to spend his vacation at the ancient city of St. Andrews, on the sea coast where there is a magnificent course for golf. On one occasion soon after these lectures were published both he and a Glasgow professor of theology, a metaphysician of the Hegelian school, were invited to a dinner in that city. Tait was very naturally drawn out to talk about the subjects on which he had been lecturing, and he did so largely and to the delight and edification of everyone except the Hegelian, who when he could stand it no longer, gravely put the question: "But, Mr. Tait, do you really mean to say that there is much value in such inquiries as you have been speaking about?" After that the subject was changed, and during the rest of the evening the mathematician and the metaphysician did little else than, as one of the company expressed it, "glour at each other."
We have seen that Tait attended the meeting of the British Association at Aberdeen in 1859; but he was not a frequent attendant, for he said that there was too much jabber and talk, and that he did not care for great "spreads." At one of the Edinburgh meetings (1871) he was president of the section of mathematics and physics, on which occasion he delivered an address on Hamilton's Calculus of Quaternions and Thomson's Principle of the Dissipation of Energy. When the Association met in Glasgow in 1876, he was requested on short notice to deliver one of the popular lectures. He took for his subject Force, he made a plea for the accurate use of terms in mechanical science, a reform which has progressed much since that time. He says that force, defined as the rate of change of momentum in a body is also the space-variation of potential energy. Another point he insisted on is that matter and energy are things—have objective existence, because their quantity in the universe is constant; force on the other hand cannot be a thing, or have objective existence, because its quantity is indeterminate. "It is only things," he said, "which can be sold." In view of this dictum it is interesting to observe that some courts have held that an electric current cannot be stolen, as it was not a thing. But what is stolen is the energy of the current, and according to Tait's ideas energy is a thing.
