318 VINCENT E. SMITH 



usable energy has a mechanical equivalent and hence can be 

 reduced to mechanical energy which in turn, when passing 

 from a potential to a kinetic stage, produces unrecoverable heat, 

 the law of entropy, though arising out of a study of heat, really 

 applies to all closed energy transformations and, if the universe 

 is finite, to all cosmic motion. 



This is why Eddington could call the law of entropy " time's 

 arrow." " Entropy is a measure of the direction in cosmic 

 processes as a whole. It reports that our universe, previously 

 claimed to be in evolution, has in reality always been going 

 downhill. Slowly it is moving toward uniformity and equi- 

 librium where all heat will have have been transformed into 

 an unusable state and where, as a result, all further motion will 

 become impossible. This is what is meant by the heat-death 

 of the universe. 



The law of entropy was being formulated at a time when, 

 despite the theory of Daniel Bernoulli and the experimental 

 evidence of Count Rumford, the caloric theory of heat was still 

 in vogue. About the middle of the nineteenth century the 

 kinetic molecular theory of matter, thanks to Joule and Max- 

 well, finally became acceptable, and in this perspective, the 

 much older idea that heat is a phenomenon of motion was 

 finally given quantitative form. The temperature of a sub- 

 stance could be correlated to the average kinetic energy of the 

 molecules, or more simply put, heat came to be considered as a 

 random motion of particles. As such, it is a problem in statistics 

 like the throwing of dice or the shuffling of cards. 



Despite all the historical and philosophical interest which the 

 study of heat can command, we are interested in the problem 

 here only to round off our discussion of entropy. Against the 

 background that heat is the random motion of molecules in 

 material substances, or that " from the standpoint of the kinetic 

 theory, heat is disorganized random mechanical energy, whereas 

 mechanical energy proper is directed, ordered," ^^ it is possible 



^- The Nature of the Physical World (New York, 1928) chap. 5. 

 "A. d'Abro, The Rise of the New Physics (New York, 1953) I, 398. 



