STEWART, BALFOUR (1828–1887), Scottish physicist, was born in Edinburgh on the 1st of November 1828, and was educated at the university of that city. The son of a tea merchant, he was for some time engaged in business in Leith and in Australia, but, returning to his studies of physics at Edinburgh, he became assistant to J. D. Forbes in 1856. Forbes was especially interested in questions of heat, meteorology, and terrestrial magnetism, and it was to these that Stewart also mainly devoted himself. Radiant heat first claimed his attention, and by 1858 he had completed his first investigations into the subject. These yielded a remarkable extension of Pierre Prévost’s “Law of Exchanges,” and enabled him to establish the fact that radiation is not a surface phenomenon, but takes place throughout the interior of the radiating body, and that the radiative and absorptive powers of a substance must be equal, not only for the radiation as a whole, but also for every constituent of it. In recognition of this work he received in 1868 the Rumford medal of the Royal Society, into which he had been elected six years before, Of other papers in which he dealt with this and kindred branches of physics may be mentioned “Observations with a Rigid Spectroscope,” “Heating of a Disc by Rapid Motion in Vacuo,” “Thermal Equilibrium in an Enclosure Containing Matter in Visible Motion,” and “Internal Radiation in Uniaxal Crystals.” In 1859 he was appointed director of Kew Observatory, and there naturally became interested in problems of meteorology and terrestrial magnetism. In 1870, the year in which he was very seriously injured in a railway accident, he was elected professor of physics at Owens College, Manchester, and retained that chair until his death, which happened near Drogheda, in Ireland, on the 19th of December 1887. He was the author of several successful textbooks of science, and also of the article on “Terrestrial Magnetism” in the ninth edition of this Encyclopaedia. In conjunction with Professor P. G. Tait he wrote The Unseen Universe, at first published anonymously, which was intended to combat the common notion of the incompatibility of science and religion.