first substances to
be driven off, so that the carbon is left behind nearly pure. In the same
way we have driven off all the gases from the half-burned match and left
the carbon. The teacher should have a piece of charcoal to show the
pupils. It still retains all the markings of the wood.

If the combustion is continued, the carbon also unites with the oxygen of
the air, till it is all converted into carbonic acid gas. This was the
case with the match where we left the glowing spark. The gray ash that was
left behind is the mineral matter contained in the wood.

(7) We can show that this gas is formed by pouring lime water into a
bottle in which a candle has been burned as in (2). The water becomes
milky from a fine white powder formed by the union of the carbonic acid
gas with the lime, forming carbonate of lime. This is a chemical test.

The wood of the match is plainly of vegetable origin; so also is the
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