FORM OF MINERALS.	25
	principal axes, eight trigonal, and twelve rhombic, or tetra­gonal with unequal angles. 7. The hexakisoctahedrons (fig. 7), bounded by forty-eight scalene triangles, vary much in general aspect, ap­proaching more or less to all the preceding forms ; but most frequently they have the face? arranged either in six groups of eight, or eight of six, or twelve of four faces. There are twenty-four long edges, often corresponding to those of the rhombic-dodecahedron ; twenty-four interme­diate edges lying in pairs over each edge of the inscribed octahedron ; and twenty-four short edges in pairs over the edges of the inscribed cube. There are six ditetragonal angles joined by the principal axes, eight hexagonal and twelve rhombic angles.—Ex., fluor spar, garnet, diamond.
	The seven forms of crystals now described are related to each other in the most intimate manner. This will appear more distinctly from the following account of the derivation of the forms, with which is conjoined an explanation of the crystallographic signs or symbols by which they are desig­nated. We have adopted these symbols throughout this work, in the belief that they not only mark the forms in a greatly abbreviated manner, but also exhibit the relations of the forms and combinations in a way which words could hardly accomplish. 2