A most interesting class of experiments can be made with an air pump, a piece of apparatus unfortunately beyond the pocket-money supply of the average boy. Nevertheless, if the following instructions are exactly followed and carefully carried out, a very excellent air pump can be made at a comparatively small cost. Some pretty, as well as interesting results will amply repay you for the trouble you take to make the pump. Although the air seems so light in comparison with water or a heavy metal like iron, you must remember that it really presses upon every square inch of the earth’s surface, aye, on every square inch of your own bodies, with a force of fourteen and a half pounds. In other words, the weight of the air at the sea level resting on each square inch of surface weighs fourteen and a half pounds.
Don’t be frightened, boys, at the explanation of one word that must be used in connection with air experiments. The word is vacuum. Vacuum really means an empty space, devoid of all matter, even of air. Although it seems easy to think of an empty space, it is quite impossible to exhaust a space of all matter, even of air. For this reason, the alchemists of the middle ages used to say: “Nature abhors a vacuum.” This was only their way of saying how impossible it was to make a space, such as the inside of a vessel, quite empty. Yet it is possible to reduce the amount of air in a vessel almost to nothing.
Fig. 1.
Fig. 2.
Now for the pump. In the first place obtain three pieces of gutta-percha tubing of the following lengths:
No. 1.—A tube twelve and a half inches long, measuring outside two and a half, inside one and a half inches in circumference.