( 138 ) 



have dh = {l>g — f>a) (f~ 'Uul according to the form of formula (4) 

 chosen for b : 



v-b ~ z ' 



rdb ri—z-" 1 



in conse(jnence ot \vhicli I passes into | — — d: ^= log z — ^c^ 



Jv—b J z 2 



Substituting into the expression for the thermodynamic potential 



we get 



a b—h, 1 fb—b,\ 

 p^ ETloq (v-b) — RTlog RT( - 



h-K 2 



If we suppose the temperature to be low, the second phase is a 



rare gas phase and we have: 



p b — &(, 



pv — RT, log {ü —b) — - log —- and - — - = 1 - 



Ri "(j—^o 



In consequence of this we get: 



or 



pv, RT-RTlo<j-^-^+-RT-^—^=RTlog^^ ' '^ 



V, bn-b., 2 v,-b, RT 



or 



a t\-b, p(i\-b,y b,-bn , 1 „ b,-b„ p 



pl^. j^RT^-^-^^-^-RTlog-^+-RT-^^RTlog-^-. 

 b. b. Oj 6„-Z>„ 2 Vj-^i a 



As yet we ha\e not applied any approximation for the liquid 

 condition. 



If in the first member we collect the terms containing j), we may 

 write them as follows : 



v,^ — 2b,v. 



The value of i\ in the liquid condition being only slightly larger 

 than 2/>i, the value of tliis expression remains below j^^i and it may 

 certainly be neglected ; if in the second member we neglect also 



a 

 p compared with —, then we may write the equation for the calcu- 

 lation of the vapour pressure at low temperatures as follows: 



a 



,„,iL^_A + !irii_,„,^+,^-Jï . . (5) 



a Rl b, b„ — o„ V, — 6, 



