246 PNEUMATICS Air- pumps. Invented by Von Guericke. Improve ments by Boyle, Hooke, Papin, &c. same succession of operations be made. Then, if p be the increase of pressure necessary to bring the mercury surface from the lower to the higher fiducial mark, we have (p + T)(v - x) = P(V + v - a-). As V can be readily determined by gauging the tube, and as P, p, p are all known, the quantities v and x can be at once found from these equations. In many experiments on the properties of gases it is necessary to have an efficient and rapid means for altering the density. Instruments for this purpose are called air- pumps, and their function may be either to rarefy or con dense the air usually the former. Otto Von Guericke of Magdeburg constructed the first air-pump about the year 1652. It was simply a spherical glass vessel opening below by means of a stop-cock and narrow nozzle into the cylinder of an " exhausting syringe," which inclined upwards from the extremity of the nozzle. The cylinder, in which a well-fitting piston worked, was provided at its lower end with two valves. One of these opened from the nozzle into the cylinder, the other from the cylinder into the outside air. During the down-stroke of the piston the former was pressed home, so that no air entered the nozzle and vessel, while the latter was forced open by the air which so escaped from the cylinder. During the return-stroke the latter was kept closed in virtue of the partial vacuum formed within the cylinder, while at the same time the former was forced open by the pressure of the denser air in the vessel and nozzle. Thus, at every complete stroke of the piston, the air in the vessel or receiver was diminished by that fraction of itself which is expressed by the ratio of the volume of the available cylindrical space above the outward opening valve to the whole volume of receiver, nozzle, and cylinder. Boyle, on hearing of Von Guericke s success in applying the expansive properties of air as a means to its rarefaction, constructed a machine essentially the same as Von Guericke s, of which no description had then been pub lished. Boyle, however, made the exhausting cylinder a continuation of the nozzle, which was thus considerably reduced in size, and worked the piston by means of a wheel and racket work. He also employed a transparent glass receiver with removable cover, so that the operator could observe what was going on inside, and more easily alter the contents. Other modifications and improvements followed rapidly ; and in the carrying out of these Boyle was greatly aided by Hooke, who conceived the happy idea of using two syringes opening into the same duct from the receiver. This improvement has recently been credited to Papin. 1 In this form the pistons are worked by the same toothed wheel, and are so adjusted that the one rises as the other falls. This arrangement not only doubles the rate of exhaustion per stroke, but vastly increases the ease of working. In the single-barrelled form the piston is drawn back against a pressure of air which is greater the more complete the exhaustion is within ; but in the double-barrelled form the downward pressures upon the two pistons to a certain extent counteract each other, producing opposite rotational effects on the toothed wheel and driving handle. Boyle also early adopted the flat plate, on which could be set receivers of various shapes and sizes. The junction of the plate and receiver he made tolerably air-tight by covering the plate with wot leather and having the receiver rim ground flat. According to Garland, the invention of the plate is due to Huygens, who constructed the first air-pump so provided in 16G1, shortly after a visit to London, where his interest in the subject was awakened by Boyle. To Huygens, prob ably in conjunction with Papin, is also due the applica- 1 Hee Gerland, in Wiedemanns Annalcn, 1 -77, 1883. tion of the mercury manometer to measure the pressure in the receiver an indispensable equipment in all serviceable air-pumps. The form of the piston plugs and valves received the special attention of Papin, who by his refined and detailed improvements did much to increase the efficiency of the apparatus. The important characteristics of an efficient air-pump are as follows. The piston must work smoothly and easily. The valves must act precisely, and be when closed absolutely air-tight. The plate on which the receivers rest must be smooth and plane, so that the ground edges of the receivers may be in close contact all round. This perfect fitting is beyond the powers of the best workmanship, so that it is necessary to press between the receiver and plate a thin layer of lard, which renders the junction air-tight. Some where in the duct leading from the receiver to the piston cylinders, a stop-cock must be fixed, so that it may be possible to shut off the receiver completely from these. Then a second stop-cock is required as a ready means for admitting air to the receiver, whenever the need should arise. A combination three-way stop-cock is a very usual form. And, finally, the apparatus should be provided with a pressure gauge a mercury manometer communicating by means of a duct with the main duct and receiver. The double-barrelled form of reciprocating air-pump, as Blanch finally employed by Boyle, is still much in use; but it is gradually being superseded by Bianchi s, which has but one cylinder and piston. The piston is, however, double- acting, as the cylinder communicates both above and below by suitable valves with the main duct which leads to the receiver. Hence, during both the up and down strokes of the piston, exhaustion is being effected, the gas which is in the diminishing chamber being driven out through a suitable valve to the open air. The chief merit of Bianchi s machine, however, as compared vith the older form, lies in the mechanism by which the piston is driven. The end of the piston rod is attached by a crank to a rotating horizontal axle, which is in gearing with the axle of a fly wheel. The piston cylinder is capable of a reciprocating oscillatory motion about its lower end, which pivots on a horizontal axis. Thus, as the crank rotates, the piston rises and falls in the cylinder, and oscillates along with it from side to side. The driving power is applied to a handle fixed to the fly-wheel. This substitution of a con tinuous rotatory motion for a reciprocating motion greatly facilitates rapid exhaustion. At every complete stroke of the piston the pressure of air in the receiver is reduced by a definite fraction of itself, which depends upon the relative volumes of the receiver and piston cylinder. Hence the absolute change of pres sure per stroke is smaller as the pressure is smaller ; and the rate of exhaustion at very low pressures becomes prac tically inappreciable. There is, in fact, a practical limit to exhaustion, the particular value of which depends upon the special characteristics of the instrument. The best air-pumps of the type described above cannot reduce the pressure to less than what would balance one or two millimetres of mercury that is, to what is technically called a pressure of one or two millimetres. To obtain a lower pressure or a higher vacuum, as it is commonly termed, requires the use of a different principle. In a well-constructed barometer the region above the mercury contains no air. It is not an absolute vacuum, but is filled with vapour of mercury at a very low pressure according to Regnault, 0372 mm. at 20 C., and 02 mm. at C. The way in which the Torricellian vacuum has been applied to give a practical air-pump has been described in MERCURIAL AiR-PuMP. So long as mercury vapour is permitted to pass freely through the exhausting tubes, it is impossible to obtain by the mercurial air-pump Comj e . x ^ ai lc Men J
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