FOURTH SERIES. LAMINATED AND SMOKED SURFACES. 7 



plates had modified and even inverted the results, and having satisfied myself of 

 that, I did not pursue the matter farther. The moment, however, that I read 

 M. MELLONI'S communication on Smoked Salt, I perceived the important light 

 which the perfectly analogous case of the split mica might throw upon the pheno- 

 menon. It was evident that the results were similar in kind, it was probable 

 that they might be made to approximate in degree. Instead, therefore, of inter- 

 posing mica piles at the great and disadvantageous obliquities which I had em- 

 ployed (when I wished simply to test their action as polarizing plates), I took a 

 split mica pile (frequently referred to in former parts of these memoirs under the 

 designation H) and placed it perpendicularly to the incident rays of heat. I 

 obtained the following results : 



11. It appears, then, very clearly, that this peculiar condition of mica in- 

 duces, in opposition to the natural quality of the substance (9), the same pecu- 

 liarity 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 lowest re- 

 frangibility, 



12. Seeing clearly from the first that the change of character 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 reflections and interferences which 

 must so take place, it occurred to me as very probable, that the effect of smoke 

 was due to the superposition of a prodigious number of minute opaque points 

 upon a transparent surface, and that not so much from any physical peculiarity 

 of its carbonaceous material, as from the mechanical distribution of opaque dust 

 over the diaphragm of rock-salt. 



13. This induced me to try the effect of mechanical alterations of the physical 

 surface of the salt, expecting to find an effect analogous to that of smoking, and, 

 guided by no other grounds of conjecture than those which I have stated, I 

 roughened 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 trans- 



