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 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