GAS

one of the three fundamental states of matter, with distinctly different properties from the liquid and solid states. one of the three fundamental states of matter. It has distinctly different properties from the liquid and solid states. A gas has no definite shape and exhibits high fluidity. It tends to expand indefinitely and readily fills any container into which it is introduced. Gases are highly compressible, and under ordinary conditions they have a density approximately 1,000 times less than that of liquids. A small change in temperature or pressure generally produces a substantial change in the volume of a gas. The relationships between the temperature, pressure, and volume of gases have been deduced and expressed in the form of equations known as the gas laws (see Boyle's law; Charles's law; Avogadro's law). Gases were studied as early as antiquity, but an understanding of the gaseous state, as of the other basic states of matter, came only with the development of the kinetic molecular theory in the 19th century. According to this theory, all matter is composed of particles (atoms or molecules or mixtures of both) in constant motion. In a gas, the particles are far enough apart and are moving fast enough to escape each other's influence (e.g., attraction or repulsion due to electrical charges). The freely moving particles constantly collide with one another, but the collisions result in no loss of energy. When a gas is cooled, its particles move more slowly, and those that are slow enough to linger in each other's vicinity tend to coalesce because a force of attraction overcomes their lowered kinetic energyi.e., energy of motion. Each particle, when it joins the liquid state with others, gives up a measure of heat called the latent heat of liquefaction, but each continues to move at the same speed within the liquid so long as the temperature remains at the condensation point. Warming up a liquid, by contrast, provides constituent particles with heat of evaporation, which enables them to escape each other and form the vapour of the liquidnamely, the gaseous state. Additional reading The best elementary discussion of the kinetic theory of gases is T.G. Cowling, Molecules in Motion (1960), clearly explaining the fundamental physical ideas without excessive mathematical manipulation. Other elementary books include Joel H. Hildebrand, An Introduction to Molecular Kinetic Theory (1963); Sidney Golden, Elements of the Theory of Gases (1964); and Walter Kauzmann, Kinetic Theory of Gases (1966). Two excellent books make greater demands on the mathematical background of the reader: James Jeans, An Introduction to the Kinetic Theory of Gases (1940, reissued 1982), which is an abridged and slightly simplified version of the author's classic The Dynamical Theory of Gases, 4th ed. (1925, reissued 1954); and Richard D. Present, Kinetic Theory of Gases (1958), an excellent though selective textbook. The historical literature is especially rich; the following works may be profitably consulted: J.S. Rowlinson (ed.), J.D. van der Waals: On the Continuity of the Gaseous and Liquid States (1988), a translation of van der Waals's 1873 Dutch thesis with a marvelous extended introductory essay by the editor that is unique in the field and surveys modern developments in the theory of liquids and solutions; Stephen G. Brush (ed.), Kinetic Theory, 3 vol. (196572), a set of famous historical papers along with introductory commentaries and summaries by the editor; Robert Lindsay (ed.), Early Concepts of Energy in Atomic Physics (1979), selections from famous historical papersmany of them omitted from the previous worktogether with the editor's comments; and Stephen G. Brush, The Kind of Motion We Call Heat: A History of the Kinetic Theory of Gases in the 19th Century, 2 vol. (1976, reissued 1986), a thorough historical account without much mathematics, and Statistical Physics and the Atomic Theory of Matter: From Boyle and Newton to Landau and Onsager (1983), covering a much broader range and requiring a thorough scientific background. More advanced professional treatments include Sydney Chapman and T.G. Cowling, The Mathematical Theory of Non-uniform Gases, 3rd ed. (1970, reprinted 1990), the acknowledged classic on the modern kinetic theory of gases based on the Enskog-Chapman approach; Joseph O. Hirschfelder, Charles F. Curtiss, and R. Byron Bird, Molecular Theory of Gases and Liquids (1954, reissued with added notes 1964), a monumental compendium of detailed results on equations of state, transport properties of gases, and intermolecular forces, especially valuable as a reference; J.H. Ferziger and H.G. Kaper, Mathematical Theory of Transport Processes in Gases (1972), successfully combining the best features of the previous two works; Carlo Cercignani, The Boltzmann Equation and Its Applications (1988), by a mathematician, one of the few books that concerns itself with free-molecule gases and the transition to continuum behaviour; and Frederick R.W. McCourt et al., Nonequilibrium Phenomena in Polyatomic Gases, 2 vol. (199091), an account of the extension of the Enskog-Chapman theory to include truly molecular shape effects, including the effects of external electric and magnetic fields and of surface collisions. Special topics are addressed in Martin Knudsen, The Kinetic Theory of Gases, 3rd ed. (1950), a series of lectures from 1933 on rarefied gas phenomena, by one of the experimental pioneers in the subject; K.E. Grew and T.L. Ibbs, Thermal Diffusion in Gases (1952), a short monograph on one of the more intriguing special topics of the kinetic theory of gases; J.S. Rowlinson, The Perfect Gas (1963), emphasizing the internal mechanics of molecules as related to the calculation of the thermodynamic properties of gases by statistical mechanics; and E.A. Mason and T.H. Spurling, The Virial Equation of State (1969), emphasizing experimental measurements and the detailed connection with intermolecular forces. Edward A. Mason