Put a lump of alum into a tumbler of water, and, as the alum dissolves,
it will assume the shape of a pyramid. The cause of the alum decreasing
in this peculiar form is briefly as follows: at first, the water
dissolves the alum very fast, but as the alum becomes united with the
water, the solvent power of the latter diminishes. The water, which
combines first with the alum, becomes heavier by the union, and falls
to the bottom of the glass; where it ceases to dissolve any more,
although the water which it has displaced from the bottom has risen
to the top of the glass, and is there acting upon the alum. When the
solution has nearly terminated, if you closely examine the lump, you
will find it covered with geometrical figures, cut out, as it were, in
relief, upon the mass; showing, not only that the cohesion of the atoms
of the alum resists the power of solution in the water, but that, in
the present instance, it resists it more in some directions than in
others. Indeed this experiment beautifully illustrates the opposite
action of cohesion and repulsion.