114 Sir W. Thomson on the Sorting Demon of Maxwell. [Feb. 28, 



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 ap- 

 proaches 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 assort- 

 ment, carried into execution by the demon, disequalises temperatui'e, 

 and undoes the natural diffusion of heat ; the former undoes the 

 natural diffusion of matter. 



By a combination of the two processes, the demon can decompose 

 water or carbonic acid, first raising a portion of the compound to 

 dissociational temperature (that is, temperature so high that collisions 

 shatter the compound molecules to atoms), and then sending the 

 oxygen atoms this way, and the hydrogen or carbon atoms that way ; 

 or he may effect decomposition against chemical affinity otherwise, 

 thus: — Let him take in a small store of energy by resisting the mutual 

 approach of two compound molecules, letting them press as it were on 

 his two hands, and store up energy as in a bent spring, then let him 

 apply the two hands between the oxygen and the double hydrogen 

 constituents of a compound molecule of vapour of water, and tear 

 them asunder. He may repeat this process until a considerable pro- 

 portion of the whole number of compound molecules in a given 

 quantity of vapour of water, given in a fixed closed vessel, are sepa- 

 rated into oxygen and hydrogen at the expense of energy taken from 

 translational motions. The motivity (or energy for motive power) in 

 the explosive mixture of oxygen and hydrogen of the one case, and the 

 separated mutual combustibles, carbon and oxygen, of the other case, 

 thus obtained, is a transformation of the energy found in the sub- 

 stance in the form of kinetic energy of the thermal motions of the 

 compound molecules. Essentially different is the decomposition of 

 carbonic acid and water in the natural growth of plants, the result- 

 ing motivity of which is taken from the undulations of light or 

 radiant heat, emanating from the intensely hot matter of the sun. 



The conception of the "sorting demon," is purely mechanical, and 

 is of great value in purely physical science. It was not invented to 

 help us to deal with questions regarding the influence of life and of 

 mind on the motions of matter, questions essentially beyond the range 

 of mere dynamics. 



[The discourse was illustrated by a series of experiments.] 



[W. T.J 



