The critical regions of ethane, carbon dioxide, and xenon have been studied, using a Schlieren optical system. The material to be studied was confined in a thermostatted high pressure cell having optically flat windows, and placed in the test section of the optical system. At temperatures over a range of about two degrees, in the critical region, refractive index gradients within the cell were determined as a function of height along the cell. From these data were calculated the density distribution, the pressure distribution, and the shapes of the pressurehyphen;volume isotherms at the various temperatures. The technique permitted detailed study of the isotherms over a narrow range of pressures, too small to be observed by usual experimental methods. Extensive visual observations were made throughout the course of the experiments. Particular attention was devoted to the question of the equilibrium state in the critical region and the process of equilibrium attainment.
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