JO SI AH WILLARD GIBBS 477 



by Trevor to the weight in a mechanical system. For instance, imagine 

 a frictionless, reversible mechanical system, such as a weight suspended 

 by a cord passing over a pulley, and let this weight by its fall to the 

 ground do a certain amount of work, such as raising a body attached to 

 the other end of the cord. The potential energy of the system is meas- 

 ured by the height of the weight above ground, and when the weight falls, 

 the available energy of the system decreases at each point and moment 

 of the descent, while the unavailable energy undergoes a corresponding 

 increase point for point. On reversing the operation and raising the 

 weight, the available energy of the system is seen to increase while the 

 unavailable energy decreases (i. e., increases in a negative direction). 

 So, in any reversible thermodynamic system, the entropy at any moment 

 is an index, determinant, or coefficient of the relative amount of un- 

 available energy it possesses. When the temperature in an isolated 

 reversible system is constant, as in jacketed steam, the system is 

 " isothermal " and the entropy may vary at any instant ; but if a 

 reversible system be so isolated that no heat can enter or leave the body, 

 the temperature might vary but the entropy would be constant, and 

 such systems, of which we have an approximation in the insulated 

 cylinder of an engine, were called " adiabatic " by Eankine and " isen- 

 tropic " by Gibbs. 



There are no mathematical or ideally reversible systems in existence, 

 although we have natural approximations to them in the motions of the 

 heavenly bodies and in certain chemical reactions, or human approxi- 

 mations in reversible heat engines or reversible electric apparatus; 

 the spontaneous processes of nature are always irreversible, proceeding 

 irrevocably in a definite direction with no negative or reversed dissipa- 

 tion of energy. In spontaneous, irreversible flow of heat from a warmer 

 to a colder body, the entropy or unavailable thermal energy of the 

 system increases inevitably to a maximum. In other words, the 

 entropy of a system is a criterion of its loss of efficiency or available 

 energy during irreversible change, and it follows, in the memorable and 

 aphoristic statement of the first and second laws by Clausius, that, while 

 the energy of the universe is constant, its entropy (or that part of its 

 energy which is unavailable) tends to a maximum and can never 



decrease : 



Die Energie der Welt ist constant, 



Die Entropie der Welt strebt einem Maximum zu. 



With this important generalization, which is the motto of Gibbs's prin- 

 cipal memoir, the first stage of thermodynamics ends. By stating the 

 second law as irreversible increase of entropy in natural processes and 

 by adopting some definite standard of the latter, all exact or scalar 

 relations in thermodynamics can be treated as shown by Eankine, 

 Clausius, and Gibbs, in a precise and definite manner.^^ But the 

 " Maxwell and Tait originally used the term " entropy " ag a synonym of 



