PROBABILITY AND ENTROPY 141 



tory of the concept of energy ; the curious and 

 almost tricky invention of potential and latent 

 energy ; how once it was tried to make the con- 

 cept of energy supreme in physics, thrusting 

 matter into a secondary place ; how the princi- 

 ple of relativity, clarifying so much that was 

 obscure, shows that energy content and mass 

 are merely different ways of stating the same 

 thing, as we express a distance in miles or in 

 meters ; how, finally, energy itself is now re- 

 garded as only an arbitrary cross section of a 

 greater entity, the tensor. 



However, it is of the second law of thermody- 

 namics that I wish to speak, and those irrever- 

 sible processes which are knowTi as processes of 

 degradation or of dissipation. A body falls to 

 the ground; it may rebound once or twice, but 

 comes eventually to rest; when hot and cold 

 bodies are brought together their temperature 

 is equalized; gases mix by diffusion and then 

 it is a difficult matter to separate them; in all 

 such processes there seems to be a general run- 

 ning down, and as a measure of the run-down- 

 ness of things Clausius coined the name en- 

 tropy, and announced the general law, "The 

 entropy of the world tends toward a maximum." 

 Each of these running-dowTi processes is said to 



