THE EXHAUSTING SYRINGE.

21

the external air cannot enter through b, because this valve opens outwards, and the atmospheric pressure upon it from without only serves to close it more securely. The valve a, however, is immediately forced open a second time by the remaining air in the vessel, which again fills the empty space that would otherwise be left by the drawing up of the piston a second time. The piston is again depressed, the valve a is again closed, and the air in the cylinder again forced out through b; and in this way the action is carried on until the air in the vessel has too little elasticity to open the valve a. The exhaustion is then said to be

complete.

Now it is evident that a perfect vacuum or empty space cannot be formed in the vessel by this contrivance. A small portion of air must always be left in the vessel. If the cylinder be of the same capacity as the vessel, and the weight and friction of the valve be regarded as nothing, one-half of the air will pass out of the vessel by the first stroke of the piston; that is, on raising the piston to the top of the cylinder, and then depressing it again to the bottom, the vessel will be deprived of exactly half of its contents; the remaining half will still completely fill the vessel, but its atoms or particles will be farther apart, its density will be diminished one-half, and, consequently, its elasticity will be diminished in the same proportion. The second stroke of the piston will again diminish the air in the vessel by one-half; that is, the air left after the second stroke will be one-fourth of its former density and elasticity. We may carry out these results to greater length, by collecting them in a tabular form. The quantity of air in the vessel before the first stroke is to be regarded as unity.

Thus, after the ninth stroke, the remaining air will only be 5th of its original quantity; and, as it still occupies the same space, it has only 51th the density and elastic force, which is equal to a pressure of only 0.028 lbs. to the square inch, which would scarcely be sufficient to raise the valve.

11. The air-pump, Fig. 6, is nothing more than a duplication

of

the exhausting syringe, with this difference, that the valve through which air is forced out of the cylinder, is not placed as at b, Fig. 5, but in the piston or plug itself. Two

of these syringes, a b, or barrels as they are called, are arranged side by side, and the motion given to their pistons is so managed (in

Fig. 6.

experimental machines by a toothed wheel and racked piston rods) that, while one piston is ascending and drawing out the air, the other is descending and expelling the air already drawn out of the vessel to be exhausted. Each barrel is furnished with a valve at the bottom, opening upwards, so that, during the ascent of either piston, the air below the valve forces it open and fills the barrel. During the descent of either piston this valve is of course closed, and another valve situated in the piston itself, and opening upwards, allows the escape of the air between the bottom of the barrel and the piston. In pneumatic experiments the vessel to be exhausted, R, is called a receiver; it is made of stout glass, its edge is formed flat, and smeared over with pomatum before it is placed on the metal plate t, called the table of the air-pump. By this means the receiver is brought into air-tight contact with the table, and forms an inclosed chamber, in which any substance or arrangement of apparatus previously placed, may be observed under any amount of rarefaction that may be given to the inclosed air. The table is perforated at its centre with a hole, which communicates by a bent metal tube c with the barrels a b. This tube is furnished with a stop-cock, which, being closed, prevents any leakage of air into the receiver from the barrels, and, when open, allows them to act upon the inclosed air. The air is readmitted into the receiver by a perforation into the bent metal tube at h. This hole is closed by a thumb-screw, made air-tight by a washer of leather. One extremity of a bent glass tube d opens into the metal tube c, while the other extremity dips into a cistern of mercury. This tube, which is more than 30 inches in length, acts as a gauge, and indicates, by the ascent of the mercury within it, the amount of rarefaction in the receiver, because,

as the rarefaction proceeds in the receiver, the elastic force of the air pressing upon the mercury in the gauge-tube is diminished. Indeed, with the first stroke of the pump, it immediately becomes less than the pressure of the external atmosphere on the surface of the mercury in the cistern: consequently this external pressure prevails, and forces mercury up to a certain height in the gauge-tube. As the rarefaction of the air in the receiver proceeds, its elastic force is diminished, the atmospheric pressure acts with increased effect, and the mercury rises higher and higher in the tube. The weight of the column of mercury thus raised, combined with the elastic pressure of the air remaining in the receiver, is equal to the atmospheric pressure, and it is evident that the elastic force of the air in the receiver must be equal to the excess of the atmospheric pressure above the weight of the column of mercury in the tube. If a common barometer hanging up in the room stands at 30 inches, and the mercury in the gauge at 26 inches, the difference between them, namely, 4 inches of mercury, will represent the elastic force of the air in the receiver. We have already given a rule for ascertaining how much air is left in the receiver after a certain number of strokes of the pistons of an exhaustive syringe of equal capacity with the receiver itself. It would be difficult to apply this rule to the air-pump, because it would be necessary, among other conditions, to know what relation in volume the barrels bear to the receiver; and, as receivers of all sizes are being constantly used, the calculation would be troublesome, if not impossible. If after working the pump some time the mercury in the gauge stands at 20 inches, and the barometer outside at 30 inches, then we know that ths or 3rds of the air remain in the receiver,

and that 3rd has been pumped out. But as the rarefaction of air is inversely as the quantity in a given space, if we invert the above fraction and make it, then we have 1.5, which is called the rarefaction; and, in such a case, the air in the receiver is said to have been rarefied 1.5 times.

In the foregoing description of the air-pump, we have omitted many details, which, although they make it a more efficient instrument in the hands of the man of science, do not affect its principle. And it is to the principles of science, rather than to their details, that the reader should constantly strive to ascend. Facts, however curious and important, are useful in proportion as they lead to the comprehension of principles; if, therefore, our little book should be found deficient in facts, our reason for the omission is, that by thus economizing space we shall be able to extend the knowledge we are now gaining to the explanation of some of the grand operations of nature.

A treatise on Pneumatics would not, however, be considered as complete without a description of such machines as the common pump, the siphon, &c., and as such machines are capital illustrations of the pressure arising from the weight, or the elasticity, or both these important properties of the air, we must find room for a few details.

12. The term suction is still applied to the common pump, and to several operations and instruments of the pump kind. This term is an unfortunate one, and requires to be explained away. When we place one end of a straw in the mouth and the other end in water, and are said to suck up the liquid, we do no such thing. We merely draw into the mouth the portion of air confined in the tube, and then the pressure of the air which is exerted on the surface of the liquid, being no

C