M. MELLONI ON THE POLARIZATION OF HEAT. 331 



eat, however constant, are subject to slight changes in their physical 

 :ate, which cause variations of the same order in the temperature ; so 

 lat it is always desirable to abridge the time that elapses between two 

 omparative experiments. The table which I have just mentioned will 

 berefore render the observations more prompt and more exact than 

 hey would be if we directly observed the fixed deviations. I have ac- 

 cordingly always employed this method in the course of this Memoir, 

 ind the force corresponding to each result will be found beside the 

 observed 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 tliat first presents itself in studying the polariza- 

 tion of heat by tourmalines, is the feeble calorific transmission of these 

 mbstances ; a circumstance which, together with the usual smallness of 

 heir dimensions, renders the intensity of the emergent rays extremely 

 eeble, and scarcely appreciable with the most delicate thermomultipliers. 

 Hence it becomes necessary to bring the source very close to the sy-^ 

 ?tem of the tourmalines, in order that they may receive the greatest pos- 

 ible quantity of the calorific rays. But this extreme proximity of the 

 -ource heats the tourmalines in a sensible degree, causes tjiem to ra- 

 liate on the pile, and, by the effect of this secondaiy heat, disturbs the 

 iction of the rays immediately transmitted by the system. 



The quantity of incident heat might indeed be augmented, without a 

 ■hange 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- 



