CRITICAL POINT. 588 CRITICISM. muHipIving the pressures given in tlie tablfi by 15); SCBSTANCE Critii-al temperature (centigrade) Critical ■ pressure (in atmos- pheres) According to : Acetic acid +321.6° , +232.8 + 37.1 -f243.G +130 + 31.1 —141 +146 +2118 +124 +194 +249,5 + 52.3 —234.5 + 36.4 — 81.8 —146 —113 +155.4 +370 +232.70 67.0 52.2 68 64.34 115 77 35 93.5 54.9 61. 7 35.65 39.66 86 20 73.07 64.9 36 60 78.9 195.5 72.85 Pawlewslii Sajoschowski Dewar Alcohol Ramsay & Young Amtnoiiia Carbonic acid.. Carbonic oxide Dewar Dewar Wroblewski Knietsch Chloroform Cyanogen Ether Denar Dewar Ramsay & Young Ethjl acetate. . Hydrochloric Nadejdin Hydrogen Laughinprdas.. Marsh Gas Dewar Janseen Olszewski Dewar Oxygen Dewar Sulphurous Water Strauss Wood Alcohol. Hannay Most of tliese figures must be regarded as correct approximately, for different investigators disagree as to their precise value. As to the critical volume, it must be remembered that when a liquid is ordinarily heated, it expand? 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 jioint of substances can bo 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 partlj' 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 jjres- sure 77 atmospheres, let it be required to trans- form continuously a given amount of the gas into liquid. To accomplish this wc may first heat the gas, say, to 35° C, raise the pressure, say, to 80 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 80 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. .t a temperature, say, of 18° C. this gas follows pretty closely the law of Boyle and JMariotte, i.e. unless the pressure is too great, the volume is inversely proportional to the pressure. But when the pressure attains 00 atmospheres partial liquefaction sets in, and then the inverse propor- tionality between pressure and volume is com- pletely destroyed; we might diminish the vol- >nne 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 woild show that there is a certain definite relation between the 'olume 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- otle. It would therefore seem that the liquid and gaseous states follow entirely different laws, separated from each other by the inteiwal 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 tem'pei'ature 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 cqiuil, 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 Waals, La continuite dcs etats gazcux et liquides (translation from Ger- man by Doimner and Pomey, Paris, 1894: Ger- man translation from original -Dutch, by Roth, Leipzig, 1881). See Gase.s, Gekeral Peo'pekties OF; Heat. CRITICISM (Fr. crlticisme, from Lat. criU- cus, Gk. kpitik6q, l-ritikos, critic, from Kpiveiv, krinein, to judge). Criticism, as the art of judg- ment, whether favorable or adverse, is applicable in all fields of human accomplishment, and all in- ventions, all institutions, all life are, broadly speaking, within its scope. It is, liowever. with literature and with art that criticism has most significantly busied it.self, with the result that the term has come to mean the interpretative study of these greatest expressions of man's na- ture. The Poetics of Aristotle has for centuries been regarded as the first important work of criticism, and .the rules there laid down have maintained their value to this day. Aristotle's manner of approach was the scientific method of induction, and his understanding of the funda- mental laws of human nature, his perception of those traits, emotions, and desires which, tran- scending any one age, belong to the men of all ages, underlaid and formed the firm basis of his criticism. Briefly sunmiarized. Aristotle's chief doctrines were that all art and literature should
