HYDROGEN INTO ATOMS 217 



evaporates so rapidly from the surface that only a negligible fraction of 

 the surface is covered at any time. This equation has already been shown 

 to be in full agreement with the experiments (Parts I and II). 



4. The conclusion that very little of the surface is covered is in good 

 accord with the fact that the accommodation coefficient of hydrogen is 

 0.19 at temperatures below 1500° K., while the "chemical" accommoda- 

 tion coefficient a2 for the dissociation of hydrogen is 0.68 at high tem- 

 peratures. 



5. The fact that the electron emission from heated tungsten is not 

 affected by the presence of pure hydrogen is additional evidence that the 

 fraction of the surface covered by hydrogen atoms or molecules must be 

 very small. 



