372 Mr. J. Brown. Some Experiments bearing 



It remained to ascertain whether the fall in potential-difference was 

 due to the gradual absorption of chemically active matters by the 

 potassium and sodium, or to the well-known effect of gradual tarnish- 

 ing of the zinc surface. If, on admitting air and moisture to the tube, 

 the potential-difference increased, the former alternative would be 

 indicated, and the absence of such increase would indicate the other 

 alternative. 



Before testing this point, it was thought desirable to ascertain 

 whether the pressure originally in the tube had changed. To measure 

 the pressure, the sealed end of the leading tube was joined by a 

 rubber tube and mercury seal to the Sprengel pump, which was worked 

 till the pump gauge showed a pressure of about 2 mm. The end of 

 the leading tube was then broken off in the inside of this rubber tube, 

 a notch having previously been filed to facilitate breaking. The pump 

 gauge then fell, and ultimately stood at 90 mm. pressure, showing that 

 a considerable amount of gas had been evolved in the tube during its 

 six months' trial. 



The leading tube was now removed from the pump, and air admitted ; 

 air was also blown in by the mouth, to introduce moisture. The differ- 

 ence of potential at once rose to 



0-39 volt, and later to 0'4S volt. 



On taking out and examining the condenser, the zinc was found to> 

 be tarnished at the edges, but not much in the middle of its surface ; 

 the sodium was scarcely altered, but the potassium had a thick coat of> 

 no doubt, oxide or hydrate, covering a core which burned on w r ater. 



Experiment II, started December 9, 1889. This was intended to be 

 practically a repetition of Experiment I. In closing the end of the 

 main tube, a considerable amount of fumes and moisture from the gas 

 blowpipe was observed to get into it, which may have affected the con- 

 dition of the zinc surface. The moisture was removed by warming 

 the tube and washing out with air. 8 grams of potassium were 

 inserted, but no sodium. The nitrogen pressure before sealing off was 

 5 mm. After sealing off, the difference of potential was found to be 



0-70 volt, 



and fell thereafter more or less regularly for a year and a half, when, 

 on June 9, 1891, it had diminished to 



0-52 volt. 



On opening the tube this value did not sensibly change. The fall in 

 difference of potential was therefore probably due to tarnishing of the 

 zinc merely. The potassium in the tube was very little altered. 



Experiment III, started June 15, 1891. The arrangement was the 



