JAMES DAVID FORBES, (1809-1868), successively pro-fessor of natural philosophy in the university of Edinburgh and principal of the United College in the university of St Andrews, was the fourth son of Sir William Forbes of Pitsligo, the lineal representative of the Forbeses of Mony-musk and Pitsligo. His mother was Wilhelmina Belches-Stuart, sole child and heiress of Sir John Stuart of Fetter-cairn. Their family consisted of two daughters and four sons, of whom James David, born at his father's town house, 86 George Street, Edinburgh, on the 20th April, 1809, was the youngest. Two years after his birth, the death of Lady Forbes took place. Sir William retired with his family from Edinburgh to Colinton, his country resi-dence, and thus it happened that, up to the age of sixteen, when he entered college, James Forbes was entirely a home-bred boy, and his only teacher the schoolmaster of the village. At an early age, however, he developed re-markable powers of self-education. Passionately fond of natural science, he stored his mind with all available knowledge of physics, constructed for himself astronomical instruments, and actually commenced a connected series of meteorological observations, which he kept up for many years. But these pursuits were carried on without the knowledge of his family; for Sir William had destined him for the bar, and Forbes loved his father too well to betray tastes and inclinations which might seem to point towards a different career. In November 1825 he entered the university of Edinburgh, and joined the classes of Latin and chemistry. He still continued his self-imposed studies, and at length forwarded anonymously to Sir David (then Dr) Brewster, who was at the time conducting the Edin-burgh Philosophical Journal, a paper on "The Apparent Number of the Stars." It was at once inserted, and further communications were requested from the anony-mous "A," The request was complied with, and during an extended tour through France, Germany, and Italy, in company with a large family party, Forbes contributed a number of papers on " The Physical Geography of the Bay of Naples," " The Horary Oscillations of the Baro-meter at Borne," and other subjects, all of which were inserted in the Journal. On his return to Scotland after a year's absence, he made himself known to Brewster as his unknown correspondent. Brewster, astonished at his extensive reading and remarkable powers of observa-tion, encouraged him in his scientific pursuits, and pro-posed him as a fellow of the; Royal Society of Edinburgh, to which, at the age of 19, he was elected. In the meantime he had re-entered college, where his subsequent career was a distinguished one. In the class of moral philosophy, taught by the celebrated Professor Wilson, he gained the highest honours, and in Sir John Leslie's natural philo-sophy class he twice carried off the gold medal. An event now occurred which changed the whole tenor of his life. His father died on the 24th of October 1828, and it became necessary that Forbes should seriously consider his future course of life. The choice lay between the bar and a scientific career, and after much consultation with his friends he chose the latter. He passed his law trials indeed, and put on his advocate's gown, but never wore it, for a competence given him by his father had rendered him independent. About this time Sir David Brewster was engaged in laying the foundations of the British Associa-tion. Forbes joined cordially in the work, and contributed much towards placing the Association on the basis it now occupies. Throughout the rest of his life he attended, with rare exceptions, all its meetings, often breaking off a foreign tour, and hastening back to do so. The presidency of the meeting held at Dundee in 1867 was offered to him, but ill health compelled him to decline it.
Immediately after his election as fellow of the Royal Society of London, in June 1832, Forbes started on an extensive scientific tour, but was suddenly recalled from Geneva by news of the death of Sir John Leslie, professor of natural philosophy at the university of Edinburgh. Forbes had left word that should this chair at any time become vacant, he desired to be put in nomination, and on his return found that this had actually been done. He also found himself, to his surprise and dismay, the rival of his old friend Sir David Brewster. But Forbes was already committed, and had received testimonials from Whewell, Airy, Peacock, Vernon Harcourt, Chalmers, and others, bearing high testimony to his attainments, and the immense results they promised. After a more than usually excited contest, into which politics to some extent entered, Forbes was elected, at the age of twenty-four, to a pro-fessorial chair of the highest distinction. The inaugural lecture with which he opened the session of 1833-34 was listened to with unusual interest. As a professor, he more than realized the high expectations which had been formed of his special capacity for the work. During the twenty-seven years of his professorship his lectures left no branch of natural philosophy untouched, and owing to his thorough acquaintance with the literature of the subject, each of them was a mass of condensed information,not, however, con-fined to old knowledge taken from books, but enriched from time to time by the results of original research, enlivened by many a happy illustration drawn from his travels and adventures, and rendered more effective by his clear ringing voice and graceful delivery. He soon gathered round him an enthusiastic body of hard-working students, and although the high standard he aimed at made him a somewhat strict disciplinarian, their personal interest in his researches and their pride in his success increased as years went on. Forbes took an interest in his students no less personal, and more than one acquaint-ance formed in the class-room ripened into a tender and affectionate friendship. But his energies were not confined to the work of his professorship; they soon made them-selves felt throughout all the machinery of the university. The system of examination for degreesalready for some time in operation at Oxford-was then much wanted at Edinburgh, where, owing to the absence of any sufficient test of proficiency, graduation was little valued and seldom sought for. Forbes became an energetic reformer in this direction, and to him is mainly owing the complete system of examination for degrees which, under his guidance as dean of the faculty of arts, greatly tended to raise the standard of Scottish education. Again, in 1841, there was much discussion in the senatus as to the disposal of a large sum of money bequeathed by General Beid for the foundation of a chair of music and for other purposes.
