the assistance of the weight of the liquid, to overcome the pressure of the air which opposes the issue of the beer from the tap. After a time the air at the top of the cask is so much expanded that it no longer exerts sufficient pressure, and so the beer will not flow. Then a little more air is let in through a hole at the top of the cask provided for the purpose, called the vent-peg, which is kept closed until it is thus necessary to open it for a short time. The gurgling sound which is heard when we pour water from a decanter that is nearly full, arises from the air forced in by the pressure of the atmosphere to supply the place of the water withdrawn; the sound continues as long as the neck of the decanter is choked by the water escaping. But as the water is gradually withdrawn room is obtained in the neck of the decanter for water to pass out through part of the neck and for air to enter through the rest of the neck. B 519. An interesting process for estimating the magnitude of the pores of bodies as compared with that of the solid parts depends on the use of the pressure of the atmosphere. Some substances, as charcoal and pumice stone, contain an immense number of small cavities, and to these the process may be applied. Take a long glass tube open at both ends, and fill a portion AB with charcoal, supporting the charcoal at B by a perforated partition which will allow air to pass through. Plunge the tube in a vessel of mercury to the level B, then cover the end A, and withdraw the upper part of the tube from the mercury. If there had been no air in the cavities of the charcoal the mercury would remain in the tube at the usual height of 30 inches above the level of the mercury in the vessel. But when the pressure of the atmosphere is diminished the air in the cavities of the charcoal issues from the cavities and expands, and by its elastic force compels the mercury to stand at a lower height than it would otherwise reach. Suppose the mercury stands at Cat the height of 15 inches above the level in the vessel. Then the air in the pores and in BC, being under half the atmospheric pressure, occupies just double the space it did formerly; and thus the space denoted by BC is just equal to the volume of all the pores. It is found in this way that the solid part of charcoal is really about four times as heavy as water, bulk for bulk, although charcoal is usually taken to be about half as heavy as water, bulk for bulk. The solid matter of pumice stone is found to be as heavy as marble, bulk for bulk. XLVII. AIR PUMPS. 520. It is important to be able to examine the consequences which result when bodies are withdrawn from the influence of the pressure of the atmosphere. Accordingly machines are constructed by the aid of which we can withdraw the air almost entirely from certain closed vessels, and perform various interesting experiments in the empty space. These machines are known as Air Pumps. 521. The construction of Air Pumps may vary a little as to details, but the principles are the same in every case. A plate of brass or other metal, made exactly plane, is provided, and on that is placed a strong glass bell with its mouth downwards; this vessel is called the Receiver. The glass at the mouth is ground very smooth, so that it may fit exactly on the metal plate. To ensure that the contact between the two shall be air-tight, it is usual to smear the mouth of the glass with lard or some other unctuous substance. The air is then withdrawn from the glass vessel by a pipe which passes through the metal plate; and we shall now describe the way in which this is effected. AB is a cylindrical vessel in which a piston can move up and down. At the bottom of the cylinder there is a valve C which opens upwards. There is also a valve D in the piston which opens upwards. A pipe E passes from the bottom of the cylinder and communicates with the receiver. Suppose the piston to be at the bottom of the cylinder, and that the receiver and the pipe contain air of B C E 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. At the end of the first we have left of the original 9 9 10 quantity; at the end of the second of what there was 10 9 9 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 tion we have left 9 10 9 81 of the quantity at the end of the 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 |