2 PROFESSOR FORBES'S RESEARCHES ON HEAT. 



facility) acquires, by being 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. 



only the more refrangible rays to pass ; and as M. MELLONI had been led by his previous experiments 

 to the same conclusion, his statement amounts to this, that, whilst rock-salt presents the analogy of 

 white glass, by transmitting all rays in equal proportions, every substance hitherto examined acts 

 on the calorific rays as violet or blue glass does on light, absorbing the rays of least refrangibility, 

 and transmitting only the others. 



" M. MELLONI believes, that the first exception to this rule, or the first analogue of red glass, 

 is rock-salt previously smoked. I desire, however, first to call attention to the fact, that, in a paper 

 published in May 1838 (Researches on Heat, Third Series), I described a substance having similar 

 properties, namely, mica split by heat to extreme thinness, such as I employ in polarizing heat. 

 In the month of March 1838, I had established by reiterated experiments, that the transmission of 

 heat through glass, far from rendering it less easily absorbed by mica in this peculiar state, had a 

 contrary effect, and also that heat of low temperature, wholly unaccompanied by light, was trans- 

 mitted 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 incandescence. 



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

 tables in the paper referred to (Art. 23, 24), properties exactly the reverse ; hence the effect is due 

 to the peculiar mechanical condition of the body, and not to its elementary composition. 



" It, therefore, at once occurred to me, on reading M. MELLONI'S communication, 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 transmitted 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; shewing, that the loss of polish affects the transmission of the more refrangible rays much 

 more sensibly than that of the others. 



" Yet this effect is not attributable to a variation in the ratio of the reflection of heat of dif- 

 ferent kinds at the surfaces of the plate. For, in the first place, I have proved, and already com- 

 municated the fact to the Royal Society (see Proceedings for April 1839), that reflection 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, re- 

 flection 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 roughened surface, as well 

 as through laminated mica and the smoky film, appears 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 modification is found to be more per- 



