OF RADIANT HEAT THROUGH CRYSTALS. Ill 



Just as little are the calorific rays which pass along the axis 

 of the tourmaline distinguishable from each other, whatever may 

 have been their plane of polarization. Nor are differences ob- 

 served among those which pass perpendicular to the axis, whatever 

 be their direction in other respects. But these two groups, when 

 compared with each other, exhibit, as already shown, striking 

 qualitative differences. 



Experiments were next conducted with two plates of tour- 

 maline, through which the calorific rays were permitted to pass, 

 first when the axes of the plates were parallel, and afterwards 

 with the axes perpendicular to each other. The following num- 

 bers exhibit the proportions in which the rays, in both cases, re- 

 spectively, pass a diathermanous body. Each number is the 

 arithmetic mean of eight distinct determinations : — 



The comparison of both series permits no doubt to exist as to 

 the different capacities of transmission of both groups of rays. 

 They are therefore to be regarded as unlike. 



Besides calcareous spar, rock-crystal, beryl and tourmaline, 

 dichroite was examined : so far as the examination of this crystal 

 extends, qualitative differences, dependent upon the direction 

 of transmission, have also been observed. 



The principal results of this investigation may be thus summed 

 up:— 



1. Radiant heat passes through certain crystals, as brown 

 rock-crystal, beryl, tourmaline and dichroite, in unequal quan- 

 tities in different directions, and shows itself (for example in its 

 deportment towards diathermanous bodies) to be of a different 

 quality, according as it has traversed the crystal in one or an- 

 other direction. These differences are connected with the polar- 

 ization of the calorific rays, and in connexion with this point we 

 find— 



2. That calorific rays whose direction is perpendicular to the 



