Electron emission---Curves showing the logarithm of the electron current per ${\mathrm{cm}}^{2}$ from tungsten and oxidized tungsten over a wide range of filament temperatures are given for several vapor pressures of caesium. At high temperatures the tungsten is covered only to a slight extent with adsorbed caesium. As the filament temperature is lowered more caesium is adsorbed. This lowers the electron work function and increases the emission many thousandfold. The process continues until a temperature is reached at which the tungsten is just covered with a monatomic layer when the work function has a minimum value. At still lower temperatures the surface is more than completely covered, the work function increases again, and the emission decreases rapidly.The positive ion emission is constant while the temperature decreases from a high value to a low critical temperature. Here the ion emission drops suddenly while some caesium sticks to the filament. Further decreases in temperature are followed by increased adsorption and decreased ion emission. If the temperature is then increased in steps the ion current retraces its path. At an upper critical temperature, about 50\ifmmode^\circ\else\textdegree\fi{} higher than the lower critical temperature, the filament cleans itself spontaneously, the caesium comes off as ions and registers as a sudden rush of current. At higher temperatures the ion current has its initial constant value which is limited by the arrival rate of caesium atoms. The critical temperatures are raised by an increase in the vapor pressure or by a decrease in the plate potential.A method of determining the amount of adsorbed caesium is developed. At a sufficiently high filament temperature the surface is clean. At a sufficiently low temperature every atom that strikes the filament sticks to it, at least until the optimum activity is reached. The product of the arrival rate, which is given by the steady positive ion current, and the time to attain the optimum activity gives the number of caesium atoms at the optimum activity. At an intermediate temperature the surface is only partly covered. If the temperature is suddenly dropped, to a low value, it takes a shorter time to reach the optimum activity. From these times the amount of adsorbed caesium at various temperatures, plate potentials, and vapor pressures can be determined. At the optimum activity there are 3.7\ifmmode\times\else\texttimes\fi{}${10}^{14}$ atoms of caesium on a ${\mathrm{cm}}^{2}$ of tungsten. This is very nearly the same as the number of caesium atoms that could be packed in a single layer, but is considerably smaller than the number of caesium ions in such a layer.The adsorption or evaporation characteristics are illustrated by curves. Caesium can evaporate either as ions or as atoms. The atomic rate depends only on the temperature and on $\ensuremath{\theta}$, the fraction of the surface covered with caesium. For a given temperature it increases very rapidly with $\ensuremath{\theta}$. The ions can permanently escape from the filament only if the potential is in the right direction. A typical isothermal ion rate curve increases rapidly with $\ensuremath{\theta}$, comes to a maximum when $\ensuremath{\theta}$ is about.01, then decreases continuously for larger $\ensuremath{\theta}$. These curves explain all the observed phenomena of these adsorbed films. They show that while the ion work function increases as $\ensuremath{\theta}$ increases, the work to remove an atom decreases with $\ensuremath{\theta}$. The ion work function for a given $\ensuremath{\theta}$ can be decreased by increasing the potential gradient at the filament.