# Dictionary of National Biography, 1885-1900/Stewart, Balfour

​STEWART, BALFOUR (1828–1887), physicist and meteorologist, born at Edinburgh on 1 Nov. 1828, was son of William Stewart, a tea merchant of Leith, and his wife Jane, daughter of the Rev. William Clouston, for sixty years minister of Stromness, Orkney. William Stewart belonged to the Stewarts of Brough, Orkney, who at one time owned the Fair Isle and other land. This property was subsequently left by a cousin of Balfour Stewart to charities, and formed ‘the Stewart Endowment,’ of which Sir Walter Scott was a trustee. According to family tradition, Scott took the characters of Minna and Brenda in the ‘Pirate’ from Jane Clouston and her sister. A brother, the Rev. Charles Clouston of Sandwick, Orkney, was a meteorologist. Balfour Stewart's grandmother belonged to the family of Balfours of Balfour.
A younger brother, William Clouston Stewart, well known in Scotland as an expert angler, was the author of the ‘Practical Angler,’ first published in 1857, and of other works on angling, and inventor of the ‘Stewart tackle’ (see Brit. Mus. Cat.)
Balfour Stewart went to school in Dundee, then for a short time to the university of St. Andrews, and then to Edinburgh, where he attended the class of James David Forbes [q. v.], the professor of natural philosophy, in 1845–6. On leaving the university at the age of 18, he entered the office of a cousin, James Balfour, a Leith merchant. He went to Australia on business about 1855, and his taste for physical science developed. His first two papers—‘On the Influence of Gravity on the Physical Condition of the Moon's Surface’ and ‘On the ​Adaptation of the Eye to the Rays which emanate from Bodies’—were contributed in 1855 to the Philosophical Society of Victoria (Transactions, i. 92, 95). On his return he gave up business, and in February 1856 joined the staff of Kew observatory as assistant observer to John Welsh [q. v.] In October 1856 he became assistant to his former teacher, Forbes, at Edinburgh. Stewart at this time also worked at pure mathematics with Professor Philip Kelland, and in 1856 wrote a paper on a theorem in the theory of numbers (Trans. Royal Society of Edinburgh, xxi. 407), his only contribution to mathematics, for which he then showed distinct aptitude (Tait). In 1857 he published an interesting paper on the relation between the density and composition of sulphuric acid solutions, deducing therefrom the existence of definite compounds of the acid and water (Proc. Royal Society of Edinburgh, iii. 482; a preliminary abstract appeared in 1855, Brit. Assoc. Report, pt. ii. p. 70). Mr. Spencer Umfreville Pickering and others have since employed Stewart's method of research.
It was under Forbes's influence that Stewart undertook the researches on radiant heat which form his most important contribution to physical science, and for which in 1868 he was awarded the Rumford medal by the Royal Society. Stewart extended the ‘theory of exchanges’ due to Pierre Prevost (1751–1839) of Geneva, and proved, in opposition to the view of Jean Baptiste Joseph Fourier (1768–1830), that radiation is not a surface phenomenon; that it depends on the thickness of the radiating body, and in general that at any given temperature ‘the absorption of a plate equals its radiation, and that for every description of heat’ (Trans. Royal Soc. of Edinburgh, xxi. 1 sqq., read 15 March 1858); and that thus ‘the streams of radiant heat crossing any point of an enclosure of uniform temperature are not altered by the interposition of a body, whether opaque or transparent.’ It is remarkable, since Forbes had proved the similarity of radiant heat to light, that Stewart did not at once extend his results to optics. He also found out later that, cæteris paribus, the internal radiation in different substances varies as the square of the refractive index (Brit. Assoc. Report, 1861, i. 107), correcting an erroneous statement made by himself previously. Meanwhile Gustav Robert Kirchhoff (1824–1887) arrived independently at results which included those of Stewart, and led to the explanation of the dark lines in the solar spectrum as due to the absorption by layers of the vapours of various elements, and to the foundation by himself and Robert Wihelm Bunsen of spectrum analysis—one of the greatest discoveries of the century (Berichte der preussischen Akad. der Wissenschaften, 11 Dec. 1859).
