98 CRYSTALLINE FORMS. 



III. The Prismatic comprises the rectangular octohedron 

 (fig. 36), the rhombic octohedron (fig. 37), the right rectan- 

 gular prism (fig. 39), and the right rhombic 

 prism (fig. 40). 



IV. The Hemi-Prismatic consists of the 

 right rhomboidal prism (fig. 41), and the 

 oblique rhombic prism (fig. 42). 



V. The Tetarto-Prismatic is composed of 

 FIG 53. * ne obtiqu 6 rhomboidal prism (fig. 43). 



VI. The Rhombohedral includes the rhombohedron (fig. 

 44), and the regular hexagonal prism (fig. 45). 



Two or more simple forms united, constitute a com- 

 bination. These combinations are of great importance in 

 the study of crystals. They are called binary, ternary, 

 quaternary, according to the number of simple forms com- 

 bined. 



The combination must be derived from the same fun- 

 damental form. They must be in such positions, with 

 regard to each other, as are peculiar to the system to which 

 they belong. 



The edges in which the faces of two different forms con- 

 tained in a combination meet, are termed edges of combi- 

 nation. 



DISCOVERIES or SIR DAVID BREWSTER. Such is a briet 

 outline of the arrangement of crystalline forms by Mohs, 

 which is extensively adopted ; the distinctions of which are 

 so far important that all the forms which a salt or other 

 mineral substance assumes, almost always belong to the 

 same system of classification. The justice of this mode of 

 arrangement has been farther proved by the optical inves- 

 tigations of Sir David Brewster, which will be best under- 

 stood by the following explanation : 



If we draw a black line on paper, and view it through a 

 piece of Iceland spar, which is the purest variety of carbonate 

 of lime,* the line will appear double in every position except 

 one, in which the one line will be observed to overlap the 

 other. In the position at right angles to that, the separa- 



* See a splendid example in the collection of the British Museum, case 43. 



