and Polarization of Heat. 1 49 



that the one fits the extremity of the thermal pile, and the other 

 slips into the first, and by turning it round we get observations 

 with plates, whose planes of incidence for rays passing along the 

 axis of the tube, are inclined 0°, 90°, 180°, or 270° to one another, 

 the direction of the ray is generally in a single straight line, and 

 the observations are made in the same manner, and with equal 

 facility as in ordinary experiments on transmission. I have little 

 doubt that, in this way, the polarization of heat might be proved 

 without the aid of the thermo-multiplier. The plates were fixed 

 at the polarizing angle for light. After what has been said, 

 art. ( 1 6), on the refrangibility of heat, it is clear, that the altera- 

 tion of the polarizing angle, in order to accommodate it to heat, 

 could hardly amount (by Sir David Brewster's law) to a sen- 

 sible quantity. 



36. I fitted up two other bundles of mica-plates, in square 

 pasteboard tubes of the kind described, which were marked E 

 and F, the other plates being occasionally substituted, in order to 

 verify the results, and to shew that no accidental peculiarity of 

 the plates could account for the differences observed. My expe- 

 riments were usually made thus. The tube E was fixed to the 

 pile ; the tube F, containing the other plate, had an index, which 

 pointed to 0°, when the two plates were parallel, to 90° when 

 they were at right angles, &c. Five observations were taken ; at 

 0°, 90°, 180°, 270°, and again at 0°. The mean of the first and 

 last were taken ; then the mean of this, and the indication at 

 1 80°, and the difference between this and the mean at 90° and 

 270°, was considered as the polarizing effect. An example will 

 best illustrate this : — 



1 834, Nov. 26. — Brass heated by Alcohol : 5|- inches from centre 



Oj A lie. Deviation. 



Analyzing plate (E) at 0° ; polarizing plate (F) at 0° 6| 



90 ........5| 



180 7 



270 6 



0° 7| 



5 