Stewart had been pushing on in the same direction, but more slowly. In 1860 he showed by experiments on tourmaline, of which the experimental arrangement was suggested by Professor (afterwards Sir George Gabriel) Stokes, that his law held good for polarised rays of light (Proc. Royal Soc. x. 503, read 22 May 1860). In the same year he also showed that red glass, when raised to a sufficiently high temperature to emit light on its own account, gives out greenish light, and similarly that a piece of platinum foil blackened appears, when so heated, brighter in the blackened part than elsewhere (ib. x. 385, read 7 Feb. 1860). In May 1861 (ib. x. 193) he wrote a paper on the theory of internal radiation in uniaxal crystals, which was developed in the same year by Stokes (ib. p. 537). By this time, however, Kirchhoff had practically exhausted the subject for the time being.
On 1 July 1859 Stewart had been appointed director of the Kew observatory in succession to John Welsh, and henceforward he devoted himself mainly to meteorology, and especially to the phenomena of terrestrial magnetism. In 1861 he was appointed additional examiner in mathematics at Edinburgh University for five years, and there made the acquaintance of his future collaborator, Professor Peter Guthrie Tait. In August and September 1859 there had been a great magnetic disturbance, accompanied by auroral displays and by marked changes in sun-spots; the analysis of the photographic records of the magnetic storm at Kew directed Stewart's attention to the subject. General Edward Sabine [q. v.] had previously shown a connection between the occurrence of sun-spots and magnetic disturbances. Stewart now put forward the view that auroræ, magnetic storms, and earth-currents are due to variations in a primary electric current in the sun (Phil. Trans. 1861, p. 423). In 1862 he was elected F.R.S., and in the same year he suggested that the ‘red prominences’ are really solar auroræ (Phil. Mag. [4] xxiv. 302). In 1863 he made a careful investigation of the increase of pressure of a given volume of air between 32° F. and 212° F., his result for this important constant agreeing closely with that of Victor Regnault (1810–1878). In 1866 he redetermined the density of mercury at 32° F. with great accuracy. As it could be shown that the law of radiation of Stewart and Kirchhoff does not hold for a moving body in an enclosure of constant temperature, he made, in conjunction with Professor Tait between 1865 and 1873, ​a number of experiments on the heating of a disc by rapid rotation in vacuo, the apparatus for which was designed by R. Beckley, engineer to the Kew observatory. The experiments have been discussed by James Clerk Maxwell [q. v.], Professor Ludwig Boltzmann, and others, but no adequate explanation of the heating effect has yet been given. In 1865–1868 Stewart published, in conjunction with Warren de la Rue [see Rue] and Benjamin Loewy, a long series of investigations on sunspots, the variation of which they attempted, though without decided success, to trace to changes in planetary configuration. Stewart showed, however, that the daily range of magnetic variation appeared to be connected with these changes. He spent much effort from this time until his death on the discovery of certain periodic inequalities in terrestrial and solar phenomena, and attempted to deduce causes for these inequalities; but these deductions, as Stewart knew, can only be regarded as valid when based on an extremely large number of observations (Schuster); and, together with William Dodgson, William Lant Carpenter, and other coadjutors, he spent a large amount of labour on the necessary calculations.
On 1 Jan. 1867 he was appointed secretary to the government meteorological committee, and in this and the following year he supervised the installation of meteorological stations all over the kingdom. He resigned the post in 1869. On 7 July 1870 Stewart was appointed professor of natural philosophy in the Owens College, Manchester, a post which he retained till his death. He continued, however, to act as superintendent of Kew observatory till 1871. In one of his journeys from London, in November 1870, he met with a railway accident in which his thigh was crushed, and for nine months lay ill at Harrow, in the course of which he passed from ‘vigorous activity … to a grey-headed old age,’ although his mental powers remained unimpaired.
In April 1875 was published anonymously a book called ‘The Unseen Universe’ by Stewart and his friend Professor Tait, in which the authors aimed at deducing from the combination of a number of theological postulates with current scientific doctrines the existence of the soul and of a transcendental universe. The book is written in a popular and picturesque style, and excited much attention, running through fourteen editions in thirteen years. The authors avowed their identity in the fourth edition (April 1876). Professor William Kingdon Clifford (1845–1879) made an attack on the book from the heterodox point of view in the ‘Fortnightly Review’ (June 1875), to which a reply was offered in the preface to the second edition. A sequel published in 1878 by the same authors, dedicated to the members of the Paradoxical Society, and entitled ‘Paradoxical Philosophy,’ portraying in dialogue form the conversion of a cynical and heterodox German mathematician to religious and social orthodoxy, proved less successful.
