Vapour Pressure and Affinity of Isotopes. 179 



• absolute zero Q being equal, the ratio of the Guldberg- 

 Waage constants K^ is the same as that o£ the vapour 

 pressures. This shows that chemical separation, though 

 possible in principle, is too small to be considered at 

 temperatures at which chemical reactions take place. 



The electromotive force of reversible cells containing two 

 isotopes as electrodes is given by wFV = A, where F is 

 Faraday's constant 96,540 coulombs, n is the valency of the 

 ion, V is the electromotive force, and A the affinity. Now 



A = RT( r iog- P f? 2 " 2 -;- -loglC) 



so that 



Ai-Ao^RTlogf^ =RTlog2l, 



whence the difference in voltage of the two otherwise 

 identical processes, e. g. Pb -f 2AgCl:^PbCl 2 + 2 Ag is 



T7 Tr RT, p 2 3 2 ,RT 



V, — V 2 = — jT log-- = — , A U?S — ™ 



2 n¥ & p x 4(T n¥ 



at ordinary temperatures. Putting p — J corresponding to 

 /3v = 95° and T = 285° and o = l per cent., one finds for the 

 above reaction, in which n=2, a difference of barely 10~ 6 volt. 

 It is not surprising that no difference has been observed if 

 the above theory is correct. If the chemical constants are 

 identical, the difference might amount to 0*18 millivolt. 



The whole theory takes a much more complicated form if 

 one does not assume continuous absorption, but takes the 



energy at temperature T to be ~ h - as in Planck's earlier 



e^-1 

 publications *. As the results are quite different from those 

 given above it may be worth while to make a rough estimate, 

 even though this basis is probably not correct. For the sake 

 of simplicity, all results will only be worked out to the first 



powers of r p- and 8. 



In this case putting 

 fl m 2 hv 



hv_ 

 * T -1 





* M. Planck, Vorlesungen uber die Theorie der Warmestrahlung 



(1906). 



N 2 



