eat, light, sound, and mechanical motion can easily be shown.

4. A weight connected by a cord with some small machine and made to run
it, will help the pupil to grasp the general principles in the storage of
energy through gravity. A vessel of water on a high support from which the
water is siphoned on to a small water wheel will serve the same purpose.

5. The storing of energy by chemical means may be illustrated by
decomposing potassium chlorate with heat or by decomposing water by means
of a current of electricity.

6. Study the transfer of energy from the body to surrounding objects, as
in moving substances and lifting weights.

Fill a half gallon jar two thirds full of water and carefully take the
temperature with a chemical thermometer. Hold the hand in the water for
four or five minutes and take the temperature again. Inference.

CHAPTER XIII - GLANDS AND THE WORK OF EXCRETION

In our study so far we have been concerned mainly with the introduction of
materials into the body. We are now to consider the removal of materials
from the body. The structures most directly concerned in this work are
known as

*Glands.*--As generally understood, glands are organs that prepare special
liquids in the body and pour them out upon free surfaces. These liquids,
known as _secretions_, are used for protecting exposed parts, lubricating
surfaces that rub against each other, digesting food, and for other
purposes. They differ widely in properties as well as in function, but are
all alike in being composed chiefly of water. The water, in addition to
being necessary to the work of particular fluids, serves in all cases as a
carrier of solid substances which are dissolved in it.

*General Structure of Glands.*--While the various glands differ greatly in
size, form, and purpose, they present striking similarities in structure.
All glands contain the following parts:

1. Gland, or secreting, cells. These are _specialized_ cells for the work
of secretion and are the active agents in the work of the gland. They are
usually cubical in shape.

2. A basement membrane. This is a thin, connective tissue support upon
which the secreting cells rest.

3. A network of capillary and lymph vessels. These penetrate the tissues
immediately beneath the secreting cells.

4. A system of nerve fibers which terminate in the secreting cells and in
the walls of the blood vessels passing to the glands.

These structures--secreting cel