AINTAB, in-tab, Turkey in Asia, town in the villayet of Aleppo on the southern foothills of Mount Taurus and on the right bank of the river Sajur; 70 miles northeast of Aleppo. Its strong mediwval castle Hamtab is the seat of an important military station. There is an active trade in the tobacco, grain and fruits of the region, and in the local manufactures of cotton goods, striped cloths and pelanez con serve made from grapes; also in antiquities from ancient Doliche, the site of the cult of Baal, to the northwest. Aintab is the seat of American, Anglican and Franciscan missions. Pop. 45,000.
AIR, the gaseous substance that envelops the earth and forms its atmosphere. (See AT MOSPHERE). It consists almost entirely of the gases oxygen and nitrogen, which are merely mixed and not chemically combined; but in addition it contains many other substances in small amounts, among which are water-vapor, carbon dioxide, nitric acid, ammonia, ozone, argon, neon and organic matter, as well as dust, germs and other solid particles held in suspen sion. In certain localities other components may occur. Near the sea, for example, salt can always be detected in it, and over the land it contains sulphates in small amounts. The quantity of water-vapor present in air varies greatly with time and place, and in all analyses and statements of its composition the water vapor is supposed to have been removed first. The quantity of carbon dioxide is subject to considerable variation also. It is very constant in the open country, where it constitutes about 0.043 per cent (by weight) of the air; in cities the percentage is higher, rising to 0.07 and occasionally to 0.10. In crowded rooms, espe cially where artificial lights are burning, the quantity of carbon dioxide present may be even greater than this. In country air the percentage of carbon dioxide is subject to a diurnal change amounting to about one-eighth of its total amount, more being present at night than in the daytime. This is undoubtedly due to the fact that plants absorb the gas by day and exhale it during the night. The proportion of nitrogen and oxygen in air is subject to variation also, though within much narrower limits. In gen eral, 100 volumes of air contain about 21 vol umes of oxygen and 79 of nitrogen. Regnault analyzed air collected in different parts of the world, and found that the volume-percentage of oxygen in the air of Europe varied from 20.903 to 21.0 per cent. The average of 17 samples collected from over the Arctic seas gave 20.91 per cent. Regnault was of the opinion that sea air contains slightly less oxygen than land air; but Lewry considered that no distinct difference could be proved except in the tropics, where sea air exhibited a slight diurnal variation. Argon constitutes about 1 per cent of air, and neon about 0.001 per cent. The nitric acid present in the air is so small in amount that it can be detected only in rainwater, by which it is dis solved and brought down. It is very likely formed partly by the direct combination of oxygen and nitrogen under the influence of electric discharges, and partly by the action of ozone upon ammonia. The quantity present is greatest in summer and least in winter. The ammonia of the air occurs partly as carbonate and partly as nitrate. Its amount is exceedingly variable, ranging from 0.1 to 135.0 parts (cal culated as carbonate) in 1,000,000 parts of air, the average amount being perhaps 6. The amount present decreases during a heavy rain, but within a few hours it returns to the normal amount again. No ozone can be detected in city air, and air over marshes and in malarial regions contains very little of it. Normal country air contains not more than one volume of this gas to 700,000 of air. It is more abun dant in summer than in winter, and is most noticeable during thunderstorms and heavy winds. In the laboratory ozone is produced by the action of electric discharges upon oxygen, and it is probably produced in the air in the same way. Hydrogen peroxide has been de tected in the air, and some authorities consider that it may be present in greater abundance than ozone, and that it may sometimes be mis taken for ozone. (For further information on the composition of the air, see Angus Smith's (Air and Rain)). According to Regnault, one cubic centimetre of air that has been freed from water-vapor, carbon dioxide, and am monia, weighs 0.0012932 gramme when the air is at the temperature 0° C. and under a barometric pressure of 760 millimetres of mer cury at Paris (lat. 48° 50' N.), and at a height of 60 metres above the sea. In English equiva lents this means that at ordinary atmospheric pressure and at the temperature of melting ice (32° F.) a cubic foot of air weighs 0.080681 pound; atmospheric pressure,* signi fying the pressure that would be exerted by a weight of 14.7 pounds, resting upon a base one
