Second Series.—Polarization by Refraction. 357 
of heat communicated from A to B, in the positions of Fig. 2. 
and Fig. 3. ‘lo any one who had tried any experiment of the 
kind, the enormous difference in the effect observed (rising to 
40 per cent. even in my first experiments), and the instantaneous 
nature of the change, would have been a sufficient answer. But 
I farther shewed that the effect was wholly independent of the 
interval between A and B, and that, therefore, the objection must 
fall to the ground. 
19. To place the matter beyond all cavil, however, I after- 
wards devised this experiment: I had a tin canister A, made, 
Fig. 4, having a surface a, similar in size, figure, and position to 
the mica plate A of Fig. 2, whose absorbed heat was supposed to 
affect the pile differently, by being placed in a rectangular posi- 
tion. The second mica plate B, was interposed as before,—that 
is, between the pile P and the heated body A, the canister being 
now filled with boiling water. The effect on the pile, of the sur- 
face a, was now very great, exceeding, perhaps some hundred 
times, that of the mica surface with its absorbed heat, which it 
replaced. Yet, upon turning the canister A into successive rect- 
angular positions, no very decided difference of effect upon the 
pile could be observed (Dec. 21. 1835.) If any, the effect indicated 
a greater supply of heat in the crossed than in the parallel positions, 
or was opposed to that which would have indicated polarization. 
20. The polarizing effect takes place only at the surfaces of 
plates,—the absorptive effect depends upon their thickness. 
Hence to polarize heat effectively, a minute subdivision of mica 
into thin lamine is essential. ‘This I formerly effected by a pen- 
knife. I have since, however, discovered a method much more 
effectual. A piece of mica, thrown into a brisk red fire, is split 
up, by the expansion of the air between its films, into a multi- 
tude of pellicles, which reflect light. with almost metallic bril- 
liancy, and polarize it intensely by transmission. Such mica 
plates I have used, one pair being marked G and H ; the other 
I and K. 
21. By experiments with both these pairs of plates I have 
