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The broad change, therefore, which has taken place since the mechanics of Newton is the advance from the consideration of masses to that of molecules of smaller and smaller size, and the truth of the former is not thereby invalidated. Newton, Descartes, Fresnel, Carnot, Joule, Mayer, Faraday, Helmholtz, Maxwell appear as one great succession of unifiers.
Our lecturer was both entertaining and instructive when he dealt with old time rations; but he naturally grew weak in approaching the physiological aspect of the question. He went through with it manfully and with a touch of humour much appreciated; whereas, for instance, he deduced facts from 'the equivalent of Mr. Joule, a gentleman whose statements he had no reason to doubt.
Joule, Clausius, and Maxwell, and no doubt Daniel Bernoulli himself, and I believe every one who has hitherto written or done anything very explicit in the kinetic theory of gases, has taken the mutual action of molecules in collision as repulsive. May it not after all be attractive?
When he addressed the British Association at Southampton, he had ventured to suggest two electrical units additional to those established at the Electrical Congress in 1881, viz.: the watt and the joule, in order to complete the chain of units connecting electrical with mechanical energy and with the unit quantity of heat.
It was in this way that he was led to the determination of the so-called mechanical equivalent of heat, shortly before the same discovery was made in a quite different manner by Joule.
And then, without pausing, we must shift yet again, this time to Germany, and consider the work of three other men, who independently were on the track of the same truth, and two of whom, it must be admitted, reached it earlier than either Joule or Colding, if neither brought it to quite so clear a demonstration. The names of these three Germans are Mohr, Mayer, and Helmholtz.
Professor Tyndall opens the question in his volume respecting the share which different investigators have had in establishing the new theory of forces, and his observations have given rise to a sharp controversy in the scientific journals. The point in dispute seems to have been the relative claims of an Englishman and a German Dr. Joule and Dr.
The conception of the law of conservation of energy arose from the discovery of the constant numerical relation between heat and mechanical work, known as the mechanical equivalent of heat. This discovery was made at about the same time by Joule in England and J. R. Mayer in Germany, although by entirely different routes. Joule, a brewer, was a man of practical bent.
First, that heat is a mode of motion was proved by Sir Humphry Davy and Count Rumford before 1820. In 1842 Joule, of Manchester, England, proved the quantitative relation between mechanical energy and heat. In 1863 note the dates Tyndall gave a course of lectures on heat as a mode of motion, and was even then sneered at by some scientific men for his temerity.
Joule and Lyon Playfair showed, in 1846, that metals in different allotropic states possess different atomic volumes, and Matthiessen, in 1860, was led to the view that in certain cases where metals are alloyed they pass into allotropic states, probably the most important generalization which has yet been made in connection with the molecular constitution of alloys.
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