148 THE ANATOMY OF SCIENCE 



made in the partition the gases mix by diffusion, 

 so that ultimately we cannot distinguish be- 

 tween the contents of the two sides. This, ac- 

 cording to the ideas of classical thermodynam- 

 ics, was a "completely irreversible phenomenon. 

 But suppose the demon stationed at the orifice 

 with a small shutter which he can open or close 

 at will, and suppose that he decides to let only 

 oxygen molecules pass in one direction and only 

 nitrogen molecules in the other. Can he not 

 through this exercise of conscious choice ulti- 

 mately restore the original condition, with all 

 the oxygen on the one side and all the nitrogen 

 on the other ? That such a reversal of a so-called 

 irreversible process may occur, even without a 

 demoniacal agency, was first recognized by Wil- 

 lard Gibbs, who announced that such a reversal 

 is not an impossibility but only an improbabil- 

 ity ; and this idea, which was thoroughly devel- 

 oped by Boltzmann a generation ago, has grown 

 to be an important, and almost the most impor- 

 tant, guiding principle of modern physics. Its 

 full utilization has been delayed owing to mis- 

 conceptions as to the nature of light, but it 

 seems to me that these difficulties are now re- 

 moved. 



In order to illustrate the modern statistical 



