On the determination of molecular fields. III.—From crystal measurements and kinetic theory data

Although in the two preceding papers it was not possible, either from viscosity measurements or from isotherm measurements, to arrive at conclusive information about molecular fields, it is shown in this paper how some finality can be reached by using as well X-ray measurements of crystals. All that was possible in the other investigations (at any rate in the case of Argon) was the determination of a series of molecular models, each of which satisfactorily explained the experimental facts. With this information it is now possible to make theoretical calculations of the interatomic distances in crystalline Argon, and as crystalline Argon has recently been obtained and its structure measured and examined, a comparison of theoretical and observed values serves to fix the molecular field of Argon. Thus here, for the first time, information derived by the methods of the kinetic theory is applied to explain interatomic distances in crystals. Attempts have already been made to determine atomic and ionic fields from crystal measurements alone, as they have from viscosity and compressibility results alone, but the results have not been wholly satisfactory. By regarding atoms as rigid spheres, Bragg and Wasastjerna have obtained an atomic “diameter” characteristic of each element. But the values so obtained are in conflict with those obtained by other methods on similar assumptions. As Rankine has observed, they are considerably less than those derived from viscosity measurements. Such representations of atoms are recognised as only rough and approximate and are used in lieu of something better.