Onsager, Lars, 1903-76, American physical chemist, b. Oslo, Ph.D. Yale, 1935. Onsager taught at Brown Univ. from 1928 to 1933 and was on the faculty at Yale from 1933 until his retirement in 1972. He received the Nobel Prize in Chemistry in 1964 for his work on the thermodynamics of irreversible chemical processes. Onsager demonstrated that reciprocal relations exist between such variables as temperature and pressure in irreversible chemical processes, and the mathematical expressions he derived to describe this behavior enabled a complete theoretical description of such processes. The Onsager reciprocal relations are now known collectively as the fourth law of thermodynamics.