PROF. H. C. UREY is known internationally for his great contributions to physical chemistry, and for his human approach to the problems of the social implications of science. His discovery of deuterium (heavy hydrogen) in water was recognized by the award of a Nobel Prize ; but most important of all, this work established his interest in the problem of isotope separation. His development of chemical exchange, distillation and other methods of separation of isotopes has enabled American firms to market such materials as N15 as commercial commodities. Urey was one of the first in the United States to recognize that atomic weapons could be made, and his influence was crucial in the early history of the American project. He was identified closely with development at Columbia University of the diffusion process for separation of the uranium isotopes, and he visited Great Britain in this connexion at the height of the War. The menace to civilization presented by the discovery of atomic explosives was a challenge to Urey's social conscience, and after the War he devoted himself wholeheartedly to the public dissemination of information designed to impress the problem on the people of America and the rest of the world. Since the War ended, Urey has joined the new Institute which has been established in the University of Chicago to carry out academic researches in nuclear physics.