M. MELLONI ON THE POLARIZATION OP HEAT. 177 



tion here do not undergo any ordinary refraction, but simply 

 two successive reflections ; and the lamina intei-posed perpen- 

 dicularly to the pencil of obscure heat which passes from one 

 mirror to the other, only reveals, so to speak, its state of po- 

 larization produced by reflection alone. Indeed this species of 

 calorific polarization may be separately developed by more direct 

 means, exactly similar to those employed to exhibit the ana- 

 logous phaenomenon of Hght; but to do this would hazard 

 displacing the source or the thermoscope in giving the perpen- 

 dicular direction to the two planes of reflection, for it may be ob- 

 jected that the calorific rays do not present themselves at the 

 opening of the thermoscopic tube, with the same directions that 

 they affisct when the two planes of reflection are parallel ; or, 

 that the intensity of the source, or its position in relation to the 

 mirrors, has been altered during the necessary movement. 



But let us return to the piles. When the planes of refraction 

 are perpendicular, the interposition of the ckcular disc of mica 

 between the two series of laminae, increases the calorific trans- 

 mission if its principal section be inchned 45° upon the first 

 plane of refraction, and leaves it in nearly its natural state if 

 the disc present its principal section parallel to that plane. 

 According to the denominations adopted in England, Mr. Forbes 

 caUs the relation of the quantities of heat transmitted through 

 the system, in these two positions of the lamina, the effect of de- 

 polarization. When endeavouring to determine a similar rela- 

 tion for heat proceeding from different sources, Mr. Forbes found 

 that it varies even when employing the same depolarizing lamina, 

 and the same system of piles arranged under a constant inclination. 

 Thus, in certain circumstances, the heat of copper at 400° gave 

 him, as the mean of several observations, 100: 118; and the 

 heat of incandescent platina 100 : 134. He thence concludes 

 that calorific rays are more or less depolarizable *, according to 

 their proper nature. 



If the tenor of the reasoning with which this second part of the 

 memoir commenced has been well understood, it wiU be easily 

 seen that Mr. Forbes's conclusion is inadmissible. Indeed we 

 have seen that in the conditions of distance which he adopted, 

 the heat proceeding fi-om the whole of the system of mica was 

 mixed in a sensible manner with the direct rays of the source 



* Trans, of the R. S. of Edin. vol. xiii., part i. p. 155, [or Lond. and Edinb, 

 Phil. Mag. vol, vi. p. 286. Edit.] 



