Gibbs free energies of mixing of methanol-water mixtures in the range of 35-65 °C and densities of the mixtures at 25 °C are measured. The enthalpies and free energies of mixing are thermodynamically consistent within the limits to which the gas imperfections are known. Earlier liquidvapor equilibrium measurements are reanalyzed, using only total pressure and liquid composition data, and with corrections for gas imperfections. Of the previously reported free energies of mixing, only the single set of transpiration measurements at 25 °C by Butler et al. is to any degree consistent with our data.

Vapour pressures and heats of mixing have been measured for mixtures of dioxan (A) and chloroform (B) at SO" C. The results have been satisfactorily interpreted by assuming the existence of the four species A, AB, A B 2 and B in mutual equilibrium and by further assuming that these four species mix ideally. Values are given for the equilibrium constants and heats of formation of AB and AB2.Several systems, such as ether + chloroform and acetone + chloroform, in which the formation of a complex of type AB is believed to occur through the formation of a hydrogen bond between the hydrogen of the chloroform and the oxygen of the other molecule, have been made the subjects of thermodynamic study.1-5 However, we know of no previous thermodynamic investigation of a liquid mixture in which any other type of complex formation would be expected.

The theory of the thermal boundary layer at the walls of a spherical acoustic resonator is discussed in detail. For gases at low pressures, the temperaturejump effect is found to make a significant contribution to the resonance frequencies of the radial modes but not to their acoustic losses. Experimental results are reported for argon at 273.16 K and pressures between 15 and 248 kPa, and compared with the theory. These were obtained using the four radial modes with lowest frequency of a spherical resonator with a radius of 60".The thermal accommodation coefficient between argon and the aluminium wall of the resonator was found to be (0.84 i 0.05). The results suggest that a determination of the gas constant with a fractional imprecision of 1 x IO-' or better should be possible using a spherical acoustic resonator.

The use of the term amount of substance of an elementary entity is expounded. Methods of measurement of the ratio of two amounts of substance are described. The SI unit of amount of substance, the mole, is introduced. Quantities involving amount of substance, including molar mass, molar quantities in general, the Avogadro constant, the molar gas constant, and the Faraday constant, are defined. An historical account is given of the notion of amount of substance and of the unit mole. Some personal views are advanced about the desirability of a new name for amount of substance and for derived quantities.

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