90 THE PHYSICAL SIGNIFICANCE OF ENTROPY 



As regards the use of helpful proposition (a) : 



We know that PLANCK'S motor statement of the Second Law 

 was grounded on the well-known irreversible passage of heat from 

 a cold to a hot body. But to show the mutual interdependence (a) 

 of one irreversible change on every other, we will instance in illus- 

 tration the case of a frictional event, or the conversion of mechan- 

 ical work into heat. 



If this frictional occurrence could by any simple or complex 

 apparatus be made completely reversible so that everywhere, in 

 the whole of Nature, the same state would be restored which 

 existed at the beginning of the frictional occurrence, then such 

 an apparatus would be the motor contemplated in PLANCK'S 

 statement of the Second Law, for this periodically running 

 apparatus would convert heat into work without any other change 

 remaining. A similar line of argument, with a similar result, 

 could be pursued with every other case of irreversibility that could 

 be adduced. It is evident that, with the help of the above-given 

 propositions (a), (b), and (c), the Second Law can be cast into 

 many other valid forms. 



We close this presentation of the meaning of the Second Law 

 by the remark that this law has no independent significance, for 

 its roots go down deep into the Theory of Probabilities. It is 

 therefore conceivable that it is applicable to some purely human 

 and animate events as well as to inanimate, natural events; 

 provided, of course, that the former possess numerous like and 

 uncontrolled constituents which may be properly characterized 

 as " elementar-ungeordnet," in other words, provided the variable 

 elements present constitute adequate haphazard for the Calculus 

 of Probabilities. 



