1850-60] ROYAL ASTRONOMICAL SOCIETY 
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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, and 
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 studj' 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. W T hile 
in another direction G. G. Stokes was cutting out new paths in the