Air and all other gases manifest a pressure in all directions not due to their weight. The power of air to keep tires and footballs inflated and that of steam in driving an engine are examples. It is this force that prevents the pressure of air from crushing in, since there are many air spaces distributed throughout the body.
COMPOSITION OF AIR
This subject and the three immediately following it have a special bearing on hygiene.
1. Invert a sealing-jar over a lighted candle. Has the candle used up all the air when it goes out?
2. Place a very short candle on a thin piece of cork afloat on water in a plate; light the candle, and again invert the jar over it. Note that the candle goes out and the water rises only a short distance in the jar; therefore all the air has not been used up.
3. Slip the glass top of the jar under the open end and set the jar mouth upward on the table without allowing any water to escape. Now plunge a lighted splinter into the jar. The flame is extinguished.
Air, therefore, contains an active part that helps the candle to burn and an inactive part that extinguishes flame. The names oxygen and nitrogen may be given. These gases occur in air in the proportion of about 1:4. (This method is not above criticism. Its advantage for young pupils lies in its simplicity.)
OXYGEN
Make two or three jars of oxygen, using potassium chlorate and manganese dioxide. (See any Chemistry text-book.) Let the pupils examine the chemicals, learn their names, and know where to obtain them. Perform the following experiments:
1. A glowing splinter relights and burns very brightly if plunged into oxygen.