1835.] Refraction and Polarization of Heat. 395 



lacy in his mode of operation, and proved the polarization of heat, 

 whether luminous or obscure, by tourmaline. 



The Third Section treats of the polarization of heat by refraction 

 and reflection. The former method the author found by far the most 

 convenient, employing thin plates of mica, arranged at the polarizing 

 angle, and through which even dark heat was very freely transmitted. 

 The results were so marked that they were verified in a great va- 

 riety of ways, and with heat from sources extremely different, as that 

 of an argand lamp, and water below 200" Fahr. The polarization 

 of non-luminous heat by reflection was also established, though with 

 much less ease and simplicity. In this form it was announced by 

 Berard about twenty years ago, but hitherto his experiment does not 

 appear to have been repeated with success. 



The Fourth Section considers the modifications which polarized 

 heat undergoes by the action of doubly refracting crystals. The ana- 

 logies here are derived entirely from those of light. Very numerous 

 experiments are quoted to shew that the effects are quite analogous, 

 even when the source of heat is water under the boiling point. The 

 doubly refracting substance used to depolarize was generally mica. 

 Out of 157 recorded experiments on depolarization, with three dif- 

 ferent mica plates, only one gave a neutral and one a negative result. 

 Yet of these 157 experiments, no less than 92 were made with heat 

 unaccompanied by any visible light. One very striking experiment 

 is quoted in illustration of the marked nature of the effects. When 

 the polarizing and analyzing plates were situated so as to transmit 

 least heat to the pile, and a thin film of mica was interposed between 

 the plates in such a position as would depolarize light under similar 

 circumstances, the film was found to stop more heat than it depola- 

 rized, or the needle moved towards zero ; but if a mica film much 

 thicker (so much thicker as to stop more than twice as much com- 

 mon heat as the first) was similarly placed, that film depolarized 

 more than it stopped, and the needle moved in the opposite direction 

 to the former one. The investigation of the laws of depolarization 

 given in this section are hardly capable of abridgment. 



The following are the general conclusions : — * 



1. Heat, whether luminous or obscure, is capable of Polarization 

 by Tourmaline. 



2. It may be polarized by Refraction. 



3. It may be polarized by Reflection. 



4. It may be depolarized by Doubly Refracting Crystals. Hence — 



5. It is capable of double refraction and the two rays are pola- 

 rized. When suitably modified, these rays are capable of interfering 

 like those of light. 



6. The characteristic law of polarization in the case of light holds 

 in that of heat; viz., that the intensities in rectangular positions of 

 the polarizing and analyzing plates are complementary to each other. 



7. As a necessary consequence of the above, confirmed by experi- 

 ment, heat is susceptible of circular and elliptic polarization. 



8. The undulations of obscure heat are probably longer than those of 

 light. A method is pointed out of deducing their length numerically. 



* These conclusions were stated nearly in these words (except the 6th) to the 

 Royal Society on the 5th January. 



