Measurements of the gas compressibility of methane were made in the region 0°to 350°, 16 to 400 atm., and 0.75 to 12.5 mole liter"1. From these results, values were derived for the compressibility factor Z = PV/RT; second, third, and fourth virial coefficients B, C, and D, respectively, in the equation PV = RT (1 + B/V + C/V2 + D/V3 + . . . ); and parameters of the Beattie-Bridgeman and Benedict-Webb-Rubin equations of state.

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~The methane-tetrafluoromethane system was examined for conformity with the principle of corresponding states in terms of the Boyle reference point, the critical reference point, and mixing rules as applied to the reference temperatures and volumes. Experimental values of gas density are given for pure methane, pure tetrafluoromethane, and three mixtures (0.25, 0.50, and 0.75 mole fraction of methane) over a temperature range from 0 to 350°, a pressure range from about 16 to 400 atm, and a density range from 0.75 to 12.5 mole L-'. Values are derived for the second, third, and fourth virial coefficients of the mixtures, B M , CM, and D M , respectively, and for the cross-term virial coeficients, BIZ, C112, C122, 01112, D1122, and D12z2. Based on the experimental results for BIZ, the geometrical-mean mixing rule was found to be unsatisfactory for a corresponding states correlation. However, in terms of the Boyle temperature, TB, and Boyle volume, VB = ( TdB/dT),T,, the experimental reduced second virial data for the methane-tetrafluoromethane system were found to follow the corresponding states function with high accuracy.

Thermodynamic, spectroscopic, and molecular structure information was used to show that internal rotation about an Si-O bond in hexamethyldisiloxane is free or nearly so. Thermodynamic functions for hexamethyldisiloxane in the ideal gas state (0 to 1500°K.) were calculated by methods of statistical mechanics. Experimental studies provided the following information: values of heat capacity for the solid (11°K. to the triple point), the liquid (triple point to 375°K.) and the vapor (363 to 500°K.); the triple point temperature; the heat of fusion; thermodynamic functions for the solid and liquid (0 to 375TÍ.); heat of vaporization (332 to 374°K.); parameters of the equation of state; and vapor pressure (309 to 412°K.). Thermodynamic functions also were calculated for the related substance tetramethylsilane.

Measurements of the gas compressibility of tetrafluoromethane were made in the region, 0–350°, 15–394 atm. From these results, values of the compressibility factor Z=PV/RT, the second, third, and fourth virial coefficients, and the parameters of the Beattie-Bridgeman and Benedict-Webb-Rubin equations of state were derived. The virial coefficients were correlated by the Lennard-Jones (12, 6), Lennard-Jones (28, 7), Stockmayer, and Kihara intermolecular potential energy functions, and numerical values for the molecular parameters were obtained. One of the first tests of the theoretical fourth virial coefficient of the Lennard-Jones (12, 6) potential was made using the experimental fourth virial coefficients of tetrafluoromethane.