Polarization hy Tourmaline. 549 



perceive where 'physical proof becomes satisfactory, though 

 yet far removed from mathematical certainty. To supply the 

 latter is a humbler and more mechanical task, which may be 

 undertaken at leisure, as in this paper we shall partly attempt 

 to do. 



§ 2. Polarization of Heat hy Tourmaline* 



On this subject I have little to add. It does not possess 

 the same theoretical importance as in the case of polarization 

 by other methods. Double refraction is better shown by the 

 phaenomena of depolarization by mica, described in my last 

 paper (art. 46 et seq.), and the phaenomena attendant on the 

 absorption of one of the doubly refracted pencils, are so ca- 

 pricious and ill understood, even in the case of light, that it 

 might hold or not for heat without altering our views as to 

 the probable identity of the cause of both those physical 

 agents. With heat from incandescent platinum the effects 

 are extremely well marked. Thus with tourmalines E and F 

 (see First Series, 22) I obtained for the ratio of the quanti- 

 ties of heat transmitted, with axes of crystals parallel, and 

 axes crossed, 100 : 76; or 24 per cent, of the heat was polar- 

 ized (Jan. 19, 1836). When one of these tourmalines was 

 combined with a mica plate, marked G, polarizing by trans- 

 mission (see below art. 20), the proportion was 100 : 62, or 

 32 per cent, polarized of heat from incandescent platinum. 



With dark heat incomparably greater difficulty was expe- 

 rienced. Excessively little heat could be obtained through 

 the combined mass of tourmaline and the glass to which it is 

 cemented, and of that little it appeared that but a minute por- 

 tion was polarized, or at least absorbed by the action of the 

 former. At one time I seriously doubted whether any per- 

 fectly dark heat came out of tourmaline polarized in one plane 

 only. I have reason to believe that, in my first experiments, 

 there was a source of error arising from the form of the plates, 

 which was not adverted to formerly. I have, however, satis- 

 fied myself that even dark heat is capable of being acted upon 

 by tourmalines in the same manner as light. In my experi- 

 ments the quantity apparently polarized did not exceed one- 

 seventh or one-eighth of the small quantity transmitted. This 

 was in combination with a polarizing mica plate (marked I). 



$ 3. On the Laws of the Polarization of Heat by Refraction 

 or Transmission. 



In my last paper on this subject, I stated the fact of the 

 polarization of all the kinds of heat which I tried by trans- 

 mission through thin bundles of mica placed obliquely. I 

 stated the difficulties which I experienced, and the quantita- 



