Air, the gaseous fluid which surrounds the earth. It is a mechanical mixture of somewhat variable composition, consisting chiefly of nitrogen, oxygen, argon, water vapor, and carbon dioxide. If freed from water, carbon dioxide, and other minor constituents, air would contain by volume approximately 78 parts of nitrogen, 21 parts of oxygen, and 1 part of argon. If gases did not possess the property of diffusion the several constituents of air would arrange themselves in layers in the order of their densities. Thus, following an illustration given by Graham, there would be the following layers resting upon one another and covering the earth's surface. Next to the surface, five inches of water; next above, thirteen feet of carbon dioxide; then a layer of argon of about ninety yards thickness; above that, one mile of oxygen, and on the top about four miles of nitrogen. The amount of carbon dioxide in the air is small, being as a rule from three to four parts in ten thousand. It is produced from combustion and decaying of organic matter, and the respiration of animals. While the percentage amount of carbon dioxide is small, the total amount is quite large, it being estimated that over each acre of the earth's surface there are about 30 tons of the gas. Water vapor is the most variable constituent, due to the changing capacity of air for moisture at different temperatures and to the character of the earth's surface. When air contains all the moisture it can hold, it is said to be saturated. Since nitrogenous animal and vegetable matters are constantly undergoing decay, traces of ammonia and ammonium compounds are always present in the air. In the neighborhood of factories, smelting-works, and of cities burning soft coal, there is a noticeable amount of sulphur acids, sometimes so considerable as to destroy vegetation. There also exists in the air suspended matter, consisting of fine volcanic dust, spores of molds and algae, small plant seeds, bacteria, soot, and finely pulverized earth. The dust and like impurities can be seen when a beam of sunlight finds its way into a room. On a larger scale, these impurities cause beautiful sunsets as well as disagreeable fogs. The theory has been advanced that fogs and clouds are to some extent the result of particles of condensed moisture adhering to the dust particles in the air. According to Tyndall, the blue color of the sky is due to the action of these particles on sunlight; above the atmosphere it is reasoned that the firmament appears inky black. Ozone, which is a condensed and more active form of oxygen, is found in very small quantity chiefly in pure air, such as country air and the air of the seaside. Oxygen is the constituent of air most necessary to animal life. The average adult human being draws about one pint of air into his lungs at each breath, the oxygen being partly taken up by the blood and the remainder passed out in the exhalation. The inhaled oxygen combines with the venous blood in the lungs and oxidizes it. The oxygenated blood passes through the body and returns to the lungs charged with carbon dioxide, which gas is exhaled into the atmosphere. Here it follows that the respiration of animals affords a constant supply of carbon dioxide. Carbon dioxide serves as a food for plants. Under the influence of sunlight the leaves of plants absorb the gas, which is decomposed within the plant tissues into carbon and oxygen. The oxygen not required by the plant is returned to the atmosphere, thus tending to maintain the conditions requisite for the life of animals. Air is also necessary for the germination of seeds. The presence of an ample supply of air in the soil is as indispensable to the life of upland plants as is that of water, and methods of tillage are adopted which facilitate soil breathing. The nitrogen of the air serves to dilute the oxygen, thus preventing too rapid oxidation or combustion. In an atmosphere containing a much larger proportion of oxygen combustion would be more rapid and intense; if there were less oxygen, breathing would be more difficult, and fires would burn more slowly. In an atmosphere containing no oxygen there could be no combustion, no growth of animals, or even of plants. The office of argon is not yet understood. Nitrogen is also a source from which is obtained plant nutriment. Certain plants, legumes like clover, are able with the aid of bacteria to obtain their nitrogen direct from the air. Fertilizers are now made from air by means of electrical methods. This industry has assumed great importance in Norway where water power is abundant and affords a cheap source of electrical energy. The so-called Norwegian saltpeter is made at a factory at Notodden in the Hitterdal, Norway. This factory has been in operation since May, 1905, and is capable of turning out from 3,000 to 5,000 tons of nitrate per