AIR-PUMP, is an instrument for removing the air out of a vessel. It effects the reverse operation to that performed by the condensing syringe, by which additional air is forced into a vessel. Both in the exhausting and the condensing syringe there is a tube closed at one end, excepting an orifice to which a valve or lid is attached. A piston, with a rod and handle, enters at the other end, and can be moved up and down the tube. The piston is not entirely closed, but has a valve opening the same way as the other valve. Both are attached to vessels, the air of which is to be rarefied or condensed. In the exhausting syringe, both valves open upwards or let air only out of the vessel and the piston : in the condensing syringe, both open downwards, or let air only into the vessel and the piston. In the exhausting syringe, every time that the piston is drawn upwards, it leaves a sort of va cuum in the barrel ; and the air in the receiver forces up the lower valve to fill up this vacuum. At the next movement, the air thus raised is driven out of the instrument altoge ther; for the valves are so placed that no air can go from above downwards. By repeated movements'of this kind, nearly all the air may be drawn out of the receiver. In the conden sing syringe, all the operations are exactly re versed ; the • downward motion of the piston being the efficient agent in forcing into the re ceiver a quantity of air many times greater than that which it originally contained.
The exhausting syringe is, in principle, the common air-pump. In most forms of air pump there is at the top a metal plate ground to a perfectly plane surface, on which is placed an inverted glass jar or receiver, whence the air is to be extracted. A hole in the plate is connected with a tube, which communicates with two pump barrels. These barrels are ex hausting syringes. One or more guages are attached to the instrument to test the degree of exhaustion of the air. The pistons which work in the two pump-barrels are connected by a rack-and-pinion movement with a handle, in such a way that when the handle is worked in semi-circular movements, the pistons are raised alternately. As the lower part of each barrel is connected with the receiver by means of the tube, the movements of the two pistons gradually draw out the air from the receiver, in the manner of the exhausting syringe ; and in this way a nearly perfect vacuum may be produced.
In most of the objects for which an air-pump is required in scientific experiments, a glass receiver, provided with stop-cocks or other adjustments, is placed on the plate of the air pump to have the air extracted ; and experi ments or observations are made in the vacuum thus produced.
If the receiver of an air- pump be open at both ends, and the upper orifice be stopped by the hand,—on exhaustion, the pressure of the exterior air will be painfully great on the hand. If a piece of bladder be tied tightly over the orifice, as the exhaustion proceeds the bladder will be pressed inwards, and will finally burst with a loud noise. The weight of the air is proved by exhausting a copper ball furnished with a stop-cock, which is shut before the ball is removed from the air-pump : it will then be found to weigh less than before the exhaustion was made. The presence of air in various substances may be detected by means of an air-pump. A glass of liquid placed under the receiver will give out bubbles of air as soon as the exhaustion begins. A shrivelled apple will be restored to apparent freshness by the expansion of the air which it contains ; but will resume its original appear ance when the air is allowed to return. The elasticity of air may be shown by placing a bladder under the receiver, not distended, and the mouth of which is tied up ; on exhausting the receiver, the air contained in the bladder will expand it more and more, as more of the pressure from the exterior is removed; and the bladder will finally burst from the interior pressure. If a hole be made in the smaller end of an egg which is placed under the re ceiver, the small bubble of air, which is always found in the larger end, will by its expansion force out the contents of the egg.
In machinery employed in manufactures, the air-pump is applied in various ways, espe cially with some forms of steam-engines. In such eases no glass-receiver is necessary, and the mechanism of the air-pump is larger and stronger.