twenty-five to electric measuring instruments, one to the electrolytic production of alkali, and two to valves for fluids. Helmholtz, visiting Thomson in 1884, found him absorbed in regulators and measuring apparatus for electric lighting and electric railways. 'On the whole,' Helmholtz wrote, 'I have an impression that Sir William might do better than apply his eminent sagacity to industrial undertakings ; his instruments appear to me too subtle to be put into the hands of uninstructed workmen and officials. . . . He is simultaneously revolving deep theoretical projects in his mind, but has no leisure to work them out quietly.' But he shortly added ' I did Thomson an injustice in supposing him to be wholly immersed in technical work ; he was full of speculations as to the original properties of bodies, some of which were very difficult to follow ; and, as you know, he will not stop for meals or any other
consideration.'
Thomson's teaching was always characterised by a peculiar fondness for illustrating recondite notions by models. The habit was possibly derived from Faraday ; but he developed it beyond precedent. 'I never satisfy myself,' he wrote,. 'until I can make a mechanical model of a thing. If I can make a mechanical model, I can understand it. As long as I cannot make a mechanical model all the way through I cannot understand it.' He built up chains of spinning gyrostats to show how the rigidity derived from the inertia of rotation might illustrate the property of elasticity. The vortex-atom presented a dynamical picture of an ideal material system. He strung together little balls and beads with sticks and elastic bands to demonstrate crystalline dynamics. Throughout all his mathematical speculation his grip of the physical reality never left him, and he associated every mathematical process with a physical significance.
In 1893 Lord Kelvin astonished the audience at the Royal Institution by a discourse on 'Isoperimetrical Problems,' endeavouring to give a popular account of the mathematical process of determining a maximum or minimum, which he illustrated by Dido's task of cutting an ox- hide into strips so as to enclose the largest piece of ground ; by Horatius Codes' prize of the largest plot that a team of oxen could plough in a day ; and by the problem of running the shortest railway fine between two given points over uneven country. On another occasion he entertained the Royal Society with a discourse on the 'Homogeneous Partitioning of Space,' in which the fundamental packing of atoms was geometrically treated, and he incidentally propounded the theory of the designing of wall-paper patterns.
In 1884 Thomson delivered at Baltimore twenty lectures 'On Molecular Dynamics and the Wave Theory of Light.' His hearers, mostly accomplished teachers and professors, numbered twenty-six. The lectures, reported verbatim at the time, were issued with many revisions and additions in 1904. They show Thomson's speculative genius in full energy and brilliance. Ranging from the most recondite problems of optics to speculations on crystal rigidity, the tactics of molecules and the size of atoms, they almost embody a new conception of the ultimate dynamics of physical nature. Thomson accepted little external guidance. He never accepted Maxwell's classical generalisation that the waves of fight were essentially electro-magnetic displacements in the ether, although in 1888 he gave a nominal adhesion to the theory, and in his preface in 1893 to Hertz's 'Electric Waves,' he used the phrase 'the electromagnetic theory of fight, or the undulatory theory of magnetic disturbance.' But later he withdrew his adhesion, preferring to think of things in his own way. Yet to the last he took an intense interest in the most recent discoveries. He discussed the new conception of electrons — or 'electrions,' as he called them — and read again and again Mr. Ernest Rutherford's book on 'Radioactivity' (1904). He objected, however, in toto to the notion that the atom was capable of division or disintegration. In 1903, in a paper called ' Æpinus Atomized,' he reconsidered the views of Æpinus and Father Boscovich from the newest standpoint, modifying the theory of Æpinus to suit the notion of 'electrions.'
Honours fell thickly on Thomson in his later life. He was thrice offered and thrice declined the Cavendish professorship of physics at Cambridge. He had been made a fellow of the Royal Society in 1851, and in 1883 had been awarded the Copley medal. He was president from 1890 to 1894. He was raised to the peerage in 1892 under the style and title of Baron Kelvin of Largs in the county of Ayr. On 15-17 June 1896 the jubilee of his Glasgow professorship was impressively celebrated by both the town and university m the presence of guests who included the chief men of science of the world. He resigned his professorship in 1899. He was one of the original members
