1879.] Sir W. Thomson on the Sorting Demon of Maxwell. 113 



WEEKLY EVENING MEETING, 

 Friday, Feb. 28, 1879. 



C. William Siemens, Esq. D.C.L. F.R.S. Vice-President, in the Chair. 

 Sib William Thomson, LL.D. F.R.S. 



The Sorting Demon of MaxwelL 



[Abstract.] 



The word " demon," wliicli originally in Greek meant a supernatural 

 being, has never been properly used to signify a real or ideal personi- 

 fication of malignity. 



Clerk Maxwell's "demon" is a creature of imagination having 

 certain perfectly well defined powers of action, purely mechanical in 

 their character, invented to help us to understand the " Dissipation of 

 Energy" in nature. 



He is a being with no preternatural qualities, and differs from 

 real living animals only in extreme smallness and agility. He can at 

 pleasure stop, or strike, or push, or pull any single atom of matter, 

 and so moderate its natural course of motion. Endowed ideally with 

 arms and hands and fingers — two hands and ten fingers suffice — he 

 can do as much for atoms as a pianoforte player can do for the keys of 

 the piano — just a little more, he can push or pull each atom in any 

 direction. 



He cannot create or annul energy; but just as a living animal 

 does, he can store up limited quantities of energy, and reproduce them 

 at will. By operating selectively on individual atoms he can reverse 

 the natural dissipation of energy, can cause one-half of a closed jar of 

 air, or of a bar of iron, to become glowingly hot and the other ice cold ; 

 can direct the energy of the moving molecules of a basin of water to 

 throw the water up to a height and leave it there proportionately 

 cooled (1 deg. Fahrenheit for 772 ft. of ascent); can "sort" the 

 molecules in a solution of salt or in a mixture of two gases, so as to 

 reverse the natural process of diffusion, and produce concentration of 

 the solution in one portion of the 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 



