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Melloiii finds, that rock-salt (which is well known to transmit rays 

 from every source with equal facility) acquires, by being smoked, 

 the power of transmitting most easily heat of low temperature, or 

 that kind of heat stopped in greatest proportion by glass, alum, 

 and (according to M. Melloni) every other substance. The ex- 

 periments contained in the Third Series of my Researches on Heat, 

 shew tliat this is equivalent to saying, that substances in general 

 allow 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 propor- 

 tions, every substance hitherto examined acts on the calorific rays 

 as violet or blue glass does on light, absorbing the rays of least re- 

 frangibility, and transmitting only the others. 



" M. Melloni believes, that the first exception to this rule, or the 

 first analogue of red glass, i'^ rock-salt previously smoked. I de- 

 sire, however, first to call attention to the fact, that, in a paper 

 published in May 1838 (Researches on Heat, Third Series), I de- 

 scribed 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 experi- 

 ments, that the transmission of heat through glass, far from ren- 

 dering it less easily absorbed by mica in this peculiar state, had a 

 contrary eflfect, and also that heat of low temperature, wholly un- 

 accompanied 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 ha/fas 

 much luminous heat from a lamp 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 efi'ect 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 efi^ect of smoking the salt might be merely 

 owing to a mechanical change in the surface affecting the trans- 

 mission. 



" Roughening the surface was the most obvious experiment, and 

 I found, as I anticipated, that heat of low temperature is very much 



