3 8 PHENOMENA, ATOMS, AND MOLECULES 



removed from the surface ; in other words, hydrogen can remove mon- 

 atomic oxygen fihns from the tungsten surface if the amount of oxygen on 

 the surface is less than a definite amount. This probably means that the 

 adsorbed oxygen molecules cannot react with hydrogen atoms directly but 

 can react with hydrogen atoms adsorbed in adjacent spaces on the tungsten 

 surface. 



Thorium on Tungsten (i6). When a tungsten filament which is made 

 from tungstic oxide containing about i percent of Th02 is heated to tem- 

 peratures of 2,800° or more, a minute fraction of the thoria is reduced to 

 metallic thorium. The thoria exists in the filament in the form of minute 

 spherical particles distributed throughout the tungsten crystals and not at 

 the boundaries of crystals. If the filament is then heated for a period of a 

 few minutes at 1900 to 2,ooo°K, the metallic thorium which has been pro- 

 duced at the higher temperatures dififuses slowly through the crystal grains 

 to the crystal boundaries, then difi;uses rapidly along these boundaries to 

 the surface of the filament and then spreads over the surface of the filament 

 by surface migration and forms a monatomic film of adsorbed thorium 

 atoms on the surface of the filament. At 2,000° the rate of evaporation of 

 thorium from the filament is so small that sufficient thorium soon ac- 

 cumulates on the filament to form a nearly complete monatomic film. If the 

 temperature is raised to 2,200 or 2,400°, the rate of evaporation of the 

 thorium from the surface increases so much more rapidly than the rate of 

 arrival from the interior by dififusion, that the actual surface concentration 

 decreases greatly. 



These changes in the thorium content of the adsorbed film can be 

 studied by measurements of the electron emission from the filament at a 

 standard low temperature, called the testing temperature, which is chosen 

 so low that neither diffusion to the surface, nor evaporation from the 

 surface, causes appreciable changes in the adsorbed film. A convenient test- 

 ing temperature is 1,500°. At this temperature the presence of adsorbed 

 thorium on the surface may increase the electron emission as much as 10^ 

 fold over that from a pure tungsten surface. 



Such experiments make it possible to investigate the electrical properties 

 of surface films having known amounts of thorium. On the assumption 

 that each thorium atom on the surface acts as a dipole having a definite 

 dipole moment, it is possible to show that the logarithm of the electron 

 emission should increase linearly with the number of thorium atoms on the 

 surface. The experiments showed that for low concentrations of thorium 

 this relation holds approximately. 



There are several indications that the adsorbed film of thorium that is 

 formed by diffusion from the interior never exceeds one atom in thickness. 



The Interaction of Carbon Dioxide unth Carbon Filaments. If a carbon 



