262 RUDOLF JULIUS EMANUEL CLAUSIUS. 



volume of the Memoirs of the Academy of Paris, describing the first 

 part of the magnificent series of researches which the liberality of the 

 French government enabled him to carry out for the solution of this 

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

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

 some years Joule had been making those experiments which were 

 to associate his name with one of the fundamental laws of thermo- 

 dynamics 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 unques- 

 tioned 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 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 

 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 flir 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," t 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 publication 

 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, 1850, and published in the March and 

 April numbers of Poggendorff" 's Annalen. 

 t Nature, vol. xvii, p. 257. 



