# Dictionary of National Biography, 1885-1900/Forbes, James David

​FORBES, JAMES DAVID (1809–1868), man of science, youngest son of Sir William Forbes, seventh baronet of Pitsligo, and Williamina Belches, sole child and heiress of John Belches of Invermay, Perthshire, afterwards Sir John Belches Stuart of Fettercairn, Kincardineshire, was born at Edinburgh on 20 April 1809. His mother had been the first love of Sir Walter Scott. Forbes was educated at home until the age of sixteen, when he entered the university of Edinburgh, with a view to joining the bar. His natural bent, however, soon drew him to the study of physics, and at a very early age he contributed anonymously some able papers to Sir David Brewster's scientific periodical, the ‘Philosophical Journal.’ He avowed the authorship after a time, when Brewster encouraged his scientific zeal, and proposed him as a member of the Royal Society of Edinburgh. He was elected at the unprecedentedly early age of nineteen. Forbes now relinquished his legal studies, in opposition to Brewster's prudent advice. In the spring of 1831 Forbes visited London, Cambridge, and Oxford, where he formed friendships with Mrs. Somerville, Herschel, Babbage, Whewell, Lyell, Airy, and Buckland. The same year he co-operated with Brewster in the foundation of the British Association. In 1832 he was elected a fellow of the Royal Society of London. Forbes had started on an extensive scientific tour in the summer of 1832, when he was suddenly recalled from Geneva by news of the death of Sir John Leslie, professor of natural philosophy in the university of Edinburgh. Sir John Herschel, in a testimonial, spoke of him ‘as marked by nature for scientific distinction.’ His friend Brewster was his chief opponent, and a temporary coolness resulted. Forbes was elected, after a very exciting contest, by a majority of twenty-seven to nine, 30 Jan. 1833. He soon justified his selection. ‘In addition to high scientific genius,’ says Principal Shairp (Life of Forbes), ‘a finely cultivated literary taste and style, and natural powers of eloquence, perfected by the best aids of art’ (he took lessons in elocution from Mrs. Siddons), Forbes had ‘a dignified and commanding presence, and gentle and refined manners, wielded by a will of rare strength, purity, and elevation.’
In his lectures Forbes traversed the whole range of natural philosophy, but the manuscripts were by his orders destroyed by his executors. His discovery of the polarisation of heat soon indicated his genius as a scientific investigator. The professorial work achieved by Forbes included the institution of a complete system of examining, which is still in force. In 1837 Forbes was appointed dean of the Faculty of Arts, in special recognition of the part which he had taken in establishing the improved system. In 1841 and subsequently Forbes was very active in the discussions arising out of a bequest by General Reid. Forbes was anxious to devote this to a superannuation fund for professors. He afterwards induced the senatus to apply this and the Straton bequest of 1842 to the foundation of fellowships. It was finally decided, however, by the law courts that the Reid fund should be devoted to the music chair. He had some sharp encounters with opponents, especially with Sir William Hamilton, but without losing their respect or friendship. Forbes meanwhile continued his experiments, and carried on a correspondence with many of the most distinguished men of science of the day.
Forbes's vacations at this time (1840–2) were spent in Alpine travels and glacier investigations, which yielded scientific results of the first importance. He married Alicia, ​eldest daughter of George Wauchope, on 4 July 1843. In consequence of ill-health Forbes was compelled to spend the winter of 1843 and the summer of 1844 in Italy, returning to Edinburgh in September of the latter year. The summer of 1845 was spent with his wife in the west highlands, in a tour ranging from Bute to Skye. In the latter island he explored the Cuchullin mountains with M. Necker, finding ‘amidst the splendid hypersthene formation indisputable traces of glaciers.’ These explorations were afterwards embodied in a paper on the geology of the Cuchullins. In September 1845 a pension of 200l. per annum was conferred upon him for the services he had rendered to science. In 1846 he visited the Alps, and again for the last time in 1850. In 1850 he put the finishing touches to his survey of the Mer de Glace, which for some years was the only correct Alpine map in existence.
