of Crystallized Bodies. 283 



able difference between compositions like Fig. 16, and such as 

 are formed from Fig. 7, by the filling up of the re-entering an- 

 gles, in the last of which the whole is bounded by the faces of 

 the original horizontal prisms, which are all inclined upon the 

 same axis. The faces of composition in the first case pass 

 through the edges, in the second through the centres of the faces 

 of the compound groupe. The regular forms of both these 

 substances are considered as belonging to the rhombohedral 

 system, in the last edition of Hauy's Traite. Professor Fuchs 

 first ascertained the actual forms of the strontianite of Leogang. 



The four species, described above, not only agree in their 

 mode of being joined in regular compositions, but they likewise 

 show nearly the same secondary faces. In every general crys- 

 tallographic consideration these four species are inseparable. 

 Even their history coincides in this important point, that — with 

 the exception of the first only — they have all been taken for 

 rhombohedral. They form a curious instance likewise of the 

 isomorphism of lead, calcium, baryum, and strontium. 



One of the most curious substances, particularly for its near 

 resemblance to certain rhombohedral species, is the prismatic 

 copper-glance, the grey copper of Cornish miners. By far the 

 greater number even of first rate mineralogists to this day con- 

 sider its form as connected with the regular six-sided prism, or 

 belonging to the rhombohedral system of Mohs ; and yet these 

 forms are derived from a rhombic prism of 11 9° 35', which is usu- 

 ally found in regular compositions of three individuals, crossing 

 each other at angles of nearly 60° and 120°. The 17th Figure, 

 the projection of a crystal in the cabinet of Mr Allan, would be 

 scarcely recognizable for what in fact it is, were we not led to it 

 by the re-entering angles or evasures along some of its edges, and 

 by the stria?, which, as in chrysoberyl and other species, are pa- 

 rallel to the intersection of the face s with the faces of the vertical 

 prisms. Such striae are usually met with, and though the entire 

 groupes are not always so well defined as in the case represented, 

 yet this peculiarity will be sufficient to mark out the extent of 

 each individual, and to ascertain the law by which it is joined to 

 another. This substance, however, occurs also in several other 

 kinds of regular composition, which, though they render it more 



