and Polarization of Heat. PSS 
stance, render it capable of reflection at a surface inclined to the 
rays of light at the polarizing angle, which they were incapable 
of doing before the crystal was interposed, or if before capable of 
reflection, they may now be partially, or wholly, incapable of it. 
Such a mode of action may in general terms be called depolariza- 
tion, an expressive term, though not quite correct, or as has more 
lately been proposed, in conformity with the more accurate views 
now entertained on the subject, D1-polarization, indicating that 
the action of the interposed crystal is to separate the incident 
polarized ray into two parts by its doubly refracting energy ; 
which parts are polarized in rectangular planes, and by their 
union produce the modified effect. But whatever be the explana- 
tion which we adopt of the curious and complicated changes 
which doubly refracting crystals exercise in the case of light, it is 
clear that the establishment of a correlative fact in regard to 
heat unaccompanied by light, must force us to admit an identity 
of the laws which combine, by a smgularly refined mechanism, 
to produce an identical result. 'The theory of undulations is 
in fact by far the simplest that we can adopt, and it requires us, 
if we admit depolarization, to admit the existence of double re- 
fraction and of interference. 'The demonstration, then, of such 
a property of heat, is one of such importance, as to require the 
fullest. proof. 
48. The power of mica to depolarize heat, I discovered on 
the 16th of December last. If in the case of polarizing light, 
whether by reflection or refraction, the planes of incidence rela- 
tively to the polarizing and analyzing plates be at right angles to 
one another, the light is wholly (or at least in great part) stopped. 
The plates remaining in this position, it is well known, that if a film 
of mica be interposed between them, so as to be perpendicular to 
the incident light, that light will no longer be stopped excepting 
in two positions, namely, when the Principal Section of the mica 
plate (or the plane containing the two axes) is parallel or perpen- 
dicular to the plane of polarization. In intermediate positions, 
light reaches the eye. This is true for all thicknesses of the film 
VOL. XIII. PART I. U 
