THERMODYNAMIC AL SYSTEM OF GIBBS 85 



i.e., the heat absorbed in a change which occurs at constant 

 pressure, when the only work done is that due to increase in 

 volume, is equal to the increase of x- 



Similarly, when a system undergoes a change at constant 

 volume, pAv is zero and, if no work is done against external 

 forces other than the pressure, the increase of energy is equal 

 to the heat absorbed: 



Ac = Q„, (34) 



so that the energy can be regarded as the heat function at 

 constant volume. 



Various names have been given to the thermodynamic func- 

 tions 4/, ^, X- Clerk Maxwell called rp the available energy, but 

 a certain amount of confusion has arisen because Helmholtz in 

 1882* used the term, free energy, for the same quantity. G. N. 

 Lewis,t in his system of thermodynamics, has made use of the 

 functions A, F and H which are identical with Gibbs's ^, f, x 

 and has used the names: 



A or \^: Available energy. 

 F or ^'. Free energy. 

 H OT X' Heat content. 



F. Massieut was the first to show that the thermodynamical 

 properties of a fluid of invariable composition may be deduced 

 from a single function, which he called the characteristic func- 

 tion of the fluid. He made use of two such functions; which, 

 in Gibbs' notation, are as follows : 



(1) 

 (2) 



— e-\- ty _ _ ]A 



t ~ ~ t 



— €-{-tv - PV _ _ f . 



t ~ ~ t 



* Sitzungsber. preuss. Akad. Wiss, 1, 22 (1882). 



t Lewis and Randall, Thermodynamics and the Free Energy of Chem- 

 ical Substances (1923). 



t Comptes rendus, 69, 858 and 1057, (1869). 



