284 Mr Haidinger on the Regular Composition 



complicated, greatly enhance the interest attached to the consi- 

 deration of the crystalline forms of the species. 



Dr Gustavus Rose assigns a regular composition also to the 

 crystals of zinkenite, a species which he first described. (See 

 this Journal) p. 17.) One of them is represented in Fig. 18. The 

 planes of junction here, as in the preceding case, pass through 

 the centres, or nearly so, of the faces, which limit the prism 

 whose angles approach to 120°. This is therefore, properly 

 speaking, the complementary law to the original one ; the axis 

 of revolution is perpendicular to a face of Pr, the plane of com- 

 position also perpendicular to it, and, at the same time, to an 

 acute edge of P. The faces P and M bounding the crystal, 

 are remnants of those of the horizontal prisms. 



The reverse, that is to say, the original law, takes place in 

 prismatic melane-glance, Fig. 19. The planes of junction pass 

 through the edges, and are parallel to the faces of Pr. The 

 crystal lographic sign, therefore, will be ... Pr-f- oo. (Pr-f go) 3 

 Pr+oo, 2{p r }. Four of the angles are =115° 39', equal 

 to the angle of the horizontal prism, and two =128° 42 ; . Not- 

 withstanding this great difference of the angles from 120°, the 

 crystals of the substance were long taken for rhombohedral ; 

 nay, the substance itself was confounded with red silver, with 

 which it agrees almost in none of its properties, except that it 

 likewise contains silver. 



The antimonial silver of the Hartz sometimes occurs in acute 

 six-sided pyramids, Fig. 20, imbedded in, or rather enveloped 

 by, coats of native arsenic. These are compound groupes, 

 formed exactly in the manner described of the preceding spe- 

 cies. There is a distinct, though not very bright cleavage 

 perpendicular to the axis of the supposed pyramid ; but there 

 are also two directions of cleavage, which produce a prism, 

 parallel to the one, whose faces are d and d, with an angle of 

 about 95 J°, contiguous to the apex of that pyramid. The 

 want of continuity of these faces of cleavage immediately be- 

 trays the compound state of the mineral. I have seen at 

 Clausthal, in the Hartz, a groupe nearly similar to Fig. 21, of 

 two crystals, leaving between them re-entering angles, the axes 

 of the prisms forming an angle of about 60°. I could not 

 obtain an exact measurement of any of the forms, and more 



