44 THE PHYSICAL SIGNIFICANCE OF ENTROPY 



SECTION F 

 MORE PRECISE AND SPECIFIC STATEMENTS OF THE SECOND LAW 



We have here classified these statements in the same way as 

 that followed in the preceding section, when grouping the general 

 equivalents of the Second Law under the head of change of entropy. 

 In making comparisons we must, here as there, bear in mind the 

 following three helpful propositions : 



(a) The summary of all the necessary prerequisites (or condi- 

 tions) for determining entropy may be regarded as a complete 

 and valid statement of the second law. 



(b) Any general consequence of any one correct statement 

 of the second law may be regarded as itself a valid and complete 

 statement of the second law. 



(c) All cases of irreversibility stand or fall together; if any 

 one of them can be completely reversed all can be so reversed. 



In the preceding section we have already given the most 

 precise physical statement of the Second Law, namely, when all 

 the participating bodies of the system are considered, every 

 natural event is marked by an increase in the number of com- 

 plexions of the system. We have numbered the following state- 

 ments of the second law, for convenience of reference: 



(1) J. W. GIBBS. "The impossibility of an uncompensated 

 decrease in entropy seems to be reduced to an improbability." 

 This of course considers all the participating bodies of the 

 system. 



(2) All changes in nature involve a net growth in entropy; 

 when such a change is measured in reversible ways, the growth 



is indicated by the summation : ^ o, when the j . [ 



sign refers to processes which on the whole are completely 



irreversible . 



Of course it is now thoroughly understood that 



J irreversible 1 

 reversible J 



