We have the air about us everywhere. A so-called empty vessel is one
where the contents are invisible. The following experiment is a good
illustration of this.
(1) Wrap the throat of a glass funnel with moistened cloth or paper so
that it will fit tightly into the neck of a bottle, and fill the funnel
with water. If the space between the funnel and the bottle is air-tight,
the water will not flow into the bottle.
[Illustration: FIG. 1.]
Do not explain this in advance to the pupils. Ask them what prevents
the water from flowing into the bottle. If they are puzzled, loosen the
funnel, and show them that the water will now flow in. In the first case,
as the air could not escape, the water could not flow in; in the second,
the air was displaced by the heavier water.
Ask the pupils why the air in a crowded room becomes so difficult to
breathe. Could a person live if he were shut up in an air-tight room for a
long time? Fresh air is necessary to life. The teacher may explain that it
is the oxygen in the air that supports life. Air is composed one-fifth of
this gas and four-fifths of nitrogen. The gases are mixed and the nitrogen
simply dilutes the oxygen, as it were.
Fresh air is necessary to support combustion as well as life. Ask them why
we put out a fire by throwing a blanket or a rug over it. The following
experiment illustrates this.
(2) Take a small, wide-mouthed bottle, covered with a card or cork.
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