Recently Fowler (1936a) and Peierls (1936) have applied the methods of Bragg and Williams and of Bethe to the study of adsorption isotherms when there is a negative interaction energy (attractive force) between neighbouring adsorbed atoms. Their results give an explanation of the critical phenomena which are observed in the condensation of metal vapours on solid surfaces, in that below a certain critical temperature each theoretical isotherm has an unstable region where the fraction of surface

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... overed changes suddenly from a small to a large value. The purpose of the present paper is to study the form of the adsorption isotherm if there is a positive interaction energy between neighbouring adsorbed atoms, and to extend the statistical calculation of the isotherm and also the thermodynamic treatment by which an equation analogous to the Clausius-Clapeyron equation is deduced to the case of adsorption of diatomic molecules with dissociation.