Stewart, who was a devoted and fervent churchman, was elected by a conference held at Lambeth Palace on 7 Jan. 1881 as member of a committee for promoting interchange of views between scientific men of orthodox views in religious matters. He was also one of the founders of the Society for Psychical Research, in whose investigations he took a deep interest; he made several short contributions to its proceedings, and was president of the society from 1885 till his death.
In February 1887 he was elected president of the Physical Society, and also of the Manchester Literary and Philosophical Society. He died on 19 Dec. 1887 of apoplexy, at Ballymagarvey, a small estate near Drogheda, which he had inherited and whither he had gone to spend his Christmas vacation. He married, on 8 Sept. 1863, Katharine, only daughter of Charles Stevens, a lawyer in London. Two sons and a daughter survived him. Stewart was a man of exceptionally modest, gentle, and kindly nature. A photograph of him is in the common-room of the Owens College.
According to the bibliography by Professor Schuster in the ‘Memoirs of the Manchester Literary and Philosophical Society,’ Stewart published about sixty-seven papers of his own composition exclusively. In collaboration with others, he published two papers with J. Brito Capello, astronomer at Lisbon, three with W. L. Carpenter, ten with De la Rue and B. Loewy, two with the Rev. Father Walter Sidgreaves of Stonyhurst, two with Professor Tait, four with William Dodgson, one with Morisabro Hiraoka, three with B. Loewy, one with Father Stephen Joseph Perry [q. v.] on the comparison of magnetic observations at Kew and Stonyhurst, and one with (Sir) Henry Enfield Roscoe. He also contributed various reports to the British Association.
In addition to the papers and books already mentioned, Stewart published a number of successful text-books, which are not only in general conscientious and accurate, but show considerable power of picturesque illustration. Their titles are:
* ‘Treatise on Heat,’ 1866; 3rd edit. 1866; 5th edit. 1888.
* ‘Lessons in Elementary Physics,’ 1870.
* ‘The Conservation of Energy,’ 1872, a popular ​exposition, translated into French, German (1875), and Czech (1885).
* ‘Lessons in … Practical Physics,’ in conjunction with Mr. William Haldane Gee, assistant lecturer in the Owens College, vol. i. 1885; vol. ii. 1887; ‘the most complete exposition of experimental methods in physics which has been written’ (Schuster).
* ‘Lessons in Practical Physics for Schools,’ 1888, also in conjunction with Mr. Gee. He also contributed an important article on ‘Terrestrial Magnetism’ to the ninth edition of the ‘Encyclopædia Britannica.’
In 1874 Stewart edited, jointly with his colleague, Professor Adolphus William Ward (later principal of the college), a series of ‘Essays and Addresses by Professors and Lecturers of the Owens College.’ He was joint-editor with Professors Thomas Henry Huxley (1825–1895) and (Sir) Henry Enfield Roscoe of a valuable series of science primers published by Messrs. Macmillan & Co., for which Stewart wrote the ‘Primer of Physics’ (1872).
[Besides the sources mentioned, see Manchester Guardian, 20 and 24 Dec. 1887; Proceedings of the Society for Psychical Research, i. 35, iii. 64, iv. 42, 262, v. 1; Thompson's The Owens College, passim; Men and Women of the Time, 12th edit. (from notes by Stewart); obituaries in Nature, xxxviii. 202, and Proc. Royal Society, xlvii. p. ix, by P. G. Tait; Memoirs of Manchester Literary and Philosophical Society [4] i. 253, and Monthly Notices of Royal Astron. Society, xlviii. 166, by Professor Arthur Schuster, F.R.S.; Proc. of the Physical Society for 1887–8, p. 10, see also p. 6; Proc. Roy. Soc. xlv. 85, xxxix. 37 et seq. (Hist. of the Kew Observatory by R. H. Scott, F.R.S.); Roscoe and Schuster's Spectrum Analysis, passim; Life and Letters of J. D. Forbes, pp. 367, 391 (a communication from Stewart); Brit. Mus. Cat.; private information from Mrs. Balfour Stewart (his widow) and Professor Schuster; Stewart's own works, and personal knowledge. Stewart published an historical account of the theory of exchanges (including spectrum analysis) in the Brit. Assoc. Report for 1861, i. 97 &c. Kirchhoff published in Poggendorff's Annalen for 1862 (vol. xviii.) an historical account of the history of spectrum analysis, containing a somewhat grudging estimate of Stewart's work.]
P. J. H.