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AIR BRAKE becomes as rare as the surrounding atmosphere, showing that it is elastic. The weight of the atmosphere makes the lower air so much denser than the upper air that one-half of the whole atmosphere is squeezed into a belt around the earth about three and one-half miles in thickness, while the upper half extends more than forty miles. Air is made up of about 78 parts by volume of nitrogen, 21 of oxygen and one of argon, in 100 parts, and with it is always mixed a variable quantity of water-vapor, which amounts to about one per cent by volume on the average. It contains also about of carbon dioxide and minute quantities of several other gases, including helium, neon, krypton and xenon, which are inactive elements resembling argon. Oxygen is the element that is necessary to animal life, while carbon dioxide is required by plants. On the other hand, animals produce carbon dioxide while plants give off oxygen, so that each supplies the other, and the composition of the air is kept nearly constant. There is air also dissolved in water, and by the same double process fishes and sea-plants keep the air pure. In the cities the air is less pure than in the country, as there are more people to breathe it and fewer plants to supply oxygen. The gases of combustion and decay, which produce much carbon dioxide, also tend to contaminate the air. In a room the breathing of a number of persons will soon make the air unwholesome if the ventilation is not good. The gas, carbon dioxide, which is commonly called carbonic acid, is often called a poison; but it is not poisonous, although the presence of a large quantity of it interferes with breathing, and if enough is present it may cause suffocation. Carbon monoxide, the gas that burns with a pale-blue flame at the top of coal fires and is present in illuminating gas, on the other hand is a deadly poison. If this escapes unburned into the air of a room, the results may be very serious. Air Brake..  Air Gun, a weapon for shooting bullets or other projectiles by means of condensed air. The air is forced into a vessel, usually in the stock, by means of a condensing syringe. When the finger touches the trigger, the air reaches into the space behind the bullet and drives it out of the barrel, and when the finger is taken away the vessel is again closed. Thus several shots may be fired, but with less force each time. At its greatest, the force is not equal to an ordinary charge of gunpowder.  Air Plants. Those plants which obtain all their food materials from the air. See.  Air Pump, an instrument used either to compress air in a closed vessel or to exhaust the air from a closed vessel. When used for the former purpose it is generally known as a "force pump," and when used for the latter purpose it is frequently called a "vacuum pump."

Air pumps, like other pumps, consist essentially of a cylinder fitted with a piston and two valves. The simplest of those which are used for compressing air is, perhaps, the ordinary bicycle pump illustrated in Fig. 1. Here the piston is provided with a more or less flexible leather collar which allows the air to pass down around it as the piston is lifted. But on the down stroke of the piston this leather collar acts as packing and prevents the air from passing up. Hence this one part acts as both piston and valve.

A second valve, V2, at the bottom of the cylinder prevents the air in the tire from getting back into the cylinder during the upstroke of the piston. The vacuum pump is built upon exactly the same principles as the force pump, only the direction in which the valves open is reversed.

The first artificial vacuum was produced by Otto von Guericke, about 1650, with a pump similar to an ordinary lift-pump used in wells, except that instead of pumping the air out of an open vessel, such as a well, he pumped it out of a closed vessel.

The immense improvement which has recently been made in the construction of air pumps, especially by the introduction of valves which open automatically, can be seen from the fact that Guericke's pump required four able-bodied men to operate it. A modern air pump is easily worked by one hand.

Ordinary vacua are thus produced by a piston working in a brass cylinder; but when it comes to "high vacua," such as those employed in the incandescent lamp and in X-ray bulbs, a much more perfect instrument is required. Here it is necessary to replace the ordinary cylinder by a mercury column in a glass tube.