1850-60] ROYAL ASTRONOMICAL SOCIETY in matical physicists. In the early years of the period Faraday was still pouring out the Experimental Researches in Electricity papers, which for clearness and charm of style, acuteness of insight,fand fertility in experiment will always remain classics, and should be read by every young scientific aspirant whatever branch of science he intends to follow. The Philosophical Transactions for 1851 contain no less than four of these memoirs out of a total of twelve papers. The last number appeared in the volume for 1856, and in the next year his Bakerian Lecture, Experimental Relation of Gold and other Metals to Light, was the last of his great memoirs. He was then sixty-six, and the remaining ten years of his life were naturally a period of diminishing activity. Thus ended the stage in which the physicist was compelled to rely mainly upon experiment, and in the next stage experiment tended to become the vehicle of verification rather than of investiga- tion. At the same time, when Faraday was approaching the end of his labours, William Thomson, then a young man under thirty but with already a continental reputation, was engaged in laying the foundations of thermodynamics and in resolutely clearing away the last difficulties that stood in the way of the full acceptance of the principle of the conservation of energy. Joule's great memoir, On the Mechanical Equivalent of Heat, had been read to the Royal Society in 1849 June, and there only remained to explain the apparent paradox that while a definite amount of heat was exactly equivalent to a definite amount of work, even a perfect engine could, as shown by Carnot's reasoning, only develop a fraction of the total. What, then, became of the energy apparently lost, and, if lost, where was the conservation of energy ? The solution was soon apparent to Thomson's acute mind, who saw that it lay in the distinction between the total energy of any system and the available energy, and with this solution the foundation-stone of thermodynamics was laid and the basis of all modern development of energy production firmly fixed. We may therefore fairly claim that in the year 1851 the two fundamental principles of physical science, principles which neither rearrangements of time and space, nor new conceptions of matter and force have yet shaken, the con- servation of energy and the second law of thermodynamics, were defined in terms which would stand to-day and were finally accepted in their present form. Just about this time another young man of the same school, James Clerk Maxwell, had taken his degree, and thinking of embarking on the study of electricity, was asking advice as to what books on the subject he should read, trying his hand in the meantime on a very difficult problem of astronomical dynamics, the constitution and stability of Saturn's rings. While in another direction G. G. Stokes was cutting out new paths in the
