s "heat stored as potential
energy," that is released by the combination of lime with water.
Slackened lime, then, is called calcic hydrate.

Very little of the limestone that we find is absolutely pure. It is
considered good when it does not contain over five or six per cent. of
foreign substance. When more than this is present the lime is considered
poor, and when it reaches fifteen per cent. or more of impurities it
assumes the property of hardening under water and is called cement.

Carbonate of lime is found in several other forms; for instance, the
various kinds of marble and chalk are carbonates of lime. The
composition of marble and chalk is exactly the same as that of
limestone. The difference is chiefly one of molecular rather than
chemical structure. Marble is what chemists would call an allotropic or
changed form of limestone; and, as before stated, the difference seems
to consist in the fact that the marble assumes a crystalline arrangement
of its atoms and will therefore take a high polish, which is not true of
ordinary limestone. Marble varies greatly in coloring and texture, all
of which differences are explainable under the one head of molecular
arrangement. Nearly pure carbon exists in three distinct forms--the
diamond, graphite, and charcoal. As is the case with marble, these
differences in the different forms of carbon are not chemical, but
molecular differences. The substances are the same, but their
infinitesimal particles are differently arranged.

Carbonate of lime--as it exists in its various forms, as limestone, from
which lime and cement are made, and marble, which is such an important
element in the arts--is a substance of great importance to man. We have
already noted some of the processes that nature uses in gathering up
these substances from the ocean by the employment of various forms of
animal life. Here is another. Whoever has visited the Bermudas has seen
an island wholly formed of what is called coral rock. Coral is a
structure produced by a peculiar form of sea animal that gathers up the
calcareous or lime-like matter floating in the sea water, and builds a
house of it in which to live during the little lifetime that is allotted
to him. When he dies his children do not occupy the old home, but build
a new one, which is a superstructure planted upon the old one as a
foundation. This process of growth sometimes takes the form of a tree or
plant, and coral trees grow upon trees and pl