by a vortex, whirling round an axis passing through the point. Helmholtz investigated the properties of a vortex-ring such as skillful smokers emit. The whirling is round the core of the ring, and is associated with a progressive motion. Thomson replaced Rankine's vortex-atmosphere with Helmholtz's vortex-ring; and showed that the properties of the vortex-ring in a perfect fluid would account for the indestructibility, elasticity and difference in kind of the atoms. The simplest kind oi vortex is the unknotted ring. Suppose that one knot is put on the ring before the ends are tied; this will give the trefoil knot. It has three crossings, and was supposed to figure an essentially different kind of atom.
Professor Tait investigated all the essentially different forms up to nine crossings, and contributed his results to the Edinburgh Royal Society. "Clever," some said, "but what is the use of it." The application was obvious; to elaborate the vortex-ring theory of atoms. Since then, however, electrical investigations have thrown more light on the subject of the atoms, so that Lord Kelvin is for going back to Lucretius.
In the discharge of his duties as a teacher, Tait was a model to his colleagues. The lecture always began punctually at seven minutes past the hour, and did not end till the clock struck the next hour. Lecturing to undergraduate students he never obtruded his own researches, still less made them the subject of lectures; he had a conscientious desire to teach them thoroughly the appointed subjects. He was also punctual in his attendance at the laboratory. In the summer term he came about 11 o'clock, would discuss results and plans with the researchers, take up his own investigation, and generally leave about 2 o'clock. In those days the physical laboratory did not remain open for long hours—from 10 to 3. He had little liking for the general business of the University, and in later years he was to be found only in his lecture room, or laboratory at the University, in his library at home, or in the hall of the Edinburgh Royal Society. For many years he was general secretary, and did Herculean work for the Society. He never sought fellowship in other scientific societies, and the scientific
