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Place the nearly empty bladder under the receiver of an air-pump, and exhaust the air. As the air becomes rarer in the receiver, the bladder will be seen to expand, showing that the air within it is expansible. In the same way, it may be shown that any gas is expansible.

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102. Air, like other bodies, has weight.

To show this, take a hollow globe of glass, fitted with a stop-cock, as shown in Fig. 71. Having attached it to one scale pan of a delicate balance, counterpoise it by weights placed in the other. Then by means of the air-pump exhaust the air from the globe; the opposite scale pan will descend, and some weights will have to be added

(102.) How is it shown that air has weight.

to the first scale pan to restore the equilibrium. The weights added will indicate the weight of the exhausted air.

103.

Composition of the Atmosphere.

It has been stated that our atmosphere is composed principally of oxygen and nitrogen, with small quantities of carbonic acid and watery vapor.

The amount of watery vapor depends upon the place, the season, the temperature, and the direction of the wind; under all circumstances it forms but a small per-centage of the entire atmosphere.

The carbonic acid in the atmosphere arises in a great measure from respiration and combustion. A continual supply of this gas is afforded by volcanoes. On the other hand, it is being continually taken up in the process of vege tation. Plants continually absorb it, appropriating the carbon, and giving off the oxygen which it contains. Another cause of diminution in the amount of carbonic acid in the air, is absorption by the water of our streams. Water absorbs large quantities of it, which thus becomes the means of dissolving earthy matters, and eventually of causing calcareous deposits.

It is the result of observation, that the supply and loss are very nearly balanced, so that the per-centage of carbonic acid in the atmosphere remains nearly constant. It amounts to about a thousandth part of the entire atmosphere.

Atmospheric Pressure.

104. The atmosphere, by virtue of its weight, exerts a force of pressure upon the surface of the earth as well as upon every object with which it is in contact. This force is called the Atmospheric Pressure.

(103) Upon what circumstances does the watery vapor in the air depend? Whence is carbonic acid supplied? What becomes of the excess of carbonic acid? How do the supply and loss compare? What is the amount in the atmosphere? 104.) What is the Atmospheric Pressure?

This pressure decreases as we ascend into the atmosphere.

If we suppose the atmosphere to be divided into layers parallel to the surface of the earth, it is evident that each layer is pressed down by the weight of all above it. Hence, the higher layers are less compressed than those below them. Being less compressed, they expand, or become rarefied. The existence of atmospheric pressure may be shown by a variety of experiments, some of which will be explained below.

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105. A glass cylinder, open at both ends, has its upper end covered by a stretched membrane, such as is used by gold-beaters,

How does it vary as we ascend? How shown that the air becomes rarer in ascending? (105.) Explain the experiment of bursting a membrane.

and its lower end is ground so as to fit the plate of an air-pump, as shown in Fig. 72.

In its natural condition, the membrane is pressed down by the weight of the atmosphere above it, and this pressure is resisted by the tension of the air within the cylinder. If now the air be exhausted from the cylinder, the membrane will no longer be pressed from within, and will finally burst with a loud report.

The bursting of the membrane shows the pressure of the air. The report arises from the sudden rush of air to fill up the exhausted cylinder.

The Magdeburg Hemispheres.

106. This apparatus, named from the city where it was invented, consists of two hollow hemispheres of brass, which are ground so as to fit each other with an air-tight joint. The hemispheres are shown in Fig. 73. One of them is so prepared that it can be attached to an air-pump, and is provided with a stop-cock, by means of which a communication with the external air can be opened or closed at pleasure.

The two hemispheres being placed one upon the other, the pressure of the external air is exactly counterbalanced by the tension of that within, and no obstacle prevents them from being drawn apart. If, however, the air be exhausted from within, the external pressure is no longer counteracted by an expansive force from within, and it requires a considerable effort to effect their separation, as shown in Fig. 74. We shall see hereafter that the hemispheres are pressed together by a force equal to 15 lbs., multiplied by the number of square inches in their common cross section.

Fig 73.

What causes the bursting? The report? (106) What are the Magdebourg Hemispheres? Describe the experiment, and explain it.

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The experiment was devised by OTTO VON GUERICKE, of Magdebourg. He constructed two hemispheres more than two feet in diameter, and after having exhausted the air, it is reported that it required several horses to draw them asunder.

Torricellian Tube.-Measure of the Atmospheric Pressure.

107.

The preceding experiments show that the atmosphere exerts a force of pressure; the intensity of that force may be measured by other means.

TORRICELLI, a pupil of GALILEO, showed in 1643, that this pressure amounts to about 15 lbs. on each square inch of surface, at the level of the sea.

What experiment was made by OTTO DE GUÉRICKE? (107.) What is the pressure of the atmosphere on a square inch?

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