Nearly one hundred years ago, T. Andrews deduced that for every gas, there should be a “critical temperature” above which the gas cannot be liquefied no matter how much it might be compressed. Further studies revealed that the critical temperature was one coordinate of a point on the P-V-T surface where differences between the intensive properties of the liquid and of the gas disappear (the P-V-T properties near the critical point are schematically diagrammed in Fig. 7.1). Since the time of Andrews, a tremendous body of literature has accumulated on the subject of critical-point phenomena, but progress in characterizing the critical point has been difficult because this point, much like the absolute zero of temperature, is approachable but not attainable.