the Separation of the Isotopes of Mercury. 33 



evaporation under usual conditions is considered, we are 

 not in the position to utilize the quicker movements oh' 

 the lighter molecules for the purpose of separating isotopes, 

 because the quicker evaporation in the case of the lighter 

 molecules is just compensated by a quicker re-condensation. 

 We can, however, prevent this compensation by suppressing 

 one of tiie two compensating processes ; and on the grounds 

 of this principle we are able to reach a simple method for 

 a partial separation of the isotopes. This is most easily 

 accomplished by allowing the liquid to evaporate in a 

 vacuum and placing a highly. cooled glass plate over it s 

 surface. Now when the vapour-pressure of the liquid 

 is sufficiently slight, each molecule which leaves the liquid 

 will reach the cooled wall before it has had an opportunity 

 of meeting other molecules and being thrown back into 

 the liquid. Having reached the cooled wall, it will be 

 held by it, transferred into the solid state, and hindered 

 in re-evaporation. It follows from the above that in the 

 "ideal distillate " obtained in this way there will be 



a/ — - times as much of the lighter isotope as in the initial 



in 



substance, and through repeated "ideal distillations" of 

 the fractions thus obtained it is possible to carry the partial 

 separation further. 



The applicability of the evaporation method depends on 

 two conditions : (a) a not too great vapour-pressure of 

 the liquid and a very good vacuum, in order to avoid 

 collision of the evaporating molecules with each other or 

 with other molecules contingently present which would 

 cause reflexion and return to the liquid phase ; (b) equal 

 composition of the surface and interior of the liquid phase. 

 If the velocity of the exchange of atoms between the liquid 

 surface and the liquid body be insufficient, then the heavier 

 isotope will be concentrated in the surface layer and soon 

 render all further separation impossible. 



As we shall see later on (§3), these conditions in mo.-t 

 cases do not in any way render the evaporation method 

 impracticable as long as the substance to be separated is in 

 a liquid state ; while, on the other hand, the requirement 

 of the condition (b) practically excludes the use of ibis 

 method in the case of a solid substance. 



The second separation method used by us is the effusion 

 process. If a molecular flow * of an isotopic vapour takes 



* Kimdsen, Ann. der PhysiJc, (4) xxviii. p. 999 (1909). R. W. Wood, 

 Phil. Mag. xxx. p. ,°,00 (1915). 



Phil. Mag. 8. 6. Vol. 43. No. 253. Jan. 1922. D 



