CRITICAL POINT. Experience shows that there is for every gas a certain temperature above which it cannot be liquefied, no matter how' great the pressure exerted upon it. Thus, above 31.1° C. (S7.9S° F.) it is impossible to liquefy carbonic-acid gas: water cannot exist in the liquid state above 370° C. (698° F.). etc. Such temperatures are termed the critical points or critical temperatures of substances. The vapo•-tension of a liquid at its critical tempera ture is termed the critical pressure, and the spe cific volume of the fluid at the critical tempera ture and under the critical pressure is termed the critical volume.
The following table gives the critical tempera tures and pressures for some of the more com mon substances (the critical pressures in terms of pounds per square inch may be obtained by multiplying the pressures given in the table by 15) : :Most of these figures must be regarded as correct approximately, for different investigators disagree as to their precise value. As to the critical volume, it must he remembered that when a liquid is ordinarily heated. it expands as the temperature rises, i.e. its density continually diminishes: at. the same time the density of its vapor continually increases; with rising tem perature, therefore, the densities of liquid and vapor tend to equalize, and finally, at the crit ical temperature, the densities become exactly equal. The surface of separation between liquid and vapor then disappears, and the substance as sumes the form of a perfectly homogeneous fluid.
The critical point of substances can be taken advantage of for passing from the gaseous to the liquid state of aggregation and conversely in a 'continuous' way, i.e. without having to deal, at any moment during the process, with a mass con sisting partly of liquid, partly of vapor, and hence having two different specific volumes. Thus, remembering that the critical temperature of carbonic acid is 31.1° C. and its critical pres sure 77 atmospheres, let it be required to trans form continuously a given amount of the gas into liquid. To accomplish this we may first heat the gas, say, to 35° C., raise the pressure, say, to SO atmospheres, and then. keeping the pressure unchanged, let the temperature fall, say, to 20° C.; we will then find the substance entirely liquid: for a sudden liquefaction of the entire mass will have taken place when, during the process of cooling. the temperature of 31.1° is reached: but at no moment will liquid have existed simultaneously with gas. Similarly, if it should be required to transform continuously a given amount of liquid carbonic acid into gas, we might proceed as follows: lower the tempera ture, say, to 20° C., raise the pressure, say, to SO atmospheres, and then allow the temperature to rise, say, to 35° C.; we would then find the
substance entirely gaseous, without, however, the mass having at any moment during the proc ess consisted partly of liquid, partly of gas.
Continuous changes like those just described have great importance in physical chemistry, be cause they permit of extending the laws of gases to liquids, and thus break down the barrier that long seemed to exist between the two states of aggregation. Consider, for example, carbonic acid gas without reference to its critical point. At a temperature, say, of 18° C. this gas follows pretty closely the law of Boyle and Alariolte, i.e. unless the pressure is too great, the volume is inversely proportional to the pressure. Bnt when the pressure attains GO atmospheres partial liquefaction sets in, and then the inverse propor tionality between pressure and volume is com pletely destroyed; we might diminish the vol ume by causing more vapor to turn to liquid, but as long as any vapor at all remains the pres sure would remain constant. if we should cause the substance to liquefy entirely, we would find that the pressure could again be raised and the volume of the liquid thus further diminished. Careful investigation would show that there is a certain definite relation between the volume of the liquid and the pressure exerted upon it, but the law expressing this relation would be seem ingly different from the law of Boyle and Mari (Ate. It would therefore seem that the liquid and gaseous states follow entirely different laws, separated from each other by the interval during which a substance is partly liquid, partly gas eous, and during which there is no connection at all between pressure and volume. But from what we said above, it may be seen that the change from gas to liquid, as well as the converse change, can be made to take place continuously, through the critical point, and that such a con tinuous process involves no interval during which the dependence of volume on pressure is destroyed; for when the critical temperature is reached during the continuous process, the sub stance is at one instant entirely gaseous and at the very next instant entirely liquid. The spe cific volumes of liquid and vapor at the critical point being equal, the sudden liquefaction in volves no change of volume, and hence the law governing the liquid must evidently form an im mediate continuation of the law governing the gas. Consult Van der Wallis, La eontinuW des (tots gazeux et liquides (translation from Ger man by Dommer and Pomey, Paris, 189-1; Ger man translation from original Dutch, by Both, Leipzig, 1SS1). See GASES, GENERAL PROPERTIES Or ; HEAT.