Vapour Pressure Equation. 657 



where c x is the critical velocity necessary for the penetration 

 of the surface. The number of vapour molecules entering the 

 liquid surface per second is on our hypothesis 



v/2 



IT 



F) 



= e 



mc\ 

 ~2R9 



For equilibrium conditions n v —■ n t . We have also k v N v = kiNi, 

 where k is the velocity constant of the reaction. Therefore 

 the equilibrium constant 



K = — = ni ^ v 

 h v nvNi 



\ ' 2Rey 



Let us write the equation in the form 



N.= N,« •■(1 + 5), 



mcj 2 

 where ri = -^=p~- 



Now NvocP and N^oc p, where p is the vapour pressure 

 and p is the density of the liquid at the absolute temperature 0. 

 Thus we get for the vapour-pressure equation 



B 



p = Ape-'(l + - g ) ; . (1) 



In the few cases where density data up to the critical 

 temperature are available this expression agrees well with 

 the experimental results over the whole range of temperature 

 through which the liquid phase exists. For moderate ranges 

 of temperature where it may be assumed that the con- 

 centration of the liquid molecules remains constant, the 

 expression 



p = A'e {l+ ) (2) 



is sufficiently good (see the following tables). 



