66 BUTLER 



ART. D 



equilibrium. Conversely, it is impossible to obtain mechanical 

 work, over and above the work expended from other sources, 

 by the change of a system, which is in equilibrium, into another 

 state. 



We have seen that the maximum work is obtained from a 

 spontaneous change when it is carried out by a reversible 

 process. But a reversible process proceeds infinitely slowly, 

 since at every stage the forces of the system are nearly balanced 

 by opposing forces. When changes occur in Nature at a finite 

 rate, the forces of the system must be appreciably greater than 

 the opposing forces. Such changes are essentially irreversible 

 and the maximum work of which they are capable, which 

 Kelvin called the available energy, is not obtained. In an 

 irreversible process only part of the available energy is obtained 

 as work, the remainder is dissipated. Kelvin (1852) therefore 

 stated the second law of thermodynamics as the Principle of 

 the Dissipation of Energy : 



"1. There is at present in the material world a universal 

 tendency to the dissipation of mechanical energy. 



"2. Any restoration of mechanical energy, without more than 

 an equivalent of dissipation, is impossible in inanimate material 

 processes, and is probably never effected by means of organised 

 matter, either endowed with vegetable life or subjected to the 

 will of an animated creature." 



To return to Carnot's cycle, Kelvin had pointed out in 1848 

 that Carnot's theorem may be employed to define an absolute 

 scale of temperature. Since the ratio of the work obtained in a 

 reversible Carnot cycle to the heat absorbed at the higher tem- 

 perature depends solely on the temperatures of the two bodies 

 between which the transfer of heat is effected, we may write 



Qt = <t>ii', i")i 



where ^{t' , t") is a function of t' and t" alone. 

 Kelvin defined absolute temperature so that 



t' — t" 

 <l>{t', t") = —^- 



