22 REPORT—1840., 
researches on the polarization of heat by refraction and re- 
flexion. In the former he employed piles of mica, and 
through these, found even dark heat very freely transmitted at 
the polarizing angle. Without (in this stage of the inquiry) 
aiming at quantitative results, he found in general that the pro- 
portion of heat polarized varied with the source in the following 
order, begining with the highest :— 
Argand lamp. 
Locatelli lamp. 
Spirit lamp. 
Incandescent platina. 
Hot brass, about 700° Fahrenheit. 
Mercury, 500° in crucible. 
Water under 200°. 
(3.) The polarization of heat by reflexion at the surface of a 
pile of plates of mica was also established; and with regard to 
the reflexion from glass, Prof. Forbes has also remarked,that 
from the known proportions of heat reflected, the quantity, 
even at the maximum which would reach the thermoscope after 
two reflexions, would be so extremely small, that no difference 
of effect in the two rectangular positions could really have been 
perceptible in the form of the experiment adopted by Berard. 
(4.) In the fourth section the author enters on the modifications 
which polarized heat undergoes by the intervention of crystal- 
lized plates between the polarizing and analysing parts of the 
apparatus ; an inquiry suggested by the obvious analogy in the 
case of light. In the crossed position, when polarized heat is 
stopped (if the analogy hold good), the intervention of a plate 
of double refracting crystal would restore the effect. This 
apparently paradoxical result was fully verified with plates of 
mica, and subsequently with selenite and other substances, not 
only in the case of luminous sources, but even with water be- 
low the boiling temperature. Of 157 experiments with three 
different mica plates, only one gave a neutral and one a negative 
result. Of these 157, 92 were made with heat below luminosity. 
The apparent paradox was increased by the circumstance 
that a thin plate of mica which “‘ depolarized’ but feebly 
seemed to stop more heat than a thick plate which depolarized 
more completely. 
The main fact was ascertained for the first time on Decem- 
ber 16, 1834. The Professor justly censures the use of the 
term “‘ depolarize,”’ and suggests ‘‘ dipolarize”’ as preferable. 
(5.) From the result thus unequivocally established, a train 
of highly curious consequences follow. We have hence, as 
direct corollaries, the dowble refraction of the rays of heat by 
