158 Professor Forbes on the Refraction 



that, by most decisive experiments, I found that heat under 200° 

 Fahrenheit, is capable of being depolarized by mica. Even where 

 I did not measure the amount, the instantaneous motion of the 

 needle in the proper direction, when the Principal Section of the 

 mica plate was parallel, or inclined 45° to the plane of primitive 

 polarization, gave as strong evidence to this fact as to any other 

 I have recorded. 



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

 count of my experiments on depolarization within moderate com- 

 pass. 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 mentioned in (56),) 

 amongst 157 numerical comparisons, for the purpose of obtaining 

 the depolarizing effect, only one gives a negative, and one a neu- 

 tral result ; and these exceptions occur in observations made up- 

 on heat of the lowest temperatures, namely, from 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 unac- 

 companied 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 laminae into which it may 

 be split, naturally suggested itself, but I found that its intercep- 

 tive power for heat is so much greater than that of mica, as to 

 render these experiments nearly abortive. With heat from in- 

 candescent platinum, however, I got tolerably 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 



