76 A POPULAR TREATISE ON GEMS.
or specific gravity; 2d. Those
phenomena called forth in minerals by the influence of some external
power or agent, as their optical, electric, or thermal relations; and, 3d. Other
characters depending on the personal sensation of the observer—on his
taste, smell, and touch. All these properties furnish useful characters
in distinguishing and describing mineral species.
Cleavage and Fracture.
In
many species there are certain planes at right' angles to which
cohesion seems to be at a minimum, so that the mineral separates along
or parallel to these planes far more readily than in any other
direction. This property is named cleavage, and these planes
cleavage-planes. They have a strictly definite position, and do not
show any transition or gradual passage into the greater coherence in
other directions. The number of these parallel cleavage-planes is
altogether indefinite; so that the only limit that can be assigned to
the divisibility of some minerals, as gypsum and mica, arises from the
coarseness of our instruments.
These
minima of coherence or cleavage-planes are always parallel to some face
of the crystal, and similar equal minima occur parallel to every other
face of the same form. Hence they are always equal in number to the
faces of the form, and the figures produced by cleavage agree in every
point with true crystals, except that they are artificial. They are
thus most simply and conveniently described by the same terms and signs
as the faces of crystals. Some minerals cleave in several directions
parallel to the faces of different forms, but the cleavage is generally
more easily obtained and more perfect in one direction than in the
others. This complex cleavage is well seen in cale-spar and fluor spar,
and very remarkably in zinc-blende, where
