170 M. MELLONI ON THE POLARIZATION OF HEAT. 



diate index between the extremes, the values of which ah'eady 

 approach so nearly. 



The variable calorific transmission of a series of numerous 

 parallel laminae, presented under increasing inclinations to the 

 radiation of flame, has recently led us to the inference that heat 

 like light is polarized by reflection; that is to say, that this 

 species of polarization takes jjlace in a plane perpendicular to 

 that of heat polarized by virtue of the refracting forces, and 

 that the angle at which it is completely effectuated differs by a 

 scarcely appreciable quantity from that given by the complete 

 polarization of light. It may here be added that this angle does 

 not undergo any sensible variation if the nature of the calorific 

 radiation be altered, either by the interposition of laminae of 

 different diathermancy, or by substituting other sources of heat 

 for flame. The emergent rays of opake black glass, trans- 

 mitted by a pile of seventy laminae, give actions upon my ap- 

 paratus, which, at an inclination of 33° 30', the moment of the 

 maximum effect, push the index to more than 30°, and allow it 

 to descend rapidly towai'ds zero when the laminae are inclined in 

 either direction. The direct rays of copper heated to 400° pro- 

 duce the same relations of intensity at different inclinations, but 

 upon a much smaller scale. 



I shall here observe, once for all, that in the greater number 

 of experiments on calorific polarization, in which rays of heat 

 unmixed with light are required, the obscure heat of bodies 

 below incandescence may be very advantageously replaced by 

 the emergent heat of perfectly opake black glass exposed to 

 the calorific fluxes of flame or incandescent platina. For this 

 sort of heat is certainly perfectly obscure, and, in addition, is 

 endowed with a diathermancy very analogous to that of mica ; 

 it consequently presents all the conditions desirable for the ve- 

 rification, upon heat alone, of the facts corresponding to those 

 obsei-vable in luminous polarization. 



The invariability manifested in the angle of the complete po- 

 larization of heat by reflection, notwithstanding the differences 

 of the mean indices of the refraction of the various incident 

 pencils, may be conceived relatively to the limits of precision 

 furnished by our actual calorimetrical instruments, from reasons 

 exactly analogous to those that have just been alleged when 

 treating of polarization by refraction. 



And even should we at some future time succeed in insulating 



