By taking advantage of the expansibility of the air, we are able to prove that it is included in the pores of many bodies. Thus, if an egg is dropped into a deep jar of water, and this covered with a receiver, as soon as exhaustion is made, a multitude of air-bubbles continually ascend through the water. (Fig. 41.) Or if a glass of porter be placed beneath such a receiver, its surface is covered with a foam, the carbonic acid gas, which is the cause of its agreeable briskness, escaping away. (Fig. 42.) And even common river or spring water treated in the same manner exhibits the escape of a considerable quantity of gas, which ascends through it in small bubbles, and gives it a sparkling appearance Fig. 42. CHAPTER VIII. PROPERTIES OF THE AIR. Loss of Weight of Bodies in the Air-Theory of Aerostation—The Montgolfier Balloon-The Hydrogen Balloon-Mode of Controlling Ascent and Descent-Artificial and Natural Currents in the Air-Resistance of the Air to Projectiles-Velocity with which Air flows into a Vacuum. Velocity of Efflux of different Gases-Principles of Gaseous Diffusion— These principles regulate the Constitution of the Atmosphere. On principles which will be fully explained when we come to speak of specific gravity, it appears that a solid immersed in a fluid loses a portion of its weight. It follows, of course, that a substance weighs less in the air than it does in vacuo. To the arm of the balance, a, Fig. 43, let there be hung a light glass globe, a с Fig. 43. c, counterpoised in the air on the other arm, b, by means of a weight, d. If the apparatus be placed beneath a receiver, and the air exhausted, the globe, c, descends, but on re-admitting the air the equilibrium is again red stored. This instrument was formerly used for determining the density of the air. A substance that has the same density as atmospheric air, when it is immersed in that medium, loses all its weight, and will remain suspended in it in any position in which it may be placed. But if it be lighter, it is pressed upward by the aërial particles and rises, upon the same principle that a cork ascends from the bottor a bucket of water. And as the density of the air continually dimḥ) as we go upward, it is evident that such a body, ascending from one si to another, will finally attain one having the same density as itse there it will remain suspended. In virtue of its elasticity, atmospheric air is the common medium for the transmission of sounds. Under the receiver of an air-pump, let there be placed a bell, a, Fig. 38, the hammer, 6, of which can be moved on its Fig. 38. h d pivot, c, by means of a lever, h, which is worked by The air also is absolutely necessary for the support of life. The higher warmblooded animals die when the air is only partially rarefied. A rabbit, or other small animal, placed under an air-pump jar, may remain there several minutes without being much disturbed; but if we commence with drawing the air, the animal instantly shows signs of distress, and if the experiment is continued, soon dies. (Fig. 39.) Fig. 39. So, too, if a jar containing some small fishes be placed under an exhausted receiver, the animals either float on their backs, at the surface of the water; or descend only by violent muscular exertions. Fishes respire the air which is dissolved in water, and hence it is somewhat remarkable that they continue to live for a considerable length of time in an exhausted receiver. The air is also necessary to all processes of combustion. If a lighted candle be placed under a receiver, it will burn for a length of time; but if the air be withdrawn by the pump, it presently dies out. The smoke also descends to the bottom of the receiver, there being no air to buoy it up. If a gun-lock be placed in an exhausted receiver, and the flint be made to strike, no sparks whatever appear; and, consequently, if there were powder in the pan, it could not be exploded. The production of sparks by the flint and steel is due to small portions of the latter which are struck off by the percussion burning in the air, and when Fig. 40. the air is removed that combustion can of course no longer take place. (Fig. 40.) Fig. 41. By taking advantage of the expansibility of the air, we are able to prove that it is included in the pores of many bodies. Thus, if an egg is dropped into a deep jar of water, and this covered with a receiver, as soon as exhaustion is made, a multitude of air-bubbles continually ascend through the water. (Fig. 41.) Or if a glass of porter be placed beneath such a receiver, its surface is covered with a foam, the carbonic acid gas, which is the cause of its agreeable briskness, escaping away. (Fig. 42.) And even common river or spring water treated in the same manner exhibits the escape of a considerable quantity of gas, which ascends through it in small bubbles, and gives it a sparkling appearance Fig. 42. CHAPTER VIII. PROPERTIES OF THE AIR. Loss of Weight of Bodies in the Air-Theory of Aerostation-The Montgolfier Balloon-The Hydrogen Balloon-Mode of Controlling Ascent and Descent-Artificial and Natural Currents in the Air-Resistance of the Air to Projectiles-Velocity with which Air flows into a Vacuum. Velocity of Efflux of different Gases-Principles of Gaseous Diffusion— These principles regulate the Constitution of the Atmosphere. ON principles which will be fully explained when we come to speak of specific gravity, it appears that a solid immersed in a fluid loses a portion of its weight. It follows, of course, that a substance weighs less in the air than it does in vacuo. To the arm of the balance, a, Fig. 43, let there be hung a light glass globe, с Fig. 43. c, counterpoised in the air on the other arm, b, by means of a weight, d. If the apparatus be placed beneath a receiver, and the air exhausted, the globe, c, descends, but on re-admitting the air the equilibrium is again red stored. This instrument was formerly used for determining the density of the air. A substance that has the same density as atmospheric air, when it is immersed in that medium, loses all its weight, and will remain suspended in it in any position in which it may be placed. But if it be lighter, it is pressed upward by the aërial particles, and rises, upon the same principle that a cork ascends from the bottom of a bucket of water. And as the density of the air continually diminishes as we go upward, it is evident that such a body, ascending from one stratum to another, will finally attain one