charcoal, which is nearly pure carbon. Coal is also carbon, the remains of
ancient forests, from which the gases have been slowly driven off by heat
and pressure. All the common fuels are composed principally of carbon and
hydrogen. When these elements unite with oxygen, carbonic acid gas and
water are formed.[1]

[Footnote 1: [Transcriber's Note: This note is missing from original
text.]]

(8) The same products are formed by respiration. We breathe out carbonic
acid gas and water from our lungs. Breathe on a cold glass. It is bedewed
exactly as it is by the candle flame. Breathe through a bit of glass
tubing into a bottle of lime water. It becomes milky, showing the presence
of carbonic acid gas. Why is this?

Every act or thought is accompanied by a consumption of material in the
body, which thus becomes unfit for further use. These waste substances,
composed chiefly of carbon and hydrogen, unite with oxygen breathed in
from the air, forming carbonic acid gas and water, which are breathed
out of the system. The action is a process of slow combustion, and it is
principally by the heat thus evolved that the body is kept warm. As we are
thus constantly taking oxygen from the air, a close room becomes unfit to
live in and a supply of fresh air is indispensable. The cycle of changes
is completed by the action of plants, which take in carbonic acid gas, use
the carbon, and return most of the oxygen to the atmosphere.

APPARATUS FOR EXPERIMENTS.[1]

[Footnote 1: The glass apparatus required, including an alcohol lamp, may
be obtained for one dollar by sending to the Educational Supply Co., No. 6