M. MELLONI ON THE POLARIZATION OF IHIEAT. 343 
source of heat, or emitted by different sources, are very unequally 
affected by the cause through which the phenomena of polarization in 
the tourmalines are rendered sensible. Some of them appear to un- 
dergo no action of this kind, others produce indices of polarization 
more or less marked, and others are, like rays of light, completely po- 
larized. Tourmalines in general, and the green tourmalines in particu- 
lar, absorb the most polarizable rays, and transmit those species which 
seem totally or partially to escape polarizing action. The consequence 
is, that their apparent index of polarization is generally very feeble and 
sometimes even inappreciable. But it rises even to +%,2,, and perhaps 
higher, in those systems of plates which are permeable to a greater pro- 
portion of heat susceptible of a high degree of polarization, as we see 
in the plates of yellow, brown, or violet tourmalines. The index of 
apparent polarization in a given system varies considerably in passing 
from one source to the other, because a change takes place in the qua- 
lity and the grouping of the rays constituting the calorific stream issuing 
from the focus of heat. In fine, this index varies, and in certain cases 
attains its two extreme limits, 0 and 100, when we introduce between 
the same source and the same system of tourmalines diathermanous 
plates of a different kind; because the particular-absorption of these 
sereens affects the relations of quantity existing between the several 
groups of rays composing the calorific stream naturally transmitted by 
the polarizing system.” 
Tn all these statements we have taken care to apply the qualifying 
term apparent to the signs of feeble polarization exhibited by the tour- 
malines ; and, in fact, all the rays of heat, whether direct or transmitted 
through a screen, might be completely polarized, as light is, in the in- 
terior of these crystallized bodies, and yet the polarization not be ren- 
dered perceptible by any diminution in the quantity of heat transmitted 
by the plates when the parallel is exchanged for the perpendicular po- 
sition of the axes. 
In order to understand this proposition it is necessary to recollect the 
phenomena which take place in the polarization of light by tourmalines. 
When a ray of natural light falls perpendicularly on a plate of tour- 
maline cut parallel to the axis of the needles, the double refraction first 
divides the ray into pencils possessing sensibly equal intensities and 
polarized at right angles; but, in proportion as these pencils penetrate 
into the substance of the tourmaline, they suffer very different degrees 
of absorption, that of the pencil which has undergone the ordinary 
refraction being much the greater ; so that, beyond a depth often vely 
inconsiderable, one of the pencils is entirely absorbed, while the other 
pursues its path, emerges from the plate, and shows itself in its proper 
direction of polarization. This inequality of absorption is proved by 
the following experiment, for which we are indebted to M. Biot. We 
