83 



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 variety 

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

 an argand lamp, and of 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 show 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 depolar- 

 ized, or the needle moved toward zero ; but if a mica film much 

 thicker (so much thicker as to stop more than twice as much common 

 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 polar- 



ized. When suitably modified, these rays are capable of 

 interfering, like those of light. 

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

 Royal Society on the 5th January. 



