SURFACE CHEMISTRY 35 



By cooling the bulb in liquid air the adsorption increased. The maximum 

 amount of hydrogen adsorbed on the glass in these experiments was 0.03 

 mm^ per cm^ measured as molecular hydrogen at atmospheric pressure. 

 This corresponds to = 1.5 X 10^^ atoms of hydrogen per cm-, which is 

 equal to the number of spheres of diameter ^ 2.8 A which can be packed 

 per cm- into a close-packed hexagonal lattice. The spacing of the adsorbed 

 atoms is probably determined by the arrangement of the atoms in the 

 underlying glass which provide "elementary spaces" in which atoms can be 

 held. If the average diameter of the atoms of the glass can be assumed to 

 be 2.8 A, the observed maximum amount of adsorbed hydrogen thus agrees 

 well with that to be expected in a monatomic film. If a glass surface satu- 

 rated with atomic hydrogen at liquid air temperature is allowed to warm 

 up to room temperature, part of the hydrogen escapes as molecular hydro- 

 gen; the rest can be driven off at 300° C. Since adsorbed hydrogen atoms 

 probably react on contact to form molecular hydrogen, then experiments 

 indicate that the surface mobility of the adatoms ^ is very small at room 

 temperature. 



It was shown that the atomic hydrogen can diffuse long distances 

 through glass tubing at room temperature (but not at liquid air tem- 

 perature) and can then reduce metallic oxides (9), such as WO3, CuO, 

 Fe203, ZnO or Pt02, and can dissolve in platinum sufficiently to raise its 

 resistance (10). It reacts at room temperature with phosphorus (3) to 

 form PH3. 



Oxygen Films on Tungsten. When a tungsten filament is heated to 

 i,500°K or more in oxygen at very low pressures, such as 100 baryes or 

 less, the oxygen reacts (11) with the tungsten to form the oxide WO3 

 which evaporates from the filament at these temperatures as fast as it is 

 produced, leaving the surface of the filament apparently clean. At tempera- 

 tures below about 2,200°K the presence of extremely small amounts of 

 oxygen (io~^ mm) decreases the electron emission from a tungsten fila- 

 ment to values that range from lo"^ to 10^^ of that from pure tungsten, 

 depending on the temperature at which the emission is measured. This 

 modification of the properties of the surface must depend upon the presence 

 of a film which contains oxygen. When the filament temperature is high, 

 such as 2,000° or more, the emission returns to a normal tungsten emission 

 as soon as the oxygen is completely consumed or is removed by vaporizing 

 into the bulb a "getter" such as magnesium. If, however, the filament tem- 

 perature is as low as 1,500°, complete removal of oxygen from the gas 



^ Atomic and molecular distances will be given in terms of the Angstrom unit 

 10"^ cm, which will hereafter be denoted by A. 



^ J. A. Becker, Trans. Amer. Electrochem. Soc. 55, 153 (1929), has suggested this 

 term for adsorbed atoms. 



