Critical Temperature

Critical Temperature is that temperature below which a substance may, and above which it cannot, be liquefied by pressure alone. This temperature for carbonic acid gas, for example, is 30.9° C.—i.e. below that temperature, the liquefaction of the substance may be easily effected if sufficient pressure be applied; but above it the substance cannot be liquefied, no matter how great be the pressure to which it is subjected. The discovery of the critical temperature by Dr Andrews in 1869 first gave the means of distinguishing between a true gas and a true vapour; for the former is a substance above, the latter one below, its critical temperature. When any substance is at this temperature it is in the critical state—i.e. its passage from liquid to gas, or vice versa, is one in which the two parts, liquid and gaseous, so merge into one another as to render them optically indistinguishable. The critical temperatures of oxygen, hydrogen, and nitrogen (formerly termed 'permanent' gases) are all extremely low, and hence liquefaction can only be effected (as it has already been) by the application of intense cold as well as considerable pressure. On the other hand, such vaporous bodies as sulphurous anhydride, alcohol vapour, ether vapour, &c. can, provided the pressure be sufficiently great, be liquefied at ordinary air temperatures, since these are much below their critical temperatures. See GAS, VAPOUR, STATES OF MATTER.