Having given this brief account of the general properties of air, we shall refer to a few experiments, and the instru ments which are commonly used in per forming these experiments ; beginning with the air-pump, which has been alrea dy described in a general way. Pig 2, isan air pump, much in use. A A are two brass barrels, each containing a piston, with a valve opening upwards. They are worked by means of the winch, 13, which has a pinion that fits into the teeth of the racks, C C, which are made upon the ends of the pistons, and by this means moves them up and down alternately. On the square wooden frame, D E, there is placed a brass plate, G, ground perfectly flat, and also a brass tube, let into the wood communicating with the two cylin ders and the cock, I, and opening into the centre of the brass plate at a. The glass vessel, K, to be emptied or exhaust ed of air, has its rim ground quite flat, and rubbed with a little pomatum, or hog's. lard, to make it fit more closely upon the brass plate of the pump. Sometimes thin slips of moistened leather are used for this purpose. These vessels are called receiv ers. Having shut the cock, I, the pistons are worked by the winch, and the air be ing suffered to escape when the piston is forced down, because the valve opens up wards, but prevented from returning in to the vessel. for the same reason the re ceiver is gradually exhausted, and will theri be fixed fast upon the pump-plate. By opening the cock, I, the air rushes again into the receiver.
To the air pump is attached the guage, z, or instrument for measuring the de gree of rarefaction, or exhaustion, pro duced in the receiver, and which is a ne cessary appendage to the air-pump. Ifa barometer be included beneath the re ceiver, the mercury will stand at the same height as in the open air, but when the receiver begins to be exhausted, the mercury will descend, and rest at a height, which is, in proportion to its former height, as the spring of the air remaining in the receiver is to its spring before ex haustion. Thus, if the height of the mer cury, after exhaustion, is the thousandth part of what it was before, we say that the air in the receiver rarefied is a thou sand times. On account of the inconve nience of including a barometer in a re ceiver, a tube, of six or eight inches in length, is filled with mercury, and invert ed in the same manner as the barometer. This being included, answers the same purpose, with no other difference, than that the mercury does not begin to de scend till after about three-fourths of the air is exhausted : it is called the short ba rometer guage. This is generally plac ed detached, but communicating with the receiver by a tube concealed in the frame, as is represented in the figure; another and better guage was invented by Mr. Smeaton, and called, from its form, the pear-guage. It consists of a glass ves sel, in the form of a pear (fig. 3.) and suf ficient to hold about half a pound of mer cury : it is open at one end, and at the other end is a tube, hermetiCally closed at top. The tube is graduated, so as to
represent proportionate parts of the whole capacity. This guage, during the exhaustion of the receiver, is suspended in it by a slip wire, over a cistern of mer cury, placed also in the receiver. When the pump is worked as much as is thought necessary, the gunge is let clown into the mercury, and the air re-admitted. The mercury will immediately rise in the guage ; but if any air remained in the re ceiver, a certain portion of it would be in the guage ; and as it would occupy the top of the tube above the mercury, it would shew by its size the degree of ex haustion ; for the bubble of air would be to the whole contents of the guage, as the quantity of air in the exhausted re ceiver would to an equal volume of the common atmospheric air. If the receiver contained any elastic vapour generated during the rarefaction, it would be con densed upon the re-admission of the at mospheric air, as it cannot subsist in the usual pressure. The pear-guage, there fore, shows the true quantity of atmos pheric air left in the receiver. Hence it will sometimes indicate that all the per manent air is exhausted from the re ceiver, except about part, when the other guages do not chew a de gree of exhaustion of more than two hundred times, and sometimes much less.
When the receiver is placed upon the plate of the air-pump without exhausting it, it may be removed again with the ut most facility, because there is a mass of air under it, that resists, by its elasticity, the pressure on the outside ; but exhaust the receiver, thus removing the counter pressure, and it will be held down to the plate by the weight of the air upon it. What the pressure of the air amounts to, is exactly determined in the following manner : when the surface of a fluid is exposed to the air, it is pressed by the weight of the atmosphere equally on every part, and consequently remains at rest. But if the pressure be removed from any particular part, the fluid must yield in that part, and be forced out of its situa tion.
Into the receiver A, (fig. 4), put a small vessel with quicksilver, or any other fluid, and through the collar of lea thers at B, suspend a glass tube, herme trically sealed, over the small vessel. Having exhausted the receiver, let down the tube into the quicksilver, which will not rise into the tube as long as the re ceiver continues empty. But re-admit the air, and the quicksilver will immedi ately ascend. The reason of this is, that upon exhausting the receiver, the tube is likewise emptied of air ; and therefore, when it is immersed in the quicksilver, and the air re-admitted into the receiver, all the surface of the quicksilver is press ed upon by the air, except that portion 3:vhich lies above the orifice of the tube : consequently, it muss rise in the tube, and continue so to do, until the weight of the elevated quicksilver press as forci bly on that portion which lies beneath the tube, as the weight of the air does on every other equal portion without the tube.