The New Student's Reference Work/Air

Air is the atmosphere in which we live. It is invisible, and has neither taste nor smell, but we know that it is all around us, for we take it into our lungs with every breath and it becomes our most important food. It has weight, which we do not feel because of the air and other gases within us that exert an equal outward pressure. Upon every square inch of the earth’s surface there rests a weight of about fifteen pounds of air, so that upon the body of a medium-sized man the air presses with a force equal to thirty thousand pounds. Air may be compressed or packed closely into smaller space than it usually fills. Thus, if a tumbler is pushed down, bottom upward, under water, the water will rise up inside the tumbler and press the air into smaller space. But as the tumbler is brought back to the surface the air again

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 1/8000 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.