and Polar ization 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 (2 1 ), not mounted on glass, and of a 

 pale amber colour. 



6 1 . From these experiments, the depolarization, or Depolariza- 

 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 rings and brushes, denned 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 Nicol's very 

 elegant single-image calc-spar prisms, but without success, as I had anticipated, from 

 the great proportion which the thickness of the spar necessarily bears to its aperture. 



