 

 ei^Jier when in combination with the body from which it einanates, 

 or which the radiated portion possesses. Though we have no 

 right to identify it with light, it is yet evident that it is in some 

 very close state of union with the light. It is so modified as to 

 have lost its power of heatjng many sorts of solid matter ; trans- 

 parent bodies for instance ; through which it is as it we;-e con- 

 veyed, without any developement of its power of increasing tem- 

 perature. Again it is not upon all opaque bodies that it can now 

 exert its influence ; and the degree in which that influence is ex- 

 erted depends upon quite different characteristics in such bodies 

 from those by which its ordinary influence is most developed. It 

 accompanies the rays of light in fheir course, however altered,. 



