Prof. Forbes on the Uefraction and Polarization of Heat, 289 



58. It would be quite impracticable to give any detailed ac- 

 count of my experiments on depolarization within moderate 

 compass. It may be satisfactory, however, to mention, that, 

 upon an examination of all the experiments I have recorded, 

 I find that (excluding those on the thin plate of mica men- 

 tioned in (56),) amongst 157 numerical comparisons, for the 

 purpose of obtaining the depolarizing effect, only one gives a 

 negative, and one a neutral result; and these exceptions occur 

 in observations made upon heat of the lowest temperatures, 

 namely, fiom mercury under 500°, and water under 200°. 

 These experiments were made with heat from the various 

 sources mentioned above (57), and with three different mica 

 plates. The comparisons were always made from alternate 

 observations, as in (20) and (52). Of these 157 comparisons, 

 no less than 92 were made with heat wholly unaccompanied 

 by visible light. 



59. These conclusions, derived entirely by the use of mica 

 as the depolarizing crystal, I endeavoured to confirm in the 

 case of some others. Selenite, from the thin lamina? into 

 which it may be split, naturally suggested itself, but 1 found 

 that its interceptive power for heat is so much greater than 

 that of mica, as to render these experiments nearly abortive. 

 With heat from incandescent platinum, however, 1 got toler- 

 ably marked indications of its action. 



60. With tourmaline I was more successful. Not only was 

 I able to obtain decisive depolarization when slightly luminous 

 heat was employed, such as that from incandescent platinum, 

 and the principal section of the tourmaline was alternately pa- 

 rallel, 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-po^ 

 larization 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 po- 

 larized in opposite planes, and that they become capable of in- 

 terfering by the action of the analysing plate*. 



62. These results we hold to be direct conclusions from the 

 establishment of the existence of a mode of action, of a very 



* I made one attempt to obtain polarizing effects by means of Mr. Nicol's 

 very elegant single-image calcspar pri;.ms, but without success, as I had an- 

 ticipated, from the great proportion which the thickness of the spar neces- 

 sarily bears to its aperture. 



Third Series. Vol. 6. No. U. April 1835, 2 P 



