Josiah Willard Gibbs. 283 



such as all mechanical engines execute. Such cycles were, in fact, 

 afterwards appealed to in mechanical statics, by the originators of the 

 doctrine of conservation of energy, to establish the existence of a 

 definite energy-function, on the ground that otherwise work could be 

 obtained in unlimited amountr~ut of nothing. In the hands of Carnot, 

 so far back as 1824, they had been already employed in a wider scope 

 to prove that all reversible heat engines are mechanically equivalent 

 if they work between the same temperatures, on the ground that 

 otherwise work in unlimited amount would be derivable as he then 

 thought from nothing. In 1848, W. Thomson, whose notice was 

 attracted to this theory by Clapeyron's graphical exposition of a part 

 of it, seized upon it as affording a new and purely dynamical con- 

 ception of the notion of temperature hitherto unconnected with other 

 physical ideas as a function specifying the capacity of heat for doing 

 work. In the following year he published an exposition of Carnot's 

 doctrine, still reposing, provisionally, on the dogma of the indestructi- 

 bility of heat ; he then professed himself at a loss* to reconcile Carnot's 

 .argument with Joule's doctrine of the transmutation of heat into 

 work, so much so as to lead to an unguarded expression of his belief 

 that no case of this had yet been made out, though further experiment 

 was urgently necessary. The difficulties that presented themselves 

 to the pioneers in this new realm are necessarily hard to appreciate 

 by us, to whom so much has, through their labours, become 

 obvious. Yet one weighty circumstance may be recalled. Before 

 Clausius' attention was attracted to the subject, the doctrine of Carnot 

 made, in the hands of James Thomson, the first of its long series of 

 predictions in the entirely new domain of change of physical statet ; 

 and no delay ensued before his propositions were verified by his 

 brother's experiments on the lowering of the freezing point of water 

 by pressure. It is in Clausius' great memoir of February, 1850, written 

 from a knowledge of Clapeyron and Thomson alone, that the recon- 

 ciliation of Joule's and Carnot's principles is effected ; so simple yet 

 significant is it, merely the change of the words " derivable from 

 nothing " above to " derivable from completely diffused heat," that 

 in the reprint of James Thomson's memoir on the lowering of the 

 freezing point, in 1851, in the " Camb. and Dub. Math. Journal," only 

 one sentence in the argument had to be altered. The doctrine of 

 Carnot, including most of the results he derived from it, might in 



* In his memoir of Joule (Manchester Memoirs, 1892) Osborne Reynolds 

 connects this difficulty with the recognition that something is lost when heat 

 merely diffuses; he considers that it was overcome only when the principle of 

 dissipation of energy clearly emerged, which asserted that while the energy is con- 

 served its mechanical availability is in part destroyed. 



f Clapeyron had already deduced the corresponding formula for change of 

 boiling point from Carnot's principles. 



