76 Professor Forbes's Researches on Heat. 



tion of mica induces, in opposition to the natural quality of 

 the substance (9), the same peculiarity which a film of smoke 

 possesses relatively to the incident heat. It is truly for heat 

 what red glass is for light, it transmits most freely rays of low- 

 est refrangibility. 



12. Seeing clearly from the first that the change of charac- 

 ter in mica was due to the splitting up into an almost infinite 

 number of minute surfaces the natural laminae of the mineral 

 mica; and attributing the character of redness (so to speak) to 

 the multiplied and irregular reflexions and interferences which 

 must so take place, it occurred to me as very probable, that 

 the erFect of smoke was due to the superposition of a prodigi- 

 ous number of minute opake points upon a transparent surface, 

 and that not so much from any physical peculiarity of its car- 

 bonaceous material, as from the mechanical distribution of 

 opake dust over the diaphragm of rock-salt. 



13. This induced me to try the effect of mechanical altera' 

 tions of the physical surface of the salt, expecting to find an ef- 

 fect analogous to that of smoking, and, guided by no other 

 grounds of conjecture than those which I have stated, I rough- 

 ened with sand-paper both sides of a polished plate of rock-salt, 

 furrowing each surface rectangularly until it was quite dim. 

 I then examined its transmissive power for heat from different 

 sources, and was gratified to find my anticipation realized. 

 The proportion of dark heat transmitted, compared to that 

 from a lamp sifted by glass, was no less than as 3 to 1*. 



* I state it as a proof of the conviction which I had of the real charac- 

 ter of split mica with respect to heat, that the reasoning stated in the text 

 was founded upon no experiments made subsequently to those of March 1 838 

 already, qnoted. The very first ent/y in my journal-book of last autumn 

 contains simultaneous experiments, (1.) on smoked salt,toverify M.Melloni's 

 observations: (2.) on split mica, to extend my own of March 1838 to per- 

 pendicular incidences : (3). on scratched surfaces, on the assumption that 

 the two former would be realized. As M. Melloni thinks that I had not 

 a clear idea of the properties of split mica, which, indeed, if I understand 

 him, he still doubts, I will quote verbatim the passage in my laboratory- 

 book alluded to. — " 1839, Nov. 12. M. Melloni having lately stated 

 (Comptes Rendus, 2nd Sept.) that smoked rock-salt is the only substance 

 known which transmits heat of low temperature easier than luminous, this 

 is in the first place contradicted by my experiments of 1838, Mar. 20. &c. 

 on mica split by heat, already published, — and in the next place, I felt [feel] 

 some doubt whether [in his experiments] it was the quality of the material 

 or only the surface which affects the result. To try this, and to verify pre- 

 vious experiments, I smoked a plate of rock-salt; I roughened another with 

 sand-paper, first on one, and then on both surfaces ; I had also the split 

 mica plate marked H placed pcrpendicidarly to the rays of heat." 



[Here follow the experiments.} 

 " It clearly appears, then, that salt simply roughened transmits most dark 

 heat. 1 presume that the effect of smoking is only superficial, and that 

 roughening stifles luminous heat faster than dark heat." 



This is the first entry in my book after the publication of M. Melloni's 

 letter in the Comptcs Rendus, and it is given entire. 