The last scientific expedition undertaken by Forbes was a journey to Norway at the close of the university session of 1850–1. He went to see the total eclipse of the sun, and to examine the Norwegian glaciers. The tour was a very fatiguing one, and Forbes returned home with his health greatly impaired. He began his classes in November 1851, but was attacked by hæmorrhage, which proved to be the precursor of a long and dangerous illness. In the succeeding January he moved from Edinburgh to Clifton, which was his headquarters for two years. His enforced leisure was employed in the composition of his ‘Dissertation on the Progress of Mathematical and Physical Science,’ principally from 1775 to 1850, for the eighth edition of the ‘Encyclopædia Britannica,’ and in preparing for the press a work on ‘Norway and its Glaciers,’ similar in character to his ‘Glaciers of the Alps.’ The university of Oxford conferred the honorary degree of D.C.L. on Forbes in June 1853. He resumed his class work in the session of 1854–5, and continued it, with but little interruption from illness, until 1859, being latterly assisted by Dr. Balfour Stewart. The foundation of the Alpine Club in 1858 was regarded by Forbes with keen interest, and he was elected an honorary member.
In 1859 Brewster resigned the principalship of the United College, St. Andrews, on becoming principal of Edinburgh University. Forbes offered himself for the vacancy, with the recommendation of Sir G. Cornewall Lewis, the Duke of Argyll, and Mr. Gladstone. He received the appointment on 2 Dec. 1859, and resigned his professorship at Edinburgh University in the following April, when he received the honorary degree of LL.D. The Scottish University Commission was sitting, and Forbes had to supply it with information and suggestions. He proved himself to be an able and a fearless reformer, and the college was also indebted to him for a laborious examination and classification of its ancient charters. The collegiate church of St. Salvator was in great part restored by his action. He gave lectures on glaciers, climate, heat, and the history of discovery, and endeavoured to complete his researches on the conductivity of iron. In consequence of continued weak health Forbes was obliged to decline the presidency of the British Association in 1864. From this time forward there was no recovery in his condition. The last public act he performed was to preside at the ceremonial of the laying of the foundation-stone of the new college hall at St. Andrews—a building which owed its existence entirely to his own exertions. By September 1867 he had to go to the Riviera for his health. His weakness obliged him to decline an offer of the presidency of the Royal Society of Edinburgh. In the summer he returned to Clifton, to be under the care of Dr. Symonds. He lingered for eight months, and died on 31 Dec. 1868.
Forbes, though somewhat cold in manner, united to a very sensitive nature a high moral courage, while his domestic affections were unusually warm. He was methodical and persevering, and his cousin, Bishop Forbes, says that his ‘sense of right amounted to chivalry.’ He was a strict disciplinarian, and somewhat over-sensitive about his claims to scientific reputation (Life, p. 467), but he was universally respected, and was beloved by his intimate friends. He left a great mass of correspondence, which is said to be of much interest, but too much concerned with personal controversy to be published at present. He was an attached member of the episcopal church of Scotland. Forbes had two sons, Edward Batton and George, and three daughters, one of whom died before, and the others soon after him.
An original experimenter upon heat, Forbes, beginning with Melloni's thermo-multiplier, measured the refractive index of rock-salt with heat from various sources, luminous and non-luminous, and was led in early life to his most brilliant discovery, viz. the polarisation of heat, by transmission through tourmaline and thin mica plates, and by reflection from the latter. ‘By employing mica for depolarisation, he succeeded in showing the double refraction of non-luminous heat—a fact of which this experiment remains the only proof. He also produced circularly polarised heat of two internal reflections, using Fresnel's rhombs made of rock-salt. He thus esta​blished by these researches the identity of thermal and luminous radiations.’ Professor P. G. Tait, in his survey of the scientific work of Forbes, observes that his ‘discovery of the polarisation of heat will certainly form an epoch in the history of natural philosophy.’ At a later stage Forbes determined the thermal conductivity of trap-tufa sandstone and pure loose sand, and finally obtained quantitative measurements of the absolute thermal conductivity of iron at various temperatures, and showed that this is diminished (contrary to the assumption of Fourier) by increase of temperature, thus following the known laws of electrical conductivity.
