194 



substance being supposed to furnish the maximum amount of radia- 

 tion which that substance was capable of affording. 



It lately occurred to the author of this paper, reasoning from the 

 theory of exchanges, that mere surface radiation is not sufficient to 

 account for the equilibrium of temperature which exists between a 

 body and the enclosure which surrounds it. 



These theoretical conclusions have been amply verified by experi- 

 ment, and the subject has been discussed in a paper published in 

 the ' Transactions of the Royal Society of Edinburgh ' for the year 

 1858. As the chain of reasoning by which this fact is deduced 

 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. 



