294 Dr. Norman Campbell on the 



8. The results for R w / and W may he dismissed shortly. 

 W was never changed by any treatment of the platinum ; 

 there was not the smallest evidence of any alteration in the 

 ionization potential. When Z was very small the precise 

 determination of: W was somewhat difficult, for the minimum 

 became very flat ; but in the most extreme case an error of 

 2 volts could not have been made, and in most cases it pro- 

 bably did not exceed ^ volt. R m ; , on the other hand, decreased 

 regularly with Z ; the smallest value found, corresponding to 

 the smallest value of Z, to be noticed immediately, was 0*302. 

 R OT ' was rather more variable than Z, probably because it 

 would be affected considerably by any small traces of gas. 

 However R„/, and the early part of the curve generally, is 

 not of much importance for our purpose. Other observers 

 have shown that it varies somewhat with the precise state of 

 polish of the surface; and in these experiments it was probably 

 also affected by the distortion of the platinum on heating. We 

 shall confine most of our attention to Z. 



9. After the state B was obtained, it was found that Z 

 was increased and the platinum tended to revert to its 

 original condition, if W was heated. W had been heated 

 continuously with P, and further heating for 24 hours caused 

 the evolution of no gas which could be measured on the 

 gauge (sensitiveness 0*0001 mm.). However, it was found 

 that P still continued to evolve gas on heating (doubtless 

 because it constituted with its leads a much more massive 

 piece of metal), and it is probable that W was still evolving 

 some gas. After the value of Z had been increased by this 

 means, it could be reduced to 0'47 again by heating P to a 

 bright red for two or three minutes. 



Further heating of P, both in a vacuum and in oxygen, 

 produced a further gradual decrease in Z to 0*29. At this 

 stage the heating of W still caused an increase in Z, which 

 was still immediately reversed by a short heating of P. The 

 effect of making P the cathode of a discharge carrying about 

 2 milliamperes in oxygen was then tried. This procedure 

 reduced Z still further, the lowest value which was noted 

 being 0*15; but the decrease was only temporary; Z increased 

 very rapidly if W were heated, so rapidly, in fact, that it was 

 difficult to measure Z, since measurements involved, of course, 

 the heating of W. In two minutes Z had returned to about 0*3. 

 This rapid variation made it impossible to determine W for 

 the platinum in this condition ; it also made it difficult to 

 determine whether Z changed when W was not heated, for 

 the time occupied by a single measurement of Z was almost 

 sufficient to restore the value 0*3. But it is probable that 



