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161. General Properties of Gases and Vapors. GASES and VAPORS are highly compressible and elastic fluids.

Their particles, like those of liquids, move freely, and transmit pressure in all directions; but they differ from liquids in the predominance of the repellent force exerted between their molecules, in consequence of which a mass of gas always tends to expand.

The force that elastic fluids exert in this way is called their tension.

The distinction between a gas and a vapor is not very clear. When a body in the gaseous form can be reduced to a liquid by cooling, or by a moderate pressure, it is usually called a vapor.

It is now known that all the gases may be reduced to the liquid form by great pressure and intense cold combined.

162. The Atmosphere. Common air possesses all the mechanical properties that belong to gases and vapors. It is therefore taken as the type of aeriform bodies.

The atmosphere that surrounds the earth is transparent, without odor, and colorless except in great masses.

In

masses it assumes a blue tint, and is the cause of the blue

color of the sky.

It is composed of oxygen, nitrogen, carbonic acid, watery vapor, and some accidental impurities.

The principal ingredients are oxygen and nitrogen, and these are mixed in the proportion of twenty-one parts by volume of oxygen to seventy-nine parts of nitrogen.

Carbonic acid forms but a small portion of the atmosphere, but it is an constant and very important element. It is continually

Fig. 113.

supplied to the air by the respiration of animals, by the combustion of coal and other fuel, and by the decay of animal and vegetable substances. The burning of a single ton of coal sends into the atmosphere more than three tons of this gas.

On the other hand, all growing plants absorb it and retain the carbon, but restore to the air the oxygen which it contains. It is found that the supply and loss are very nearly balanced, so that the proportion of carbonic acid in the atmosphere remains nearly

constant.

It amounts, in volume, to about one part in twenty-five hundred of the whole atmosphere.

163. Expansive Force of Air.- Air and the gases always tend to assume a greater volume.

To show this property, take a bladder or rubber bag, fitted with a stop-cock, as shown in Fig. 113. Press out nearly all the air, then close the stop-cock and place the bag under the receiver of an air

pump. Then pump the air out of the receiver, and the elastic force of the air in the bag will cause it to expand.

In the same way it may be shown that any gas is expansible.

164. Weight of Air. Air, like other bodies, has weight.

To show this, take a hollow globe of glass, fitted with a stop-cock, as shown in Fig. 114. 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 to the first scale-pan to restore the equilibrium. The weights added will indicate the weight of the exhausted air.

165. Atmospheric Pressure. - Since the atmosphere has weight it exerts a pressure on all bodies upon which it rests. creases as we ascend into the atmosphere.

Fig. 114.

This pressure de

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.

166. Bursting a Membrane. A glass cylinder open at both ends, has its upper end covered by a piece of oiled silk or a stretched membrane, such as is used by gold-beaters, and its lower end is ground so as to fit the plate of an air-pump, as shown in Fig. 115.

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.

If a piece of thin sheet rubber be used in place of the membrane, it will be gradually forced inward as the air is exhausted, and will be stretched in proportion to the degree of exhaustion.

167. The Magdeburg Hemispheres. 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. 116. 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