vf Carlonale of Baryles. 121 



II. Carbonate of Barytes. 



The ncitlve carbonate of barytes possesses, like the agate, the 

 remarkable faculty of forming two images, one of which is bright, 

 and the other nebulous. The shapeless appearance of the agate; 

 its heterogeneous and imperfect structure, and its anomalous 

 character in *he mineral kingdom, corresponded well with the 

 singularity of its optica! properties, and discouraged the antici- 

 pation of analogous phsenomena, in minerals of a more perfect 

 structure. I was, therefore, surprised to find the same charac- 

 ter in carbonate of barytes, a mineral vvhich has a regular cry- 

 stalline form, and possesses two distinct refractive powers. The 

 index of refraction for the perfect or least refracted image is 

 1-540; and its dispersive power 0'0285. 



In order to observe with accuracy the phenomena presented 

 by the carbonate of barytes, I formed nine prisms, cut in different 

 directions, from the same specimen. In one of these prisms, 

 which was bounded by planes parallel to the striae or longitu- 

 dinal joints, the least refracted image was extremely distinct, 

 while the other was a faint nebulous image, of a bro\niish-red 

 hue. It was small and round, and the intensity of its light was 

 inconsiderable, when compared with that of the bright image. 



When the image of a candle polarized by reflection, was 

 viewed through this prism, having the longitudinal joints pa- 

 rallel to the plane of reflection, the light which formed the bright 

 image of the candle was wholly reflected, while the nebulous 

 light alone penetrated the mineral. But when the longitudinal 

 joints were perpendicular to the plane of reflection, th'e light 

 image became extremely distinct, in consequence of the nebulous 

 light having novy refused to penetrate the prism. 



In a second prism, the nebulous or most refracted image was 

 more luminous than in the first, and approached to a definite 

 form ; the general shape of the candle l)eing distinctly visible. 



In a third prism, the nebulous image was more luminous than 

 in the last case, and the form of the candle still more distinctly 

 seen ; but it had nov/ the appearance of an assemblage of in- 

 coincident images. 



In a fourth prism, in wliich the plane of refraction was pa- 

 rallel to the longitudinal joints, both the images were imperfect, 

 and the most refracted image was extremely faint. By inclining 

 the prism, an image appeared on each side of the least refracted 

 image ; but they were polarized in the same manner, and were 

 probably analogous to the two images which are frequently seen 

 in specimens of calcareous spar. 



In a fifth prism, which was formed by jdanes nearly perpendi- 

 cular to the longitudinal joints,yb?/r images were plainly visible, 

 all of which were imperfect, and consisted of circular arches of 



nebulous 



