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Stewart, Balfour


(b. Edinburgh, Scotland, 1 November 1828; d. Drogheda, Ireland, 19 December 1887)

physics, meteorology, terrestrial magnetism.

Son of William Stewart, a tea merchant, and named Balfour after his grandmother’s family, Balfour Stewart was educated at the universities of St. Andrews and Edinburgh, and then embarked upon a mercantile career. His interest in the physical sciences had, however, been sparked in the natural philosophy class of James D. Forbes at Edinburgh; and after ten years in the business world, Stewart sought a career in science–first, briefly, as an assistant at the Kew observatory and then as an assistant to Forbes at Edinburgh, where he soon made original contributions to the study of radiant heat. In 1859 Stewart returned to Kew as director; and for twelve years he was involved with the continuing missions of that institution, including the study of meteorology, solar physics, and terrestrial magnetism. During this period was elected a fellow of the Royal Society of London; he married Katharine Stevens, daughter of a London lawyer; and he was awarded the Rumford Medal by the Royal Society for his earlier work on radiant heat. In 1870 Stewart was appointed professor of natural philosophy at Owens College, Manchester. He was subsequently president of the Manchester Literary and Philosophical Society, the Physical Society, and the Society for Psychical Research.

The heart of Stewart’s contributions to the study of radiant heat (infrared radiation), and to thermal radiation in general, came in “An Account of Some Experiments on Radiant Heat, Involving an Extension of Prévost Theory of Exchanges” (1858). Building in part on suggestions by Forbes and results of Macedonio Melloni and others, he had investigated the abilities of various materials to emit and absorb radiation of various wavelengths. Stewart found, in particular, that a material that radiates heat of a certain wavelength preferentially tends also to absorb heat of that same wavelength preferentially. He dealt with these results by extending a theory that had been developed by Pierre Prevost in the years around 1800. Considering thermal equilibrium to be dynamic, involving continual exchange of the heat substance between the bodies in equilibrium, Prevost had been able to derive relationships between the emissive and reflective powers of surfaces. Extending these arguments from surfaces to volumes, and specializing to individual wavelengths, Stewart derived from the “theory of exchanges” a result in accord with his experimental data on the emissive and absorptive powers of thin plates of various materials: “The absorption of a plate equals its radiation [emission], and that for every description [wavelength] of heat” (“Account,” Brace ed., p. 39).

Unfortunately, this original and significant work had little influence on the subsequent development of science. Two years after Stewart’s paper had been submitted to the Royal Society of Edinburgh, Gustav Kirchhoff, not knowing Stewart’s work and proceeding from experiments on optical spectra, came to similar conclusions. Kirchhoff’s results were more solid: the experiments were cleaner, the derivations more rigorous, and the results more clearly and generally stated. Moreover, the particular experimental context of Kirchhoff’s work suggested immediate and extremely important applications in chemistry and astronomy. Thus, although Stewart’s work was meritorious and prior, as he and his supporters vigorously argued for decades after, it was Kirchhoff’s work that had decisive influence throughout the physical sciences.

Upon becoming director of the Kew observatory, Stewart turned his attention to the continuing missions of that institution. (In 1867 Kew was designated the central meteorological observatory of Great Britain; and the annual operating grant from the British Association was then augmented by direct governmental support of the meteorological activities, bringing the yearly budget over £1,000.) Extending investigations of the relationships between terrestrial magnetism and the sunspot cycle, he also studied correlations of these phenomena with various other terrestrial and “cosmical” cycles. Later, in an influential review article on terrestrial magnetism (published in the Encyclopaedia Britannica), Stewart proposed mechanisms for some of these correlations. In particular, certain variations in the geomagnetic field were to be referred to varying electric currents in the upper atmosphere: Assuming that the rarefied air of the upper atmosphere possessed appreciable electrical conductivity, thermal and tidal air currents, moving the conducting air through the earth’s magnetic field, would generate electric currents. By this mechanism the well-known daily variation of the geomagnetic field was explained. Also explained were seasonal variations of the geomagnetic field, analogies between global wind patterns and global magnetic patterns, and lunar correlations. Stewart’s hypotheses informed subsequent research in geomagnetism, and strong confirmation ensued.

