286 Obituary Notices of Fellows deceased. 



Joule had been making those experiments which were to associate his 

 name with one of the fundamental laws of thermodynamics and one of 

 the principal constants of Nature. In 1849 he made that determination 

 of the mechanical equivalent of heat by the stirring of water, which for 

 nearly thirty years remained the unquestioned standard. In 1848 and 

 1849, Sir William Thomson was engaged in developing the consequences 

 of Carnot's theory of the motive power of heat ; while Prof. James 

 Thomson, in demonstrating the effect of pressure on the freezing point 

 of water by a Carnot's cycle, showed the flexibility and the fruitfulness 

 of a mode of demonstration which was to become canonical in thermo- 

 dynamics. Meantime, Rankine was attacking the problem in his own 

 way, with one of those marvellous creations of the imagination, of which 

 it is so difficult to estimate the precise value. 



" Such was the state of the question when Clausius published his 

 first memoir on thermodynamics, ' Ueber die bewegende Kraft der 

 Warme, und die Gesetze, welche sich daraus f iir die Warmelehre selbst 

 ableiten lassen.' This memoir marks an epoch in the history of 

 physics. If we say, in the words used by Maxwell some years ago, 

 that thermodynamics is * a science with secure foundations, clear 

 definitions, and distinct boundaries,' and ask when those foundations 

 were laid, those definitions fixed, and those boundaries traced, there 

 can be but one answer ; certainly not before the publication of that 

 memoir. The materials indeed existed for such a science, as Clausius 

 showed by constructing it from such materials, substantially, as had 

 for years been the common property of physicists. But truth and 

 error were in a confusing state of mixture. Neither in France, nor in 

 Germany, nor in Great Britain, can we find the answer to the question 

 quoted from Regnault. The case was worse than this, for wrong 

 answers were confidently urged by the highest authorities. That 

 question was completely answered, on its theoretical side, in the 

 memoir of Clausius, and the science of thermodynamics came into 

 existence. And, as Maxwell said in 1878, so it might have been said 

 at any time since the publication of that memoir, that the foundations 

 of the science were secure, its definitions clear, and its boundaries 

 distinct. The constructive power thus exhibited, this ability to bring 

 order out of confusion, this breadth of view which could apprehend 

 one truth without losing sight of another, this nice discrimination to 

 separate truth from error these are qualities which place the possessor 

 in the first rank of scientific men. 



"In the development of the various consequences of the funda- 

 mental propositions of thermodynamics, as applied to all kinds of 

 physical phenomena, Clausius was rivalled, perhaps surpassed, in 

 activity and versatility by Sir William Thomson. His attention, 



