342 RADIANT HEAT. 



transmission of radiant heat. He found that rock salt acquires, by 

 being smoked, the power of transmitting most easily heat of low 

 temperature, or of that kind which is stopped in the greatest propor- 

 tion by glass, alum, and (according to his view) all other substances. 



Upon this point Professor Forbes was led to some further considera- 

 tions, and thence to fresh series of researches " On the Effect of the 

 Mechanical Textures of Screens on the Immediate Transmission of 

 Radiant Heat," an account of which he communicated to the Royal 

 Society of Edinburgh December 16, 1839. 



Upon the above-mentioned result of Melloni, Professor Forbes re- 

 marks that, according to the conclusions indicated in his own Re- 

 searches, (third series,) Melloni' s view of the interception of heat of 

 low temperature by all substances alike is equivalent to saying that 

 substances in general allow only the more refrangible rays to pass, 

 or that while rock salt presents the analogy of white glass, by trans- 

 mitting all rays in equal proportions, every other 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. And to this rule Melloni now makes out the first exception, 

 or the first analogue of red glass, to be rock salt, having its surface 

 smoked. 



Now, Professor Forbes, in his third series, had also pointed out 

 another substance having the same property, viz: mica split by heat. 

 In March, 1838, he had established, by repeated experiments, that 

 the previous transmission of heat through glass, far from rendering 

 it less easily absorbable by mica in this state, had a contrary 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. 



Mica not laminated possesses no such property; hence the effect is 

 due to the peculiar mechanical condition of the substance, and hence 

 it occurred to the author that the effect of smoking the rock salt was 

 owing merely to a mechanical change in the surface; he therefore 

 proceeded to try the effects of surfaces altered by mechanical means. 



The surface of rock salt being roughened by sand-paper, it trans- 

 mitted non-luminous heat more copiously than luminous. Mica 

 similarly scratched showed the same result. 



This effect is not attributable to differences in the proportions of 

 heat reflected, for in this respect, at a polished surface, all kinds of 

 heat are alike, as he had before shown; whilst by direct experiment 

 he found that, at least for the higher angles of incidence, reflection 

 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 ine- 

 quality. 



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 some direct experiments on 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 the more perceptible as the character of the heat is more removed 



