338 M. MELLONI ON THE POLARIZATION OF HEAT. 
and the feeble absorption which they suffer from white surfaces* ; and 
this analogy is completed here by the almost total polarization of the 
same rays under the influence of the tourmalines. 
It will now be easy to account for the differences between the indices 
of polarization produced by the different tourmalines. All the calorific 
rays emitted by the flame of the lamp do not indiscriminately pass 
through the tourmalines, each of which, according to its nature, is per- 
meable to particular quantities and qualities of heat. This fact, which 
is observable in diathermanous substances in general, is so true in the 
particular instance under consideration, that each of the plates composing 
the polarizing system indicated in the first table by the numbers 1, 2, 3, 4, 
when combined with a plate of alum, fails to afford an appreciable trans- 
mission ; an evident proof that the heat which alum is capable of trans- 
mitting is not to be found in the calorific stream emerging from these 
four systems. Now we have just seen that the different species of heat 
contained in the radiation of flame give very different indices of polari- 
zation. The calorific stream admitted by each polarizing pair will 
therefore necessarily have a mean index of polarization varying with the 
quality of the tourmalines. 
If we place outside the polarizing system a screen indued with the 
same diathermancy as the plates composing this system, namely, a screen 
permeable to the same species and the same portions of calorific rays, 
then the effect of absolute transmission will, no doubt, be more or less 
diminished in proportion as this screen is more or less diathermanous, 
but the tourmalines will present no change in their index of polarization: 
such is the case with respect to the white, the red, the orange, the yel- 
low, the blue, the indigo, and the violet glass. Water, oil, amber, alum, 
green, or opake-black glass affect this index in a greater or less degree, 
because their diathermancy differs from that of the tourmalines em- 
ployed. 
But let the polarizing system be changed. It is clear that, if the new 
system has not the same diathermancy, the order and the direction of 
the variations produced by the different screens in the value of the in- 
dex of polarization will be no longer the same. The following is in fact 
a series of observations made on the pair of green tourmalines marked 
No. 5 in the first table: 
* Ann. de Chim. et de Phys., p, 390. 
a 
