and Polarization of Heat. 149 
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 “ight. After what has been said, 
art. (16), 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 Davip Brewsrer’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 
180°, and the difference between this and the mean at 90° and 
270°, was considered as the polarizing effect. An example will 
best illustrate this :— 
1834, Nov. 26.—Brass heated by Alcohol : 53 inches from centre 
of P. ile. Deviation. 
Analyzing plate (E) at 0°; polarizing plate (F) at 0°.......... 63 
90 site. 52 
180... oe ar 
270 ..00...... 6 