But Forbes is equally well known by his glacier theory, which he summed up in the statement that ‘a glacier is an imperfect fluid or viscous body which is urged down slopes of a certain inclination by the mutual pressure of its parts.’ The analogy between glaciers and viscous bodies had been vaguely noticed by previous observers, such as Bordier (1773), Basil Hall, and especially Bishop Rendu of Annécy. Forbes was undoubtedly the first to obtain accurate measurements, and to establish a definite base for future theories. He was, as Professor Tait says (ib. p. 511), ‘the Copernicus or Kepler of this science.’ He announced facts, though he did not properly give a physical theory. The facts were sufficient to explode the so-called gravitation and dilatation theories previously current, and they have been partly explained by theories of W. Hopkins, Faraday, James and Sir William Thomson, and Professor Tyndall. Forbes's substantial originality is unquestionable, and Professor Tyndall says that his book was ‘worth all other books on the subject taken together.’ Some unfortunate discussions arose as to his relations to other inquirers. His first observations were made during a visit to Agassiz's hut on the lower Aar glacier in 1841. Forbes claimed to have been the first to notice the ‘veined structure’ in glaciers, and it seems that he was certainly the first to recognise its importance and publish an account of it. Professor Guyot of Neufchatel had noticed it previously, but his notes remained in manuscript. Agassiz had also apparently seen it, but without attaching importance to it. Two honourable men were alienated by the discussions arising out of this, and by an alleged want of recognition on Forbes's part of Agassiz's previous work. Professor Tyndall, in his ‘Glaciers of the Alps’ (1860), gave an account of Rendu's speculations, which Forbes and his friends considered to attribute too much to the earlier inquirer. Forbes wrote a ‘reply,’ now appended to his ‘Life.’ He had certainly himself called attention to Rendu's work in his first book, and Rendu afterwards wrote to him in the friendliest terms, showing no sense of injury. He must be acquitted of any intentional unfairness, and may fairly claim to have founded the scientific study of the phenomena. Full information may be found in Forbes's ‘Life’ and in the papers there referred to, with which should be compared Professor Tyndall's ‘Principal Forbes and his Biographers’ (1873). Forbes's chief work, ‘Travels through the Alps of Savoy and other parts of the Pennine Chain, with Observations on the Phenomena of Glaciers,’ appeared in 1843. It is the most charming, as well as most scientifically important of all books of Alpine travel. A list of 149 publications of various kinds, chiefly papers in the ‘Proceedings’ of the Royal Society of Edinburgh, is appended to his ‘Life,’ besides which he contributed articles to the ‘Quarterly,’ ‘Edinburgh,’ and other reviews upon scientific subjects.
The Royal Society of London awarded to Forbes the Rumford medal for his discovery of the polarisation of heat, and the royal medal for a paper on the influence of the atmosphere on the sun's rays. The Keith medal of the Royal Society of Edinburgh was thrice presented to him, and he occupied the post of secretary to that society from 1840 till the failure of his health in 1851. Besides being a fellow of the Royal Societies of London and Edinburgh, and of the Geological Society, he was corresponding member of the Institute of France, and associate or honorary member of the Bavarian Academy of Sciences, of the Academy of Palermo, of the Dutch Society of Sciences (Haarlem), of the Helvetic Society, of the Pontifical Society, of the Pontifical Academy of Nuovi Lincei at Rome, and of the Natural History Societies of Heidelberg, Geneva, and Vaud; and honorary member of the Royal Medical Society of Edinburgh, of the Cambridge, Yorkshire, St. Andrews, and Isle of Wight Philosophical Societies, and of the Plymouth and Bristol Institutions.
G. B. S.