The statistical treatment which was given previously to determine the variation of the heat of adsorption of polar molecules is extended to show how the variation of the dipole moment with the fraction of the surface covered can be taken into account. The equations have been used to determine the variation of the heat of adsorption of ammonia on a non-conducting surface. The contributions to the heat of adsorption due to the van der Waals and to the electrostatic forces are of the same order of magnitude and of opposite signs so that the resultant variation in the heat of adsorption is very much less than would be expected from a consideration of forces of one kind only. The contributions to the heat of adsorption, which are made by the electrostatic forces when the dipole moment is constant and equal to M0, when it is constant and equal to M1, and when its variation with the fraction of the surface covered is taken into account, are compared for the whole range of values of θ. It is shown that the mutual depolarizing action of the molecules reduces the magnitude of the total variation in the heat of adsorption by a considerable amount. The heat curve which is calculated from the equations given by the statistical analysis is compared with that which is obtained when it is assumed that the particles form a random distribution over the surface, and the effect of the clustering of the adsorbed particles on the surface on the variation of the heat of adsorption is determined.

The number of configurations of mixtures of dimer and single molecules, of trimer and single molecules and of trimer and dimer molecules is examined by using the Bethe technique when there may be some vacant sites. This condition provides a check on the internal consistency of the Bethe method through the integrability of the resulting partial differential equations.

Exchange energy between electrons moving in a conductor is assumed to be such that the mean contribution per electron is proportional to the product of the total current within a defined range and the speed of the electron in the direction of the total current.

The ideal resistance of a conductor can be neutralized by such an exchange energy at sufficiently low temperatures.

The surface of the conductor, and internal surfaces of discontinuity if sufficiently marked, are regarded as sites for two-dimensional resonance states in equilibrium with the three-dimensional resonance states in the metal.

The superconducting transition occurs when, at the critical temperature, the neutralization of the ideal resistance of the surface by-passes the residual resistance of the body of the metal.

At the same critical temperature, equilibrium conditions ensure that exactly the right number of electrons enter the surface states to give the whole metal an ideal diamagnetic susceptibility.

These equilibrium conditions are disturbed by a magnetic field, and the transition temperature is shown to depend on the magnetic field in the observed manner.

The intermediate state is explained as one in which the number of electrons in the superconducting surface states is constrained by a non-uniform magnetic field to remain at an intermediate value which is inadequate to produce ideal diamagnetism.

By regarding either the outer surface or internal surfaces of discontinuity as the only regions becoming perfectly conducting, the details concerning restoration of resistance by a current are well explained, as also are the observed size effects in thin films, and the phenomena of ‘non-ideal’ superconductors among the alloys.

Finally, the observed distribution of superconductors in the periodic table receives a satisfactory qualitative explanation.

A simple model of independent curled-up molecules is examined theoretically. The critical temperature is found to be of the right order of magnitude but does not increase sufficiently rapidly with the size of the molecule. The Onnes constant and the critical density show the discrepancies expected from the theoretical equation of state, and the former also has an incorrect dependence on molecular weight. In the critical region, therefore, the paraffins cannot be regarded as an assembly of independent molecules, curled up in the way expected at high temperatures and low densities.