468 Professor ForBes’s Researches on Heat. 
an equal quantity of heat reaching the pile in all positions of the 
analyzing plate. 
39. Last summer it occurred to me that it was probable that 
rock-salt, refracting heat almost as it does light, would cause it 
to undergo total reflection at a proper incidence. Supposing 
this to be the case (and I had afterwards reason to believe that 
such had been shewn to be the fact by M. Mexxon1), I then 
foresaw the possibility of trying an experiment of the most con- 
clusive character, as to the nature of heat,—its susceptibility of 
becoming circularly polarized by means of two total internal re- 
flections, as in the admirable experiment of F'RresNnex in the case 
of light. 
40. Various circumstances prevented me from trying this ex- 
periment until the end of January last, when I procured a rock- 
salt rhomb, similar to that of glass used by Fresnex, but having 
its angles calculated by Fresnex’s formula, for the refractive in- 
dex for light of rock-salt. I took the smaller of the two angles 
which the double solution of the quadratic equation gives, on 
account of the smaller dimensions required for the rhomb. ‘This 
angle is nearly 45°. On the Ist of February I performed the 
experiment with complete success, though with an apparatus less 
perfect than I afterwards procured. The arrangement represent- 
ed in Fig. 9. proved exceedingly convenient. R is the rhomb 
of salt laid on its side, so that the plane of reflection within it is 
horizontal ; S the source of heat, P the pile, I the polarizing mi- 
ca bundle, K the analyzing mica bundle. The following facts 
were observed : 
41. When the plane of reflection coincided with, or was per- 
pendiculur to, the plane of primitive polarization, the heat (whe- 
ther wholly dark, or derived from incandescent platinum) came 
out unchanged, that is, on placing the analyzing plate in azimuth 
0° and 90° relatively to the polarizing plate, the ratio of the ef- 
fects was the same as if no reflection had taken place. 
42. When the plane of first polarization was inclined + 45° 