inch square at sea level in the latitude of Wash ington. When a mass of air, originally at at mospheric pressure and at the freezing-point (32° F.), is heated to the boiling-point F.) without changing its volume, its pressure becomes 1.36728 atmospheres according to Bal four Steward, or 1.36706 according to Wiebe and Bottcher. The average of these is 1.36717, which agrees well with the value 1.36719 as given independently by Morley and Miller. The older estimates of Regnault and Magnus are probably too small. The specific heat of air (the pressure being kept constant) is 02375 according to Regnault, and 02389 according to Wiedemann. The specific heat (the volume being kept constant) is 0.1715 according to Jolt's direct measurement with the steam calori meter. Air cannot be liquefied by any pres sure whatever so long as its temperature is higher than about 220° F. below zero (-140°C.) ; but if it be first cooled to a temperature slightly below this it condenses to a liquid upon the application of a pressure of 39 atmospheres. (See CRITICAL POINT). If it be cooled to a temperature materially lower than 220° F. below zero, it can be liquefied by a correspond ingly smaller pressure. Liquid air is opalescent at first, probably from particles of solid carbon dioxide held in suspension. These can be sepa rated by filtration, or they will rise to the surface in a short time, leaving the clear, transparent air beneath. When liquid air is ex posed in a glass vessel it absorbs heat rapidly from surrounding objects, and boils actively until it has entirely evaporated. • The nitrogen that it contains evaporates faster than the oxygen, however, and the liquid remaining in the vessel becomes increasingly rich in oxygen until toward the last it consists almost entirely of that gas. Liquid air may be frozen to a clear, transparent solid by surrounding it with liquid oxygen and then forcing the evaporation by means of an air-pump. Liquid air is of great interest to the physicist for many reasons; but its importance in the arts has been grossly ex aggerated. In particular, the process that is put forth from time to time, to utilize liquid air for running a motor that shall condense more liquid air than it consumes, is impossible of realiza tion, because although such an action would not necessarily imply perptual motion it would violate the second law of thermodynamics. (See THERMODYNAMICS). If liquid air is confined and allowed to become warm through the ab sorption of heat from its surroundings its expansion gradually generates an enormous pressure. This fact, together with the safety with which liquid air can be handled, has led to its use to a limited degree for blasting in tunnels and mines, where the presence of the irrespirable products of combustion of ordi nary explosives is objectionable ; but even this application has been discontinued, owing to certain grave and apparently insuperable prac tical difficulties that were encountered. See LIQUID AIR.
The scientific study of the air has been much stimulated in recent years by the estab lishment of the Hodgkins Fund. In October 1891 Mr. Thomas George Hodgkins of Se tauket, N. Y., made a donation to the Smith sonian Institution, the income from a part of which was to be devoted to the °increase and diffusion of more exact knowledge in regard to the nature and properties of atmospheric air in connection with the welfare of man.* The first prize of $10,000 from this fund was awarded on 6 Aug. 1895 to Lord Rayleigh of London, and Prof. William Ramsay of University Col lege, London, for their discovery of the pre viously unknown element argon in the atmos phere. (See ARGON). A prize of $1,000 was also awarded at the same time to Dr. Henry de Varigny of Paris, for his 'L'Air et la Vie' ("Air and Life"), which was considered to be the best treatise upon atmospheric air, its prop erties and relationships. Further information concerning the Hodgkins Fund may be had from the Smithsonian Institution, Washington, D. C. For information concerning dust and germs, consult Tyndall's 'Fragments of Science,' and Dr. T. Mitchell Prudden's 'Dust and Its Dangers) Dephlogiaticated Air, in the old chemistry, was air that .had been deprived of phlogiston (q.v.) ; in modern terminology it is called oxy gen. Fixed air was Dr. Black's name for carbon dioxide, suggested by the fact that cer tain alkaline substances can °fix° this gas, or combine with it to produce a solid substance.
The word °air° also occurs as an element in a host of compound words. The significance of many of these is evident, but some few call for special mention and they will be found below in their respective order. See AERO DYNAMICS ; AEROSTATICS; AERONAUTICS; ATMOS PHERE; BAROMETER.