1048 Mr. A. Fairbourne on Restricted Movements 



possibility of continuous creation of potential is denied by 

 the second law of thermodynamics. 



It must be agreed, however, that any such continuous 

 experiments as this, whether actual or hypothetical, fall 

 definitely beyond the jurisdiction of the second law of 

 thermodynamics, since they depend entirely upon the 

 deliberate selection of particular molecules with specially 

 desired velocities, while the second law considers and deals 

 only with the average effects of large numbers of molecules 

 of every variety of haphazard movement, acting together 

 merely under the play of chance ; and the conclusion stated 

 by this law is consequently entirely subject to the condition 

 that no such deliberate selections of molecules with particular 

 movements shall be made. 



No record of any experimental verification of the effects of 

 such selective restriction of certain molecular movements as 

 are involved in these considerations has ever been put 

 forward, nor has any means for performing such ever been 

 suggested, and experimental verification has come in con- 

 sequence to be regarded as unlikely, or, perhaps, impossible. 



Although it may be impossible to construct an apparatus 

 capable of selecting molecules travelling with certain desired 

 speeds, yet it can be shown that it is not impossible to 

 construct an apparatus capable of selectively affecting 

 molecules travelling in certain desired directions, provided 

 that portions of the apparatus to be used for this purpose are 

 made smaller than the free paths of many of the molecules of 

 the gas employed ; and, since the mean free path of certain 

 common gaseous molecules at the easily realizable pressure 

 of 1 x 10 -2 mm. is calculated to be in the order of a centi- 

 metre in length, this requirement can be satisfactorily 

 fulfilled without such difficulty as might have been anticipated. 



Just as the deliberate segregation of molecules of parti- 

 cular speeds has been considered by Maxwell to cause the 

 creation of potentials of temperature between the different 

 portions of a system, so the continuous affecting or re- 

 directing of molecules moving in certain desired directions 

 only will cause corresponding differences Cor potentials) of 

 pressure on certain surfaces in the system. 



The principle by which such selective directional inter- 

 ferences may be accomplished can be readily indicated by 

 considering a long and narrow tube, ideally smooth and open 

 at both ends, suspended in a gas whose mean free path is 

 much greater than the length of the tube. Molecules enter 

 freely at both ends, and most of those which happen to be 

 moving in directions parallel or substantially parallel with 



