On repeating the same process, that is, drawing up and forcing down the piston, the air at each time in the receiver, will become less and less in quantity, and in consequence, more and more rarefied. For it must be understood, that although the air is exhausted at every stroke of the pump, that which remains, by its elasticity expands, and still occupies the whole space. The quantity forced out at each successive stroke is, therefore, diminished, until, at last, it no longer has sufficient force before the piston, to open the valve, when the exhausting power of the instrument must cease entirely.

Now it will be obvious, that as the exhausting power of the air pump depends on the expansion of the air within it, a perfect vacuum can never be formed by its means, for so long as exhaustion takes place, there must be air to be forced out, and when this becomes so rare as not to force open the valves, then the process must end.

A good air pump has two similar pumping barrels to that described, so that the process of exhaustion is performed in half the time that it could be performed by one barrel.

Fig. 101.

b

m

air from the vessel on the plate, suppose, that as the two pistons

The barrels, with their pistons, and the usual mode of working them, are represented by fig. 101. The piston rods are furnished with racks, or teeth, and are worked by the toothed wheel a, which is turned backwards and forwards, by the lever and handle b. The exhaustion pipe, c, leads to the plate on which the receiver stands, as shown in fig. 100. The valves v, n, u, and m, all open upwards.

To understand how these pistons act to exhaust the through the pipe c, we will now stand, the handle b is

Is the air pump capable of producing a perfect vacuum? common air pumps have more than one barrel and piston? the pistons of an air pump worked?

Why do

How are

turned towards the left. This will raise the piston A, while the valve u will be closed by the pressure of the external air acting on it in the open barrel in which it works. There would then be a vacuum formed in this barrel, did not the valve m open and let in the air coming from the receiver through the pipe c. When the piston, therefore, is at the upper end of the barrel, the space between the piston and the valve m, will be filled with the air from the receiver. Next suppose the handle to be moved to the right, the piston A will then descend, and compress the air with which the barrel is filled, which, acting against the valve u, forces it open, and thus the air escapes. Thus it is plain, that every time the piston rises, a portion of air, however rarefied, enters the barrel, and every time that it descends, this portion escapes, and mixes with the external atmosphere.

The action of the other piston is exactly similar to this, only that B rises while A falls, and so the contrary. It will be obvious, on an inspection of the figure, that the air cannot pass from one barrel to the other, for while A is rising, and the valve m is open, the piston B will be descending, so that the force of the air in the barrel B, will keep the valve n closed. Many interesting and curious experiments, illustrating the expansibility and pressure of the atmosphere, are shown by this instrument.

If a withered apple be placed under the receiver, and the air is exhausted, the apple will swell, and become plump, in consequence of the expansion of the air which it contains within the skin.

Ether placed in the same situation, soon begins to boil without the influence of heat, because its particles, not having the pressure of the atmosphere to force them together, fly off with so much rapidity as to produce ebullition.

The Condenser.

The operation of the condenser is the reverse of that of the air pump, and is a much more simple machine. The air pump, as we have just seen, will deprive a vessel of its ordinary quantity of air. The condenser, on the contrary, will double,

While the piston A is ascending, which valves will be open, and which closed? When the piston A descends, what becomes of the air with which its barrel was filled? Why does not the air pass from one barrel to the other, through the valves m and n? Why does an apple placed in the exhausted receiver grow plump? Why does ether boil in the same situation? How does the condenser operate?

Fig. 102. or treble the ordinary quantity of air in a close vessel, according to the force employed.

This instrument, fig. 102, consists of a pump barrel and piston, a, a stop cock b, and the vessel c furnished with a valve opening inwards. The orifice d, is to admit the air, when the piston is d drawn up to the top of the barrel.

To describe its action, let the piston be above d, the orifice being open, and therefore the instrument filled with air, of the same density as the external atmosphere. Then on forcing the piston down, the air in the pump barrel, below the orifice d, will be compressed, and will rush through the stop cock b, into the vessel c, where it will be retained, because, on again moving the piston up. ward, the elasticity of the air will close the valve through which it was forced. On drawing the piston up again, another portion of air will rush in at the orifice d, and on forcing it down, this will also be driven into the vessel c; and this process may be continued as long as sufficient force is applied to move the piston, or there is sufficient strength in the vessel to retain the air. When the condensation is finished, the stop cock b may be turned, to render the confinement of the air more secure.

