148 M. MELLONI ON THE POLARIZATION OF HEAT. 



heated as the temperature of the source is reduced, since, Hke 

 glass, it transmits immediately quantities of heat decreasing 

 with this same temperature. If, therefore, the proper radiation 

 of the mica piles exerts an appreciable action, the index of po- 

 larization will, in appearance, undergo a greater diminution for 

 the sources at low temperatures than for those at elevated ones. 



By the same principle of the secondary radiation of the piles 

 another experiment of Mr. Forbes's may be explained, which, 

 according to him, proves the unequally polarizable nature of 

 calorific rays. 



The radiant heat of copper raised to a temperature of 400°, 

 by means of an alcohol lamp, gave him, as we have seen 

 above, ---^-^ of polarization for the action of a certain system of 

 mica piles. By interposing a lamina of glass between the same 

 source and the same system of piles, the proportion of heat 

 polarized increased ten hundredths, that is to say, when the 

 heat traversed the glass lamina before being submitted to the 

 polarizing action of the piles, seventy- three rays in a hundred, 

 instead of /^^g, disappeared in consequence of the intersection of 

 the planes of refraction. The heat of incandescent platina ha- 

 ving given him -^^-^ of polarization, without the interposition of 

 the lamina, Mr. Forbes concludes from it that " the heat from a 

 dark source, after transmission through glass, became as polari- 

 zable as that from incandescent platinum*." But it is easy to 

 see that the lamina of glass interposed between the source and 

 the piles of mica itself absorbed the greater part of the rays 

 which previously heated these piles in the experiment of direct 

 heat ; so that the perturbing cause being considerably enfeebled, 

 the apparent effect of polarization was increased to the point of 

 becoming sensibly equal to that given by the rays of incandes- 

 cent platinum, the passage of which through the mica excites in 

 it a very slight elevation of temperature, on account of their 

 great transmissibility through that substance. 



The experiment shows that the radiant heat of incandescent 

 platinum, and that of flame, traverses the thin mica leaves in 

 nearly equal proportions f. This equaUty of heat transmitted 

 being accompanied by an equality of heat absorbed, the piles 

 must necessarily exert the same perturbation upon the imme- 



* Researches on Heat, second series, by J. D. Forbes, p. 11. London and 

 Edinburgh Philosophical Magazine, vol. xii. p. 551. 

 •f Annales de Chimie et de Physique, vol. Iv. p. 346. 



