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.

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.

No. 2.—This must be seven and a half inches long, one and a half inches outside, and an inch inside.

No. 3.—This is a length of tubing about sixty inches long, two and a half inches in outside circumference, and at least an inch thick. If an inch and a half thick all the better, as it will be more air-tight.

Divide tube No. 2 into two equal parts, cutting from right to left at an angle of 45 degrees. Into one of the parts fit a plug of hard wood pierced lengthwise by a red hot wire (fig. 1); the figure shows the shape of it sufficiently. In the hollow side cut a small opening, and over this tie very tightly a band of flexible india-rubber (fig. 3). This band will serve as the valve of the piston of the pump. Figs. 3 and 4 give a side and front view of this valve. Great care must be taken neither to split the plug in boring the hole nor to cut the tube.

This valve must now be inserted in the large tube No. 1, as seen in fig. 2.

At the other end of the large tube, which will serve as the body of the pump, at B fig. 2, fix a similar valve to the above, but the india-rubber band must be fixed on the other side of the valve as at B fig. 2. The fitting A will serve for escape, the second for withdrawing the air from the space to be exhausted. Finally fix tube No. 3 on valves A or B, fig. 2, according to your wish to produce a vacuum or to compress the air.

By means of a pedal made simply with two boards put together on hinges (fig. 5), one pressed with the foot, the air contained in the body of the pump (fig. 2) tends to escape. It therefore lifts the valve of the fitting fixed at A, and escapes through the flexible elastic band tied over the hole in the hollow side of tube No. 2. If the pressure ceases the big tube, on account of its own elasticity, takes its former form and sucks in the air. This time it is the valve at B which is lifted and lets pass the air which fills the body of the pump. If one has fixed on to the fitting at B, the long india-rubber tube No. 3, which is plunged in a receiver—a receiver is any vessel in which the air is exhausted, or into which it is forced—it is easily understood that after a few moves of the pedal, the air is drawn out, and a vacuum is obtained.

If one wishes to have a force-pump one has only to modify slightly the construction of the valve. Instead of a band of india-rubber fixed as shown in fig. 3, it is altered as in fig. 4, that is to say the valve is formed by a band of supple india-rubber fastened by two tacks only on one side of the opening in the side of the plug. For this object it is also necessary to take stronger tubes.

Let us now review the few experiments that can be made with this machine.

In order to conduct experiments a receiver must be obtained. The best vessel for your purpose is a large bell-jar with a ground glass stopper and neck to insure absolute tightness. Such a jar may be cheaply obtained at a scientific instrument maker’s for about seventy-five cents. If you cannot get a bell-jar procure a 4-lb. jam pot and a tightly-fitting bung. In the middle of the latter bore a hole to admit a glass tube, some six inches long and an inch in diameter, and then sealing-wax the whole of the upper surface of the bung so that air cannot enter. Over the projecting end of the glass tube, bind very tightly the free end of the long tubing affixed to the pump. To ensure tight binding, waxed thread should be used.