In

183. The Mercurial Gauge. — In order to measure the degree of rarefaction produced, a glass cylinder, F (Fig. 128), is connected with the pipe leading from the receiver. this cylinder is a glass tube bent into the form of the letter U, one branch being closed at the top, and the other open. The tube has its closed branch filled with mercury, and is called a siphon gauge.

The mercury, under ordinary circumstances, is kept in the closed branch by the atmospheric pressure, but as the air becomes rarefied in the receiver, the tension of the air becomes less and less, and finally the mercury falls in the closed branch and rises in the open one. The difference of level between the mercury in the two branches is due to the tension of the rarefied air, and if this difference is determined by means of a proper scale attached to the gauge, the tension can be found. Thus, if the difference of level is reduced to one inch, the tension of the air in the receiver will be only one-thirtieth part of the tension of the external atmosphere.

The siphon gauge is sometimes connected with the receiver in a different way; as seen in Fig. 113 and 125. It is only necessary that it should be so placed that the air will be exhausted from it at the same time, and to the same degree as from the receiver.

184. Sprengel's Air-Pump. - Various methods have been employed for obtaining a more complete vacuum than can be produced by the ordinary air-pump. One of the most effective instruments for this purpose is Sprengel's Air-Pump, represented in Fig. 130.

To the funnel, A, is attached a glass tube, longer than a barometer tube. Its lower end enters the glass vessel, B, and reaches nearly to the bottom. The upper part of the tube branches off at x, and is connected with the receiver that is to be exhausted.

Mercury is poured into the funnel, A, and as it flows down the tube, air from the receiver enters at x, and is carried along with it. The tube below is then seen to be filled with cylinders of mercury separated by cylinders of air, all moving downwards.

The mercury in the bottom of the vessel, B, prevents the air from passing back into the tube, and it escapes while the mercury flows into the vessel, H.

As the process goes on, the cylinders are seen to be separated by smaller and smaller spaces of air, till it apparently passes down as a solid column, no air spaces appearing. This indicates the completion of the process.

The only labor required is that of lifting and pouring the mercury back into the funnel after it flows out.

The operation is very slow, but it produces a vacuum so nearly perfect that less than one-inillionth part of the original quantity of air remains in the receiver.

By employing tubes of sufficient length water can be used instead of mercury.

B

Fig. 130.

H

The filter-pumps, now much used in chemical laboratories, are constructed on the same principle.

185. Experiments with the Air-Pump. - Several experiments requiring the use of the air-pump have already been described. Most of these serve to show the pressure of the

atmosphere. Fig. 131 shows the elastic force of a confined body of air.

Two bottles, A and B, are connected by a tube which is fitted air

Fig. 131.

tight into A, but loosely into B. The tube extends nearly to the bottom of A, which is partly filled with water. When both are placed under the receiver, and the air exhausted, the elastic force of the air in A causes it to expand and drive the water over into B.

If a lighted candle be placed under a receiver, and the air exhausted, the candle will go out and the smoke will sink, showing that it is heavier than the rarefied air of the receiver.

If an animal or bird be placed under the receiver, and the air exhausted, it will struggle and soon die. This experiment is shown in Fig. 132.

186. Practical Uses of the Air-Pump. -The most important practical application of the air-pump is in diminishing the pressure of the atmosphere to facilitate evaporation of liquids.

In order to concentrate the syrup of sugar without employing a high degree of heat, it is placed in closed vessels called vacuum pans, and the air and the steam that rise are removed by powerful air-pumps driven by steam-power. By this method the watery vapor is rapidly carried off, and the syrup brought to the proper degree of concentration without employing a degree of heat that would burn or discolor the syrup.

Fig. 132.

The same process is employed in making or concentrating a great variety of syrups and extracts that are used in medicine.

The air-pump has also been employed for exhausting long tubes that are used for transmitting letters, messages, and various small packages. These are called Pneumatic Tubes.

In London, where these tubes are extensively used, they are made of lead enclosed in tubes of iron. They are made smooth on the inside, and fitted with pistons consisting of cylinders of gutta-percha, in which the articles to be transmitted are placed. The air is then exhausted, and the pressure of the atmosphere drives the piston through the whole length of the tube. The tubes used for this purpose are about 2 inches in diameter; and some of them are more than two miles in length.

lateral tube, is an admission valve which opens inward. Ris a strong copper vessel, which is screwed upon the lower part of the pump-barrel.

When the piston is forced downward the air enters the receiver through the valve, b, which prevents its return. At the upward stroke the air enters the cylinder, through a. As the movement