"M. MELLONI ON THE POLARIZATION OF HEAT. 331 
-eat, however constant, are subject to slight changes in their physical 
tate, which cause variations of the same order in the temperature ; so 
aat it is always desirable to abridge the time that elapses between two 
- omparative experiments. The table which I have just mentioned will 
herefore render the observations more prompt and more exact than 
hey would be if we directly observed the fixed deviations. I have ac- 
-ordingly always employed this method in the course of this Memoir, 
ind the force corresponding to each result will be found beside the 
ybserved arc of impulsion. It is almost unnecessary to add, that the 
force to which all the others are referred is that which causes the 
needles to describe the first degree of the scale. 
Having by various contrivances provided myself with a thermoscopic 
instrument of very great sensibility, promptitude, and certainty in its 
indications, I proceeded to the experiments on polarization by com- 
mencing with tourmalines. 
The great difficulty that first presents itself in studying the polariza- 
tion of heat by tourmalines, is the feeble calorific transmission of these 
substances ; a circumstance which, together with the usual smallness of 
sheir dimensions, renders the intensity of the emergent rays extremely 
feeble, and scarcely appreciable with the most delicate thermomultipliers. 
Hence it becomes necessary to bring the source very close to the sy- 
stem of the tourmalines, in order that they may receive the greatest pos- 
-ible quantity of the calorific rays. But this extreme proximity of the 
source heats the tourmalines in a sensible degree, causes them to ra- 
liate on the pile, and, by the effect of this secondary heat, disturbs the 
vction of the rays immediately transmitted by the system. 
The quantity of incident heat might indeed be augmented, without a 
change in the ordinary distance of the source, by concentrating it on 
the tourmalines by means of a rock-salt lens. But then the plates be- 
come still more heated, and the pile must necessarily be placed at a 
great distance behind the tourmalines, in order to withdraw it from the 
disturbing force of this second calorific source. Now the distance of 
the pile from the tourmalines cannot be thus increased without sub- 
jecting us again to that very inconvenience of an over-feeble radiation, 
which it is our purpose to avoid ; for the rays, after having crossed each 
other in the focus, undergo a considerable divergence and a rapid de- 
crease of intensity as they proceed to a distance from the plates. In 
order to avoid both these inconveniences, and to obtain a calorific 
stream consisting solely of the rays directly transmitted by the tourma- 
lines and yet powerfully acting on the thermoscope, I first receive the 
pencil of calorific rays on a large rock-salt lens, after having made them 
parallel by means of a reflector. The concentrated heat reaches the. 
tourmalines ; a great portion of it is absorbed, and converted into ordi- 
