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Air-pump.

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through an aperture, r, in the cover. From this position of the two valves, it will be seen that, as the piston rises, the external pressure of the atmosphere cannot act in the bottom of the barrel, but the air of the receiver, in virtue of its elasticity, expands and passes by the conduit, I and A, into the barrel. The receiver is still full of air, but it is more rarefied; it is less dense.

When the piston descends, the rod which bears the valve, a, descending with it, communication between the receiver and the

barrel is cut off. The air in the barrel becomes more and more compressed, its elastic force increases, and finally overcomes the atmospheric pressure; so that the valve t, being pressed upwards by the elastic force of the air in the interior more strongly than it is pressed downwards by the atmosphere, is raised, and allows the air of the barrel to escape into the upper part of the barrel, and thence into the atmosphere. Thus a certain quantity of air has been removed. A fresh quantity is removed at a second stroke of

the piston, another at the third, and so on. The air in the receiver is thus gradually more and more rarefied; yet all the air cannot be entirely extracted, for it ultimately becomes so rarefied both in the receiver and in the barrel, that when the piston P is at

the bottom of its stroke, the compressed gas below the piston has no longer sufficient force to overcome the resistance of the atmosphere and force open the valve t. The limit of rarefaction has then been attained, and it is useless to work the pump any longer.

What has been said in reference to one barrel applies also to the other. The machine works with one; and the first air-pumps had but one. The advantage of having two is that the vacuum is more rapidly produced. The use of double-action air-pumps was first introduced by Hawksbee.

138. Measurement of the degree of rarefaction in the receiver.--Since a perfect vacuum cannot be obtained in the receiver, it is useful to have a mean sof ascertaining the degree of rarefac

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Uses of the Air-pump.

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tion at any particular time. This is effected by means of a glass cylinder, E, connected by a brass cap with the conduit in the column I (fig. 114). In this cylinder is placed a bent glass tube, closed at one end and open at the other. This is called the airpump gauge. It is fixed against a plate, on which is a graduated scale. The closed branch being at first full of mercury, so long as the air in the receiver P and in the cylinder E has sufficient tension, it sustains the mercury in the tube; the height of which is from six to eight inches. But as the machine is worked the air becomes more and more rarefied, and has no longer the elastic force sufficient for retaining the column of mercury in the closed limb. It accordingly sinks in this limb and rises in the other. The greater

20

the rarefaction, the smaller the difference of the level in the two limbs. They are, however, never exactly equal; which would correspond to a perfect vacuum. The mercury is always at least the th of an inch higher in the closed branch. This is expressed by saying that a vacuum has been created within th of an inch. 139. Uses of the air-pump.-A great the air-pump have been already described. rain (fig. 14), the fall of bodies in vacuo (fig. 87), the bursting of a bladder (fig. 89), the Magdeburg hemispheres (fig. 90), and the baroscope (fig. 132).

many experiments with Such are the mercurial (fig. 37), the bladder

The fountain in vacuo (fig. 117) is an experiment made with the air-pump, and shows the elastic force of the air. It is a flask containing water and air; the neck is closed by a cork, through which passes a tube dipping in the liquid. The flask being placed under the receiver, a jet of water issues from the top of the tube as soon as the air is sufficiently rarefied. This is due to the elastic force of the air enclosed in the flask.

By means of the air-pump it may be shown that air, by reason of the oxygen it contains, is necessary for the support of combustion and of life. For if we place a lighted taper under the receiver and begin to exhaust the air, the flame becomes weaker as rarefaction proceeds, and is finally extinguished. Similarly an animal faints and dies, if a vacuum s formed in a receiver under which it is placed. Mammalia and birds soon die in vacuo. Fishes and reptiles support the loss of air for a much longer time. Insects can live several days in vacuo.

140. Application of the vacuum to the preservation of food. An important application has been made of the vacuum in preserving food. In air, under the influence of heat, moisture, and

K

oxygen, animal and vegetable matters rapidly ferment and putrefy; but if the oxygen be removed, either by exhausting or by other means, they may be kept fresh for many years.

Appert of Paris was the first in 1809 to devise a means of preserving food in vacuo, or rather in a space deprived of oxygen, which practically amounts to the same. This method consists in placing the substances to be preserved in tin vessels, which are closed hermetically, and then heating in boiling water for some time, under the influence of heat the small quantity of oxygen left in the vessel is absorbed by the substance placed there, so that only nitrogen is present in the free state, a gas which cannot induce fermentation. Substances properly prepared in this manner may be kept for years without alteration.

Appert's method is modified in England in the following manner.

Instead of boiling the food while contained in the closed vessel, a small hole is left in the lid, through which escape the air and vapours produced during ebullition. When it is supposed that all the air has been expelled, a drop of melted lead is allowed to fall on the small hole in the cover which completely closes it. This method is practised on the D large scale in preserving food and vegetables for the use of sailors, and also in preserving Australian meat, which is now consumed in large quantities.

141. Condensing pump. The condensing pump is an apparatus for compressing air or any other gas. The form usually adopted is the following: In a cylinder, A, of small diameter (fig. 118), there is a solid piston, the rod of which is moved by the hand. The cylinder is pro

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Condensing Pump.

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that the lateral valve, o, opens from the outside, and the lower valve, s, from the inside.

When the piston descends, the valve o closes, and the elastic force of the compressed air opens the valve s, which thus allows the compressed air to pass into the receiver. When the piston ascends, s closes and o opens, and permits the entrance of fresh air, which in turn becomes compressed by the descent of the piston, and so on.

This apparatus is chiefly used for charging liquids with gases. For this purpose the stopcock B is connected with a reservoir of the gas, by means of the tube D. The pump exhausts this gas, and forces it into the vessel K, in which the liquid is contained. The artificial gaseous waters are made by means of analogous apparatus.

142. Hero's fountain.-Hero's fountain is an arrangement by which a jet may be obtained, which lasts for some time. It derives its name from its inventor, Hero, who lived at Alexandria, 120 B.C., and depends on the elasticity of the air. It consists of a brass dish (fig. 120), and of two glass globes. The dish communicates with the lower part of the globe by a long tube; and another tube connects the two globes. A third tube passes through the dish to the lower part of the upper globe. This tube having been taken out, the upper globe is partially filled with water, the tube is then replaced, and water is poured into the dish. The water flows through the long tube into the lower globe, and expels the air, which is forced into the upper globe; the air thus compressed, acts upon the water, and makes it jet out as represented in the figure. If it were not for the resistance of the atmosphere, and friction, the liquid would rise to a height above the water in the dish equal to the difference of the level in the two globes.

143. Intermittent fountain.—The intermittent fountain depends partly on the elastic force of the air and partly on the atmospheric pressure. It consists of a stoppered glass globe a, (fig. 121), provided with two or three capillary tubulures. A glass tube, d, open at both ends, reaches at one end to the upper part of the globe, a; the other end is fitted in a support, c, placed in the middle of the dish, m, which supports the whole apparatus. The support, c, is perforated with small holes, which allow air to pass into the tube just above a little aperture in the dish, m.

The water, with which the globe, a, is nearly two-thirds filled,