656 



Prof. G. W. Todd and Mr. S. P. Owen on a 



Taking the coordinates as in the figure, P (r, #, <£) being the 

 position of the volume element, the number of molecules 



having velocities between c and dc passing through unit ares 

 of the surface ds from P is 



ea 



Br 



— B . dN . e c dr dO d<j> sin 6 cos 6. 



Hence the total number with velocities between c and c + dc 

 passing through unit area from all directions is 



— ] J) Be c sin cos ddrd0d<j) = 





The number of molecules which have velocities greater 

 than a critical value c 1 is therefore 



e ' 2m r 3 . dc 



MX™)' 



Now let us consider the reversible " reaction/' 

 liquid ^ vapour. 



The number of molecules passing in unit time from the 

 liquid to the vapour is 



