1. Air takes up space. 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 glass flask with a tightly fitting rubber stopper having a short glass tube passing through it. To the glass 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 glass globe provided with a stop-cock (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?