^6 On CrysiaUographij. 



similar precision. Carbonated lime, for instance, assume?'^, 

 accordin"- to circtnnsfaiiccs, the form of a rhomboid ; of a 

 reo'ular hexahedral prism ; of a solid terminated by twelve 

 scalene angles ; or of a dodecahedron with pentagonal fsces, 

 &c. Siilphurelled iron, or ferruginous pyrites, produces 

 Jonietiines cubes and sometimes regular octahedror-s ; here 

 dodecahedrons with pentagonal faces, there icosahcdrons 

 with triangular faces, &c. 



It is true that, among the varieties of one and the same 

 species, it often happens that a more compound form differs 

 from a more simple form only by certain facets, similar to 

 those which would result from sections made on the solid 

 angles, or on the ridges of the latter*. Pyrites, for in- 

 stance, sometimes assumes the form of a cube, the eight 

 solid angles of which iicing beaten down would expose to 

 view so many triangular facets, in such a manner tiiat this 

 form may be considered as the passage from the cube to the 

 octahedron, with which it is connecTed by its eight equila- 

 teral triangles, which are situated like the faces of this se- 

 cond solid. 



But in addition to these transitions being already very 

 singular in themselves, as appertaining to modifications muck 

 more sensible than seems to be necessary to distinguish sim- 

 ple varieties, we find on the other hand certain crystalline 

 forms, which, by a singularity still more remarkable, do not 

 exhibit any vestiges of common parts, and present the ap- 

 pearance of a complete metamorphose of the mineral from 

 which they derive their orio;in. And in order to cite a new 

 example, let us place by the side of each other the regular 

 hexahedral prism of carbonated lime, (PI. I., fig. 1.) and 

 the scalene dodecahedron with scalene triangular faces 

 (fig. C). We can scarcely conceive how two polyhedrons, so 

 dissimilar at first view, come to touch together, and are akS 

 if confomukd in the crystallization of one and the same 

 mineral. , 



In short, as if the results of this operation of Nature were 

 destined to excite astonishment of every kind, while one and 

 , the same substance lends itself to so many transformations, 

 we meet very different substances, which present absolutely 

 the same form. Thustluated lime, muriated soda, sulphu- 

 retted iron, sulphuretted lead, Sec, crystallize in cubes un- 

 'tlcr certain circuinstauces ; and in other cases, the same 



* This idea sng'Sfcsfed to M. Roitil' de I'lsle tlio method of truncatiires, 

 for enabling us to derive from each other tlic diill'rent varieties of crystalline 

 f«rn*» which should bclonjj to one and the same substance. 



minerals. 



