70 Professor Forbes's Researches on Heat. 



smoked, the power of transmitting most easily heat of low 

 temperature, or that kind of heat which is stopped in greatest 

 proportion by glass, alum, and (according to M. Melloni) 

 every other substance. 



2. In the Third Series of these Researches, § 3, I have at- 



in polarizing heat. In the month of March 1838, I had established by re- 

 iterated experiments, that the transmission of heat through glass, far from 

 rendering it less easily absorbed by mica in this peculiar state, had a con- 

 trary effect, and also that heat of low temperature, wholly unaccompanied 

 by light, was transmitted almost as freely as that from a lamp previously 

 passed through glass. 



" It even appears, from experiments I have since made with the same form 

 of mica, that some specimens transmit scarcely half as much luminous heat 

 previously passed through glass, as that from a body below visible incan- 

 descence. 



" Mica itself, not laminated by the action of fire, possesses, as I have 

 shown by contrasted tables in the paper referred to (Art. 23, 24), properties 

 exactly the reverse ; hence the effect is due to the peculiar mechanical con- 

 dition of the body, and not to its elementary composition. 



"It, therefore, at once occurred to me, on reading M. Melloni's com- 

 munication, that the effect of smoking the salt might be merely owing to a 

 mechanical change in the surface affecting the transmission. 



" Roughening the surface was the most obvious experiment, and I found, 

 as I anticipated, that heat of low temperature is very much easier trans- 

 mitted by salt scratched by sand-paper in two directions at right angles, 

 than luminous heat. Thus, a plate of salt which, when well polished, 

 transmits 92 per cent, of heat derived from a lamp, and sifted by a glass plate, 

 and also 92 per cent, of heat wholly unaccompanied by light, transmitted, 

 when roughened, only 17 per cent, of the former and 45 per cent, of the 

 latter. 



" A thin plate of mica, when similarly scratched with emery-paper, so as 

 merely to depolish it, transmitted much more nearly the same per-centage of 

 heat from different sources than when bright ; showing that the loss of polish 

 affects the transmission of the more refrangible rays much more sensibly 

 that that of the others. 



" Yet this effect is not attributable to a variation in the ratio of the re- 

 flexion of heat of different kinds at the surfaces of the plate. For, in the 

 first place, I have proved, and already communicated the fact to the Royal 

 Society (see Proceedings for April 1839), that reflexion takes place at a 

 polished surface, with almost, if not exactly, the same intensity for all kinds 

 of heat ; and, secondly, I have found, by direct experiment, that, at least 

 for the higher angles of incidence, reflexion is most copious from rough 

 surfaces for heat of low temperature, or the same kind which is most freely 

 transmitted, proving incontestably that the stifling action of rough surfaces 

 is the true cause of the inequality. 



". That there is a real modification of the heat in passing through a rough- 

 ened surface, as well as through laminated mica and the smoky film, ap- 

 pears from direct experiments which I have made on the heat sifted by 

 these different media; which, when transmitted by any one of these, is 

 found in a fitter state to pass through each of the others ; and this modi- 

 fication is found to be more perceptible as the character of the heat is more 

 removed from that which these media transmit most readily, that is, as the 

 temperature of the source is higher. Thus, heat derived from a lamp, has 

 36 per cent, transmitted by a certain smoked plate of rock-salt. But if 

 the heat transmitted by the smoked salt has previously been sifted or ana- 



