of Crystallized Bodies. 65 



The only kind of composition hitherto observed in pyramidal 

 Scheelium-baryte is the one represented in Fig. 16. The face of 

 composition is parallel, the axis of revolution perpendicular to 

 a face of the rectangular four-sided prism, which is in parallel 

 position with the pyramid of 107° 27' and 113° 85', that is of 

 P-j-oo. The individuals are continued beyond the face of 

 composition, so that the result assumes the appearance of a 

 crystal belonging to the prismatic system. We are prevent- 

 ed, however, from being led into error, by the observation of 

 the striae upon the faces of P, which are parallel to the 



(P + l) 3 

 edges of combination between this form and — «—-(&). They 



terminate abruptly at a certain line upon faces which else 

 might be taken for such as belong to the same individual. 

 This kind of composition is not unfrequently met with among 

 the large yellowish-white crystals from Schlaggenwald in Bo- 

 hemia, and has been first mentioned by Mr Mohs.* A fine 

 specimen of this variety is in the cabinet of Mr Allan. The 

 form of each of the individ rials taken separately is that of 

 Fig. 17, which, in regard to the general distribution of its 

 faces, much resembles the rhombohedral species of apatite. 



The simple crystals of the Cyanide of Mercury which join 

 in regular composition may be traced in general to the form 

 of Fig. 18. It may be conceived to arise from Fig. 19, the 

 same combination with the full number of its faces, by the en- 

 largement, first of the alternating faces contiguous to both the 

 apices, and secondly, of two of the remaining faces to the ex- 

 clusion of the rest. Two of these crystals now are joined in 

 one of the faces of [P-f-oo ], in an inverse position, and com- 

 pressed between the two faces, so that the transverse section of 

 the compound crystal is again a square, or nearly so. The 

 result is Fig. 20, of which Fig. 21 is a projection upon a plane 

 perpendicular to the axis. The face of P — oo , which is like- 

 wise often found in the varieties of this species, is striated as 

 in the projection Fig. 22, parallel to its edges of intersection 

 with the plane in which the two individuals meet. The faces 

 s belong to the isosceles four-sided pyramid P-l- l,a form which 



" Treatise on Mineralogy, trans], vol. ii. p. 115. 

 VOL. III. NO. I. JULY 1825. E 



