66 Mr Haidinger on the Regular Composition 



produces horizontal edges of combination with [P+oo ], and is 

 frequently found in the crystals of Cyanide of Mercury. The 

 following are the dimensions of its forms : P=134° 36', 66° & ; 

 P+l = 122 / 46', 85° IT. The axis of P is = ^0.424. Cleavage 

 takes place pretty distinctly parallel to [P+oo ]. I have been 

 indebted to Dr Turner for a great number of crystals in 

 which I observed this regular composition ; they had been ob- 

 tained by the slow cooling of a solution concentrated at a pretty 

 high temperature, but their inside was opaque, only the exter- 

 nal coats, formed during the last stage of cooling, and the sub- 

 sequent process of spontaneous evaporation, were perfectly 

 transparent. 



Pyramidal Copper-pyrites presents a kind of regular com- 

 position, somewhat analogous to that of the preceding species. 

 The individuals, however, are here continued beyond the 

 face of composition, and the hemi-pyramidal character is ex- 

 pressed only in the alternating enlargement of the faces of P, 

 the fundamental pyramid. Pig. 23 represents the result of 

 this law, if the form of the individuals contains nothing but 

 the faces of P, alternately larger and smaller. This is the 

 rarest among the regular compositions of the species. I have 

 observed it in some varieties from the mine of Kurprinz near 

 Freiberg, generally associated with composition in other di- 

 rections. 



By far the most frequent among them is the composition 

 parallel to one of the faces of P, the isosceles pyramid of 109° 

 53' and 108° 40', which, if it takes place in crystals having the 

 form of this pyramid, produces an appearance very much re- 

 sembling the twins of the regular octahedron in Spinelle, in 

 octahedral Iron-ore, and other minerals, similar to Fig. 24. 

 The hemi-pyramidal character of the combinations of this spe- 

 cies causes them, however, generally to assume a shape some- 

 what different from this figure, which, nevertheless, also fre- 

 quently occurs, and, besides, supposes the individuals to ter- 

 minate at the face of composition. The 25th figure, which re- 

 presents a crystal in the cabinet of Mr Allan, is intended to 



P P 

 convey an idea of this composition. The combination ^ ■ — <f ■ 



[P+ oo] (Fig. 26.) is the form of the individuals, one of which 



