The so-called "Law of Equal Volumes" is a result of differences in bulk density and molar volume relationships between initial and final replacement reaction products. The replacement of calcite by whewellite, for example, proceeds in a kinetic system until the large positive volume change from calcite to whewellite begins to affect the permeability of the original calcite body. Eventually the flow of oxalate-containing solution may cease entirely and the resulting "whewellite replacement body" apparently represents volume replacement of calcite by whewellite. Replacement reactions and initial rates are determined by the activities of the system constituents in relation to their activity product ratios. Large positive or negative volume changes between initial and final products do not initially prevent or cause replacement reactions, but are a result of replacement reactions.