354 



PHENOMENA, ATOMS, AND MOLECULES 



Mathematical analysis ^^ shows that the condition for a stationary boundary 

 (when D, the surface diffusion coefficient, can be taken to be independent 

 of 6) is that Xi = X^ when (v^ + Vp) is plotted as a function of B. The 

 dotted line in Fig. 19 has been so drawn. If ^a is raised, the concentrated a 

 phase will appear. The velocity of motion depends on the displacement 

 of [Xo. 



On 



Fig. 19. Atom (va) and ion (v;,) evaporation rates for zero field at the lower values 

 of 0. (Calculated at 848° K.) Circles give observed data under conditions as 



described in text. 



Boundaries are established by the formation and growth of nuclei at 

 slight inhomogeneities of the surface on which Q man increase or decrease 

 more rapidly than on neighboring areas. Nucleus growth is not possible 

 above point A in Fig. 19 since the dilute /5 phase is unstable. It may and 

 does occur for any value of fio producing a B between points A and B. The 

 \Xa or Q at which formation and growth begin depends on the surface con- 

 ditions and on how rapidly \Xn (or ^) is varied. 



In particular, in Fig. 18 the points where a discontinuity sets in depend 

 entirely on this accidental nucleus formation and have no other significance. 

 A detailed comparison of these points in Fig. 18 with the curves of Fig. 19 

 is difficult since Vp in the former is affected by the field, whereas the latter 

 curves represent field free emission of ions. 



As previously discussed (Section IV), a series of ion evaporation ex- 

 periments were carried out with the bulb in liquid air so that [Xa was 



^^ I. Langmuir, Jour. Chem. Phys., i, 3 (1933). 



