572 JAMES CLERK MAXWELL. 



opened or closed without the expenditure of energy. Each 

 trap door he supposed placed in charge of a "demon" that is a 

 creature whose eyes are sharp enough to see the molecules 

 and estimate their velocities, and hands agile enough to 

 open and close the trap doors in time to allow or prevent 

 the passage of any particular molecule which is approaching 

 the partition. The operation depends on the difference of 

 the velocities of the particles in the same mass of gas, and 

 the office of the demon is purely selective, so that any mechan- 

 ism which could be devised to sort the molecules in the 

 same way would be equally effective. Suppose each demon 

 to open his trap door when a molecule is approaching the 

 partition from A with a velocity above the average, but to 

 keep it closed when the velocity of the molecule approach- 

 ing from A is below the average ; while in the case of a 

 molecule approaching the partition from B the door is 

 opened if the velocity be small, and closed if it be great. 

 In this way all the slowly moving molecules will gradually 

 be sorted into the compartment A, while the rapidly moving 

 particles will be accumulated in B. Thus the temperature 

 of the gas in B will be raised, and that in A lowered with- 

 out any loss of energy or any work being done by an 

 external agent. A heat engine may now be employed, 

 using B as the source and A as the condenser, and doing 

 work at the expense of part of the heat of A until equi- 

 librium of temperature between B and A has been produced, 

 when the services of the demons may be again utilised, and 

 the process repeated until the whole of the heat of the gas has 

 been converted into work. This is contrary to the principle 

 of dissipation of energy, which has thus been circumvented 

 by intelligence. No corresponding method of overcoming the 

 principle of conservation of energy can be devised, and this 

 principle is thus shown to rest on an entirely different kind 

 of footing from that of the second law of Thermodynamics. 



The following extract from Maxwell's- article "ATOM " in 

 the ninth edition of the Encyclopaedia Britannica is character- 

 istic. Speaking of the teaching of molecular science re- 

 specting the size of atoms, he says : 



