From these experiments the pupils have seen that the life-sustaining
quality of the air is used up by combustion and respiration. To bring in
the subject of purification by plants, ask them why all the oxygen in
the world is not exhausted by the people and the fires in it. After the
subject has been explained, the following experiment can be prepared and
put aside till the next lesson.
(4) Fill two bottles with air from the lungs, as in (3) having previously
introduced a cutting from a plant into one of the bottles. Allow them to
stand in the sun for a day or two. Then test both bottles with a burning
match. If properly done, the result will be very striking. The end of
the cutting should be in the water of the dish. This experiment will not
succeed excepting with bottles such as are used for chemicals, which have
their mouths carefully ground. Common bottles allow the air to enter
between the bottle and the glass.[1]
[Footnote 1: See note on page 13.]
[Illustration: FIG. 3.]
4. _Fuel_.--Light a match and allow it to burn until half charred. Blow it
out gently, so as to leave a glowing spark. When this spark goes out it
will leave behind a light, gray ash. We have to consider the flame, the
charred substance, and the ash.
Flame is burning gas. In all ordinary fuels, carbon and hydrogen, in
various combinations and free, make the principal part. The first effect
of the heat is to set free the volatile compounds of carbon and hydrogen.
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