pressed through the pores of the wood and descend in a silver shower. The jar, c, should be so placed as to prevent any of the quicksilver getting into the interior of the pump. A Fig. 28. d There are many substances which exist in the liquid condition, merely because of the pressure of the air. Take a glass tube, A, Fig 28, closed at one end and open at the other, and having filled it with water, invert it in a jar, B; introduce into it now a little sulphuric ether, which will rise, because of its lightness, to the top of the tube, at a. Place the apparatus beneath the receiver of the air-pump, and exhaust. The ether will now be seen to abandon the liquid, and assume the gaseous form, filling the entire tube and looking like air. On allowing the pressure again to take effect, it again relapses into the liquid form. The following experiments illustrate the elasticity of the air: Take a glass bulb, a, Fig. 29, which has a tube, b, projecting from it, the open extremity of which dips beneath some water in a cup, c; the tube and the bulb being likewise full of water, except a small space which is occupied by a bubble of air at a. Invert over the whole a jar, d, and, placing the arrangement on the pump, exhaust. It will be found, as exhaustion goes on, that the bubble, a, steadily increases in size until it fills all the bulb, and even the tube. On re-admitting the pressure, the bubble collapses to its original size. The air is, Fig. 29. therefore, dilatable and condensible—that is, it is elastic. If a bottle, the sides of which are square and the mouth hermetically closed, be placed beneath a receiver, and the pressure removed, the air imprisoned in the interior, exerting its elastic force, will violently burst the bottle to pieces. It is, therefore, well to cover it with a wire cage, as represented in Fig. 30. The elastic force of the air increases with its density. Powerful effects therefore arise by condensing air into a limited space. The condenser, which is an instrument for this purpose, is represented in Fig. 31. It conFig. 30. sists of a tube, a, b, in which there moves by a handle, g, a piston, f. In one side of the tube, at c, there is an aperture, and at the lower part, d, there is a valve, e, opening downward. On pushing the piston down, the air beneath it is compressed, and, opening the valve, e, by its elastic force, accumulates in the receiver, R. When the piston is pulled up, a vacuum is made in the tube; but as soon as it passes the aperture, c, the air rushes in. Another downward movement drives this through the valve into the receiver, and the process may be continued until the elastic force of the included air becomes very great. f R Fig. 31. If the receiver be partly filled with water, and there be placed in it a piece of wax, an egg, or any yielding or brittle bodies, it will be found impossible to alter their figure by condensing the air to any extent whatever. And this arises from the circumstance already explained-that the pressure generated is equal in all directions. [A common toy, known by the name of the water balloon, is constructed upon the following scientific principles:-The figure of a balloon is blown in glass, and the lower part, which is curved for the car to be attached, has a small hole in it. A car is then suspended to it, and this is of such a weight that it is exactly balanced by the balloon full of air placed in a tall vessel of water, covered with a piece of bladder or india-rubber, which is firmly tied over the top. By pressing the finger on the cover, the balloon will descend to the bottom, and on removing the finger, the elastic force of the air in the balloon drives out the excess of water which had previously entered, and the balloon becoming lighter, re-ascends. Therefore, we say that the ascent and descent of the balloon depends upon the condensation and expansion of the air contained in it.] Fig. 32. On precisely the same principle, if a small bladder, only partly full of air, be sunk by a weight, Fig. 32, to the bottom of a deep glass of water, on covering the whole with a receiver, and exhausting the elastic force of the included air, dilates the bladder, which rises to the top, carrying with it the weight. When the pressure is re-admitted, the bladder collapses and descends again to the bottom of the jar. There are numerous machines in which the elastic force of air is brought into operation, such as the air-gun, blowing-machines, &c. Indeed, the various applications of gunpowder itself depend on this principle-that material on ignition suddenly giving rise to the evolution of an immense quantity of gas, which exerts a great elastic force. CHAPTER VII. PROPERTIES OF THE AIR. Marriotte's Law-Proof for Compressions and Dilatations—Case in which it Fails-Resistance of the Air to Motion-The Parachute-The Air transmits Sound; supports Animal Life, Combustion, and Ignition— Exists in the pores of some bodies and is dissolved in others. ATMOSPHERIC air being thus a highly compressible and expansible substance, we have next to inquire what is the amount of its compressibility under different degrees of force? This has been determined experimentally by different philosophers, the true law, having first been discovered by Boyle and Marriotte. The density and elasticity of air are directly as the force of compression. The volume which dir occupies is inversely as the pressure upon it. To illustrate, and at the same time