annum. The atmosphere plays an important part in controlling the general temperature at the earth's surface. The direct rays from the sun passing toward the earth are considerably weakened by absorption on their way through the air. It has been estimated that a vertical ray passing through clear air reaches the earth with a loss of about one-fourth of its original intensity. The amount of radiant heat absorbed is all applied in raising the temperature of the air; the amount of heat transmitted is partly absorbed and partly radiated at the earth's surface. The heat radiated from the earth by day also aids in raising the temperature of the air. At night the earth cools and the air near it is cooled by radiation to the cooled surface. The greatest control of air temperatures by radiation takes place in the lower air, over the land, and in valleys. If the air becomes dusty, as in desert regions, or smoky, as in the neighborhood of forest fires, or cloudy, as in stormy weather, the lower strata are shielded from warming by day and from cooling by night. Under the dense fogs of London, hardly any perceptible rays from the sun reach the ground. In a certain sense the atmosphere acts as a blanket, absorbing and retaining heat and serving to lessen the extremes of temperature from day to night and from summer to winter. Professor Langley ventures the statement that, if we had no atmosphere, the noonday temperature in the midst of what is now at times the hottest part of Africa would not be above 328 degrees below zero Fahrenheit-a degree of cold almost inconceivable. The air being a compressible fluid under the action of gravity does not lie in a layer of uniform density throughout, but diminishes rapidly in density from the surface upwards. The greater portion of the atmosphere is estimated to lie within four or five miles above the earth. At the sea level, air presses downward at the rate of about 14.7 pounds to the square inch, or, technically speaking, exerts a pressure of "one atmosphere." The barometer on the summit of Pike's Peak stands at about 17 inches, equivalent to a pressure of 8.3 pounds, and water boils at 184 degrees Fahrenheit. Direct observations with the barometer have been made by aeronauts at a height of 31,500 feet, and there are reasons to think that for at least fifty miles above the surface of the earth there is an appreciable atmosphere. Mountain climbers and balloonists experience increased difficulty in breathing as they ascend to higher altitudes. Investigators who have attained heights approximating 30,000 feet have found temperatures far below zero. At such heights the air within the body, being unsupported by outside pressure, expands and causes swelling of the tissues. One observer states that his hands turned black and that he became unconscious. A balloon carrying self-registering instruments was launched from Pittsfield, Massachusetts, in the summer of 1908. At the ground the temperature was 40 degrees Fahrenheit, and the temperature decreased as the balloon rose to the base of a cloud. Above the cloud the temperature continued to fall with increasing rapidity to a height of nearly eight miles, where the minimum temperature of 66 degrees below zero was registered. Here a great warm stratum of air was entered and penetrated farther than ever before in America, namely to the height of eleven miles, where the temperature was about 52 degrees below zero. One cubic foot of air at 32 degrees Fahrenheit, and under one atmosphere pressure, weighs about one and a quarter ounces; in other words, air is about 773 times lighter than water. Under a pressure of 39 atmospheres, and at a temperature of 220 degrees below zero, Fahrenheit, air is liquefied. Liquid air is very mobile and has a bluish tint. It boils at about 310 degrees below zero. In spite of its low temperature liquid air can be poured upon the hand without danger; it does not even feel cold. The liquid may be frozen at a somewhat lower temperature, under which condition it looks like ordinary ice. Liquid air boils vigorously and changes rapidly to a gas. This tendency to change to a gas makes the liquid available for such purposes as running a motor. As compared with ordinary fuels, one great advantage is the small weight of the liquid required to perform a unit of work, but the cost of producing the liquid is a serious drawback. Nearly all liquid and gaseous bodies become solid when cooled in liquid air, and chemical changes are retarded or arrested. Cartridges made of granular charcoal and cotton waste, when saturated with liquid air, have been used as an explosive in mining. The largest liquid air factory in the world is near Munich, at which place as high as fifty quarts an hour have been produced.-JULIUS HORTVET. See AIR-PUMP; AIRSHIP; ARGON; BALLOON; BAROMETER; CYCLONE; FROST; MARS; NITROGEN; OXYGEN; VENTILATION.