294 Sir William Thomson on the Kinetic 



previously described. Let the orders now he that each demon 

 is to stop all molecules from crossing his area in either 

 direction except 100 coming from A, arbitrarily chosen to be 

 let pass into B, and a greater number, having among them 

 less energy but equal momentum, to cross from B to A. Let 

 this be repeated over and over again. The temperature in A 

 will be continually diminished and the number of molecules 

 in it continually increased, until there are not in B enough 

 of molecules with small enough velocities to fulfil the con- 

 dition with reference to permission to pass from B to A. If 

 after that no molecule be allowed to pass the interface in 

 either direction, the final condition will be very great con- 

 densation and very low temperature in A ; rarefaction and 

 very high temperature in B ; and equal pressures in A and B. 

 The process of disequalization of temperature and density 

 might be stopped at any time by changing the orders to those 

 previously specified, and so permitting a certain degree of 

 diffusion each way across the interface while maintaining 

 a certain uniform difference of temperatures with equality of 

 pressure on the two sides. 



If no selective influence, such as that of the ideal u demon," 

 guides individual molecules, the average result of their free 

 motions and collisions must be to equalize the distribution of 

 energy among them in the gross ; and after a sufficiently long 

 time, from the supposed initial arrangement, the difference of 

 energy in any two equal volumes, each containing a very great 

 number of molecules, must bear a very small proportion to the 

 whole amount in either ; or, more strictly speaking, the pro- 

 bability of the difference of energy exceeding any stated finite 

 proportion of the whole energy in either is very siimlL 

 Suppose now the temperature to have become thus very 

 approximately equalized at a certain time from the beginning, 

 and let the motion of every particle become instantaneously 

 reversed. Each molecule will retrace its former path, and at 

 the end of a second interval of time, equal to the former, 

 every molecule will be in the same position, and moving with 

 the same velocity, as at the beginning ; so that the given 

 initial unequal distribution of temperature will again be 

 found, with only the difference that each particle is moving 

 in the direction reverse to that of its initial motion. This 

 difference will not prevent an instantaneous subsequent com- 

 mencement of equalization, which, with entirely different 

 paths for the individual molecules, will go on in the average 

 according to the same law as that which took place immediately 

 after the system was first left to itself. 



By merely looking on crowds of molecules, and reckoning 



