OF CARBONATE OF BARYTES. 289 



yet be found to consist of ingredients as remarkable as those 

 which enter into the. composition of sulphuret of carbon. 



H. Carbonate of Barytes. 



The native 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 ano- 

 malous character in the mineral kingom, corresponded well 

 with the singularity of its optical properties, and discouraged 

 the anticipation of analogous phenomena^ in minerals of a more 

 perfect structure. I was, therefore, surprised, to find the same 

 character in carbonate of barytes, a mineral which has a re- 

 gular crystalline form, and possesses two distinct refractive 

 powers. The index of refraction for the perfect or least refrac- 

 ted 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 

 longitudinal joints, the* least refracted image was extreme- 

 ly distinct, while the other was a faint nebulous image, of 

 a brownish-red ' hue. It was small and round, and the intensi- 

 ty of its light was inconsiderable, when compared with that of 

 the bright image* 



When the image of a candle polarised 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 



