54

M

FIG. II a.

A

the orifice under the mercury in the vessel at the bottom. Many alterations have since been made in the shape of the apparatus, and that shown in fig. 11 a, has been found very convenient. The mercury is supplied by the funnel, ▲, the flow being regulated by the compression-cock, B, from which it descends through a narrow tube hanging loosely in a wider tube, c, any air bubbles that may be carried down the narrow tube escape through the annular space between the two tubes. From c the mercury traverses a caoutchouc tube covered with canvas or tape into the air-trap, D (the action of which will be described directly), from this it again descends, and, by means of another covered caoutchouc tube, is conducted into the tube, E, through which it rises to supply the fall-tube, F, which is the active portion of the pump. The vertical distance between the lower end of c and the bend of the air-trap, D, must not be less than 800mm. When the apparatus is first filled with mercury, the compression-cock on the second caoutchouc tube is closed, the stopper, d, at the top of the air-trap removed, and the mercury allowed to flow gradually through B, until the air is entirely expelled from the air-trap, which is facilitated by removing the latter from the screw on which it rests, and lowering it until the mercury reaches the orifice, d, when the stopper is replaced and the air-trap again suspended from the screw. The lower compressioncock may now be opened, which permits the mercury to rise in E, and enter the top of the fall-tube. When the tubes are of the proper diameter (about 1mm.), and the bend at the top of the fall-tube rightly constructed, the mercury passes down the falltube in separate drops, forming pistons of equal length, and carrying the gas down quite regularly. The receiver to be exhausted is connected by a glass ball and socket-joint, or otherwise, to the tube, G; and the gas pumped out is delivered through the mercury in the funnel, H. After the pump has been at work for some time, the fall-tube is liable to become soiled, which much interferes with the efficient action of the apparatus. The tube may be cleaned by the introduction of concentrated sulphuric acid, which can be admitted through the tube, J, the lower end of which passes through the cork of a wide tube containing mercury, covered with a layer of sulphuric acid; to introduce the acid, the block, K, is removed, and the wide

K

F

31.]

PRESSURE OF AIR.

55

tubes lowered until the end of J is above the mercury, the acid then passes up the tube into the pump. The second tube, L, contains sulphuric acid, to dry the air before it comes in contact with that which is to be used for the pump. The fannel, H, is arranged to deliver mercury from the narrow bent tube attached to it, the sulphuric acid passing over the lip. When the pressure in the receiver is reduced to about 50mm. a vacuum is formed in the air-trap, D, and the mercury thus falls through a vacuum before reaching the fall-tube, any air which may be dissolved in the mercury is thus given up in the trap, and not into the receiver. By using caoutchouc connexions at the bottom of the apparatus, rigidity is avoided, which is of great value when fitting up the pump, the fall-tube merely rests in the funnel, being held upright by a loose support at M. There is no danger of leakage of air through the caoutchouc, since there is always a considerable internal pressure of mercury, and air cannot possibly penetrate into the receiver if the supply funnel becomes empty, for the difference of level between the mercury in the bend of D and in the tube c, cannot exceed the length of the barometric column. By means of an instrument similar to this, Mr. W. F. Donkin has obtained some very perfect exhaustions (Chem. News, 1874, xxix. 125). Without the air-trap, and by connecting the receiver to the pump by an india-rubber joint surrounded by glycerine, the pressure was reduced to 10770000 of that of the atmosphere. When the air-trap was employed, and the receiver hermetically sealed on to the pump, the best exhaustion obtained was 537920000 of the atmospheric pressure.

(31) Pressure of Air.-The increase in volume of the inclosed air, and consequent decrease in its pressure, may be illustrated by inserting a tube, blown into a bulb at one end, full of air, and with its open mouth downwards in a vessel containing water, which is placed under the receiver of the pump. With each movement of the piston, the air in the bulb expands, while a portion of it in the act of expanding escapes, and bubbles up through the water. An amusing variation of this experiment may be made by placing a number of shrivelled apples in the receiver, and then working the pump. The apples contain air in their pores, which is prevented from escaping by the rind; on working the pump the diminished pressure causes this imprisoned air to expand; in consequence, the apples swell up, and regain their fresh and plump appearance. The illusion vanishes the moment the atmospheric air is readmitted, because the pressure of the external air reduces that in the apples to its former bulk. The pressure thus exhibited is very considerable, as may be shown by the following experiment. Take a thin vessel, such as a light flask, and seal it up full of air; now if the air be exhausted from a receiver placed over it, the flask will be burst into fragments. The great pressure which air exerts against the internal surface of the vessels in which it is contained, may also be exhibited by allowing a weight of several pounds to rest upon a bladder placed under the receiver of the air-pump; on exhausting the air from the receiver, the air in the bladder expands, and lifts the weight.