In later years at Owens College, Stewart lectured on elementary physics and established a teaching laboratory; J.J. Thomson, one of his most illustrious students, traced his initial enthusiasm for research to that laboratory. Textbooks and popularizations by Stewart were widely read; Ernest Rutherford, at age ten, owned one of them. Another widely read book, written in collaboration with Peter Guthrie Tait, was intended to demonstrate the compatibility between science and religion. Entitled The Unseen Universe, the book argued that the individual soul was immortal, existing after death in the context of the subtle media of nineteenth-century physics, including the luminiferous ether, the ultramundane (gravitation-causing) particles of Le Sage, and the ubiquitous fluid substratum of William Thomson’s vortex atoms; thermodynamics and evolution also supported the argument.

“Have great [physical] vitality, restless,” but “no power of great amount of [mental] work,” Stewart wrote of himself (Hilts, “Guide”). In the domain of science broadly defined, he participated in many institutions, programs, and discoveries but left no single monument.


I. Original Works. The following are representative of Stewart’s publications: “An Anccount of Some Experiments on Radiant Heat, Involving an Extension of Prévost’s Theory of Exchanges,” in Transactions of the Royal Society of Edinburgh, 22 (1857–1861), 1–20, repr. In D. B. Brace, ed., The Laws of Radiation and Absorption: Memoirs by Prévost, Stewart, Kirchhoff, and Kirchhoff and Bunsen (New York, 1901), 21–50; The Unseen Universe: or Physical Speculations on a FutureState (London, 1875), written with P. G. Tait; and “Terrestrial Magnetism.” an appendix to the article on meteorology in Encyclopaedia Britannica, 9th ed., XVI, 159–184.

II. Secondary Literature. A list of Stewart’s publications, as well as a review of his scientific career, is furnished in Arthur Schuster, “Memoir of the Late Professor Balfour Stewart, LL.D.,F.R.S.,” in Memoirs and Proceedings of the Manchester Literary and Philosophical Society, 4th ser., 1 (1888), 253–272. See also Philip J. Hartog, “Balfour Stewart,” in Dictionary of National Biography.

Aspects of Stewart’s activities are treated in the following: Joseph Agassi, “The Kirchhoff-Planck Radiation Law,” in Science, 156 (1967), 30–37; Sydney Chapman and Julius Bartels, Geomagnetism, II (Oxford, 1940), 750–752; P.M. Heimann, “The Unseen Universe: Physics and the Philosophy of Nature in Victorian Britain,” in British Journal for the History of Science, 6 (1972), 73–79; Victor L. Hilts, “A Guide to Francis Galton’s English Men of Science, in Transactions of the American Philosophical Society (in press): Hans Kangro, “Kirchhoff und die spektralanalytische Forschung,” editor’s Nachwort in Gustav Robert Kirchhoff, Untersuchungen über das Sonnenspectrum und die Spectren der chemischen Elements und weitere ergänzende Arbeiten aus den Jahren 1859–1862, Milliaria, no. 17 (Osnabrück, 1972), 17–26; Robert H. Scott, “The History of the Kew Observatory,” in Proceedings of the Royal Society, 39 (1885), 37–86; Daniel M. Siegel, “Balfour Stewart and Gustav Kirchhoff: Two Independent Approaches to ’Kirchhoff’s Radiation Law,’” in Isis (in press); and J.J. Thomson, Recollections and Reflections (New York, 1937), 18–22.

A large collection of unpublished source materials is held at the Royal Society.

Daniel M. Siegel

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Stewart, Balfour (1828-1887)

Stewart, Balfour (1828-1887)

Professor of natural philosophy at Owens College, Manchester, England, who received the Rumford Medal of the Royal Society for his discovery of the law of equality between the absorptive and radiative powers of bodies. He occupied the presidential chair of the Society for Psychical Research, London, from 1885 to 1887.

Stewart was born on November 1, 1828, in Edinburgh, Scotland. He was educated in Dundee and the Universities of St. Andrews and Edinburgh. He traveled to Australia, where he acquired a reputation as a physicist. After returning to Britain in 1856, he joined the staff of Kew Observatory, becoming a director in 1859. He also made important scientific contributions in mathematics and radiant heat.

Stewart was interested in the phenomena of the medium Daniel Douglas Home, of whom he commented to Sir William Crookes:

"Mr. Home possesses great electrobiological power by which he influences those present however susceptible the persons in the room to that assumed influence, it will hardly be contended that Mr. Home biologized the recording instrument."

Stewart coauthored with Professor Tait the anonymously published The Unseen Universe (1875), a book that created a stir as the first serious scientific attempt to establish a spiritual view of the universe to oppose the prevailing materialistic one. He died suddenly on December 19, 1887, of a cerebral hemorrhage.


Berger, Arthur S., and Joyce Berger. The Encyclopedia of Parapsychology and Psychical Research. New York: Paragon House, 1991.

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