the Separation of the Isotopes oj Mercury. 37 



already attains the value of 0*7 mm., and the mean free 

 path is accordingly only 015 mm. — only quite a small 

 fraction of the distance between the mercury and the 

 cold class surface. That the separation succeeded in spite 

 of this is explained by the fact that the current of the 

 mercury vapour from the warm to the cold surface is 

 onesidedly directed, and that the presence of the steep 



Fia-. 2. 



gradient hinders the retreat of the evaporated molecules 

 into the liquid. Under these conditions " the mean free 

 path " surmounts many times the mean free path of the 

 mercury molecules experienced under normal conditions^ 



From the fact that even by rather rapid evaporation 

 a partial separation of the mercury isotopes can be achieved, 

 we are justified in concluding that the exchange of the 



