Although the machine just described is equally adapted for condensing air as for rarefying it, by merely connecting the bent pipe with the oil vessel R, instead of the lower part of the pump, yet as the former operation seldom requires the same delicacy of process, it is frequently effected by a simpler machine, termed a condensing syringe. The following description will give an idea of the ordinary construction of this instrument, a is a cylindrical tube of small diameter, open at one end, the other end being perforated with a very small hole b, and turned externally to a very small cylinder. A strip of bladder, or of thin leather, soaked in a mixture of oil and tallow, must be tied over the hole. On the end of the cylinder is cut an external screw, to attach it to the vessel in which the air is to be condensed, c is the piston moving air-tight in a; d, the piston rod screwing into c; this rod is perforated with a small hole throughout its length, the lower end of the hole having a valve e, similar to the valve b in the cylinder, and opening into the cylinder.
To the upper end of the rod is fixed a handle, g is the neck of the receiver, having a hollow screw fitting the solid screw on the end of the cylinder.
Now when the piston is drawn up, a void is left below it, and the external air rushing through the perforation of the piston rod, opens the valve e, and fills the cylinder. Then, on pushing down the piston, the valve e closes; the air being compressed into less space, presses on the valve g, shuts it, and none escaping through the piston, it is gradually condensed as the piston descends till it opens the valve b, and is added to the air already accumulated in g. We may thus force into the vessel any quantity of air consistent with its strength; and if the receiver be furnished with a stopcock, the cock may be turned, and the receiver be detached from the syringe, and thus be in a state to be transferred to any other purpose required. In all cases where considerable force is required, and, consequently, a great condensation of air, it will be requisite to have the condensing syringe of small bore, perhaps not exceeding half an inch diameter, otherwise the force requisite to produce the compression will become so great, that the operator will not be able to work the machine; for as the pressure against each square inch is about 15 lbs. for each additional volume of air forced into the receiver, 121bs. upon each circular inch, if the syringe be of 1-inch diameter, it will require a force of 120 lbs. to condense ten volumes of air into the barrel, whereas with a half-inch bore it would only require 30 lbs.
We insert the following description of an air-pump on account of its extreme simplicity. It is the invention of Mr. W. Ritchie, and has no artificial valves, which, as commonly constructed, are very liable to be deranged, and the repairs are attended with considerable trouble and expense. The machine consists of a barrel shut at the lower end, and having a small aperture at c, forming a free communication with the receiver at f; the piston d is solid, and stuffed in the usual way. The piston rod works in a small stuffing-box at a, so as to render it completely air-tight. There is a small aperture at e, in the top of the barrel, to allow the air to make its escape when the piston is raised. The air-pump may be worked in the usual way, or by the method of continued motion. In commencing the exhaustion of the receiver, the piston is supposed to be below the small aperture at c. The piston is then raised, and the air which occupied the barrel is forced out through the aperture at e. The point of one of the fingers is applied to the perforation in the same manner as in playing the German flute.
The air easily passes by the finger which, when the piston begins to descend, shuts the opening, and completely prevents the entrance of the external air.
The piston is again forced down below the opening c, the air in the receiver rushes into the barrel, and is again expelled by the ascending piston. Since the air in the receiver has no valve to open by its elasticity, it is obvious that there is no limit to the degree of exhaustion, as in the common construction. The simplicity and economy of this arrangement alike recommend it.
For a long time the use of the air-pump was confined to the purposes of philosophical experiment: the first application of it to mechanical purposes was made by Bolton and Watt, in their steam-engines, where it is used for drawing off the air extricated from water by boiling, as also the water used for condensing the steam; and about twenty years ago, the Hon. E. Howard, availing himself of a well-known principle, that liquids enter into ebullition at a much lower degree of temperature in vacuo, than when under the pressure of the atmosphere, introduced a most important improvement in the process of sugar-refining, by concentrating the syrup in close vessels, in which a vacuum is maintained by means of an air-pump; and subsequently, Messrs. Allen & Co. of Plough Court, have adopted the principle in preparing the more delicate medicinal extracts. A patent has also been taken out for more speedily tanning leather, by enclosing the skins in boxes, and exhausting the air from the upper surface of the skins whilst the lower surface is exposed to the tanning liquor, which is forced into the pores by the pressure of the atmosphere.
Dr. Church, of Birmingham, also has a patent for improvements in casting metals, which consists in exhausting the air from the moulds; and patents have been taken out for distillation in vacuo, all of which applications of the air-pump we propose to explain in detail when we come to treat of the above-named manufacturing processes.
In the description of Cuthbertson's air-pump it was stated that it was equally applicable to the condensation of air, as to its exhaustion. We now present to our readers (page 46) a condensing air-pump of a different description, which is the invention of the late Mr. D. Gordon, of the Portable Gas Works, where it was used for compressing the gas into the portable gas-holders, and will be found very effective in subjecting any air or gas to a great compression, as the whole volume of gas taken into the pump at each stroke is effectually discharged, the mercury employed in the pump completely excluding every portion from below the delivery valve. The figure represents a vertical section of one of the pumps with the plunger a at its lowest immersion, or down-stroke, as it is called. At this moment of time every part of the syphon is completely filled with the fluid it contains to the entire exclusion of atmospheric air, the deeply shadowed part c being water, and the lighter tint, quicksilver.
It will therefore be easily conceived that when the plunger is withdrawn by the up-stroke, an empty space or vacuum, equal to the cubic admeasurement of the plunger, will be left in the syphon; and as mercury is a much heavier body than water, the latter is pushed up by the former, and follows the plunger as it ascends. The mercury, consequently, sinks below its present level, which causes the "suction valve" e to open, and to let in a volume of gas at the ordinary pressure, which flows from a gasometer, or gas-holder, through the pipe f. At the down-stroke of the plunger, the gas is then compressed and forced through the discharge valve, (which opens only outward) into the pipe h, which leads to a strong recipient into which the gas is condensed. The action of the pumps being continued, the compression of the gas is effected to whatever degree may be required, provided the power of the gas engine, or other first mover, be adequate. In the Portable Gas Works above mentioned, there were recently nine pumps in operation, worked by a 10-horse engine.
The quicksilver and water are poured into the syphon by means of the basin and perforation at i, the aperture at k being opened to allow the air to escape; the aperture k is next made perfectly tight by a plug, which is screwed into the orifice, during which time the water is continually being poured in at i, to expel the air completely, and fill up every crevice with the fluid; another plug is then screwed into the orifice at i, with the water above it. After this, the first down-stroke of the plunger expels the atmospheric air on the surface of the mercury, in the short leg of the syphon, and the pump is then ready to perform the office of alternately drawing in the gas from the gas-holder, and compressing it into the portable lamps. The gas sent out by the company was compressed into 1/30 of its original volume.