in Gases to the Contact Potential Series. 393 
gas-pressure, 1s strikingly less than for hydrogen. This is 
evident when the curves for oxygen (fig. 4), taken at pressures 
0°8 and 1:17 mm., are compared with those for the same 
metals in hydrogen (fig. 3) at 1 mm. pressure. This increase 
for oxygen is also smaller than for nitrogen, though the 
minimum fall for nitrogen is about 50 per cent. less than for 
oxygen. 
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Of present interest, however, is the fact shown by the 
experiments with all three gases. The series of metals 
arranged according to the magnitude of the cathode-fall is the 
