CRYSTALLINE STRUCTURE 15
This
is not so laborious a matter as would appear, for if we take a
substance which crystallises in a cube we find it is possible to draw
nine symmetrical planes, these being called " planes of symmetry," the
intersections of one or more of which planes being called "axes of
symmetry." So that iti the nine planes of symmetry of the cube we get
three axes, each running through to the opposite side of the cube. One
will be through the centre of a face to the opposite face ; a second
will be through the centre of one edge diagonally ; the third will lie
found in a line running diagonally from one point to its opposite. On
turning the cube on these three axes—as, for example, a long needle
running through a cube of soap—we shall find that four of the six
identical faces of the cube are exposed to view during each revolution
of the cube on the needle or axis.
These
faces are not necessarily, or always, planes, or flat, strictly
speaking, but are often more or less curved, according to the shape of
the crystal, taking certain characteristic forms, such as the square,
various forms of triangles, the rectangle, etc., and though the
crystals may be a combination of several forms, all the faces of any
particular form are similar.
All
the crystals at present known exhibit differences in their planes, axes
and lines of symmetry, and on careful comparison many of them are
found to have some features in common : so that when they are sorted
out it is seen that they are capable of being classified into
thirty-three groups. Many of these groups are analogous, so that on
analysing them still further we find that all the known crystals may be
classed in six separate systems
