On CrystallographT/, 269 



tigiious to ihe grand diagonal are intercepted by two 

 facets, which make it apj)ear diminished in the di- 

 rection oF its length. Ex. Recurved sidphated barytes, 

 h. Retreated {retreci)^ when the primitive form being a 

 prism with rhombic bases, the longitudinal ridges 

 contiguous to the small diagonal are intercepted by two 

 facets which make it appear diminished in the direction 

 of its breadth. Ex. Retreated sulphaled barytes. 



2. Secondary forms considered in themselves, and as being 

 p2trely geometricaL 



The crystal is called, 



a. Cubical, when it presents the form of the cube, which in 



this case is always secondary. Ex. Cubical flualed lime. 



b. Ciihoidal, when its form differs a little from the cube. 



Ex. Cuboidal carbonated lime. 



c. Tetrahedral, when it presents the form of the regular 



tetrahedron, as a secondary form. Ex. Tetrahedral 



sulphurated zinc. 

 d» Octahedral, when it presents the form of this soiid_, as 



secondary. Ex. Octahedral muriated soda. 

 e. Prismatic, when it has the form of a straight or oblique 



prism, the panes of which are inclined li'O'^ amonj 



each other- Ex. Prismatic carbonated lime, prismatic 



feldspar. 



f. Dodecahedral, when its surface is composed of twelve 



triangular, quadrangular, or pentagonal faces, all equal 

 and similar, or solely ot" two measurements of different 

 angles. Ex. Dodecahedral quartz, dodecahedral zir- 

 con, dodecahedral sulphurated iron. 



If the dodecahedron had not all itslaces of the same 

 number of sides, it would be sufficient to brin2:thcmto 

 this aspect in imagination, by varying its dimensions. 



g. Icosahedral, when its surface is composed of twenty 



triangles, of which twelve are isosceles and eight equi- 

 lateral. Ex. Icosahedral sulphurated iron. 



h. Trapezoidal, when its surface is composed of twenty-four 

 equal and similar trapezoids. Ex. Trapezoidal garnet» 



i, Triacontahedral, when its surface is composed of thirty 

 rhombuses. Ex. Triacontahedral sulphurated iron. 



k, Enneaco?itahedral, when its surface is composed of 90 

 faces. Ex. Enneaeoniahedral idiocrasis. 



/. BirhomboidaL when its surface is composed of twelve 

 faces, which being taken by sixes, and lengthened in 

 imagination until they intersect, would form two difr- 

 fereut rhomboids. Ex. Birhomboidal carbonated lime. 



Wc 



