there are formations built also from
the lime in the dropping water on the floor of the cave, and these are
called stalagmites. In time the stalactites and the stalagmites will
meet, forming a great column reaching from floor to ceiling. Some of
these formations, when they are free from foreign substances, are very
beautiful. They are also very hard, giving off a metallic musical tone
when struck by any hard substance.

We have already stated that limestone is a compound of ordinary lime and
carbon dioxide, forming a carbonate of lime. This statement does not
give a complete analysis of all the elements entering into limestone. In
the first place lime itself is a compound formed of two elementary
substances, calcium and oxygen. The lime molecule is composed of one
atom of calcium and one of oxygen. Neither calcium nor lime is found
pure in nature. Inasmuch as carbon dioxide is composed of one atom of
carbon and two of oxygen, and lime is composed of one atom of calcium
and one of oxygen, when we have the two combined the molecule of
carbonate of lime, or, as it is technically called, calcic carbonate, is
composed of one atom of calcium, one of carbon and three of oxygen,
(lime plus carbon dioxide).

As before stated, lime is not found un-combined with other substances in
nature. And as it is of great economic importance, it will be profitable
to know how it is formed. Lime is produced from ordinary limestone by
burning it in kilns where it is subjected to a heat of a certain
temperature for a number of hours. The heat drives off the carbon
dioxide, which, as we have seen, has taken away from each molecule of
the compound all of the carbon and two atoms of the oxygen, while all
of the calcium is retained with one atom of oxygen, leaving ordinary
lime. Lime, then, is simply oxide of calcium.

As all know, it is used almost exclusively for making mortar for
building purposes. In order to do this we have to put it through the
process of "slacking," by pouring water upon it, and here another
chemical change takes place. The water unites with the lime, when
immediately the heat that was expended in throwing off the carbon
dioxide and was stored in the lime as energy is now given up again in
the form of heat. When a considerable bulk of lime is slacked very
rapidly the heat that is given off is so great that it will produce
combustion. Here is a beautiful illustration of what has been
erroneously called "latent heat." It i