PRESSURE, VOLUME, TEMPERATURE RELATIONS OF ETHYLENE IN THE CRITICAL REGION. I.

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Abstract
Pressure isothermals of the system were determined both above and below the temperature of disappearance of the meniscus. Regions of constant pressure with changing volume were observed with all isothermals, in agreement with the prediction of Mayer and Harrison (2, 3). The complete envelope of this region above the critical temperature has not as yet been determined. The pressure of the heterogeneous system at 9.50 °C. was found to correspond within experimental error to the pressure of the flat portion of the homogeneous isothermal at the same temperature. The pressure of this heterogeneous system was found to be independent of the mass-volume ratio; this is in agreement with the experimental behaviour previously discovered by the authors (5) for such a system at 9.80 °C. Moreover, the pressure of the heterogeneous system was found to be identical with that of the system which had been heated at constant volume to a temperature at which the density difference in the tube is known to have been destroyed, and then cooled to the same temperature of 9.50 °C.The isothermal at 9.60 °C. was studied, and shown to be the equilibrium curve at that temperature. A hysteresis was observed on reversing the direction of measurement, i.e., from "vapour" to compressed "liquid". This hysteresis was found to be caused by the time lags which are observed in passing from the "vapour" region of an isothermal to regions of high density. Certain apparent discrepancies between the work of the present authors and that of Geddes and Maass have been observed and cannot as yet be explained.The phenomena observed are interpreted on the basis of a difference between the gaseous and liquid states of aggregation, with a structure assigned to the latter.

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