356 Mr. B. Stewart's Reply to some remarks by M. Kirchhoff 



proved (with perfect right, as Kirchhoff himself allows) that 

 radiation proceeds from the interior of bodies as well as from 

 their surface, the question then occurs, " Are we to suppose each 

 particle of each substance to have at a given temperature an in- 

 dependent radiation of its own, equal, of course, in all direc- 

 tions?'' It might have been answered to this, that radiation is 

 a property of the particles of a body which we cannot well con- 

 ceive of as being influenced by the position of those particles in 

 the substance of the body ; so that perhaps it was hardly neces- 

 sary to prove that the radiation of a particle or plate is indepen- 

 dent of its distance from the surface. Nevertheless I have shown 

 experimentally that the heat from two plates of rock-salt placed 

 the one behind the other is the same as that from a single plate 

 equal in thickness to the two. 



Assuming it therefore as proved that the radiation of a par- 

 ticle or plate is independent of its distance from the surface, the 

 proof of the law which asserts " that absorption is equal to radia- 

 tion, and that for every description of heat," may for convenience' 

 sake be carried into the interior of the body, by which means we 

 are able to rid ourselves of surface reflexion. Let us therefore 

 suppose that in the interior a stream of radiant heat is constantly 

 flowing past a particle A in the direction of the next particle B. 

 Now, since radiation is independent of distance from the surface, 

 the radiation of A is equal to that of B ; and since absorption is 

 equal to radiation, the absorption of A is therefore equal to that 

 of B. Again, as the stream of radiant heat passes A, part of it 

 will be absorbed by A ; but since the radiation of A is equal to 

 its absorption, this stream will be as much recruited by the one 

 as it is diminished by the other, so that when it has passed A it 

 will be found unaltered by its passage with regard to quantity. 



Of this heat it has already been shown that B absorbs as much 

 as A ; and in order that this may be the case, the quality as well 

 as the quantity of the heat which impinges upon B must be the 

 same as those of the heat which impinged upon A. For, suppose 

 that the heat, by passing A, had changed its quality though 

 not its quantity, and that it had been transformed into a descrip- 

 tion of heat scarcely absorbed at all by the substance in question ; 

 then the absorption of B would manifestly be less than that of 

 A, and this we have already shown cannot be the case. We 

 conclude therefore that the stream of heat, in passing A, has 

 neither altered its quantity nor its quality, and hence we argue 

 that radiation is equal to absorption, and that for every descrip- 

 tion of heat. 



This is the whole proof; and I am quite at a loss to know 

 in what respect it is deficient, especially since Kirchhoff has 

 not definitely stated his objections. Having gone so far, we 