having the same density as itself, and there it will remain suspended. On these principles aerostation depends. Air balloons are machines which ascend through the atmosphere and float at a certain altitude. They are of two kinds: first, Montgolfier, or rarefied air balloons; and second, Hydrogen gas balloons. The Montgolfier balloon, which was invented by the person whose name it bears, consists of a light bag of paper, or cotton, which may be of a spherical or other shape; in its lower portion there is an aperture, with a basket suspended beneath for the purpose of containing burning material, as straw or shavings. On a small scale, a paper globe two or three feet in diameter, with a piece of sponge soaked in spirits of wine, answers very well. The hot air arising from the burning matter enters the aperture, distending the balloon, and makes it specifically lighter than the air, through which, of course, it will rise. (Fig. 44.) Fig. 44. The hydrogen gas balloon consists, in like manner, of a thin, impervious bag, filled either with hydrogen or common coal gas. The former, as usually made, is from ten to thirteen times lighter than air; the latter is somewhat heavier. A balloon filled with either of these possesses, therefore, a great ascensional power, and will rise to considerable heights. Thus, Biot and Gay Lussac, in 1804, ascended in one of these machines to an elevation of 23,000 feet. When the balloon first ascends, it ought not to be full of gas, for as it reaches regions where the pressure is diminished, the gas within it is dilated, and though flaccid at first, it will become completely distended. [A balloon which is only half full at the surface of the earth, becomes quite full when it has risen three miles and a half; because at that altitude, air from below doubles its volume on account of the diminished pressure.-Dr. Arnott's "Elements of Physics," 3rd ed. p. 401.] If it were full at the time it left the ground, there would be risk of its bursting open as it rose. The gas balloon requires a valve placed at its top, so that gas may be discharged at pleasure, and the machine made to descend. The aeronaut has control over its motions by taking up with him a quantity of sand in bags, as ballast. If he throws out sand, the balloon rises; and if he opens the valve, and lets the gas escape, it descends. The rarefaction which air undergoes by heat makes it, of course, specifically lighter. Warm air, therefore, ascends, and cold air descends. When the door of a room which is very warm, is open, the hot air flows out at the top, and the cold enters at the floor; these currents may be easily traced by holding a candle near the bottom and top of the door. In the former position the flame leans inward, in the latter it is turned outward, following the course of the draught. The drawing of chimneys, and the action of furnaces and stoves, depend on similar principles; the column of hot air contained in the flue ascending, the cold air replacing it below. When the sun Similar movements take place in the open atmosphere. shines on the ground or the surface of the sea, the air in contact becomes warm and rises; it is replaced by colder portions, and a continuous current is established. The direction of these currents is changed by a variety of circumstances, as the diurnal rotation of the earth, and other causes less understood. On these depend the various currents known as breezes, tradewinds, storms, hurricanes. A The atmosphere does not rush into a void space instantaneously; but, under common circumstances of density and pressure, with a velocity of about 1,296 feet in one second. Its resisting action on projectiles moving through it with great velocities is intimately connected with this fact. cannon ball moving through it with a speed of two or three thousand feet leaves a total vacuum behind it. and condenses the air correspondingly in front. It is, therefore, subjected to a very powerful pressure continually tending to retard it. [Though we should be led, as in hydraulics, to conclude that the resistance which air makes to moving bodies is as the square of their velocities, experiment appears to prove, especially when the velocity is great, that the resistance is partly proportional to the square, and partly to the simple power of the velocity.-Playfair's "Nat. Philosophy," vol. 1, page 276.] The rush of the air flowing into the vacuous spaces left by moving bodies is the cause of the loud explosions they make. When gases of different densities flow from apertures of the same size, the velocities with which they issue are different, and are inversely as the square roots of their densities. The lighter a gas is, the greater is its issuing velocity; and, therefore, hydrogen, which is the lightest body, moves, under such circumstances, with the greatest speed. The experiment represented in Fig. 45, illustrates these principles. Let there be a tube, a b, half an inch in diameter and six inches long, the end, b, being open and a closed with a plug of plaster of Paris, which is to be completely dried. Counterpoise this tube on the arm of a balance, and fill it with hydrogen gas, taking care to keep the plug dry, letting the open end, b, of the tube dip just beneath the surface of some water contained in a jar, C. In a very short time it will be discovered that the hydrogen is escaping through the plaster of Paris, and the tube, filling with water, begins to descend; and after a few minutes much of the gas will have gone out, and its place be occupied partly by atmospheric air, which comes in the opposite direction, and partly by the water which has risen in the tube. Fig. 45. Even when gases are separated from each other by barriers, which, strictly speaking, are not porous, the same phenomenon takes place. Thus, if with the finger, we spread a film of soap-water over the mouth of a bottle, a, and then expose it under a jar to some other gas, such as carbonic acid, this gas percolates rapidly through the film, and, accumulating in the bottle, distends the film into a bubble, as represented in Fig. 46. Meanwhile, a little atmospheric air escapes out of the bottle through the film in the opposite direction. Fig. 46. This propensity of gases to diffuse into each other is clearly shown by filling a bottle, H, Fig. 47, with a very light gas, as hydrogen; and a second one, C, with a heavy gas, as carbonic acid, and putting the bottles mouth to mouth. Diffusion takes place, the light |