' 
OF RADIANT HEAT THROUGH DIFFERENT BODIES. 59 
interior of one continuous medium. This transmission we have ex- 
amined, and, as we have just seen, it presents nothing contrary to its ana- 
logy with the transmission of light through coloured media. There is 
however a particular case in which two homogeneous screens act in so 
singular a manner in respect to light that it must be interesting to know 
whether something analogous does not take place in respect to caloric. 
The optical phenomena presented by most of the slices of tour- 
maline cut parallel to the axis of crystallization are universally known. 
If these slices are placed one over the other and their axes laid in the 
same direction, they transmit light in considerable quantities. But if 
they be laid at right angles to one another, the light is totally intercepted. 
Do these phenomena, arising, as is well known, from the polarization of 
the light in the interior of the slices, take place in respect to calorific 
rays also; or, in other words, is radiant heat capable of being polarized 
in its passage through tourmaline ? 
In order to ascertain this I have taken two square plates of the same 
dimensions. I have made an aperture in the centre of each. This 
aperture was likewise a square having its sides parallel to those of the 
plate and each equal to the least breadth of the two polarizing slices. 
I then took some soft wax and attached a tourmaline to each aperture, 
holding the axis of the former parallel to one of the sides of the latter. 
These two plates being laid one over the other, it evidently depended on 
one of the sides of the one plate being placed parallel or perpendicular 
to a side of the other whether the light was to be transmitted or inter- 
cepted. Yet this pair of plates being placed vertically on the stand of 
my thermoelectric apparatus and exposed to the radiation of a lamp or 
incandescent platina, uniformly produced the same calorific transmission, 
whatever might be the relative direction of the sides of each plate. 
That this fact might be put beyond the reach of doubt the galvano- 
metric index was carried to the 18th or 20th degree, and the calorific 
communication now established was suffered to remain while we placed 
one of the plates on each of its sides in succession. The flame or the 
incandescent platina was then observed to appear and disappear alter- 
nately while the magnetic needle continued invariably at the same point 
of deviation. ; 
This experiment was repeated many times with several tourmalines, 
and the angle formed by the intersection of their axes varied. The re- 
sult was in all cases the same. The quantity of calorific rays trans- 
mitted through the two polarizing slices is then independent of the re- 
spective directions given to their axes of crystallization; that is to say, 
the heat radiating from terrestrial sources is not polarized in its passage 
through tourmalines*. 
* This result seems opposed to the experiments of M. Bérard on the polari- 
zation of reflected heat; but, ignorant as we are of the nature of the relations 
