Mr. F. S. Spiers on Contact Electricity. 85 



(Dr. Erskine-Murray does not mention that the high 

 potential produced by burnishing is merely a temporary 

 effect), and remained practically constant at that value. I 

 then exhausted and washed out the apparatus with hydrogen 

 and repeated the process three times just as described in § 12. 

 The tube was then heated with a blowpipe-flame and kept at 

 a bright red heat for about 20 minutes. It was then, of 

 course, full of hydrogen, at about an atmosphere pressure 

 (when hot). Knowing the pressure of the gas in the tube 

 both when it was hot and cold I was able, by making certain 

 assumptions as to the distribution of heat, to roughly estimate 

 the maximum temperature of the gas in the tube. After 

 keeping the outside copper tube at a bright red heat for 

 about 40 minutes the maximum inside temperature was not 

 less than 800° C. 



The next morning the P.D. was 



— 0*40 volt (platinum positive to iron). 



I then heated again, as before, for 33 minutes ; when cold, 

 after about three hours, the P.D. was 



-0'06 volt. 



I again heated for half an hour. The next morning the 

 reading was 



-0*28 volt. 



I heated again for half an hour. After three hours the P.D. 

 was 



-0-05 volt. 



I repeated the operation. The next day the P.D. was 



-0-13 volt. 



On again heating for half an hour the P.D., as measured 

 after two hours' cooling, was 



+ 0-05 volt. 



1 further heated for 35 minutes. In two hours and a half the 

 P.D. was 



-0-02 volt. 



I heated again for half an hour. The next morning the 

 voltage stood at 



-0-16 volt. 



I had hoped that after so many successive heatings and cool- 

 ings the P.D. would have settled down to a steady value after 

 all the oxygen had been got rid of, and that this value would 

 be the true contact P.D. in an atmosphere of hydrogen. But, 



