182 M. MELLONI ON THE POLARIZATION OF HEAT. 



tourmalines. In these phaenomena the two pencils into which a 

 ray of light is divided, in penetrating into the interior of the 

 plates, are so modified in their progress, that the ordinary- 

 pencil is completely absorbed during its passage, and the ex- 

 traordinary pencil presents itself alone completely polarized at 

 the emergence ; and that whatever be the coloui- of the incident 

 light. The case is different with radiant heat, the two pencils 

 of which produced at the entrance of the same polarizing plates, 

 undergo absorptions sometimes extremely diverse, sometimes 

 perfectly equal, which occasions great variations in the appear- 

 ances of polarization, according to the quality of the calorific 

 rays. 



Polarization becomes equal for radiations of every kind, if it 

 be produced by the forces of refi^action and reflection, which 

 are perfectly independent of the absorption of the media. 



It is similar when this latter force has no longer any influ- 

 ence upon the pha^nomenon of reflection. Indeed, we have just 

 seen above, that diffuse reflection, in which absorption acts a 

 part so important, varies considerably from one ray of heat to 

 the other ; but the portion of incident radiation, which is re- 

 flected in a regular manner at the poUshed surface of rock salt, 

 and other diaphanous substances, is equal for every species of 

 heat and light. 



All bodies exposed to radiant heat become hot, and, when 

 withdrawn from the action of radiation, presence for some time 

 the heat acquired ; but very few substances, after exposure to 

 light, retain it so as to become luminous in dai'kness : in general 

 the light disappears even at the moment of absorption. 



In short, the heat absorbed is found, so to speak, to have 

 changed its nature. It then forms a homogeneous flux, and 

 the mode of its transmission acquires characters quite oppo- 

 site to those effected by calorific or luminous radiation. This 

 absorbed heat makes its way, in the body, in every direction, 

 is propagated in it slowly, like heat communicated by contact, 

 and its propagation is considerably modified by the displace- 

 ment of the different parts, of which the body is composed. 

 Light, and radiant heat, on the contrary, are composed of he- 

 terogeneous fluxes, they move only in a rectihnear direction, 

 travelling over any interval whatever in an imperceptible space 

 of time, and do not receive any influence fi'om the agitation, 

 whether more or less violent, of the media which transmit them. 



