116 PHENOMENA, ATOMS, AND MOLECULES 



Substituting these values in (lo) we get (for Wd in watts, P in atmos., 

 and 6" in cm.) 



v,,=\o.7 sVT.P^lC^+x) (13) 



TABLE I 

 THE DEGREE OF DISSOCIATION OF HYDROGEN INTO ATOMS 



At To = 3600 deg. and one atmosphere, x = 0.361 and Wd is then 

 170 S, while from (8) we find at this temperature that JVc is 22.4 5. Thus 

 the heat conductivity of the hydrogen between 3600 deg. and 300 deg. is 

 increased 8.6 fold by the dissociation. At 5000 deg. the factor is about 11. 



ARCS IN HYDROGEN AT LOW PRESSURES 

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

 tamination by the secondary spectrum, R. W. 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 had suggested ^^ 

 that the eflfect of moisture is to poison the catalytic activity of the dry glass 

 *3R. W. Wood, Proc. Roy Soc, 102, i (1922) and Phil. Mag., 44, 538 (1922). 



