36 THE PHYSICAL SIGNIFICANCE OF ENTROPY 



can represent all of them completely and uniquely determines 

 the sequence of their events." They are useful for theoretical 

 demonstration and for the study of conditions of equilibrium. 



There is a certain, limited, incomplete sense in which we say 

 that we can change from one state of equilibrium to another in 

 a reversible manner. For example, we can, considering only the 

 one converting (or intermediate) body, effect said change by a 

 successive use of isentropic and isothermal change. But this 

 ignores all but one of the participating bodies and this is not 

 permissible if we strictly adhere to the true definition of complete 

 reversible action. 



We must remember too that no other universal measure of 

 irreversibility exists than entropy. "Dissipation" of energy has 

 been put forward as such a measure, but we know already of 

 two irreversible cases where there is no change of energy, namely, 

 diffusion and expansion of a gas into a vacuum. [Unavailable, 

 distributed, scattered energy are terms which could be used here, 

 free from all objection.] 



But of course, the full equivalent of entropy can be substituted 

 as a universal measure of irreversibility. On p. 2 7 we have pointed 

 out that the number of complexions included in a given state can 

 be defined as the probability W of the state, then in a footnote, 

 attention is called to the identity of entropy with the logarithm 

 of this state of probability = logarithm of the number of complexions 

 of the state. This makes entropy a function of the number of 

 complexions, so that one may in this sense be regarded as the 

 equivalent of the other. We may now properly speak of the 

 number of complexions of a state as the universal measure of 

 its irreversibility. The physical meaning of irreversibility becomes 

 apparent when put in this form. The greater the number of com- 

 plexions included in a state the more disordered is its elementary 

 condition and the more difficult (more impossible, so to speak) , is 

 it to directly so influence the constituents of the whole that they 

 will reverse the sequence of the mean values the aggregate tends 



