Thermal Diffusion of Gasts of Equal Molecular Weight. 147 



The object of the present note is to point out that " thermal 

 diffusion " offers a physical means of partially separating two 

 gases of equal molecular weight, provided that their diameters 

 or laws o£ inter-action are not identical. This property may 

 be of much or little practical importance, but it seems worth 

 while to call attention to it on account of its theoretical 

 interest. If we have two vessels containing numbers of 

 identical molecules, and open a channel of communication 

 between them, the random molecular motions will soon 

 bring about a mixture of the two sets of molecules such as 

 is beyond our powers to disentangle. If, however, the two 

 sets of molecules are unlike, in such wise that we can apply 

 unequal external forces to the members of the two sets, 

 a partial separation by means of "forced diffusion"" can be 

 effected. This case is realized in practice when a vertical jar 

 containing a mixture of two gases of unequal molecular weight 

 is left to itself under the action of gravity, which acts unequally 

 on the molecules of the two kinds. If one set of particles in 

 a gas-mixture be charged, an electrical force will similarly 

 effect a partial separation. In my memoir already cited I 

 have shown that a pressure gradient, in whatever way pro- 

 duced, also results in diffusion of the two gases, so that 

 if a force similar to that of gravitation (but acting equally 

 on all the molecules) could be used to establish such a 

 gradient, the molecules might be partially sorted out by 

 u pressure diffusion." This effect, however, is directly pro- 

 portional to the difference between the molecular masses, so 

 that actually gravitational separation hardly affords an in- 

 stance of this phenomenon as distinct from that of forced 

 diffusion. A better example is to be found in a horizontal 

 layer of gas contained in a vessel which is rapidly revolving 

 about a vertical axis : the heavier molecules will then, of 

 course, be proportionately more numerous at greater than at 

 smaller distances from the axis. 



When the two sets of molecules are of equal molecular 

 weight, however, and when they are electrically neutral, 

 there has hitherto seemed to be no means of gaining any 

 control over their relative proportions in a mixture, after this 

 has once become uniform. The equality of mass requires an 

 equality of the mean random motion of the molecules, and 

 pressure diffusion, which depends upon an inequality of this 

 random motion, is unable to act. The discovery of thermal 

 diffusion, however, opens a mode of possible action which is 

 more subtle than any of those yet mentioned, in that it 

 depends essentially (when the molecular masses are equal) 

 on the nature of the inter-actions between molecules during 



