and the Is other mals of Vapours. 443 



Substituting t- for Vu -r f° r v 2, and -7 r for 6, this 



becomes 



di+rf., 



^=RT(rf 1 + rf 2 )(|j Jl " 2 (1) 



We thus obtain an equation for the saturated vapour 

 pressure in terms of the densities of the liquid and its 

 saturated vapour, the absolute temperature, and the gas 

 constant. The equation contains no adjustable constants 

 whatever. 



The constant b of Dieterici's equation is here replaced by 



-j — , which varies slowly with the temperature. Similarly, 



d>i ~\~ «2 



Dieterici's A now becomes 



2RT di 



di + d 2 ge d 2 ' 



also a function of temperature. For the critical temperature, 



2 

 when d 1 — d 2 — d c =j-, we find 



Oc 



P> 



= 2RT r d c e~ 2 . 



The equation, like Dieterici's, thus gives the satisfactory 



value ie 2 = 3'695 for at the critical point. 



pv 



The agreement of the values given by the equation (1) 



with these directly observed has been tested by calculation 



for the substances * whose densities in liquid and vapour 



states are given in the tables of L. Graetz's article on 



vapour pressures in Winkelmann's Handbuch der Pln/slk, 



2nd ed. vol. iii. pt. 2, pp. 962-1086. The approximation to 



actual values is about the same for all these substances, 



whether associated or non-associated. Table I. is typical : 



* Ammonia, Carbon Dioxide, Water, Pentane, Hexane, Heptane, 

 Octane, Isopentane, Di-isopropyl, Di-isobutyl, Hexamethylene, Ethyl 

 Ether, Methyl Formate, Ethyl Formate, "Propyl Formate. Methyl 

 Acetate, Ethyl Acetate, Propyl Acetate, Methyl Propionate, Ethyl 

 Propionate, Methyl Butyrate, Methyl Isobutyrate. 



