Heat radiated by Rough and by Bright Surfaces. 93 



Concluding Remarks. 



It is well known that Melloni, at the beginning of his inves- 

 tigations, concluded from the fact which he was the first to ob- 

 serve, namely that light and heat are transmitted in quite unequal 

 proportions by one and the same substance, that these two agents 

 were not identical. Later on, this view was abandoned by him ; 

 nevertheless I am not of opinion that Melloni regarded heat and 

 light as identical, for he speaks only of the " lien le plus puissant 

 qui reunisse ensemble ces deux grands agents de la nature"*. 



If it is desired to raise the question of the identity of heat and 

 light, it appears to me that the points of difference between heat 

 and light must first of all be distinctly stated. 



It is well known that, in order to account for the propagation 

 of heat across empty space, it is necessary to assume, as in the 

 case of light, the existence of an aether which pervades all space, 

 and by the motions of which this propagation takes place; and 

 the simplest assumption is that both light and heat are propa- 

 gated by means of the same sether. When the heat-motions 

 have arrived at a distant body, they set the aether which this con- 

 tains in corresponding movement, provided it is capable of assu- 

 ming it. The questions as to what is the nature of these move- 

 ments, and how they differ from those which constitute light, 

 must for the present remain unanswered. The heat-movements, 

 however, have the effect of altering the position of the parts of 

 the body to which they are communicated. For every body upon 

 which rays of heat fall is thereby caused to expand, and thus the 

 relative position of its parts is changed. The only case in which 

 the rays would occasion no .elevation of temperature, and conse- 

 quently no change in the position of the parts, would be if the rays 

 fell upon a perfectly diathermanous substance. But a perfectly 

 diathermanous body does not exist ; for even atmospheric air does 

 not allow all the rays of heat to pass through it perfectly. If 

 we compare herewith the motions which constitute light, we find 

 that their effects are quite different. It is true that there is no such 

 thing as a perfectly transparent substance, and it would hence 

 appear possible for light to act on the particles of bodies ; but 

 no such action has hitherto been observed, except in so far as light 

 can occasion chemical combination, and can produce fluorescence 

 and phosphorescence. In these phenomena, the motions of the 

 material particles, although there can be no doubt of their exist- 

 ence, have as yet entirely eluded direct observation. The cha- 

 racteristic distinction between the movements which constitute 

 light and those which constitute heat may accordingly be stated 



* ha Thermochrose, p. 333. 



