Mr. B. Stewart on Internal Radiation in Uniaxal Crystals. 329 



theoretically from the law of exchanges, and the experimental evi- 

 dence upon which it rests, are both of a very simple nature, it has 

 been thought well to restate them here before proceeding further in 

 this investigation. 



Let us imagine to ourselves an enclosure of lamp-black kept at a 

 constant temperature, and containing two pieces of polished rock- 

 salt similar to one another, except that the thickness of the one is 

 greater than that of the other. 



Now it is evident that since the thick piece absorbs more of the 

 heat which falls upon it from the sides of the enclosure than the thin 

 piece, it must likewise radiate more in order that it may always re- 

 main at the same temperature. Here then we have the fact of in- 

 ternal radiation in the case of rock-salt deduced as a theoretical conse- 

 quence of the law of exchanges ; experimentally it is found that a thick 

 piece of rock-salt radiates very considerably more than a thin piece. 



The fact of internal radiation being conceded, it is easy to see 

 that the amount of heat which a particle radiates must be indepen- 

 dent of its distance from the surface. For besides that this is the 

 simplest hypothesis, the absorption, and consequently the radiation 

 of two similar plates of rock-salt placed with their surfaces together, 

 ought to be the same as from a single plate of double the thickness ; 

 and experiment shows that this is the case. 



It being therefore supposed that the internal radiation of a particle 

 is independent of its distance from the surface, let us imagine a row 

 of particles A, B, C, D in the midst of a substance of constant tem- 

 perature which extends indefinitely on all sides of them. There will 

 be a certain stream of radiant heat constantly flowing past any such 

 particle A to go in the direction AB. 



Now, since the radiation is supposed to be the same for the different 

 particles A, B, C, D, it follows that the absorption of the stream of 

 heat by these particles must also be the same for each ; and in order 

 that this may be the case, it is necessary that the stream which im- 

 pinges on one particle be the same in quantity and in quality as that 

 which impinges upon another. This consideration leads us to a me- 

 thod of viewing internal radiation, which is wholly independent of the 

 diathermanous or athermanous character of the body. For whatever 

 be the absorption of a particle for any description of heat, its radia- 

 tion must necessarily be precisely the same in order that the stream 

 of heat in passing the particle may be just as much recruited by its 

 radiation as it is reduced by its absorption ; in other words, we may 

 regard the substance through which the heat passes as perfectly 

 diathermanous. 



We gain another advantage by this method of viewing the subject : 

 for, in the law which is expressed by saying that the absorption of a 

 particle is equal to its radiation, and that for every description of 

 heat, the word description is used to define and separate those rays 

 of heat which are absorbed in different proportions by the same sub- 

 stance. Therefore in any problem connected with this subject we 

 may suppose that a separate equilibrium holds for every such ray. 



Now it is well known that rays of different wave-lengths are ab- 

 sorbed in different proportions by the same substance. We are 

 therefore entitled to suppose that a separate equilibrium holds for 



Phil Mag. S. 4. Vol. 23. No. 154. April 1862. Z 



