Vapour Pressure and Affinity of Isotopes. 181 



Putting T = 290° and«S~l per cent., this would lead to a 

 difference in electromotive force of almost exactly one milli- 

 volt. This should be perceptible if due precautions were 

 taken. 



The general conclusion may be formulated ns follows : 

 Isotopes must in principle be separable both by fractionation 

 and by chemical means. Th \ amount of separation to be 

 ■expected depends upon the way the chemical constant is 

 calculated and upon whether "Nullpunktsenergie" is assumed. 

 At temperatures large compared with /3v, which are the only 

 practicable temperatures as far as lead is concerned, the 

 difference of the vapour pressure and of the constant of the 



Bv 

 law of mass action may be expanded in powers of ^ . The 



most important term of the type log 7^- is cancelled by the 



chemical constant if this is calculated in what seems the 



Bv 



only reasonable way. The next term in -^ is cancelled by 



the " Nullpunktsenergie " if this exists. All that remains 



are terms containing the higher powers of -^p. In practice 



therefore fractionation does not appear to hold out prospects 

 of success unless one of the above assumptions is wrong. 

 If the first is wrong a difference of the order of 1\ per cent, 

 -should be found. If the second is wrong a difference of as 

 much as 3 per cent, should occur at 1200° and a difference 

 of electromotive force of one millivolt might be expected. 

 Negative results would seem to indicate that both assumptions 

 are right. 



Summary. 



It is shown that though isotopes cannot be identical 

 chemically there is only a second order difference if 

 "" Nullpunktsenergie " is assumed and if the chemical constant 

 is derived from m, k, h, and 6. The difference should be 

 measurable if there is no " Nullpunktsenergie/' and it is 

 suggested that experiments on the vapour pressure and 

 affinity of isotopes would give valuable information on this 

 important point. 



Sidmouth, 

 May 1st, 1919. 



