SURFACE CHEMISTRY 59 



The experiments showed, however, that this conclusion, aUhough true 

 for over 99 percent of tlie surface, did not hold exactly. Deviations at very 

 low values of indicated that about 0.5 percent of the surface consisted of 

 what we may call active spots which adsorbed caesium far more strongly 

 than the rest of the surface, so that the heat of evaporation of these areas 

 was about 37 percent greater than that for atoms on the normal part of the 

 surface. Experiments also showed that adsorption on this part of the sur- 

 face was of the type indicated by Eq. (17) : the simple type of adsorption 

 isotherm. This means that the atoms adsorbed in the active areas did not 

 exert appreciable forces on one another. Evidently the active spots consist 

 of separated elementary spaces distributed over the surface, probably near 

 grain boundaries or at steps in the crystal faces. 



A variation of the dipole moment M with results from the depolariz- 

 ing effect of neighbouring dipoles. The electric fields due to these dipoles 

 can be calculated by an integration process similar to that used in calcula- 

 tion of F, and are of the order of 5 X 10''' volts cm~^. 



When a tungsten filament is heated to about 1,100° in caesium vapor 

 and there is an accelerating field drawing ions from its surface, two co- 

 existent surface phases may occur. For one of these is about 0.15, while 

 in the other it is very close to zero. The atoms that strike the less con- 

 centrated phase evaporate as ions, while those that strike the concentrated 

 phase evaporate as atoms. The phase boundary is perfectly stable and 

 stationary at a definite temperature in the presence of a given pressure of 

 caesium. If the temperature is raised slightly, the phase boundary moves 

 towards the concentrated phase, so that the whole filament gradually be- 

 comes bare ; a lowering of the temperature causes the whole surface to 

 become covered with the concentrated phase. A detailed analysis of the 

 mechanism at the boundary between the phases which involves diffusion 

 from one phase to the other, makes it possible to measure the surface 

 diffusion coefficient D. Results have shown that 



log D = — 0.70 — 3060/T. 



From the temperature coefficient of D it can be calculated that the surface 

 diffusion activation energy is about 0.6 volts. This means that between each 

 of the elementary spaces there is a potential barrier of the height of 0.6 

 volts over which the adatoms must hop in order to migrate over the surface. 

 The experimental methods involved in these studies of caesium films 

 on tungsten are capable of very great accuracy and sensitiveness, so that 

 they seem to offer a rich field for the detailed study of adsorption phe- 

 nomena. Dr. Taylor and I are continuing work in this field. We are plan- 

 ning to make accurate comparisons between the properties of films of 

 adsorbed caesium, rubidium and potassium. We have developed a method 