56

CONDENSING SYRINGE-WEIGHT OF AIR.

[32.

(32) Condensing Syringe.-If the valves in the syringe be made to open in the direction opposite to those of the air-pump, the instrument constitutes the condensing syringe. By attaching it to a reservoir capable of resisting the pressure, as shown in fig 12, air may be compressed without difficulty, and stored up as a mechanical power; the pressure of air so compressed is

capable of being brought suddenly into exercise.

Instances of this kind are furnished in the compressed air-fountain, and in the common forcing-pump, one variety of which constitutes that invaluable machine, the fire-engine. A still more striking illustration is seen in the air-gun, where the power of compressed air is made to execute the office of ordinary gunpowder, a substance which may be regarded as a magazine of condensed air which can be brought into action at will.

(33) Weight of the Air.-By means of the air-pump it is easy to show by direct experiment, that air, in common with every form of matter, has weight, and even to measure its weight. For

33.]

WEIGHT OF THE AIR.

57

this purpose a well-shaped globular flask, F, fig. 13, furnished with a small stop-cock, is screwed to the plate of the pump, and the air is exhausted. In this state it is transferred to a good balance and accurately counterpoised; it is then attached to a graduated jar, G, filled with air, also provided with a stop-cock, and standing over water; the moment that, by opening the stop-cocks, a communication is made between the jar and the flask, the air rushes into the exhausted vessel. The amount that thus enters is read off by noticing the level of the water before the stopcocks are opened, and then estimating its rise afterwards by the marks on the side of the jar. On transferring the flask back to the balance, it will be found to have increased in weight several grains.

Minute attention to a variety of circumstances is required to insure a correct result in this experiment. It is by experiments conducted on this principle that the density of the air has been well ascertained (146).

According to Prout, 100 cubic inches of air at a temperature of 60° F., when the column of mercury in the barometer stands at 30 inches, are 31'0117 grains. Regnault found that I litre of air at o° C., was (barometer 760mm,, also at o° C.) 1*293187 grammes. This, if reduced to the English standards, would make the 100 cubic inches of air amount to 30938 grains,* or 13 cubic feet of air would be about 1 lb.

We may form some notion of the density of the air, by calculating the quantity contained in a given space. Take, for example, a room 10 metres square, and 5 metres in height, offering a cubic content of 500 cubic metres: since a litre of air is 1293187 grammes, a cubic metre, or 1000 litres, will be 1293187 kilogrammes, and 500 cub. metres of air will be 646 kilos. Such a room in English measures would be about

* According to Regnault, the density of mercury at o° C. is 13'596, water at 4° C. being taken as I; consequently the relative densities of air, water, and mercury will be

under a barometric pressure of 760mm,, or 29′922 English inches at o° C. Calculating these values all at the temperature of 60° F., and at the barometric pressure of 30 inches, the mercurial column being also at 60° F., and allowing for the relative expansion of water and mercury by heat, the ratios will be the ollowing

58

HOUSEHOLD PUMP.

[33.

32.809 feet square, and 16'4045 feet high, with a cubic content of 176583 cub. feet, and the air that it contains at 60° F. and 30 in. barometer, would be about 1349 lb. or 12 cwt.

It is obvious that if, in the experiment with the flask, just described, the graduated jar had contained any other gas instead of atmospheric air, it would be possible to ascertain the weight of a given quantity of such gas; and by comparing this weight with that of an equal volume of air, to ascertain its density.

FIG. 14.

(34) Household Pump.-The air is the power which raises water in the bore of an ordinary pump. The construction of this very useful machine will be at once understood from the description of the airpump which has been already given; the arrangement of the valves being similar. Suppose that at first there is no water in the body of the pump. On depressing the piston rod (fig. 14), air escapes through the upper valve, A, and on raising it again, a fresh portion enters from the pipe attached below the second valve, B. The pressure of the atmosphere upon the surface of the water in the well forces up a portion of this liquid till it reaches a height at which the pressure due to the column raised together with the diminished pressure of the air in the barrel is equal to that of the atmosphere; on again depressing and raising the piston several times successively, the whole of the air at length has its place supplied by the water thus raised from the well below, the pressure of the atmosphere being partly removed from the surface of the water contained in the pipe beneath the valves. It is manifest, however, that there must be a limit in the height to which water can be raised in this way. As soon as the column of water in the pump above the level of that in the well is long enough to balance the pressure of the air, the water will rise no higher. Such a column of water is about 33 feet, 10'33 metres in height. If a tube 40 feet (or nearly 12 metres) long be closed at its upper end, filled with water, and then placed mouth downwards in a vessel of water, the water in the tube will fall till it stands about 10 metres

B