M. MELLONI ON THE POLARIZATION OF HEAT. 175 



by taking the mean of several observations. But these expedients 

 were not sufficient in the case under consideration, as is even 

 evident from the nature of the results obtained upon the rays of 

 obscure heat, which, though giving a tolerably considerable dif- 

 ference, and always in the same direction, would yet be far from 

 proving the equality of the two actions, if it were not deducible 

 from the analogous case of light, in which this equality is esta- 

 blished from inductions that cannot admit of the least doubt. 



To render the experiment conclusive of itself, it must be per- 

 formed upon an obscure calorific flux, very intense and very trans- 

 missible through mica, in order to be able to polarize it almost 

 completely by piles of numerous laminae, while still presen'ing 

 a notable portion of its energy, and to render it, thus strongly 

 polarized, more sensible to the doubly refracting action of the 

 interposed laminae. It must also be secured from the heating 

 of the mica system, which always tends to diminish the apparent 

 effects of polarization. Nothing is more effectual for satisfying 

 these conditions than our calorific rays rendered parallel by the 

 rock-salt lens, and completely separated from light, and from the 

 greater portion of the heat absorbable by the mica, by their pre- 

 vious tx'ansmission through opake black glass. I therefore 

 caused a pencil of this obscure heat to fall upon my two piles of 

 twenty laminae inclined 33° 30' upon the axis of radiation ; I 

 placed between them the perpendicular lamina of mica, and 

 after ascertaining, by the means indicated above, that the proper 

 heat radiated by the last pile upon the thermoscopic body was 

 insensible, I proceeded to the measurement of the two variations, 

 which were then very considerable, as may be seen from the 

 following table. 



Each of the three sources of heat was placed at the centre of 

 a spherical reflector ; the calorific pencil of parallel rays, after 

 having traversed the black glass and the system of mica laminae, 



M 2 