Forbes strongly opposed the idea of a musical professor-ship, urging that the money would be more usefully employed in providing retiring allowances for superan-nuated professors. But controversy ran high, litigation ensued, and after years of contention a part of the Beid bequest was assigned by the law courts to the chair of music. Next year another large sum, the Straton fund, had to be dealt with, and here again Forbes was instru-mental in inducing the senate to devote what was left of the Reid bequest, combined with the Straton fund, to the foundation of fellowships to be held by distinguished students after graduation. In these and other controversies the uncompromising energy with which he strove to carry out his views of what was right could not fail to bring him into collision with men whose opinions were as decided, and their wills as strong as his own. When two such unbend-ing natures as Sir William Hamilton and James Forbes came into contactas they did more than oncethe shock was a rough one ; but although they differed widely in opinions, each has left a permanent mark for good on the university.
It would be impossible to enter here into the subjects or method of Forbes's class lectures; nor is it necessary to give more than a brief notice of the important researches which shared with his class work the winter months. They were the subject of voluminous private correspond-ence with such men as Whewell, Brewster, Peacock, Airy, Faraday, Arago, Melloni, Cauchy, and many others, and their results were embodied in a long list of communi-cations to various scientific societies, ranging over a great variety of subjects.
For a paper on " The Transparency of the Atmosphere and the Laws of Extinction of the Sun's Rays passing through it," the Royal Society of London awarded him their Royal Medal. Another, on " The Selective Absorp-tion of the Sun's Light in passing through Steam," was one of the first steps in the direction of spectrum analysis. In 1832 he described the experiments by which he suc-ceeded in producing a spark by means of a natural magnet. He was the author of valuable memoirs on the thermal springs of the Pyrenees, the extinct volcanoes of the Vivarais (Ardeche), and the geology of the Cuchullin and Eildon hills. Soon after his elevation to the chair Forbes resumed his researches in heat, begun some years before. He commenced experiments with Melloni's thermo-multi-plier, measured the refractive index of rock salt with heat from various sources, luminous and non-luminous, and pursued a course of investigations which led him to his most brilliant discovery, the polarization of non-luminous heat, by transmission through tourmaline and thin mica plates, and by reflection from the latter. By employing mica for depolarization he succeeded in showing the double refraction of non-luminous heat, a fact of which this experi-ment remains the only proof. He also produced circularly polarized heat by two internal reflections, using Fresnel's rhombs made of rock salt. By these researches the identity of thermal and luminous radiations was finally established. In 1846 he made careful arrangements for the measurement of underground temperatures, and by sinking his thermo-meters in three different sets of surface materials, he obtained an absolute determination of the thermal conductivity of trap-tufa, sandstone, and pure loose sand. His experiments on the conductivity of metals occupied the closing years of his life. By a thoroughly original method he obtained quantitative measurements of the absolute thermal conduc-tivity 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.
Forbes generally devoted a part of each vacation to excursions with his family in various parts of Scotland, and to geological tours among his native mountains, in the course of which, at times in company with some scientific friend, more often alone, he traversed on foot the greater part of the Highlands. In 1856 he acquired a beautiful cottage near Pitlochrie in Perthshire, where the succeeding summers were chiefly spent. From 1835 to 1851 Forbes usually passed some months on the Continent, travelling, as he expressed it, not as an amusement, but as a serious occupation, with De Saussure before him as a model. And it may be almost doubted whether science did not profit as much by his summer travels as by his winter work.
His first tour was devoted to the geology of the Pyrenees and of the Vivaraisthe latter of which he afterwards revisited. In 1837, after a short sojourn at Bonn, for the purpose of study, he travelled throughout Germany and Austria, making frequent experiments on terrestrial mag-netism and other subjects. In 1839, among the crags and glaciers of Monte Viso, of the Pelvoux range, and of the Alps of Cogne, he learned to be an active and intrepid moun-taineer ; and during the intervals of his glacier investiga-tions he did good service in opening up Alpine districts before scarcely known to Englishmen, by a series of excur-sions extending over the whole of the Pennine chain.
