Polarization by Refraction, 551 



Mica Plates I and K. Argand Lamp, with Chimney and Re- 

 Jlector : 16 Inches from Centre of Pile. 



Dynamical Effect. Mean. Ratio. 



Parallel 15.6 . 



Crossed 4.2 i 15.67 . . . 27 : 100 



Parallel 15.75^ 



Crossed 4.4 i 15.85 . . . 28 . 100 



Parallel 15.95-' 



Dark Heat from Brass, warmed by Alcohol: 16 Inches. 



Parallel 10.55. 



Crossed 4.05 \ 10.57 • . . 38 : 100 



Parallel ..... 10-6 ^ 



Crossed 3.9 i 10.55 ... 37 : 100 



Parallel 10.5 ^ 



These are given as examples of the usual mode of pro- 

 ceeding with such experiments, the zero point being ascer- 

 tained between each observation, and the dynamical effect 

 reckoned from it. On the whole, I obtained for the propor- 

 tion of heat polarized, or stopped in the transverse position 

 of the plates, the following numbers : 



Source of heat. Rays out of 100, polarized. 



Plates I and K. Plates G and H. 



Argand lamp 72 to 74 82 



Incandescent Platinum, 72 79 



Brass heated to about 700°, .... 63 68 



Heat from the same source transmitted \ ^^ ^r» 



through glass, ] '-^ "* 



Mercury in a crucible at 410°, ... 48 



Boiling Water, 44 49 



It thus appears that the Plates G and H are capable of polar- 

 izing no less than 82 per cent, of some kinds of incident heat : 

 these plates I began to use in the commencement of Decem- 

 ber last. 



The unequally polarizable nature of different kinds of heat 

 having been controverted, I took several methods of assuring 

 myself that the observed effects were not due to inequalities 

 in the dimensions of the sources of heat employed, or to their 

 variable distances from the pile. A multitude of proofs might 

 be given, but I will content myself with stating one or two of 

 the most decisive. 1. Incandescent platinum and dark hot 

 brass were successively placed at the same distance of twelve 

 inches from the pile ; a thin plate of glass being placed be- 

 tween the latter and the pile, and two thick plates of glass 

 between the former and the pile. The quantities of heat 

 reaching the pile were thus almost equalized, and the result 

 was, that the heat from a dark source, after transmission 

 through glassy became as polarizable as that from incandescent 



