Second Series.— Polarization by Refraction. 355 
out altering our views as to the probable identity of the cause of 
both those physical agents. With heat from incandescent pla- 
tinum the effects are extremely well marked. Thus with tour- 
malines E and F (see First Series, 22) I obtained for the ratio of 
the quantities of heat transmitted, with axes of crystals parallel, 
and axes crossed, 100: 76; or 24 per cent. of the heat was 
polarized (Jan. 19. 1836). When one of these tourmalines was 
combined with a mica plate, marked G, polarizing by transmis- 
sion (see below art. 20), the proportion was 100: 62, or 32 per 
cent. polarized of heat from incandescent platinum. 
16. With dark heat incomparably greater difficulty was ex- 
perienced. Excessively little heat could be obtained through 
the combined mass of tourmaline and the glass to which it is ce- 
mented, and of that little it appeared that but a minute portion 
was polarized, or at least absorbed by the action of the former. 
At one time I seriously doubted whether any perfectly dark heat 
came out of tourmaline polarized in one plane only. I have rea- 
son to believe that, in my first experiments, there was a source 
of error, arising from the form of the plates, which was not ad- 
verted to formerly. I have, however, satisfied myself that even 
dark heat is capable of being acted upon by tourmalines in the 
same manner as light. In my experiments the quantity appa- 
rently 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. 
17. In my last paper on this subject, I stated the fact of the 
polarization of all the kinds of heat which I tried by transmis- 
sion through thin bundles of mica placed obliquely. I stated 
the difficulties which I experienced, and the quantitative errors 
to which the results were liable. I shewed at the same time that 
these errors were of a kind calculated to mask the effects of po- 
