ON THK PHYSICAL VIEW OF NATl'ItE. ITo 



ture reinaiuing constant), lie gave the name of free energy. «' 



Ilelmholu'* 



He showed that in a state of equilibriiun the free or "'"* 

 available energy must he a minimum. He also showed 

 the connection in which the available or free energy 

 stands to the quantity introduced l)y liankine and 

 Clausius, the entropy wliich measures the unavailable or 

 hidden energy. \'>y making chemical changes depend 

 on the increase or decrease of a definite measurable 

 quantity a parallel was established between chemical and 

 mechanical processes, the latter always taking place in 

 the direction of a decrease of potential energy. Free 

 energy has thus been appropriately termed Ity M. Duhem 

 the thermo-dynamic potential. 



Helmholtz did not apply this fruitful view to chemical 

 processes on any extensive scale, but his explanations 

 have done much to establish that correcter and more 

 comprehensive way of treating such questions which has 

 since become general, llorstmann had indeed leil up 

 to this view, Willard Gibbs had applied it before, and 

 Lord liayleigh had suggested it.^ The conception of 



^ The general use of the concep- and 11), and more definitely ex- 

 tion of useful or free energy must jilaincd and aijplied to the phy.s- 

 be dated from the remai'kably lucid ical phenomena of dissociation by 

 expositions of Helmholtz, though Oibbs (' Thermodynamisclie Stud- 

 it is now recognised by all who ien,' ed. Ostwald, p. 66, kc. ; 

 have studied the history of this j ' Amer. Journ. of Sciences and 

 fertile conception that the physi- t Arts,' 1879) ; and that it is ea- 

 cal notion of available energy goes i)ecia!ly owing to the labours of 

 back to Thomson (see Tait, ' Ther- Duliem that the subject has 

 modynamics,' 1868, p. 100) and received the attention of oliemists. 

 Maxwell {' Heat,' p. 187, 8th ed. ; M. Duhem, in the introduction to 

 Duhem, ' Mdamique chimi(|ue,' the work of 1886, gives a very 



vol. i. p. 92 ; Le Chatelier in 

 'Journal de Physique,' 1894, p. 

 291) ; that the mathematical 

 formulic were given by Massieu 

 (quoted by Duhem, ' Le Potential 



valuable and lucid historical ex- 

 position, and subsequently in liis 

 large work in four volumes ('M<5- 

 canique chin)i(|ue,' 1897-1900) a 

 vast number of applications. Kor 



Thermodynamique,' 1886, pp. v I the history of thought the imiH.rt- 



