32 Prof. J. N. Br/z(nsted and Prof. G. Hevesy on 



velocities) appearing as a consequence of the mass difference 

 of the isotopes. That is for instance the case with an incom- 

 plete reaction between a solid or liquid and a highly diluted 

 gaseous phase *", and thus also with a partial evaporation or 

 condensation which belong to the most simple heterogeneous 

 reactions. If a liquid is in equilibrium with its vapour, then 

 in unit time a certain number of vapour, molecules will hit 

 its .surface and adhere ; in order to keep up the state of 

 equilibrium, it is necessary that in unit time the same number 

 of molecules pass from the liquid into the vaporous phase. 

 The velocity of exchange of molecules between the two phases 

 depends upon the density of the vapour and the velocity of 

 the vapour molecules ; the quicker the latter, the more vapour 

 molecules will reach the surface in the time-unit and the 

 livelier will be the exchange of molecules between the two 

 phases. 



We will now consider a liquid containing two isotopes in 

 equal amounts. As in this case the vapour-pressure of the 

 two isotopes is equal, the vapour phase will have the same 

 composition as the liquid. Notwithstanding this equality in 

 the composition of the two phases, a livelier exchange will 

 take place between the lighter than between' the heavier 

 atoms in the two phases. The former have, namely, in 

 accordance with their smaller mass (?%), a velocity (y L ) 



\/ — times as great as the latter — the mass and velocity of 



which we will denote by m. 2 , respectively v 2 — and so in the 



time-unit \7 — times as manv molecules of the lighter as 



V m l " ° 



of the heavier isotope will be transferred from the liquid 

 in the vaporous stage, and vice versa. As long as an 



* A concrete example best explains these conditions. If chlorine 

 gas is passed through a silver tube and only a small fraction of the 

 molecules is assumed to react with the metal, then on account of the 

 greater molecular velocity of the lighter isotope the molecules of this 

 will have a greater probability of hitting the wall of the silver tube 

 than the molecules of the heavier one have. More of the lighter 

 chlorine will therefore be bound as silver chloride than of the heavier, 

 and — as a simple estimate shows — the ratio will under the most 

 favourable conditions equal vfr times the ratio of the isotopes in 

 the gaseous phase. 



By this estimate, however, the possibility of the result of an impact 

 being influenced by the velocity of the molecules is neglected, as well as 

 the fact that besides (01 35 ) 2 and (C1 3T J 2 , also (CPCl 37 ) molecules are- 

 present in the gaseous phase. 



