RUDOLF JULIUS EMANUEL CLAUSIUS. 459 



question, was published in 1847. In the same year appeared Ilelra- 

 holtz's celebrated memoir, " Ueber die Erhaltung der Kraft." For 

 some years Joule had been making tbose experiments which were to 

 associate his name with one of the fundamental laws of thermody- 

 namics and one of the principal constants of nature. In 18-19 he 

 made that determination of tbe mechanical equivalent of heat by the 

 stirring of water which for nearly thirty years remained the unques- 

 tioned standard. In 1848 and 1849 Sir William Thomson was en- 

 gaged in developing the consequences of Carnot's theory of the motive 

 power of heat, while Professor 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 thermodynamics. Meantime Ran- 

 kine 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 fiir 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," f and ask when those foundations were laid, those defini- 

 tions 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 com- 

 mon 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 publica- 

 tion of that memoir, that the foundations of the science were secure, 

 its definitions clear, and its boundaries distinct. 



* Read in the Berlin Academy, February 18, I860, and published in the 

 March and April numbers of PoggendurlT'.s Annalen. 

 t Nature, vol. xvii. p. '1'ii. 