But of all his journeys and discoveries, those made by him among the mountains of Savoy are most popularly known; and their results have associated his name for ever with the glaciers of the Alps. In 1840, while presiding over the physical section of the British Association at Glasgow, he met M. Agassiz of Neuchatelwho, after spending several years in the study of glacial phenomena, had just published his, Etudes sur les Glaciers,and agreed to visit Switzerland with him the following year. Accord-ingly, at the close of the session of 1840-41, Forbes joined Agassiz and his party at the well-known " Hotel des Neu-chatelois," a small hut on the moraine of the Lauter Aar Gletscher. After a fortnight spent in exploration and observation, in the course of which Forbes called attention for the first time to the veined structure of glacier ice, they ended the campaign by the passage of the Ober Aar Joch, and the ascent of the Jungfrau. This was Forbes's appren-ticeship in glacier observation,a field of inquiry then jointly occupied by two rival theories, the gravitation or sliding theory of De Saussure, and the dilatation theory of Charpentier. After a close analysis of these Forbes not only showed conclusive physical reasons for rejecting both, but pointed out the impossibility of forming a sound theory of glaciers until the internal structure of these anomalous bodies had been investigated, and the character and velocity of their motions ascertained.
These important data Forbes resolved to obtain for him-self, and the summer of 1841 found him at Chamounix, engaged in a close examination of the Mer de Glace. He attacked the problem for the first time as a question of pure physics, executed accurate measurements with instru-ments of precision, and at once established the fact that the motion of a glacier averages roughly a foot in 24 hours, and that such motion is continuous. As his observations went on, he successively discovered that tire centre of the glacier moves faster than the sides, and the surface than the ice vertically below it, that its velocity increases directly with the steepness of its bed, and that the motion of a portion of the glacier when. embayed behind a rock is greatly diminished, and the whole mass distorted, without any solution of continuity taking place. Convinced by these discoveries that the resemblance between a glacier and a river, already noticed by Captain Hall, Msgr. Bendu, and others, was more than a mere analogy, he deduced from the above mentioned data the following theory of glacier motion : A glacier is an imperfect fluid or a viscous body, which is urged down slopes of a certain inclination by the mutual pressure of its parts. In reply to the strongly urged objection that ice is by its nature a brittle solid, and not sensibly possessed of any viscous quality, he asserted that ice is only hard and crystalline at a temperature of 30° F. or lower, and that at 32° it is a plastic solid possessing a slight but perfectly sensible genuine molecular plasticity. When the urgency of the forces to which it is subjected causes this limit to be over-powered, a general bruise takes place, producing innumer-able internal fissures, with finite displacements in certain planes, and subsequent restoration of continuity by " time and cohesion"; hence the blue bands of the conchoidal veined structure. When the urgency is still greater the mass acts like a solid, and crevasses open. In the case of glacier ice an additional element of plasticity exists, for glacier ice is scarcely ever coherent. It is usually a congeries of irregular polyhedrons tightly wedged together, possessing a rude flexibility, and always traversed by an infinity of capillary fissures. Hence a glacier is really a compound of ice and water, more or less plastic according to its wetness and infiltration,a fact proved by its more rapid motion in summer when saturated with water, and its relative retar-dation in winter when the surface water remains frozen. Forbes's " viscous theory " was sharply criticized, but it is now universally accepted, subject to some modifications which do not affect the groundwork of the structure, and rather elucidate than disprove his conclusions. But besides scientific criticism, he was compelled to defend himselfwhich he did with complete successagainst in-sinuations, not only of plagiarism, but of an endeavour to suppress the rights of others to priority of discovery.
