288 Prof. Forbes ofi the Refraction and Polarization of Heat, 



and with that from incandescent phitinum. When I pro- 

 ceeded to estimate its depolarizing power by the usual method 

 of placing the Principal Section at 0° or at 45°, I totally failed 

 in obtaining a sensible effect with non-luminous heat, and 

 with incandescent platinum it was extremely faint. My sub- 

 sequent experiments gave for the proportion of the depolari- 

 zing effect to the whole heat which reached the pile when the 

 plates E and F were crossed, 



Non-luminous Heat. Incandescent Platinum. Argand. 



•00 -OK) -03 



But upon performing this experiment with a thicker plate, 

 namely, that before alluded to in (53) and (54), I found that 

 where it was interposed between the crossed polarizing and 

 analysing plates, the quantity of heat which reached the pile 

 was increased by that interposition by about 0°*5. Hence we 

 have the singular spectacle of the transmission of heat being 

 greater when a thick obstacle is interposed, whilst the direct 

 effect is actually diminished by the interposition of a thin one. 

 This effect was of the most marked character with heat from 

 incandescent platinum; with dark heat the result was quite 

 analogous, but within narrower limits. With unpolarized dark 

 heat, 1 found that the thin plate stopped 30 out of 100 rays, 

 whilst the thick one stopped 65^ or more than twice as much. 

 57. The depolarizing effect of mica was tried under every 

 variety of circumstance, and with the most conspicuous and 

 coincident results. The quantity of light accompanying the 

 heat, appeared by no means to regulate the quantity of heat 

 depolarized. The heat emitted from platinum, of a full red, 

 (and therefore not vividly incandescent,) was one of the most 

 fiivourable. Heat from an Argand lamp, with glass chimney, 

 was also employed, and absolutely non-luminous heat from 

 brass about 700°. I also employed mercury in an iron ves- 

 sel, at about 500°, and found the results admirably marked. 

 Pursuing the experiment as the temperature of the mercury 

 descended, I found the effect still very sensible at 220°, and 

 then lhou-;ht of trying hot water, which 1 had not done since 

 I devised the telescopic method of observing the galvanome- 

 ter (6). The result was, that, by most decisive experiments, 

 I found that heat under 200° Fahrenheit^ is capable of being 

 dejwlarized 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. 



