THE SORTING DEMON OF MAXWELL. 139 



water, leaving pure water in the remainder of the 

 space occupied ; or, in the other case, separate 

 the gases into different parts of the containing 

 vessel. 



" Dissipation of Energy " follows in nature from 

 the fortuitous concourse of atoms. The lost 

 motivity is essentially not restorable otherwise 

 than by an agency dealing with individual atoms ; 

 and the mode of dealing with the atoms to restore 

 motivity is essentially a process of assortment, 

 sending this way all of one kind or class, that way 

 all of another kind or class. 



The classification, according to which the ideal 

 demon is to sort them, may be according to the 

 essential character of the atom ; for instance, all 

 atoms of hydrogen to be let go to the left, or 

 stopped from crossing to the right, across an ideal 

 boundary ; or it may be according to the velocity 

 each atom chances to have when it approaches 

 the boundary : if greater than a certain stated 

 amount, it is to go to the right ; if less, to the left. 

 This latter rule of assortment, carried into execu- 

 tion by the demon, disequalises temperature, 



