If two solvents in which any number of substances are dissolved are brought into contact with one another, then the concentrations slowly balance out by diffusion so that in the equilibrium state each kind of molecule has a constant division ratio between the individual solvents independently of the presence of other categories of molecules and of whether the category of molecules under consideration chemically reacts with the others or not. Riesenfeld and Zermelo 1909 presents solutions to the then unanswered question as to how great the concentrations are that occur at the dividing surface and how they change during the process of diffusion.