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Equations are derived for the variation of potential with distance and for the components of charge and capacity in the diffuse double layer at a plane interface for electrolytes of the 1:2 and 2:1 valence types. Some additional relations are also deduced for other valence types.

The theory of the diffuse double layer is developed for the case of a dielectric of variable dielectric constant. Using experimental data by Malsch, calculations have been made of the properties of the diffuse double layer. It is found that those properties which are experimentally significant are almost unaffected by the onset of dielectric saturation. The Born equation for the free energy of hydration of ions is elaborated to include the effects of dielectric saturation, and it is found that these effects are negligible in the case of the alkali halide ions.

The rate of change with concentration of the capacity of the electrical double layer on mercury has been measured. From this there have been calculated Γ+, the excess of cations in the double layer, and Γ—, the excess of anions. The latter has been subdivided into η—d, the excess of anions in the diffuse double layer, and η—i, the excess of anions in the inner part of the double layer. From these data it has also been possible to evaluate ψ0, the potential of the outer Helmholtz plane. It is found that fluoride ion remains unadsorbed on mercury at all potentials investigated, including potentials anodic to the e.c.max. Other anions are strongly adsorbed (chemisorbed) when the mercury is positively charged, and sometimes even when it is negatively charged. The ``hump'' in the capacity curves of salts of such anions as chloride, bromide, acetate, and nitrate, are found to be present in curves of C—, the capacity attributable to anions alone. No chemisorption of monatomic cations could be detected. The kinetic theory of the diffuse double layer with constant dielectric constant is found to fit the experimental results within the expected accuracy. This fit extends to electrolytes of the 1:2 and 2:1 types also. A new method of evaluating salt adsorption at the potential of the electrocapillary maximum (e.c.max.) is described and results are given. Likewise a new method of determining C+ at the e.c.max. is described and used.

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