to prove these laws, we make use of a tube, a d c b, so bent that it has two parallel branches, a and b. It is closed at b, and has a funnel-mouth at a. Sufficient mercury is poured into the tube to close the bend and to insulate a volume of air in b d. Of course this air exists under a pressure of one atmosphere equal to a column of mercury thirty inches long. Through the funnel, a, mercury is now to be poured; as it accumulates it presses on the air in d b, and reduces its volume to c. If, in this manner, a column thirty inches long be introduced, it will be found that the air in b d is reduced to half. There are, therefore, now two atmospheres pressing on the included air-the atmosphere itself being one, and the thirty inches of mercury the other. Two atmospheres, therefore, reduce a given quantity of air into half its volume. Fig. 33. In the same manner it could be proved, if the tube were long enough, that the introduction of another thirty inches of mercury, giving a pressure of three atmospheres, would condense the air to one-third, that four would compress it to one-fourth, five to one-fifth, &c. [The truth of this law may be proved for rarefactions as well as condensations. Arago and Dulong have demonstrated that Marriotte's law does not vary for atmospheric air in its application to a pressure of twenty-seven atmospheres.] For this purpose let there be taken a long tube, a b, Fig. 34, open at the end, b, and closed at a, with a screw; a jar, B, filled with mercury to a sufficient height, is also to be provided. Now let the screw at a be opened and the tube depressed in the mercury until the metal, by rising, leaves in the tube a few inches of air. The screw is now to be closed and the tube lifted. The included air at once dilates, and a column of mercury is suspended. It will be found that when the air has dilated to double its volume, the length of the mercurial column in the tube will be fifteen inches-that is, half the barometic length. B B By such experiments it therefore appears that Marriotte's law holds both for condensations and rarefactions. This law has been verified until the air has been condensed twentyseven times and rarefied one hundred and twelve times. În the case of gases, which easily assume the liquid form, it is, Fig. 34. however, departed from as that point is approached. Besides the properties already described, atmospheric air possesses others which require notice. Among these may be mentioned resistance to motion. This property may be exhibited by means of the two wheels, a b, Fig. 35, which can be put in rapid rotatory motion by the rack, d, which moves up and down through an air-tight stuffing-box, e. The wheels are so arranged that the vanes of a move through the air edgewise, but those of b with their broad faces. On pushing down the rack, d, and making the wheels rotate with equal rapidity in the atmospheric air, one of them, a, will be found to continue its motion much longer than the other, b; and that this arises from the resistance which b experiences from the air is *proved by making them rotate in the receiver from which the air has been exhausted, when b will continue its motion as long as a, both ceasing to revolve simultaneously. The water-hammer affords another instance of the same principle. It consists of a tube a foot or more long, and half an inch in diameter. In it there is included a small quantity of water, but no atmospheric air. When it is turned upside down the water drops from end to end, and emits a ringing metallic sound. If there were any air in the tube, it would resist or break the fall of the water. A well-made mercurial thermometer exhibits the same fact. If there is a perfect vacuum in its tube, on turning the instrument upside down the metal drops like a hard, solid body against the closed end. THE PARACHUTE is a machine by which aëronauts may descend from a balloon to the ground in safety. It bears a general resemblance to an umbrella, and consists of a strong but light surface, a a, Fig. 36, from which a car, b, is suspended. When it is detached from the balloon it descends at first with an accelerated velocity; but this is soon checked by the resistance of the air, and the machine then falls at [A very simple experiment willenable any person to illustrate the principle of the parachute. Take a piece of paper about three feet square, and attach a piece of thread to each corner of it; then fasten a proportionate weight to the lower part, and let it drop from a window, or even from your hand, held as high as you can. It will descend slowly to the ground.] [Zacharia of Rosleben conceived that it would be possible to construct a flying boat, Fig. 37. and, as an experiment he made a case of light wood, covered with linen, in the shape of a flat obtuse-angled keel, five and a half feet in diameter, and half a foot deep, weighing fourteen and a half pounds. He launched this machine from a scaffold twenty-seven feet high, which was erected on a rock one hundred feet above the racecourse of Wendelstein, 17th of September, 1822, and the boat flew 153 feet. He made another experiment upon the same day, and from the same spot, with another boat, weighing twenty-five pounds, which landed after a flight of 158 feet. The experiments were not repeated, on account of the expense and their partial failure.-Abridged from the " Elements of Natural Philosophy," by Prof. Viest, of Anhalt-Dessau, page 207.] |