poles,
and the edges connected with them polar edges. The two other axes are
named subordinate or lateral axes, and a plane passing through them is
named the basis of the crystal. The two planes that pass through the
principal and one of the lateral axes are named normal chief sections,
and a plane through the chief axis intermediate to them, a diagonal
chief section. The name tetragonal is derived from the form of the
basis, which is usually quadratic.
There are eight tetragonal forms, of which five are closed, —that is, bounded on all sides by planes, and of definite extent,—and three open, which in certain directions are not bounded, and consequently of indefinite extent.
The
description of the varieties is as follows, it being premised that a
crystallographic pyramid is equivalent to two geometrical pyramids
joined base to base.
