FLAMES OF ATOMIC HYDROGEN 131 



not in the form of liquefied or solidified atomic hydrogen, but is held there 

 rather as an adsorbed film. Hydrogen atoms striking a bare glass surface 

 can condense, but if they strike atoms already on the surface they combine 

 with these and the molecules escape. 



The maximum amount of hydrogen that can be adsorbed by glass cooled 

 in liquid air was found to be 0.03 cu. mm. per sq. cm., the hydrogen being 

 measured at atmospheric pressure and temperature. This corresponds to 

 1.5 X 10^^ atoms per sq. cm. If these are arranged in a close-packed lattice 

 on the surface their distance between centers would be 2.78 X io~* cm. 

 The diameter of the electron orbit in the Bohr hydrogen atom is 1.06 X 

 io~^ cm. This result confirms the theory that impinging hydrogen atoms 

 combine with adsorbed hydrogen atoms which they strike. If the glass 

 surface is allowed to warm up to room temperature some of the atoms can 

 react with adjacent atoms and thus escape as molecules. 



The atomic hydrogen adsorbed on glass is capable of reacting rapidly 

 with oxygen at room temperature and slowly even at liquid-air tem- 

 perature. Thus if hydrogen is cleaned up by a heated filament on to a bulb 

 cooled in liquid air, and oxygen is admitted after cooling the filament, the 

 oxygen disappears very slowly; but when the liquid air is removed an 

 amount of oxygen suddenly disappears which is the chemical equivalent of 

 the hydrogen that was previously cleaned up. This removal of the adsorbed 

 hydrogen makes it then possible to clean up another portion of hydrogen 

 on to the bulb when the filament is lighted. 



Atomic hydrogen from a filament in hydrogen at low pressure, even 

 after dififusing through several feet of glass tubing at room temperature, 

 can manifest itself by reducing such metallic oxides as WO3, CuO, FcaOs, 

 ZnO, or Pt02.-^^ The blackening of WO3 or ZnO thus serves as a means 

 for detecting atomic hydrogen. An oxidized piece of copper rapidly ac- 

 quires a metallic luster. The fact that the atoms can pass through a long 

 crooked tube proves that adsorbed hydrogen atoms can evaporate from 

 glass surfaces at room temperature. Experiments showed that no atomic 

 hydrogen passed through a trap cooled in liquid air, so that at this tem- 

 perature the evaporation of the atomic hydrogen does not occur. A plug 

 of glass wool in a tube at room temperature also stops the passage of the 

 atoms. 



A well-exhausted tungsten filament lamp was connected through a glass 

 tube to a Hale-Pirani vacuum manometer ^^ which contained pure platinum 

 wire 0.028 mm. in diameter and 45 cm. long heated to about 125° C. by 

 the passage of a current of 0.0092 ampere. The resistance of the platinum 



'* Langmuir, Proc. Amer. Inst. Elec. Eng., 33, 1894 (1913) ; Trans. Amer. 

 Electrochem. Sqc, 2g, 294 (1916). 



^'^ Hale, Trans. Amer. Electrochem. Soc, 20, 243 (1911). 



