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Critical Point

temperature, gas, pressure, °critical, liquid, liquefaction and called

CRITICAL POINT (or CRITICAL STATE), in physics a state or condition at which the physical properties of a given sub stance undergo some important and more or less hidden modification. The term is seldom ap plied to a state or condition at which an easily visible change (such as freezing) takes place, but is usually reserved, by common usage (though not of necessity nor by any formal ag:reement), for limiting conditions of a less obvious nature. For example, the temperature (ranging, for different samples of the metal, from 690° C. to 870° C.) at which iron ceases to be susceptible of magnetization, is called the °critical temperature' of iron with respect to magnetism. At or near this same temperature the iron also undergoes important changes in its thermal and electrical properties.

The term °critical point" is most familiarly applied to the state that a gas is in when any further rise in temperature would preclude the possibility of liquefying it by pressure alone. It was long known that reduction of tempera ture facilitates the liquefaction of a gas, but it was nevertheless believed that a sufficient pres sure would effect the liquefaction at any tem perature whatever. Dr. Thomas Andrews, in the Bakerian Lecture for 1869, entitled the Continuity of the Gaseous and Liquid States of Matter' (see

accompanying table gives some of the values of the critical constants of gases. The critical temperatures are given on the Centigrade scale, and the critical pressures in atmospheres.

One important and curious fact that follows from the existence of a critical point is that the gaseous and liquid states of a given substance may be regarded as continuous with each other, inasmuch as it is possible to cause a substance to pass from one of these states to the other by a continuous process, and without any abrupt change of condition such as is apparent when ordinary condensation takes place. For exam ple, if we heat a cubic foot of carbon dioxide gas up to C., we can then compress it all that we please without producing the least sign of liquefaction; because the critical temperature of this gas is 31° C., and hence liquefaction cannot be induced at any temperature higher than 31° C. Let us now compress it at this temperature until its pressure is (say) 150 at mospheres. It is still a gas, for the reason just given. Finally we cool the gas, still constantly maintaining its pressure at 150 atmospheres, until its temperature becomes 15° C. There can be no doubt that it has now become liquid, and in fact actual experiment proves this to be the case. If the temperature had been maintained at 15° C. throughout it would not have been possible to compress the gas into the liquid con dition without a visible, discontinuous passage from the one state to the other; but by the proc ess described above it is possible to convert the substance from the gaseous state into the liquid state in such a manner that the transition is imperceptible to the senses, and is not accom panied by any sudden change of density. (For further discussion of the theoretical principles involved in the consideration of the critical state, see MOLECULAR THEORY; THERMODY tuatics). Consult also Maxwell, 'Theory of Heat' ; Preston, 'Theory of Heat' ; and modern books on thermodynamics.