FLAMES OF ATOMIC HYDROGEN 133 



the potential gradient was 150 volts per centimeter, making a power dis- 

 sipation of 1500 watts per centimeter of length, this being about fifteen 

 times as great as in nitrogen or argon. This abnormal behavior of hydrogen 

 was attributed to the dissociation which carried energy so rapidly out of 

 the arc. 



ARCS IN HYDROGEN AT LOW PRESSURES 



In attempting to obtain the Balmer spectrum of hydrogen without con- 

 tamination by the secondary spectrum. Wood ^^ built very long vacuum 

 tubes of moderate bore, in which he passed currents as large as 20 amperes 

 through moist hydrogen at about 0.5 mm. pressure. He observed many 

 remarkable phenomena. Short pieces of tungsten wire projecting into the 

 discharge were heated to incandescence, although a fine thread of glass 

 or a platinum wire in a similar position was apparently not heated by 

 the discharge. On drying the hydrogen with phosphorus pentoxide the 

 secondary spectrum (due to molecular hydrogen) appeared strongly and 

 the Balmer spectrum (due to atomic hydrogen) nearly disappeared. The 

 heating of the tungsten wire was also prevented by drying the hydrogen. 



In correspondence with Professor Wood, the writer pointed out that 

 oxygen and water vapor decrease the rate of dissociation of hydrogen in 

 contact with tungsten and must thus also tend to prevent the recombination 

 of hydrogen atoms on a tungsten surface. He also suggested that moisture 

 poisons the catalytic activity of the dry glass surfaces that otherwise con- 

 verts atomic into molecular hydrogen. Thus with moist hydrogen the tube 

 becomes filled with nearly pure atomic hydrogen and the diffusion of this 

 to the catalytically active tungsten wire causes the heating of the latter. 

 Calculations based on the measured heat of dissociation proved that a 

 pressure of only 0.16 mm. of atomic hydrogen at 500° C. would suffice to 

 maintain a tungsten filament at 2400° K. 



These conclusions were confirmed by Wood's observations that the 

 walls of the tube became only slightly heated if the hydrogen was moist, 

 whereas they were strongly heated with dry hydrogen. A tungsten wire was 

 heated red hot even when mounted in a side tube (of 5 mm. diameter) at a 

 distance of 4 cm. from the discharge tube, showing that the hydrogen atoms 

 could diffuse in relatively large quantities out of the discharge. 



It occurred to the writer that it should be possible to obtain even higher 

 concentrations of atomic hydrogen by passing powerful electric arcs 

 between tungsten electrodes in hydrogen at atmospheric pressure and this 

 atomic hydrogen could be blown out of the arc by a jet of molecular 

 hydrogen directed across the arc. 



14 



Proc. Roy. Soc, 102, i (1922) ; Phil. Mag. 44, 538 (1922). 



