exactly,
as explained in the last chapter, showing its parent form, shape and
characteristics with microscopic perfection, but more and more in
miniature as its size is reduced.
This may clearly be seen by taking a very small quantity of such a substance as chlorate of potash. If a crystal
of this is examined under a magnifying glass till its crystalline form
and structure are familiar, and it is then placed in a test-tube and
gently heated, cleavage will at once be evident. With a little
crackling, the chlorate splits itself into many crystals along its
chief lines of cleavage (called the cleavage planes), every one of
which crystals showing under the microscope the identical form and
characteristics of the larger crystal from which it came.
The
cleavage of minerals must, therefore, be considered as a part of their
crystalline structure, since this is caused by cleavage, so that both
cleavage and crystalline structure should be considered together. Thus
we see that given an unchangeable crystal with cleavage planes evident,
it is possible easily to reproduce the same form over and over again by
splitting, whereas by simply breaking, the form of the crystal would be
lost; just as a rhomb of Iceland spar might be sawn or broken across
the middle and its form lost, although this would really be more
apparent than real, since it would be an alteration in the mass and
not in the shape of each individual crystal. And given further
cleavage, by time or a sudden breaking down, even the mass, as mass,
would eventually become split into smaller but perfect rhombs.
Much skill is, therefore, required in cutting and