The magazines of air guns are filled in the manner above described. The air gun is shaped like other guns, but instead of the force of powder, that of air is employed to project the bullet. For this purpose, a strong hollow ball of copper, with a valve on the inside, is screwed to a condenser, and the air is condensed in it, thirty or forty times. This ball or maga. zine, is then taken from the condenser, and screwed to the gun, under the lock. By means of the lock, a communication is opened between the magazine and the inside of the gunbarrel, on which the spring of the confined air against the leaden bullet is such, as to throw it with nearly the same force as gun-powder.

Explain fig. 102, and show in what manner the air is condensed. Explain the principle of the air gun.

Fig. 103.

C

a

Barometer.

Suppose a, fig. 103, to be a tube, thirty feet long, and the piston b, to be so nicely fitted to its inside, as to work air tight. If the lower end of the tube be dipped into water, and the piston drawn up by pulling at the handle c, the water will follow the piston so closely as to be in contact with its surface, and apparently to be drawn up by the piston, as though the whole was one solid body. If the tube be thirty-five feet long instead of thirty, the water will continue to follow the piston, until it comes to the height of about thirty-three feet, where it will stop, and if b the piston be drawn up still further, the water will not follow it, but will remain stationary, the space from this height, between the piston and the water, being left a void space, or vacuum.

The rising of the water, in the above case, which only involves the principle of the common pump, is thought by some to be caused by suction, the piston sucking up the water as it is drawn upward. But according to the common notion attached to this term, there is no reason why the water should not continue to rise above the thirty-three feet, or why the power of suction should cease at that point, rather than at any other. Without entering into any discussion on the absurd notions concerning the power of suction, it is sufficient here to state, that it has long since been proved, that the elevation of the water in the case above described, depends entirely on the weight and pressure of the atmosphere, on that portion of the fluid which is on the outside of the tube. Hence, when the piston is drawn up, under circumstances where the air cannot act on the water around the tube, or pump barrel, no elevation of the fluid will follow. This will be obvious, by the following experiment.

Suppose the tube, fig. 103, to' stand with its lower end in the water, and the piston a to be drawn upward thirty-five feet, how far will the water follow the piston? What will remain in the tube between the piston and the water, after the piston rises higher than thirty-three feet? What is commonly supposed to make the water rise in such ca ́ses? Is there any reason why the suction should cease at 33 feet? What is the true cause of the elevation of the water, when the piston fig. 103, is drawn up?

Fig. 104.

Suppose fig. 104, to be the sections, or halves of two tubes, one within the other, the outer one being made entirely close, so as to admit no air, and the space between the two being also made air-tight at the top. Suppose, also, that the inner tube being left open at the lower end, does not reach the bottom of the outer tube, and thus that an open space be left between the two tubes every where, except at their upper ends, where they are fastened together; and suppose that there is a valve in the piston, opening upwards, so as to let the air which it contains, escape, but which will close on drawing the piston upwards. Now let the piston be at a, and in this state pour water through the stop cock, e, until the inner tube is filled up to the piston, and the space between the two tubes up to the same a point, and then let the stop cock be closed. If now the piston be drawn up to the top of the tube, the water will not follow it, as in the case first described, but will only rise a few inches, in consequence of the elasticity of the air above the water, between the tubes, and in the space above this, there will be formed a vacuum between the water and the piston, in the inner tube.

The reason why the result of this experiment differs from that before described, is, that the outer tube prevents the pressure of the atmosphere from forcing the water up the inner tube as the piston rises. This may be instantly proved, by opening the stop-cock c, and permitting the air to press upon the water, when it will be found, that as the air rushes in, the water will rise and fill the vacuum, up to the piston.

For the same reason, if a common pump be placed in a cistern of water, and the water is frozen over on the surface, so that no air can press upon the fluid, the piston of the pump might be worked, in vain, for the water would not, as usual, obey its motion.

It follows, as a certain conclusion from such experiments, that when the lower end of a tube is placed in the water, and the air from within is removed by drawing up the piston, that

How is it shown by fig. 104, that it is the pressure of the atmosphere which causes the water to rise in the pump barrel? Suppose the ice prevents the atmosphere from pressing on the water in a vessel, can the water be pumped out?