Appreciation of the physical characteristics of pure substances and mixtures in the vicinity of the critical state is difficult to obtain without direct observation. Demonstration equipment is described that permits the semiquantitative observation of the changes in state encountered during changes in volume of heterogeneous and homogeneous systems along a defined path in the vicinity of the critical state. The apparatus was so arranged that the appearance of the system and semiquantitative values of specific volume, temperature, and pressure may be projected on a screen for observation.