(Consult _Science of Common Life_, Chaps. VIII, IX, X.)
1. Air takes up s.p.a.ce. Put a cork with one hole into the neck of a flask or bottle. Insert the stem of a funnel and try to pour in water. Try with two holes in the cork. When we call a bottle "empty" what is in it?
2. Air is all around us. Feel it; wave the hands through it; run through it; note that the wind is air; inhale the air and watch the chest.
3. Air has weight. This is not easy to demonstrate without an air-pump and a fairly delicate balance.
Fit a large gla.s.s flask with a tightly fitting rubber stopper having a short gla.s.s tube pa.s.sing through it. To the gla.s.s tube attach a short rubber one and on this put a clamp. Open the clamp and suck out all the air possible. Close the clamp and weigh the flask. When perfectly balanced, open the clamp and let the air enter again. Note the increase in weight.
If an air-pump is available, procure a gla.s.s globe provided with a stop-c.o.c.k (see Apparatus). Pump some of the air from the globe, then weigh and, while it is on the balance, admit the air again and note increase in weight.
Tie a piece of thin sheet rubber over the large end of a thistle tube; suck the air out of the tube and note how the rubber is pushed in. This is due to the weight or pressure of the air. Turn the tube in various positions to show that the pressure comes from all directions. To show that "suction" is not a force, let a pupil try to suck water out of a flask when there is only one opening through the stopper. If two holes are made, the water may be sucked up, that is, _pushed_ up by the weight of the air.
Fill a pickle jar with water. Place a piece of writing paper on the top and then, holding the paper with the palm of the hand, invert the jar.
The pressure of the air keeps the water in.
A cubic foot of air weighs nearly 1-1/4 oz. Find the weight of the air in your school-room.
The atmosphere exerts about fifteen pounds pressure on every square inch of the surface it rests against. Find the weight supported by the top of a desk 18 inches by 24 inches. If the surface of the body is eight square feet, what weight does it have to sustain? Why does this weight not crush us?
THE BAROMETER
The experiments immediately preceding will have paved the way for a study of the barometer.
1. Fill a jar with water and invert it, keeping its mouth below the surface of the water in another vessel. If the pupils can be led to see that the water is sustained in the jar by the air pressing on the water in the vessel, they can understand the barometer.
2. Fill a tube about 30 inches long, and 1/4 inch inside diameter with water, and invert it over water, as with the jar in the previous experiment.
3. Use the same tube or one similar to that in 2 above, but fill with mercury and allow the pupils to notice the great weight of the mercury.
Holding the mercury in with your finger, invert the tube over mercury.
This time the fluid falls some distance in the tube as soon as the finger is removed. A tube of this size requires 1 lb. of mercury.
Lead the pupils to see that the mercury remaining in the tube is sustained by the air pressure, and that any increase or decrease of the atmospheric pressure will result in the rise or fall of the mercury column. Leave the barometer (made as in 3 above) in the room for a few days and note whether its weight changes. The use of the instrument in predicting weather changes should be emphasized. Compare your barometer with the records in the daily papers.
The average height of the barometric column is 30 inches at sea-level.
Explain how you could estimate heights of mountains and balloons with a barometer.
THE COMMON PUMP
This is a valuable application of air pressure. A gla.s.s model will prove useful, but a model made by pupils will be much more so. (See _Laboratory Exercises in Physics_ by Newman.)
The water rises in the pump because the sucker lifts the air from the water inside, allowing the air outside to push the water up. A common pump will not lift water more than about 30 feet. Why is this? Compare the pump to a barometer. (See _The Ontario High School Physics_.)
EXPANSIVE FORCE OF AIR
Air and all other gases manifest a pressure in all directions not due to their weight. The power of air to keep tires and footb.a.l.l.s 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 s.p.a.ces 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 gla.s.s 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 pota.s.sium 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.
2. A piece of picture wire tipped with sulphur burns with great brightness.
3. Burn phosphorus or match heads in a spoon. A spoon may be made by attaching to a wire a bit of crayon having a hollow scooped on its upper surface. A clay pipe bowl attached to a wire will answer.
From these experiments pupils will learn the value of nitrogen as a diluent of the oxygen. Pure oxygen entering the lungs would be just as destructive as it would be entering the furnace.
CARBON DIOXIDE
1. Make a jar of this gas. Washing soda and vinegar will answer if hydrochloric acid and marble are not obtainable. (Consult the _Science of Common Life_, Chap. XIII, and any Chemistry text-book.)
2. Lower a lighted candle first into a jar of air then into the jar of carbon dioxide.
3. Make some lime-water by stirring slaked lime with water and allowing the mixture to settle. Shake up some clear lime-water with a jar of the gas. Pupils will be made to understand that the milky colour will in future be considered the test for carbon dioxide.
4. Have one of the pupils cause his breath to bubble through some clear lime-water for a minute. Using a bicycle pump, cause some fresh air to bubble through lime-water.
5. Hold a clear jar inverted over the candle flame for a few seconds, then test with lime-water.
6. Invert a large jar over a leafy plant for a day. Keep in the dark and test the jar with lime-water.
Is this gas likely to be in the air? Set a plate of lime-water in the school-room for a day or two, and then examine it. Try to pour the gas from jar to jar and use a candle as a test. Is the gas heavier than air?
On account of its weight, the gas often collects in the bottoms of old wells, mines, and tunnels. It is dangerous there since it will not support life.
USES:
1. Add a little water to some baking powder and cause the gas that forms to pa.s.s through lime-water. What causes the biscuits to "rise"?
2. Mix flour and water in a jar, add a bit of yeast cake and a little sugar, and let stand in a warm place. Test the gas that forms, for carbon dioxide. What causes bread to rise?
3. Uncork a bottle of ginger ale, shake the bottle, and lead the gas that comes off through lime-water.
4. Most portable fire extinguishers depend on the generation of carbon dioxide.