Fourth Series. — Laminated and Smoked Surfaces. 75 



Table of the proportion of Heat from different sources trans- 

 mitted by the Polarizing Mica Plates I and K, contrasted 

 with the transmissions by Mica in its usual state, and with 

 Black Glass. 



10. This singular result of the mechanical condition of the 

 mica did not fail to strike me greatly at the time, and was not 

 published until after careful repetition. It afforded a triumph- 

 ant reply to an objection against my experiments which I was 

 then combating, that the quantity of heat absorbed by the po- 

 larizing plates had modified and even inverted the results, and 

 having satisfied myself of that, I did not pursue the matter 

 further. The moment, however, that I read M. Melloni's com- 

 munication on Smoked Salt, I perceived the important light 

 which the perfectly analogous case of the split mica might 

 throw upon the phenomenon. 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 interposing 

 mica piles at the great and disadvantageous obliquities which 

 I had employed (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 : — 



Transmission through Split Mica H, at a Perpendicular Incidence. 



Source of Heat. 



Locatelli, with glass 



Locatelli 



Dark hot brass 



Hot water 



* This observation having been made at a different time from 

 the others, and probably not under exactly the same circum- 

 stances, I have stated it in the way least favourable to the views I 

 entertain : the per-centage actually observed was 19. 



11. It appears, then, very clearly, that this peculiar condi- 



