L 655 ] 



LXVI. .4 Vapour Pressure Equation. By George W. Todd, 

 D.Sc. (Birm.), B.A. (Cantab.), Professor of Experimental 

 Physics, Armstrong College, and S. P. OwEN,i?.<$c. ( Wales)*. 



IN a previous paper (Phil. Mag. xxxvii. p. 1^24, 4949) 

 the authors have considered the influence of tempe- 

 rature on homogeneous reactions from the point of view 

 of the kinetic theory, and the usual form of expression for 

 the variation of the equilibrium constant with temperature 

 was obtained. 



In this paper we have investigated the simplest type 

 of heterogeneous reaction — the equilibrium of a vapour in 

 contact with its liquid — making the assumptions (1) that 

 all vapour molecules striking the liquid surface penetrate it, 

 and (2) that only those liquid molecules which have a 

 velocity greater than a definite critical value, whatever 

 their angles of impact with the surface, penetrate it and 

 get clear. 



The number of molecules in 4 c.c. having velocities 

 between c and c + dc is 



A. / \ "I mcl 



ajs - %/7r ^ \*B,d)- e ' c ' clc > 



the symbols having the same significance as in the afore- 

 mentioned paper. 



If B denotes the collision frequency of a molecule whose 

 velocity is c, then the probability that the particle will travel 

 a path greater than r without collision is given by 



Br 



e 



In a volume element r 2 sin 6 dr dO dcf> there are 

 dN r 2 sin 6 dr d6 d<f> molecules having a velocity between 

 c and c + dc. On the average each of these collides B 

 times per second and hence begins B new paths. Therefore 

 the number which proceeds from the volume element in 

 unit time is B . c/N . r 2 s'm 6 dr d0 dxf). These proceed in all 

 directions ; hence the number passing through unit area of 

 a sphere of radius r with the volume element as centre is 



1 _ B r 



: B .dN .e c sin 6drd Odxb. 



47T r 



* Communicated b} r the Authors. 



