position of the parts; and crystallized, when it also produces a determinate external form, or a crystal.
CRYSTALS.
Faces, Edges, Angles, Axes of Crystals.—The word crystal in
mineralogy designates a solid body exhibiting an original (not
artificial) more or less regular polyhedric form. It is thus bounded
.by plane surfaces, named -faces, which intersect in straight lines or.edges, and these again meet in points. and form solid angles, bounded by three or more faces. The space occupied by a crystal is often named a form of crystallization, which
is thus the mathematical figure regarded as independent of the matter
that fills it. Crystals bounded by equal and similar faces are named simple forms ; while those in which the faces are not equal. and similar are named compound forms, or combinations, being
regarded as produced by the union or combination of two or more simple
forms. The cube or hexahedron (fig. 1), bounded by six equal and
similar squares; the octahedron (fig. 2), by eight equilateral
triangles; and the rhom-bohedron, by six rhombs,—;are thus simple
forms. An axis of a crystal is a line -passing through its
centre and terminating either in the middle of two faces, or of two
edges, or in two angles; and axes terminating in similar ]jarts of a
crystal are named similar axes. In describing a crystal, one of its
axes is supposed to be vertical or upright, and is then named the principal axis, and
that axis is chosen which is the only one of its kind in the figure. A
few other technical terms used in describing crystals will be
explained as they occur.
Systems of Crystallization.—The
forms of crystals that occur in nature seem almost innumerable. On
examining them, however, more attentively, certain relations are di*
