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SLATES. 385

on all sides by the liquid, or, if it rest upon the surface, it
must be turned daily so as to present all its faces in succes-
sion to the working builder. .

In building up crystals these little atomic bricks often
arrange themselves into layers which are perfectly parallel
to each other, and which can be separated by mechanical
means; this is called the cleavage of the crystal. The crys-
tal of sugar I hold in my hand thus far escaped the solvent
and abrading forces which sooner or later determine the
fate of sugar-candy. ' I readily discover that it cleaves with
peculiar facility in one direction. Again, I lay my knife
upon this piece of rock-salt, and with a blow cleave it in
one direction. Laying the knife at right angles to its
former position, the crystal cleaves again; and finally,
placing the knife at right angles to the two former positions,
we find a third cleavage. Rock-salt cleaves in three di-
rections, and the resulting solid is this perfect cube, which
may be broken up into any number of smaller cubes. Ice-
land spar also cleaves in three directions, not at right angles,
but oblique to each other, the resulting solid being a rhom-
boid. In each of these cases the mass cleaves with equal
facility in all three directions. For the sake of complete-
ness I may say that many crystals cleave with unequal fa-
cility in different directions: heavy spar presents an ex-
ample of this kind of cleavage.

Turn we now to the consideration of some other phe-
nomena to which the term cleavage may be applied. Beech,
deal, and other woods, cleave with facility along the fibre,
and this cleavage is most perfect when the edge of the axe
is laid across the rings which mark the growth of the tree.
If you look at this bundle of hay severed from a rick, you
will see a sort of cleavage in it also; the stalks lie in paral-
lel planes, and only a small force is required to separate
them laterally. But we cannot regard the cleavage of the

tree as the same in character as that of the hay-rick. In
17

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