42 On the Homceomorphism of Mineral Species 



the species with the other allied sulphates ; but the three octa- 

 hedral planes are then \ f , f y, and £ \ 7 ; and in any other 

 view that recognises the homoeomorphism, the expressions for 

 the planes are scarcely less complex. 



We cannot be too guarded, therefore, when deducing the 

 form for comparison with another species, in relying on the 

 prevalence of certain planes. Valuable hints are often thus 

 given, but they may lead to error. 



The lustre or smoothness of planes is a better guide, though 

 far from certain. The fundamental vertical prism in Barytes is 

 generally less highly polished than many other faces; and as we 

 have above remarked, the octahedrons of Fluor have often rough 

 surfaces. 



The prevailing direction of the more extended zones of planes, 

 especially the octahedral, often suggests rightly which is pro- 

 perly the terminal plane of the prism, these zones rising towards 

 that plane ; and they thereby afford a hint as to which is the ver- 

 tical axis. In dimetric and hexagonal species, this criterion is 

 a sure guide (except sometimes in hemihedral forms) ; but here 

 it is not needed, as the basal plane is fixed from the nature of 

 the prism. The principle holds true for Topaz and many trime- 

 tric species. In the rhombic octahedron of Sulphur, in which 

 either axis might be made the vertical, the apical angles, in 

 which the true vertical axis terminates, are at once distinguished 

 in modified crystals, by the cluster of planes about them. But 

 the ambiguous cases are numerous, and this criterion, like 

 others, is not an unfailing reliance. 



When we may succeed in fixing upon the vertical axis in a 

 species, and also the unit vertical prism, it is often difficult to 

 determine which planes about the base should be taken as the 

 unit domes or octahedron ; and often there is a choice between 

 two or three planes equal in lustre and size; and consequently 

 it may be altogether doubtful whether the vertical axis equals 

 la, or Ja, or fa. Crystallographers may take whichever is most 

 convenient without any important objection. But when look- 



