and Polarization of Heat. 141 
the refrangibility of the rays diminished with their temperature. 
I also obtained a slight refraction of non-luminous heat through 
a glass prism. 
17. But if heat be capable of refraction by the ordinary 
agents, an important question arises, Is the phenomenon of dou- 
ble refraction common to heat and light? Rock-salt, the only 
substance yet discovered, which transmits dark heat in large 
quantity, does not possess this power. To attempt it with Ice- 
land spar would certainly be fruitless, from the very small trans- 
mitting power which it possesses, besides some other practical 
difficulties which suggest themselves. It must be by more re- 
fined processes that we can detect this property. Such will be 
stated in the sequel. 
§ 2. On the Polarization of Heat by Tourmaline. 
18. It is well known that two slices of tourmaline cut parallel 
to the axis of the crystal, as they are looked through with their 
axes parallel or perpendicular to one another, transmit a great 
portion of the incident light in the one case, and almost wholly 
intercept it in the other. 
19. It occurred to me as a curious question, at an early pe- 
riod of my researches, whether non-luminous heat would undergo 
any similar change in similar circumstances. I made a prelimi- 
nary experiment with heat from an oil-lamp (not an argand), and 
though, when the axes were crossed, the whole light was stopped, 
the heat transmitted appeared to be as intense as before. ‘The 
tourmalines which I employed were mounted on glass, and were 
kindly lent to me by the Reverend Mr Craic. Struck with the 
singularity of the result, I repeated the experiment with addi- 
tional precautions, and I found that some circumstance prevented 
this statement from being true in all its generality. The quan- 
tity of heat transmitted being very small, the lamp, the tourma- 
lines, and the pile were very near to one another; and, as the 
tourmaline absorbs heat with great rapidity, I found that a mi- 
nute difference might exist if the experiment was made first with 
