operation, and submarine work are other common applications of the use of compressed air. 165. Weight. Though gases are the lightest forms of matter, each has weight, as may be found by weighing. Demonstrations. - Weigh on a delicate balance a light glass flask that is fitted with a stopcock. Exhaust the air from the flask and weigh it again, and the flask and contents will be found to be lighter than before. Weigh carefully an incandescent lamp bulb. One with a broken filament will answer, and one with a light base is desirable. Direct the point of a blowpipe flame upon one side of the bulb. As soon as the glass becomes redhot it is forced in by the pressure of the atmosphere, and if only the point of the flame is used, a small, round hole will be blown in the bulb. The filament will probably be blown in pieces, but the pieces will all be inside the bulb, and there will be no loss of weight on account of losing any of them. Weigh the bulb a second time, and the difference in weight will be the weight of the air that has entered the bulb. FIG. 147 By an extension of these methods the weight of air and other gases has been found. The weight of 1 c.c. of dry air at 0° C. and the barometric pressure of 760 mm. is 0.001293 g. Since 1 c.c. of water at 0° C. weighs practically 1 g., the weight of air is of the weight of water. Hydrogen, the lightest known gas, weighs 0.0000899 g. per cubic centimeter; hence air is about 14.4 times as heavy as hydrogen. The weight of air in English measure is 0.33 grain per cubic inch, of hydrogen 0.0228 grain, and of carbon dioxide 0.5046 grain. 166. Composition of the Atmosphere. The air composing the atmosphere of the earth is a mixture of gases. by volume is oxygen, nitrogen, 2 a variable proportion aqueous vapor. About carbon dioxide, and The quantity of aqueous vapor depends greatly upon the temperature; it varies from 4.835 g. or less per cubic meter at 0° C., to 22.796 g. or less per cubic meter at 25° C. Besides the above, there are traces of other common gases, as ammonia and ozone, and small quantities of several recently discovered gases. Argon was discovered in 1895 by Lord Rayleigh and Professor Ramsay as the result of an admirable course of scientific research. It forms nearly of the atmosphere. After its discovery, Professor Ramsay continued his researches, and discovered four other gases in the atmosphere, - helium, neon, krypton, and xenon. These are present in minute quantities only, and are isolated by the employment of low temperatures. 167. Pressure of the Atmosphere. - Since air has weight, the layers of air near the surface of the earth are subject to a pressure due to the weight of the air above. Demonstrations. Tie a sheet of thin rubber over one end of a bladder glass (Fig. 148) and place it on the plate of an air pump. different positions (Fig. 149), and it will be seen that the air presses equally in all directions. Fill a tumbler full of water. Slide a heavy card over the top of the tumbler, being careful that no air bubbles are left below it. Hold the card while you invert the tumbler. Remove the support from the card, and it will remain pressed against the tumbler, holding in the water. Hold the card on and turn the tumbler so that the card is vertical; when the hand is removed, the card still remains. FIG. 149 Select a long, clear glass bottle and fit to the neck a rubber stopper with one hole. Pass through this a short glass tube with the inner end drawn down to a fine opening. Fit one end of a rubber tube to the outer end of the glass tube and connect the other end to the air pump. Exhaust the air. Pinch the tube together; hold the bottle as in Fig. 150, and pull the rubber from the glass tube when it is below the surface of the water in the dish A. The pressure of the air upon the surface of the water will force a stream through the tube, forming a fountain inside the bottle, and the water will continue to flow until the amount of water in the bottle is equal to the volume of air taken out. This is a very old experiment, called the FIG. 150 fountain in vacuo. Procure two small bottles and put into each the end of a U-tube, fitting loosely in B (Fig. 151), and air-tight through a rubber stopper in A. Fill A half full of water; place both under the receiver of an air pump and exhaust the air. At the first stroke the water from A will begin to run into B. Explain. FIG. 151 Exhaust until nearly all the water is drawn over, and then let air into the receiver. The water will run back into A. Explain. 168. The Buoyant Force of the Atmosphere. A body submerged in water is buoyed upward by a vertical upward force that is equal to the weight of the displaced water. This pressure amounts to 62.5 lb. for each cubic foot of the volume of the submerged body. Since the air has weight, a body is buoyed up by it in the same way. The amount of the lifting force is as much less than that produced by water as the air is lighter than water. Since air weighs the weight of water, the weight of a cubic foot of air is 0.08 lb. This means that the lifting power of the air on a spherical balloon 20 ft. in diameter is over 338 lb. at the surface of the earth. The increasing rarity of the air as the distance from the earth increases makes the lifting power less at high altitudes. of The aeroplane, which is a heavier-than-air machine, is lifted by the thrust of the air upon the under side of its planes when they are driven through it at high speed. This speed is secured by the push of a light screw propeller that rotates very rapidly, being driven by a special form of gasoline engine. In order to 169. Magdeburg Hemispheres. demonstrate the pressure of the atmosphere, Otto von Guericke, the burgomaster of Magdeburg, devised what are called the Magdeburg hemispheres. A form used for classroom demonstration is shown in Fig. 152; it consists of FIG. 152 the two halves of a hollow brass sphere, with edges fitting air-tight. One handle can be unscrewed, and the hemispheres can then be attached to an air pump. |