64 Mr Haidinger on the Regular Composition 



the fundamental pyramid =105° 45' and 117° 30'.* Also in 

 the Wernerian system,f the pyramidal form of the crystals 

 was described in the Black Manganese, but too little attention 

 had been paid to this description, and it had not been receiv- 

 ed till very lately by the Abbe Haiiy. Manganese crystalliz- 

 ed in octahedrons from Piedmont was analysed by Berzehus, 

 and a similar variety described by Sage. \ The octahe- 

 drons engaged in heavy spar, like those of Sage, quoted 

 by Count Bournon, if regular, evidently cannot belong to 

 the pyramidal species ; which cannot be maintained with the 

 same degree of probability of the varieties mentioned by 

 Sage and Berzelius. The regular composition, very frequent 

 in this species, takes place parallel to one of the faces of P — 1 ; 

 the result being similar to Fig. 13, if the form of the individu- 

 als is the fundamental pyramid. It is designated by P, {-^r }• 

 The composition repeated on all the homologous terminal 

 edges of the pyramid yields a form similar to Fig. 14. where a 

 central individual is surrounded by four others as in Tin-ore 

 and Titanium-ore, with this difference, that here the apices of 

 the pyramids can be more generally observed than in either of 

 the two preceding species. Generally the surrounding indi- 

 viduals are of a much smaller size than the central one, and 

 appear only sticking on the terminal edges, much in the man- 

 ner represented in Fig. 15 ; the occurrence of the same law on 

 all the terminal edges of the pyramid being of itself sufficient 

 to prove that the forms belong to the pyramidal system, as in 

 the compositions of the two preceding species. 



The mineral species possessing hemi-pyramidal forms, which 

 have been observed in regularly composed varieties, are the 

 pyramidal Scheelium-baryte and the pyramidal Copper-pyrites 

 of the system of Mohs. Also the Cyanide of Mercury belongs 

 to this class of forms, and in the last species, in particular, the 

 whole disposition of faces and the regular compositions are 

 highly interesting. 



* Phill. 3d ed. p. 387. 



•f Hoffm. Handb. by Breithaupt, iv. 1. sect. 149. Jameson's System. 

 Ed. I. vol. ii. p. 460. It is said here to occur in octahedral crystals hav- 

 ing a single cleavage. 



+ M. Sage ( EUm. de Min. ii. 136.) cite encore une mnnganaise noirdtre 

 octaedre, dans un spath seleniteux blanc. Rome de I' Isle, vol. iii. p. 101. 



