EVAPORATION AND CONDENSATION 301 



molecules from heated tungsten does not exceed 30 per cent. It is probable 

 in this case also, that the small values of e at low temperatures are caused 

 by an inactive condition of the greater part of the surface, rather than by a 

 change in the reflectivity. 



Experiments on the oxidation of molybdenum filaments have led to 

 similar conclusions in regard to the reflection of oxygen molecules. 



When a wire of tungsten, or other metal capable of withstanding high 

 temperature, is heated to 1500° K. in hydrogen at low pressure, a part 

 of the hydrogen is converted into atomic hydrogen. ^'^ At very low pressures 

 this active modification of hydrogen can diffuse long distances through 

 glass tubing at ordinary temperatures,^^ but cannot pass even short dis- 

 tances through tubing cooled by liquid air. 



When one considers that the pressures employed in these experiments 

 were so low that the normal free path would be of the order of several 

 meters, it is evident that the reflectivity of hydrogen atoms from glass 

 surfaces cooled in liquid air must be very small, if not actually zero. With 

 the glass at room temperature the reflectivity is probably also negligible, 

 but the atoms then reevaporate more rapidly and are thus able to travel 

 considerable distances. 



In connection with a quantitative study of the degree of dissociation of 

 hydrogen at various temperatures, it has been shown ^^ that every hydrogen 

 atom striking a pure tungsten surface (at 2000-3500° K.) condenses and 

 that 68 per cent, of all hydrogen molecules striking the surface condense. 



When the tungsten filament was heated to a temperature over 2700° K. 

 in hydrogen at a pressure below 200 bars, it was found that 68 per cent, 

 of all the molecules striking the filament were dissociated and that this 

 fraction remained unchanged even when the filament temperature was 

 raised several hundred degrees higher. 



The fact that this coefficient (0.68) is larger than the accommodation 

 coefficient found for a tungsten wire at 1500° in hydrogen (0.19) requires 

 some explanation. It seems at first sight paradoxical that 68 per cent, of 

 the molecules reach chemical equilibrium and are dissociated, whereas at 

 lower temperatures only 19 per cent, reach thermal equilibrium before 

 leaving the surface. In a recent paper -° the writer gives independent 

 evidence that at lower temperatures the tungsten surface is practically 

 completely covered with a layer of hydrogen molecules, whereas at higher 

 temperatures only a minute fraction of the surface is so covered. Thus the 

 coefficient 0.19 applies to hydrogen molecules striking a surface alreadv 



^^ Langmuir, Jour. Amer. Chetn. Soc, 34, 1310, 1912. 

 '^ Freeman, Jour. Amer. Chcni. Sac, 35, 927, 1913. 

 *® Langmuir, Jour. Amer. Ghent. Soc, 37, 457, 1915. 

 ^° Jour. Amer. Chem. Soc., 38, 1145, 1916. 



