1868.] 



President's Address. 



149 



The Kumford Medal has been awarded to Mr. Balfour Stewart, for his 

 researches on the qualitative as well as quantitative relations between the 

 powers of emission and absorption of bodies for heat and light, published 

 originally in the Transactions of the Koyal Society of Edinburgh and 

 in the Proceedings of the Royal Society of London, and now made 

 more generally accessible by the publication, in 1866, of his treatise on 

 heat. 



When a body is placed within an opaque envelope which is kept at a 

 constant temperature, it soon acquires the temperature of the envelope — 

 and that, whatever be the nature or form of the envelope or of the body. 

 The same is true if any number of bodies of different kinds be placed 

 within the envelope ; in the permanent state each of the bodies attains a 

 fixed temperature, the same as that of the walls of the envelope. The 

 equilibrium of temperature is not, however, of the nature of statical equi- 

 librium; according to the theory by which Prevost so beautifully ex- 

 plained the apparent radiation of cold, each body radiates heat all the 

 while, at a rate depending only on its nature and temperature, and not at 

 all on its environment ; and it is because the other bodies and the enve- 

 lope are also radiating heat, and the first body absorbs a portion of the 

 radiant heat thus falling upon it, that its temperature remains unchanged. 

 The equality of radiation and absorption follows as a simple corollary. 



It had long been known that rock-salt is remarkable for its trans- 

 parency for obscure radiant heat. According to Melloni, a plate of rock- 

 salt of the thickness of three or four millimetres transmits 92 per cent, 

 of heat-rays from whatever source. Wow, on measuring by the thermo- 

 pile the radiation from thin and thick plates of rock-salt, as well as from 

 two or more plates placed one behind the other, all being heated up to a 

 definite temperature, Mr. Stewart found that the radiation from a thick 

 plate, or from many plates, was, indeed, greater than from a thin plate or 

 from a single plate, but that the difference was not by any means so great 

 as it ought to have been on the supposition that the heat radiated by the 

 hinder portion of a thick plate, or by the hinder plates of a group, passed 

 through the front portion of a thick plate, or through the front plate of a 

 group, as freely as obscure heat would have passed which was radiated by 

 lampblack or most other substances. It thus appeared that rock-salt at 

 any temperature is by no means transparent to heat radiated by rock-salt 

 of the same temperature — that it exerts a preferential absorption on rays 

 of the quality of those which it emits. This conclusion was confirmed by 

 using a plate of cold rock-salt as a screen by which to sift the heat-rays 

 falling on the thermopile. It was found that a much larger proportion of 

 the heat was stopped by the screen when the source of heat was a plate 

 of heated rock-salt than when it was a body coated with lampblack. The 

 proportion stopped was also sensibly greater when the source of heat was 

 a thin than when it was a thick plate of rock-salt, the reason being that 

 the heat radiated from the hinder portion of a thick plate was partially 

 sifted, in passing across the front portion, before it reached the rock-salt 



