and Polarization of Heat. 159 
principal section of the tourmaline was alternately parallel, and 
inclined 45° to the plane of primitive polarization, but also when 
dark heated brass was used (at 700°). The tourmaline was one 
of those marked C and D (21), not mounted on glass, and of a 
pale amber colour. 
61. From these experiments, the depolarization, or Di-polariza- 
tion of heat seems unquestionably established, whence admitting 
that it depends on the same mode of action as the corresponding 
facts in the case of light, which seems certain, we are bound to 
admit that heat (even that from warm water), is susceptible of 
double refraction, that the two pencils are polarized in opposite 
planes, and that they become capable of interfering by the action 
of the analyzing plate.* 
62. These results we hold to be direct conclusions from the 
establishment of the existence of a mode of action, of a very com- 
plicated character, which nothing but an acquaintance with the 
corresponding facts with regard to light could have taught us how 
to look for, and which, by coinciding with these, indicate a com- 
mon mechanism. Hence, too, were our senses or our instruments 
capable of perceiving them, we should necessarily discover, by 
the passage of heat along the axes of doubly refracting crystals, 
all the elegant forms of rmgs and brushes, defined by heating, in- 
stead of luminous rays. 
63. But this analogy may be carried still farther. So definite 
are the experimental results in depolarization, that I thought of 
comparing the intensities of the effects with those produced in 
light ; and for this purpose, our methods of estimating heat is far 
more satisfactory than those for estimating the intensity of illu- 
mination. ‘The fundamental law, which I felt most anxious to 
* I made one attempt to obtain polarizing effects by means of Mr Nico1’s very 
elegant single-image cale-spar prisms, but without success, as I had anticipated, from 
the great proportion which the thickness of the spar necessarily bears to its aperture. 
