An empirical rule suggested by Paul Walden (1863–1957) in 1906; it concerns ions in solutions, stating that the product of the molar conductivity, Λ‎_m, and the viscosity, η‎, is approximately constant for the same ions in different solvents. Some justification for Walden’s rule is provided by the proportional relationship between Λ‎_m and the diffusion coefficient, D; as D is inversely proportional to the viscosity, Λ‎_m is inversely proportional to η‎, which is in accordance with Walden’s rule. However, different solvents hydrate the same ions differently, so that both the radius and the viscosity change when the solvent is changed. It is this fact that limits the validity of the rule.