a new Mineral Species. 5 



with a drusy surface. Massive varieties usually present the ap- 

 pearance of certain kinds of mica. 



The crystals subjected to measurement were taken from Mr 

 NEUMANN'S specimen. Owing to the striae upon the crystalline 

 faces, parallel to the intersections of these faces with the face , 

 and to the great flexibility of the lamina?, the angles could not 

 be ascertained with the utmost degree of exactness. The di- 

 mensions of the forms were calculated from the admeasurement 

 of the angle at the base of P = 118, and of the angle a b c in 

 Fig. 7., shewing the inclination of two faces parallel to its short 

 diagonal in a twin-crystal, the latter of which was found to be 

 equal to 119|. The remaining measurements which were taken, 

 agreed with the angles obtained by calculation, as well as could 

 be anticipated from the nature of the substance. There is no 

 mineral, however, which could be confounded with it amono- 



O 



those of a similar aspect; if we except, perhaps, the flexible sul- 

 phuret of silver, first described by Count BOURNON *, a sub- 

 stance which I never had an opportunity of examining. The 

 angles given by Mr BROOKE f being 125 instead of 119V, and 

 the character of symmetry itself, since he considers a rhomboidal 

 prism, and not a rhombic one, as the type of the forms of 

 the species, sufficiently establish a crystallographic difference 

 between the two substances. The difference among them is 

 strengthened even by the difference in the shade of colour, 

 said to be black in the flexible sulphuret of silver, where- 

 as Sternbergite is decidedly brown, although the characters of 

 flexibility and hardness pretty nearly agree. The remaining 

 properties, particularly the specific gravity, which w r ould be of 

 great importance, have not been ascertained in the flexible sul- 

 phuret of silver. 



* Catalogue, p. 209- f Phillips' Mineralogy, p. 289. 



