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. To
this cover fasten a piece of bent wire with a taper on the end. Light the
taper and lower it into the jar. It will burn a few seconds and then go
out. Raise and light it again, and it will be extinguished as soon as it
is plunged into the bottle. This shows that the oxygen of the air is used
up by burning substances, as it is by breathing animals.

[Illustration: FIG. 2.]

The following experiment shows that fire will not burn in an atmosphere of
gas from our lungs.

(3) Fill a bottle with gas by breathing into it through a bit of glass
tubing, passed through a card or cork, and reaching to the bottom of the
bottle. The bottle will be dimmed with moisture, showing the presence of
aqueous vapor. A lighted match plunged into the bottle will be immediately
extinguished. A better way, which, however, takes some skill in
manipulation, is to fill the bottle with water, cover it with a flat piece
of glass, and invert the bottle in a dish of water, taking care that no
air bubbles enter. Then, through a bit of glass tubing, blow into the
bottle till the water is expelled. Cover the mouth with the glass under
water, and holding it tightly down, invert the bottle quickly. Set it
down, light a match, take away the glass, and at the same instant plunge
in the match. If no air has been allowed to enter, the match will go out
at once. No animal could live in an atmosphere which could not support
combustion.

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 ex