94 On Heat Radiated by Rough and by Bright Surfaces. 



thus : — the* latter cause perceptible alterations in the relative 

 positions of the parts of bodies upon which they act, while the 

 former do not produce this effect. 



The longer the rays of heat act upon a body, the more it ex- 

 pands. The change in the position of its particles is accordingly 

 a function of the time. But for a given value of the motion im- 

 parted to a body, that is, for a given intensity of the radiating 

 body, the position of the parts of the body on which the radia- 

 tion falls can only be altered up to a certain limit. For the body 

 which receives the radiation can never become hotter than that 

 from which it receives its heat. In most cases, indeed, it only 

 acquires a lower temperature than the latter, since it is constantly 

 radiating off into the surrounding space some of the heat it 

 receives. It is only in the case where this heat is given out ex- 

 clusively towards the radiating body — when, for instance, the 

 latter as it were completely surrounds the body towards which it 

 radiates — that this acquires the same temperature as the radiating 

 body, since in this case it gives out no more heat than it receives. 



A portion of the motion which is imparted to a heated body 

 is then constantly being given off to the surrounding space, or, 

 more correctly, to the surrounding sether ; and its temperature 

 can only remain constant when the circumstances are such that 

 a body loses exactly the same quantity of motion as it receives. 



We are not acquainted with any similar communication of 

 motion in the case of light. The illuminated body may reflect 

 light, or it may absorb it, but it does not thereby become self- 

 luminous, with the exception of such bodies as become phospho- 

 rescent or fluorescent by insolation. 



The motion of the parts of a body, which the rays of heat fall- 

 ing upon it produce, consequently always reproduces motion of 

 material parts ; for the heated body, by giving out heat again to 

 other bodies, warms them and occasions motion of their parts. 

 It is precisely in consequence of this communication of motion 

 to the material parts of bodies that heat can transform itself into 

 work, and can conversely be reproduced from work. 



In this communication of motion, it is remarkable that, how- 

 ever various may be the heat-colour of the rays which fall upon 

 a body, it always gives out, when thus heated, the heat-colours 

 which are peculiar to itself. In the case of light, we find some- 

 thing analogous in relation to fluorescence; for a body fluoresces 

 with the colour peculiar to its own substance, and therefore dif- 

 ferent from the colour which falls upon it. 



Hence, in discussing the identity of heat and light, we must 

 first of all keep in mind that heat depends upon motions of ma- 

 terial particles, or of the aether which intervenes between them, 

 thus causing a separation of the material particles of bodies o 



