the density of the atmosphere. Raise the piston to the top of the cylinder; the valve D is kept closed by the pressure of the atmosphere above it, while the pressure of the air in the receiver and the pipe opens the valve C, and the air diffuses itself throughout the receiver, the pipe, and the cylinder. Push the piston down to the bottom of the cylinder; then the valve C closes and the air in the cylinder is expelled through the valve D. Then the operation may be repeated. When the piston ascends the air in the receiver and the pipe is diffused through the receiver, the pipe, and the cylinder; and when the piston descends so much of the air as was in the cylinder is expelled. Thus the air in the receiver is gradually diminished. 522. A valve is a contrivance which allows a current of fluid to pass through a tube or aperture in one direction but not in the other. The valves in the air pump are commonly formed of a triangular piece of oiled silk, stretched over a grated orifice in a piece of metal, to which the corners of the triangle are fastened. When air presses on the upper surface of the silk it is brought into contact with the edge of the orifice, and the passage of air prevented; when air presses on the lower surface of the silk it is raised from the edge of the orifice, and air is allowed to pass. 523. It must be observed that we cannot remove all the air from the receiver. Let us suppose for example that the volume of the receiver and the pipe together is nine times that of the cylinder. Then when the air is diffused through the receiver, the pipe, and the cylinder, that in the 1 cylinder is of the whole. Thus by the up and down stroke 10 can never remove all the air; for at the end of an up and 9 down stroke of the piston we leave in of what there 10 was at the beginning of the stroke. It is easy to find by arithmetic what fraction of the original quantity will remain after any number of what we will call operations. 9 At the end of the first we have left of the original quantity; at the end of the second 9 10 9 9 10 of what there was at the end of the first, that is X of the original, 81 100 10 10 of the original; at the end of the third opera that is 9 tion we have left of the quantity at the end of the 9 10 second, that is X of the original quantity, that is 10 100 of the original quantity; and so on. 729 1000 524. Thus we see that even if there were no practical difficulties in the machine itself we could never draw out all the air from the receiver; but there are various practical difficulties which also limit the degree of exhaustion attainable. Thus however light the valve may be made it has some weight, and when the air in the receiver and pipe becomes so attenuated that it has no longer sufficient force to raise the valve C the exhaustion of the receiver cannot be carried further. Again, the valve D has the pressure of the atmosphere above it; if the piston could be pushed down to the bottom of the cylinder the air between D and C, however attenuated it might be at the beginning of the downward stroke, would become sufficiently condensed to overcome the pressure of the atmosphere. But practically the piston cannot be pushed close to the bottom of the cylinder, and hence it might happen that the valve D would finally remain closed, and so prevent the exhaustion of the receiver from being carried further. There are two ways in practice by which this difficulty is met. One way consists in closing the top of the cylinder, leaving only a valve opening upwards and a hole through which the piston-rod works in an air-tight collar. In consequence of this the valve D, when it descends, is relieved from the pressure of the atmosphere, and so can be opened by a very small force from below. Another advantage gained is that the removal of the pressure of the atmosphere from the upper surface of the piston diminishes the labour of the upward stroke. The air pump with this modification is called Smeaton's Air Pump. There is another way of securing the same advantage as by Smeaton's Air Pump. Instead of the cylinder being open to the atmosphere at the top it communicates with the receiver of an auxiliary air pump; and then by occasionally giving a few strokes to this we can always keep the pressure above D considerably less than that of the atmosphere. 525. We have thus sufficiently explained the principle of the air pump; in practice various details are regarded for the sake of convenience, at least when the machine is on a large scale. Thus we have spoken of one cylinder, but there are usually two, side by side; by means of a toothed wheel and rack-work the two pistons are moved simultaneously, one going up while the other goes down, so that the exhaustion proceeds twice as rapidly as with a single cylinder. Moreover the labour of working the pump is diminished; for while one piston is being drawn up the pressure of the atmosphere above it produces a great resistance to be overcome; but when two pistons are used this resistance is balanced by an equal pressure on the surface of the descending piston, which assists the motion. Thus the pump may be worked by a force which is sufficient to overcome the friction together with the difference of the pressures on the lower surfaces of the ascending and descending pistons. Instead of the valve C some instrument makers substitute a stopper, which is raised when necessary by a rod passing through the piston and working tightly in it, so as to be carried up and down by the motion of the piston-rod. In spite of all the care with which the instrument is made it is found that there is always some leakage at various parts, and although the quantity of air which thus enters is small compared with that drawn out by the early operations, yet it may be as much as is drawn out by the later operations; so that finally the exhaustion reaches a point beyond which it could not be carried were it for this reason alone. It is found that a diminution of the density of the air to one thousandth of its original value is practically almost as much as can be obtained. 526. The air pump is usually furnished with an appendage by which the degree of exhaustion can be ascertained. One such appendage is called the barometer gauge. The upper end of a barometer tube, instead of being closed, is allowed to be open and to communicate with the receiver. If all the air could be withdrawn from the receiver the mercury in the barometer gauge would then stand at the ordinary height; but some air will always remain, and thus the mercury in the barometer gauge will not reach to the ordinary height. Suppose, for example, that it stands at the height of 28 inches instead of 30 inches; this shews that the pressure of the air in the receiver is measured by the height of 2 inches of mercury, so that the density of the air in the receiver is 2 30 of the density of the ordinary air. The siphon gauge is another contrivance for ascertaining the degree of exhaustion. This is a bent tube A the air in the receiver is sity of the ordinary air. XLVIII. 3 30 of the den B AIR-PUMP EXPERIMENTS. C 527. Numerous interesting experiments are performed by the aid of the air pump; they enable us to understand the important functions of the atmosphere by shewing us how very different the phenomena would be if that atmo sphere were removed. The experiments have the great merit of being very successful; the spectator can easily watch them and will admit that their testimony is decisive. 528. The experiment called the guinea and feather experiment is intended to shew that if the resistance of the air is removed all bodies will fall to the ground from the same starting point in the same time. A tall receiver is provided, furnished with a small platform at the top on which a coin and a feather are placed. After the receiver has been sufficiently exhausted of air the platform is removed by turning a screw provided for the purpose; the coin and the feather fall, and reach at the same instant the plate which supports the receiver. 529. The pressure of the atmosphere is illustrated in the following way. A jar open at both ends is converted into a receiver by fastening a piece of bladder over one end, and the other end is placed on the plate. After one or two strokes of the air pump the bladder becomes much stretched and bent inwards, so as to take a cuplike shape; the pressure of the atmosphere above is not fully balanced by the pressure of the attenuated air below the bladder, and so the bladder is forced inwards. By continuing the exhaustion the bladder is urged still further, until at last it bursts. 530. Let a little air be put into a bladder, and let the bladder be closed in an air tight manner and placed under the receiver of an air pump. As the receiver becomes gradually exhausted the air inside the bladder, having little pressure to constrain it, expands, and the bladder swells and appears to be fully inflated. In like manner some fruits when dried and shrivelled retain within them a little air which expands when the pressure is removed from their surfaces; thus when a shrivelled apple is placed under a receiver and the air withdrawn it is restored apparently to a plump and fresh condition; raisins in like manner expand to the size of the grape from which they were originally derived. On the re-admission of air to the receiver the fruits become again shrivelled as at first. 531. When we say that water or any other liquid boils we mean that it passes from the liquid to the gaseous T. P. 15 |