Multilayer isotherms and heats are reported for helium adsorbed on an argon surface at 4.2deg;K. The data show that, on the uniform surface used in this work, the layer capacities and energies of the first two or three layers of adsorbed helium can be estimated directly from the heats without recourse to the isotherm data. The adsorption energies are shown to be in good agreement with theoretical estimates. Two alternate approaches to the theoretical calculation of layer capacity are presented: one of the methods involves a computation of the perturbation of the density of the bulk liquid due to the presence of the surface; the other calculation is based on a computation of the density dependence of the lateral interaction energy in a layer plus the surface interaction energy of the layer. The density of a completed layer can be estimated by equating the total energies per atom in successive layers. It is shown that the results of these calculations are in good agreement with each other, and with the experiments. The high coverage isotherm data are analyzed in terms of a modified form of the Frenkelhyphen;Halseyhyphen;Hill isotherm equation, and it is shown that the parameters required to fit the experiments to this equation are consistent with other estimates. Finally, the data are fitted to the B.E.T. equation, and to several of the modified B.E.T. isotherms which have been suggested as more realistic representations of helium adsorption data. It is concluded that no B.E.T.hyphen;type model is completely successful, but that the isotherm equation proposed by Steele gives the most accurate results.
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