JO SI AH WILLARD GIBBS 479 



vanish, and nothing would remain but the motions of material systems 

 and the laws of mechanics. Hence the second law must either be 

 obtained from our actual experience " with real bodies of sensible mag- 

 nitude/' or else derived a posteriori, as shown by Boltzmann, Helmholtz 

 and Gibbs, from averages of the hypothetical motions of mechanical 

 systems. In aid of this conception of the problem, Maxwell intro- 

 duced his whimsical notion of the " sorting demon," a being endowed 

 with molecular vision, who would be able through intelligence alone 

 to sort or direct the molecular movements at will and so reverse the 

 action of the second law on occasion.^'* 



The second stage of thermodynamics begins in 1872-3 with August 

 Friedrich Horstmann's application of the entropy principle to prob- 

 lems of chemical dissociation.^^ In October, 1873, Horstmann an- 

 nounced the condition for chemical equilibrium to be that of maximum 

 entropy,^ 2 and in December of the same year Gibbs, in a modest foot- 

 note, stated that the condition for thermodynamic equilibrium in a 

 chemical system at constant temperature and pressure is that the func- 

 tion now universally known as the thermodynamic potential should be 

 a minimum.^^ In 1875 Lord Eayleigh stated that dissipation of 

 energy is a sufficient if not a necessary condition for chemical change,^* 

 and in October, 1875, appeared the first installment of Gibbs's memoir 

 of three hundred pages on chemical equilibrium, which, by its applica- 

 tions of the entropy principle to all physico-chemical or energetic 

 phenomena, has become a true scientific classic doing for the second 

 law what Helmholtz, in his treatise on the conservation of energy, had 

 previously done for the first. 



Gibbs began his work in thermodynamics in 1873, with two impor- 

 tant papers on diagrams and surfaces. ^^ In the first of these he made 

 a careful and thoroughgoing study of all the diagrams that might be 

 of use or value in thermodynamics, the best known being that upon 

 which volume and pressure are erected to scale as coordinates, derived 

 from the familiar "Watts' indicator diagram found upon every steam 

 engine. Of the new diagrams which Gibbs introduced, he attached 

 most importance to the volume-entropy diagram, because it tells more 

 about the physical properties of a working substance than about the 

 heat employed or the work done. But the most important of Gibbs's 

 innovations for practical engineering purposes is the temperature- 



^' For a description of the Maxwell demon and the powers ascribed to him 

 see Lord Kelvin's paper in Nature, 1879, 126. 



^'Horstmann, Ann. d. Chem. u. Pharm., 1872, 8. Suppl.-Bd., 112-33. 



^= Horstmann, Ibid., 1873, CLXX., 192-210. 



^"^ Gibbs, Tr. Connect. Acad., Dec, 1873, II., foot-note to p. 393. 



"* Lord Rayleigh, Proc. Roy. Inst., 1875, VII., 388. 



«= Tr. Connect. Acad., 1873, II., 309-42, 382-404. Translated into French as 

 " Diagrammes et surfaces thermodynamiques," Paris, C. Naud, 1903. Trans- 

 lated into German by Ostwald in 1892. 



