of Crystallized Bodies. 67 



appears as if engaged in the centre of the other in a reversed 

 position, easily discovered at least in the present instance, 

 if we attend to the faces of the four-sided prism [P-f- ccl, 

 which are striated in a horizontal direction. Except the dif- 

 ference in the system of crystallization, we have here a case 

 extremely resembling the twins of Grey copper, represented 

 Vol. I. Plate III, Fig. 18, of this Journal, and also some of 

 Blende, a species whose crystallizations agree in general very 

 nearly with that of the Grey copper. This composition is of- 

 ten repeated, either in parallel plates, as in Fig. 27, which is 

 very frequent among almost all varieties of the species, and 

 may often be observed even in massive specimens, by the want 

 of continuity in the cleavage, or it takes place at the same time 

 parallel to two faces of the fundamental pyramid. A group, 

 resulting in the manner last mentioned, is represented in Fig. 

 28. On account of the hemi-pyramidal character, the indivi- 

 duals in the composition possess a different appearance from 

 each other, so that it requires some attention to find out their 

 shape to be that of Fig. 33.,* a combination of P, P-f-1, and 

 P+ Qc ; particularly as the relative irregular enlargement of 

 the faces adds to the difficulty. Here, as in many other com- 



• The specimen in Mr Allan's cabinet, in which this variety was ob- 

 served, is peculiarly interesting on account of the distribution of the tar- 



P 

 nished colours on the surface of its crystals. The tint of — (P) is gene- 

 rally the violet or purple, frequently inclining to the yellow colours of the 

 scale, while the tint of P+ 1 (c) is a distinct, and often very deep blue, that of 

 P+ <» {I) being a fine green. The lustre of / is not so bright as that of the 



p 

 other faces ; — — (P') generally agrees in colour with c. In some crystals 



where the tint of P is a brownish-yellow, that of c has not gone beyond the 

 purple. This difference is probably owing to a slight difference in hard- 

 ness upon the faces of crystallization belonging to different forms ; it is in 

 close connection with the physical quality of these faces themselves ; and 

 corresponds in some respect to the phenomenon of cleavage, which often 

 takes place parallel to two forms at once, but with different degrees of fa- 

 cility. Lead-glance and the rhomboidal Iron-ore from Elba have been de- 

 scribed as presenting a difference in the tarnish of their faces of crystal- 

 lization. The hexahedron in the former, and the face perpendicular to the 

 axis(R — x) in the latter, retain their natural colour and brightness, 

 while the rest of the faces assume the tints of tempered steel. 



