M. MELLONI ON THE POLARIZATJON OF HEAT. 181 



two positions, parallel or perpendicular, of the planes of polari- 

 zation, is concealed by the enomious quantity of heat which the 

 polarizers radiate upon the thermoscope equally in both cases. 

 The manifestation of this difference commences, if the action of 

 the secondary radiation of the mii'rors or piles upon the thermo- 

 scope be comparatively feeble to that of the calorific pencil, 

 which undergoes immediate reflection or transmission. Lastly, 

 it attains its normal state, when, by a suitable arrangement of 

 the apparatus, the thermoscope is completely secured from the 

 sensible effect of this radiation, Avhilst left exposed to the action 

 alone of reflected or refracted heat. 



If we take a general survey of the whole of the facts which, 

 at the present day, compose the science of radiant heat, it will 

 be seen that this agent is propagated, reflected, refracted, and 

 polarized, in absolutely the same manner as light. If these pro- 

 perties often remain unperceived, it is to be attributed to a de- 

 fect of diathermaneity in the greater number of bodies, or to 

 the particular manner, according to which their absoi-ption is 

 manifested upon the radiation of heat. 



Some media, such as air and rock-salt, transmit equally all 

 sorts of calorific or luminous rays ; but others act in a different 

 manner upon the rays of the two agents, extinguishing some- 

 times more fight than heat, at others more heat than light. We 

 have thus the singular spectacle of bodies which completely ab- 

 sorb the luminous rays, and admit the passage of certain calo- 

 rific rays ; and of substances pei'meable to light, completely ar- 

 resting every species of heat. 



Analogous differences are produced in the diffuse reflection 

 which the two radiations experience at the surface of opake 

 and athermanous bodies ; for perfectly white substances reflect 

 or absorb extremely diverse projDortions of heat, according to 

 the quality of the calorific rays ; and yet the same white sur- 

 faces absorb all the rays of light in equal proportions. It is 

 deducible even, with the clearest evidence, from the absence of 

 any coloration whatever, which would not fail to appear when 

 these surfaces were exposed to ordinary light, if, by a difference 

 of absorption, the coloured rays, which enter into the composi- 

 tion of light irregularly reflected, had not between them exactly 

 the same relations of intensity of the incident rays. 



Other inequalities, also deriving their origin from absorption, 

 are manifested in the phaenomena of polarization presented by 



