The surface chemistry of naturally occurring solids was initiated in its modern sense by soil chemists about one hundred five years ago. Jakob Maarten van Bemmelen, in his studies on soils sampled from polders in The Netherlands, published the first solute adsorption isotherm, proposed the first adsorption isotherm equation (now referred to as the Freundlich isotherm), and distilled from his results that the adsorptive powers of ordinary soils depend on the colloidal silicates, humus, silica, and iron oxides they contain. This profound first effort established one of the two great leitmotivs in the surface chemistry of soils: the use of adsorption isotherm equations to characterize equilibria between constituents of the soil solution and the solid phases in soil. The second great theme was developed thirty-five years later in the papers of Robert Gans (Ganssen),
who suggested that ion exchange reactions in soils could be described by a mass-action coefficient (conditional equilibrium constant). Once the con- cept of thermodynamic activity was developed, the rigorous extension of the ion exchange mass-action coefficient to an exchange equilibrium constant became possible. This last foundational step was taken by Albert P. Vanselow, a student of G. N. Lewis.