Lars Onsager (1903-1976), a Norwegian-born US physical chemist whose pioneering work in non-equilibrium thermodynamics earned him the 1968 Nobel Prize for Chemistry. Onsager also formulated a general mathematical expression about the behavior of nonreversible chemical processes that has been described as the “fourth law of thermodynamics.” He formulated the Onsager reciprocal relations, which express the equality of certain relations between flows and forces in thermodynamic systems out of equilibrium, but where a notion of local equilibrium exists.