An equation of state was developed which represented experimental methane P-V-T data over the temperature range of 130 to 625°K. for pressures to 10,000 Ib./sq. in. obs. with densities to 0.36 g./cc. For 598 experimental points the mean square percent deviation in the predicted pressures was 0.07% with only 10 points having deviations above 0.2%. The No attempt is made in this section to describe all of the equations of state that have been proposed. Literally hundreds of equations have appeared. Attention is restricted to those equations which have been most useful in describing the behavior of nonpolar gases. A few historical equations are also mentioned to emphasize the changes in the basic nature of the equation with time.Soon after it was discovered that real gases did not obey the perfect gas law, attempts were made to explain the deviations by simple modifications of the perfect gas expression. Modifications were proposed by van der Waals (32) in 1873, by Clausius (7) in 1880, by Berthelot (5) in 1889, by Dieterici ( 9 ) in 1899, and by Wohl (33) in 1914. These equations had two points in common; the number of constants were limited to two or three and the equations were inaccurate over any extended temperature range.With the advent of modern calculating machines, Beattie and Bridgeman (1) in 1928 proposed an equation of state having five adjustable constants. This was one of the first equations of state that did not demand that the isochores be linear. The pressure-temperature variation along a line of constant density was expressed asA, B, and C were then expressed as functions of density.The Beattie-Bridgeman equation fits experimental data for most gases at temperatures above the critical temperature but becomes unreliable in the region of the critical point or the saturation curve.Because the Beattie-Bridgeman equation was unable to represent phase behavior, Benedict and associates (2, 3, 4 ) developed a refinement to this equation. The new equation, referred to as the BWR equation, retained the same isochoric expression as proposed by Beattie and Bridgeman, but utilized eight constants in expressing the density dependence of A, B, and C. This equation described the volumetric behavior of pure substances and mixtures in the gas phase down to molal volumes on the order of the critical volume. When the coefficients were evaluated correctly, the equation predicted the molal volume at the bubble point but was not useful in describing the volumetric behavior of liquids. Nevertheless, the equation possessed many useful characteristics and it was the first expression which was successful in predicting the vapor pressure and thermodynamic properties of the coexisting After the publication of the BWR equation, no significant new equations of state appeared until 1955. This coincided with the emergence of digital computers as commercial calculating machines. Previous to this time the investigations had become as complex as was feasible with the computing tools available. With electronic digital computers it w...

on 0. However, in these experiments, approximately constant boil-up rates were maintained. To be completely quantitative, the contact angle should be determined at a specific vapor velocity. DISCUSSION An interesting phenomenon was observed while measuring the contact angles under conditions of mass transfer for the fluids under test. For all the positive systems studied, the sessile drop remained immobile, with no indication of interfacial turbulence occurring at any stage in the experimental procedure. However, for the negative systems, a considerable amount of agitation was evident in the droplet, which resulted at some concentrations in the production of droplets ejected from the main sessile body. This phenomenon explains why distillation efficiencies are often high for negative systems, although the contact angle would suggest a poor wetting performance and hence low transfer area. The behavior has an analogy in liquid-liquid extrac-

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