Under certain conditions, interaction between positively charged metal ions in an alloy may be considered to be of minor importance as compared to interaction between conduction electrons and metal ions. Then it follows that the activity of a solute metal 2 dissolved in metal 1 will be increased by a third component if metals 2 and 3 change the valence electron/atom ratio in the same direction. Conversely, the activity of a solute metal 2 will be decreased by a third component if metals 2 and 3 change the valence electron/atom ratio in opposite directions. Reference is made to the change in the solubility of hydrogen in copper by zinc, tin, aluminum, platinum, and nickel as alloying elements, to the change in the vapor pressure of zinc dissolved in copper by aluminum and nickel, and to the change in the activity of alkali metals dissolved in mercury by thallium.In addition, an equation is derived with the aid of which one may calculate the change in the activity of metal 2 dissolved in metal 1 due to the addition of a solute metal 3 if the changes in the activity coefficients with composition for the binary systems 1–2 and 1–3 are known and interaction between positively charged metal ions can be disregarded. This relation has been confirmed for the systems Hg&sngbnd;Li&sngbnd;Tl, Hg&sngbnd;Na&sngbnd;Tl, Hg&sngbnd;K&sngbnd;Tl, and Hg&sngbnd;Na&sngbnd;K.
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