SURFACE CHEMISTRY 



37 



which is characteristic of the clean-up of hydrogen in absence of oxygen. 

 Repeated experiments with different amounts of oxygen and hydrogen 

 prove that hydrogen does not interfere with the clean-up of oxygen but 

 that a minute trace of oxygen of about io~^ barye completely prevents the 

 dissociation of hydrogen into atoms by a tungsten filament at i,500°K. The 

 oxygen thus acts as a catalytic poison. Experiments of this kind can be 



10 MINUTES ZO JO 



Fig. I. Clean-up of Hydrogen and Oxygen in a Tungsten Filament lamp. 



used to make quite accurate chemical analyses of mixtures of oxygen and 

 hydrogen. .^ 



The fact that a monatomic film of oxygen on tungsten does not react 

 with hydrogen at 1,500° is a striking indication that the oxygen is in a con- 

 dition far different from that of gaseous oxygen. The results, however, 

 confirm our conclusion that the film consists of oxygen atoms which are 

 chemically saturated by the tungsten atoms with which they are in contact. 



In view of the fact that an oxygen film on tungsten at i,500°K does not 

 evaporate in less than a year, it is remarkable that in the presence of 

 hydrogen the beginning of the hydrogen clean-up (Curve V) is so sharp. 

 Since Curve V is exactly like the lower part of Curve I, it appears that 

 once the hydrogen begins to dissociate, the oxygen is suddenly completely 



