132 PHENOMENA, ATOMS, AND MOLECULES 



wire was used to measure the pressure of the residual gas in the lamp. 

 After the lamp had run about 50 hours at normal efficiency (2400° K.) 

 the resistance of the platinum wire had increased 0.12 ohm, the original 

 resistance having been about 96 ohms. Investigation showed that this in- 

 crease of 0.125 per cent in resistance was caused by atomic hydrogen which 

 diffused into the platinum wire. The increase in resistance persisted for 

 days after the tungsten filament was turned off, but the resistance returned 

 to normal either after heating the platinum wire or after admitting a little 

 oxygen and then pumping it out again, the platinum wire being kept 

 at 125° C 



The dissociation of hydrogen by a tungsten filament at 1500° K., which 

 causes a fairly rapid clean-up of dry hydrogen, is entirely prevented by 

 pressures of oxygen or water vapor of an order of magnitude of io~^mm.^^ 

 A very remarkable phenomenon occurs if a mixture of oxygen and 

 hydrogen at low pressure is admitted to a bulb containing a filament at 

 1500° K. The oxygen reacts with the filament rapidly to form WO3, 

 which evaporates at this temperature as fast as formed. The oxygen thus 

 cleans up at a rate proportional to its own pressure and the pressure of 

 oxygen thus falls to half value about every 2 minutes in a bulb of ordinary 

 size. All this occurs exactly as though no hydrogen were present. During 

 this time there is no measurable disappearance of hydrogen. After 10 or 

 15 minutes the oxygen is nearly all gone and then for 5 or 10 minutes more 

 the gas pressure remains apparently constant and corresponds exactly to 

 that of the hydrogen which was admitted. Then suddenly, when the 

 pressure of oxygen is low enough (io~^ mm.), the hydrogen begins to 

 disappear by dissociation and in a few minutes the pressure falls practically 

 to zero. Water vapor has an effect similar to oxygen in preventing the dis- 

 sociation of hydrogen. At filament temperatures of 1750° K. some of the 

 hydrogen disappears while the oxygen is cleaning up, but the kink in the 

 curve still occurs when the oxygen is gone. Water vapor and oxygen are 

 thus powerful catalytic poisons for the reaction of the hydrogen dis- 

 sociation. 



ARCS IN HYDROGEN AT ATMOSPHERIC PRESSURES 



Several years ago in this laboratory studies were made of arcs between 

 tungsten electrodes in various gases. Arcs in hydrogen were remarkable 

 because of the high voltage drop and small cross section. A lo-ampere, 

 direct-current arc between heavy tungsten electrodes about 7 mm. apart in 

 a bulb containing pure hydrogen at atmospheric pressure appeared as a 

 sharply defined, brilliant red line about 0.5 mm. in diameter along which 



^^Langmuir, Jour. Amcr. Chcm. Soc, 38, 2271 (1916) ; Trans. Amer. Electro- 

 chem. Soc, sg, 261 (1916) ; Gen. Elec. Rev., 25, 445 (1922). 



