316 VINCENT E. SMITH 



is that it simply rules out a perpetual motion machine of 

 this type, 



Carnot likened the behavior of a water system to the flow 

 of heat, but the full meaning of his achievement came only 

 when Clausius *^ formulated the Carnot principle to read that 

 heat, of itself, cannot pass from a cooler body to a hotter one — 

 any more than the water in our analogy can flow " uphill." 

 But what did Clausius mean by " entropy," — the term he 

 introduced to clarify and generalize the second law of thermo- 

 dynamics which, in the reading he gave it, simply states: the 

 entropy in any closed mechanical system always tends to 

 increase to the maximum? 



The strictly mathematical physicist will want to regard 

 entropy as " a variable of state " as as " a function of state." 

 But this definition, valid as it is within a strictly mathematical 

 physics,*^ cannot supply the fundamental physical meaning we 

 would like to find. Clausius himself wrote that if we want to 

 assign to entropy a proper name, we can 



say of it that it is the transjormation content of the body, in the 

 same way that we can say of the quantity TJ that is the heat 

 and work content of the body. However, since I think it better 

 to take the names of such quantities as these, which are important 

 for science, from the ancient languages, so that they can be intro- 

 duced without change into all the modem languages, I propose to 

 name the magnitude S the entropy of the body, from the Greek 

 word e trope a transformation. I have intentionally formed the 

 word entropy so as to be as similar as possible to the work energy, 

 since both these quantities which are to be known by these names 

 are so nearly related to each other in their physical significance that 

 a certain similarity in their names seemed to me advantageous.^" 



But what is the " physical significance " of " transformation 

 content "? Perhaps we may take a cue from Lindsay and 

 Margenau who write, "the quantity we are seeking will be 



Cf. W. Wilson, A Hundred Years of Physics (London, 1950) pp. 37-39. 

 For the nature of mathematical physics, cf. In II Phys., lect. 3 (passim) . 

 Cf. W. Magie, ed. Source Book in Physics (New York, 1935) p. 234. 



