ENCYCLOPEDIA INTERNATIONAL

 V.2, P.165-167, 1979

From the biological viewpoint both nitrogen and oxygen are highly important. Atmospheric nitrogen, which comes mostly form volcanic eruptions, goes into the making of plant and animal proteins. It moves in various chemical forms from the air into soil, there into plants, from them into animals, and back into the soil, from which it returns to the air. This cycle involves the activity of soil bacteria called nitrogen-fixing bacteria. All living things, except the most primitive one-celled organisms, require oxygen for many important chemical and biological processes. For example, man has to breathe oxygen in order to live, and its pressure must be within a certain range. The lower limit of pressure is generally equivalent to that prevailing at 10,000 ft. above sea level. Above this height, up to about 25.000 ft., the respiratory and related functions can, with some difficulty, adapt themselves. At higher elevations adjustment fails, often resulting in death from "oxygen starvation". The source of oxygen in the atmosphere is open to question, but most of it probably is given off by plants. Carbon dioxide is taken from the air in the course of photosynthesis, the wonderful process by which plants use the energy of sunlight to combine carbon dioxide with water, yielding food for themselves and releasing oxygen to the air. Atmospheric carbon dioxide comes mainly from volcanoes and gas sources within the earth. It is also exhaled by animals, and some is formed as a by-product of fuel consumption or by other industrial processes. Consequently it is found only in the lower atmosphere. Ozone (O4) is the triatomic form of oxygen, produced by the action of ultraviolet radiation. Some ozone is found at ground level, but most of it occurs in the "ozone layer," which fluctuates in elevation but is centered at an average height of about 30 mi.

ENCYCLOPEDIA AMERICANA

V.1, P. 394, 1986

AIR PRESSURE, âr presh'ar, is the force per unit area exerted by the atmosphere. The atmosphere, or air, exerts a pressure because of its weight (gravitational force between the air and the earth). The fact that air has weight was first demonstrated in 1644 by the Italian scientist Evangelista Torricelli, who invented the mercury barometer to prove his point. At sea level, the pressure of the atmosphere is approximately equal to the weight per unit cross-sectional area of a column of mercury 76 centimers high. At any altitude, the weight of the air is proportional to the mass of air above the altitude. Therefore, air pressure, like air mass, decreases with increasing height. This fact, which was first proved by Blaise Pascal in 1648, is the basis for the pressure altimeter, an instrument that determines altitude by measuring pressure. Mercury barometers, aneroid barometers, and other types of barometers are used to measure air pressure. Pressure often is expressed in inches, centimeters, or millimeters of mercury. However, the correct unit of pressure should have the dimensions of force divided by area. In meteorology, the unit of air pressure is the millibar, which is defined as 1.000 dynes per square centimeter. (The dyne is the unit of force in the cgs system.) At sea level, the air pressure is approximately equal to 1,000 millibars. At 6 kilometers above sea level the pressure is about 500 millibars. A pressure of 1,000 millibars corresponds to approximately 76 centimeters (30 inches) of mercury, which is equivalent to a pressure of about 14.5 pounds per square inch. In high-altitude commercial airplanes, the pressure inside the plane is maintained at a value that is safe and comfortable for human activity. Without this pressurization the air pressure would be too low to provide sufficient oxygen for life, and the rapid pressure changes accompanying changes in altitude would produce serious physical discomfort as well as "bends" (aeroembolism). This condition is caused by the release of nitrogen bubbles into the blood when pressure drops too rapidly.