During the winter which followed this eventful summer, Forbes was busily engaged in writing his Travels through the Alps of Savoy, a charming contribution to Alpine literature, in which graphic descriptions of scenery and mountaineering are happily blended with accounts of hi3 scientific researches. Its last lines were written almost on the eve of his marriage with Alicia, eldest daughter of the late George Wauchope, Esq. They started on their mar-riage tour to Switzerland, but had only got so far as Bonn, when Forbes was attacked by a dangerous illness, and it was not until August that they reached the Alps. He was then so far restored as to be able to resume his obser-vations on the glaciers of Chamounix and Grindelwald, but as the winter approached he was pronounced unequal to the work of the following session at Edinburgh. His request for permission to appoint a substitute was readily granted, and he set out with Mrs Forbes for Italy. Naples and Rome were their headquarters until the end of May, and the summer months were passed at the Italian lakes and in Switzerland. Forbes returned to Edinburgh in the autumn of 1844, and carried on his work during the winter months, but his health had suffered a rude shock, from the effects of which he never entirely recovered, The succeeding summer was spent with his family in the Highlands of Scotland; but the vacation of 1846 found him again on the Mer de Glace, engaged in observations on the ablation of the surface of the glaciers, and the dif-ference of movement between the surface and the under-lying ice. His next and last visit to the Alps in 1850 was entirely occupied in putting the finishing touches to his survey of the Mer de Glace, which was for some years after its publication the only correct Alpine map in existence. At the close of the session of 1850-51 he started for Norway with the double purpose of observing a total eclipse of the sun visible at Bergen, and of comparing the phenomena of the arctic glaciers with those of the Alps. These objects were not attained without great fatigue and exposure; Forbes returned from Norway with health much impaired, and had scarcely recommenced his work when he was attacked by haemorrhage, the precursor of a long and dangerous illness. All through December he lay between life and death, but in January he was able to be removed to Clifton, which he made his headquarters for two years. During this enforced leisure he was con-stantly engaged in writing a very able " Dissertation on the Progress of Mathematical and Physical Science," for the pages of the Bncyclopcedia Britannica, and in preparing for the press a work on Norway and its Glaciers, similar in character to his Glaciers of the Alps, and scarcely yield-ing to it in picturesqueness of narrative or careful observa-tion. In the summer of 1853 he was called from Clifton to Oxford to receive the honorary degree of D.C.L. Dur-ing the three sessions of Forbes's absence, his class work had been conducted by Professor Kelland, but the session of 1854-1855 found him again at his post, and he worked with but little interruption from illness until 1859, assisted latterly by Dr Balfour Stewart. In that year the current of his life was turned into another channel. Owing to the translation of Sir David Brewster to the principalship of the Edinburgh university, the principalship of the United College of St Andrews became vacant; Forbes offered him-self as a candidate, and was successful. His commission bears date December 2, 1859, but he did not resign his professorship until the following April. The degree of LL.D. was then conferred on him by the university which his own labours had so greatly benefited and adorned.
His new post, although it relieved him of the fatigue of constant lecturing, was no sinecure. The Scottish University Commission were then in full session, and in their investigations of the affairs, financial and other, of his own college, and of the university, the responsibility of supplying information and suggesting reforms fell largely on him. A zealous reformer he always was, and he had not left behind him at Edinburgh that dauntless spirit of " Thorough " which saw only one straight road to a right principle, and pursued it, not without collisions, and often painful ones. The duties of his office were performed with scrupulous industry. His own college is indebted to him for a laborious examination and classification of its ancient charters. He found the collegiate church of St Salvator in a condition of tasteless neglect; he left it partially restored and greatly beautified. He occasionally lectured on glaciers, climate, heat, and others of his favourite subjects, and even strained his failing powers to complete his researches on the thermal conductivity of iron. For the latter, shortly before his death, the Keith medal was awarded to him.
In the spring of 1867 Forbes's health gradually declined, and towards the end of September he set out with his wife and three daughters to Cannes, and afterwards went on to Hyeres. His health now continued to improve until the middle of January, when he was again attacked by haemor-rhage brought on by fatigue. He never sat up again, and while in that painful state the presidency of the Royal Society of Edinburgh was offered to him and declined. It was evident that a summer at Hyeres was impossible, so he was conveyed, together with one of his daughters who was almost in the same sad condition as himself, to Clifton, where he lingered for eight months, awaiting his end with patient calmness, sustained by the deep and fervent religious faith which had characterized his boyhood, and which a life of active scientific research had strengthened rather than impaired. He died on the 31st of December 1868,
Forbes received the honorary degree of D.C.L. from Oxford, and that of LL. D. from the university of Edinburgh. The Royal Society of London awarded him their Rumford medal for his discovery of the polarization of heat, and their Royal medal for a paper on the influence of the atmosphere on the sun's rays. By the Royal Society of Edinburgh the Keith medal was thrice presented to him, and he filled the post of secretary to that body from 1840 till the failure of his health obliged him to resign it. In 1845 a pension of £200 a year was granted to him for his services rendered to science. He was fellow of the Royal Societies of London and Edinburgh, and of the Geological Society, corresponding member of the Imperial 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 Heidel-berg, 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.
A list of his scientific writings is to be found in the Royal Society Catalogue, but the following books may be mentioned as bearing more particularly on his glacier researches :Travels through the Alps of Savoy, Black, Edin., 1843, 1845 ; Norway audits Glaciers, Black, Edin., 1853 ; Occasional Papers on the Theory of Glaciers, Black, Edin., 1859. See also Theorie des Glaciers de la Savoie, par M. le Chanoine Rendu (translated by Alfred Wills, edited by Professor George Forbes), Macmillan, Lond., 1874.
Forbes's Life and Letters, by Principal Shairp of St Andrews, Prof. P. G. Tait, and A. Adams-Reilly, was published by Macmillan, London, 1873. (A. A. R.)