James Alan McLennan Jr died on 18 November 2002 in Bethlehem, Pennsylvania. An emeritus professor of physics at Lehigh University, he had been actively involved in the life of the physics department and the world of physics until the last few weeks of his life.

Al was born in Decatur, Georgia, on 24 November 1924. He was raised in Birmingham, Alabama, and entered Auburn University in 1942. After serving in the US Army Air Corps as a bomber navigator stationed in Labrador from 1943 to 1946, he entered Harvard University to complete his university education and earned a BS in physics in 1948. He entered Lehigh, then a hotbed of physics staffed with physicists who had returned from war research efforts, and received his PhD in 1952 under Peter Havas. After spending the following year at the General Electric Co, where he carried out theoretical modeling studies of the neutron transport processes within various nuclear reactor fuel rods, Al returned to Lehigh as an instructor. He was promoted to professor in 1962 and served as chairman of the physics department from 1968 until 1978.

His earliest work was in general relativity, with Havas, on conformal invariance and conservation laws for relativistic wave equations. Al also made important early contributions to parity nonconservation and the theory of the neutrino. His interests then shifted to nonrelativistic statistical mechanics and the origins of hydrodynamics. In 1959 and 1960, he provided one of the first formal derivations of the nonlinear Navier–Stokes equations and associated Green–Kubo expressions for transport coefficients. His review article, published in Advances in Chemical Physics in 1963, remains a primary source for the formal theory of transport.

Al’s subsequent studies, which centered on describing the context of macroscopic equations and on methods to evaluate the formal expressions for transport coefficients, span physical systems: solids, gases, critical fluids, plasmas, and most recently, chemically reacting systems. His early period studies include, in 1966, his first proof of convergence for the Chapman–Enskog method to solve the Boltzmann equation. That same year, he demonstrated that it is always possible to construct a symmetric stress tensor for systems with internal angular momentum; that result was subsequently rediscovered by others in materials science and solid-state physics. With Guillaume DeVault in 1965, he developed the statistical mechanics of viscoelasticity for a stressed medium and the corresponding positivity conditions for entropy production in such systems. These are but a few examples of his wide-ranging contributions to non-equilibrium statistical mechanics.

Al’s work was characterized by sharp physical insight translated into careful mathematical analysis. Those characteristics are particularly well illustrated by the tight reasoning and its faithful mathematical presentation in his monograph, Introduction to Non-Equilibrium Statistical Mechanics (Prentice Hall, 1989). He continued his scientific work for most of the years following his retirement from Lehigh in 1991, when he became an emeritus professor of physics.

Al was a leader in the founding of the Sherman Fairchild Laboratory for Solid State Studies at Lehigh in the 1970s. He provided liaison with Lehigh physics alumni who had risen to senior executive positions of the Sherman Fairchild organizations. He oversaw the construction of the laboratory and presided over its first staffing.

With his wide disarming smile, Al was approachable and generous with his time to students and colleagues alike. He was a great sounding board. His uncanny ability to see the larger picture and to separate the inconsequential from the essential helped bring clarity to whatever issue was on the table. His comments, such as “hire the brightest regardless of specialty” and “physics is waged globally,” were often referred to as the “McLennan principles.” Al believed that the often-cited tension between teaching and research was ill-conceived: To him, research and teaching were a seamless continuum, one feeding the other.

Al remained an active bridge player and an avid gardener to the end, even after the loss of his wife, Betsy, five years before his death. His appreciation for music included operas and classical compositions; he sang in the men’s choir Bethlehem Mannechor in earlier years. His devotion to his two children and his grandchildren led to his working with the Boy Scouts for years. His two sabbatical leaves, one at the University of Brussels in 1960–61 and the second at the University of Zürich in 1966–67, enriched him and his family with lasting interest in sensible living and adventures of outdoors and skiing.

Al’s life will be an enduring testimony of what is really possible as an academic physicist. We take great satisfaction in having shared some part of our lives together as colleagues and friends.

James Alan McLennan Jr