and Polarization of Heat. 157 
non-luminous heat, and with incandescent platinum it was ex- 
tremely faint. My subsequent experiments gave for the propor- 
tion of the depolarizing effect to the whole heat which reached 
the pile when the plates E and F were crossed, 
Non-luminous Heat. Incandescent Platinum. Argand. 
.00 .016 03 
But upon performing this experiment with a thicker plate, namely 
that before alluded to in (53) and (54), [found that where it was 
interposed between the crossed polarizing and analyzing plates, the 
quantity of heat which reached the pile was increased by that inter- 
position 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, I 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 co- 
incident results. The quantity of light accompanying the heat, 
appeared by no means to regulate the quantity of heat depola- 
rized. The heat emitted from platinum, of a full red, (and 
therefore not vividly incandescent), was one of the most favour- 
able. 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 vessel, at about 500°, 
and found the results admirably marked. Pursuing the experi- 
ment as the temperature of the mercury descended, I found the 
effect still very sensible at 220°, and then thought of trying hot 
water, which I had not done since I devised the telescopic 
method of observing the galvanometer, (6). The result was, 
