Fig. 439.—Cube with angles removed.
The cube, or hexahedron, the octohedron, and the rhombohedron are all simple forms, being each bounded by equal and similar faces, or surfaces. We can thus understand how certain primary or original natural forms of crystals can be changed in appearance by connection. Of the various substances crystallizing in this system we find salt, iron pyrites, gold, silver, copper, and platinum, and the sulphide of lead called galena, in the cube or hexahedron form. The diamond and fluor-spar, alum, etc., appear in the first form (I), fig. 436 (octohedron). The cube, we see, has six equal faces, eight equal angles, and twelve equal edges. Galena, as will be observed from the illustration herewith, shows this peculiarity in a very marked manner (fig. 446).
Fig. 440.—Another intermediate form of octohedron between figs. 436 and 438.
Fig. 441.—Cube deprived of edges and angles.
Fig. 442.—Rhombic dodecahedron (garnet crystal).
