342 M. MF.LLONI ON THE POLARIZATION OF HFAT. 



Let US first direct our attention to the two last systems of tourma- 

 lines. Their indices undergo a gradual increase in passing from the 

 copper to the Argand lamp. This shows that the radiation of each of 

 the four sources contains a greater quantity of heat capable of being 

 polarized by the tourmalines in proportion as the temperature of the 

 source is more elevated. 



Yet the indices of polarization of the systems 1 and 5 undergo, from 

 the action of the first three sources, changes completely opposed to 

 those which we have been considering. In order to account for such an 

 anomaly, we must keep in view the very imperfect diathermancy of this 

 sort of tourmalines. It is true that the calorific streams of the Argand 

 and the Locatelli lamps contain rays more capable of being polarized 

 by the tourmalines than any of those contained in the calorific streams 

 from the inferior sources ; but in the present case such rays scarcely 

 contribute to increase the index of polarization ; for we have seen that 

 they cannot penetrate the plates which form the polarizing systems. The 

 green tourmalines, however, are permeable to several species of heat; 

 and, as in the radiation of each source there are several of these species, 

 it is easy to see that if a certain group of rays, possessing an index of 

 polarization inferior to those of the excluded but superior to those of 

 the transmitted rays, is more abundant in the calorific stream of the in- 

 candescent platina than in that of the Locatelli, the index of polariza- 

 tion in the two systems of green tourmalines will, in this case, suffer 

 a decrease in passing from the first to tlie second source. The same 

 reasoning will apply to the Locatelli as compared with the Argand 

 lamp ; so that, notwithstanding the fact that the rays are more suscep- 

 tible of polarization as we proceed from the incandescent platina to the 

 higher sources, the two systems of green tourmalines will give lower 

 indices of polarization. 



Without a knowledge of the laws of calorific transmission and the 

 analytical resources they afibrd, we should perhaps find it impossible 

 to extricate ourselves from the perplexing difficulty presented by these 

 singularly anomalous phaenomena of polarization. We are now able to 

 offer the following brief recapitulation of our observations on them. 

 " The different calorific rays coexisting in the radiation of the same 



Argand lamp, gives but a difference of 0°1 in the two positions of the axes, 

 the arcs of impulsion being from 2G° to 27° ; a difference whicii could not be 

 rendered perceptible otherwise than by taking the mean of 10 observations ; and 

 I am not yet quite sure that it would not vanish altogether if the experiments 

 were more numerous, for I frequently obtained a stronger transmission with 

 the axes pei-pendicular. The face is that in operating upon arcs of from 15° to 

 20° the transmission of this system of tourmalines seemed to undergo no varia- 

 tion whatsoever in consequence of the axes being crossed ; and indeed, after the 

 preceding experiments, there is nothing more surprising in the existence of tour- 

 malines that give no sign of calorific polarization than there would be in the 

 discovery of tourmalines capable of completely polarizing heat. 



