on the Theory of Voltaic Action. 373 



same as in Experiment I, except that, after the tube had been exhausted 

 and finally sealed off, the 9 grams of potassium and 3 grams of sodium 

 which had been introduced, were fused together, forming the alloy that 

 is liquid at ordinary temperatures. The difference of potential was at 

 first about 



0-75 volt. 



During the first year, the whole tube (except when being examined) 

 was kept immersed in a bath of petroleum, to prevent leakage of air, 

 in case of minute imperfections at the sealed-in wires or elsewhere. 

 The difference of potential on July 22, 1892, was about 



0-67 volt. 



The tube, no longer kept in petroleum, was examined occasionally 

 for the next six and a half years, till November 4, 1898, when it was 

 opened. The pressure had risen to about 59 mm. of mercury. The 

 difference oi potential just before opening was about 



0-49 volt; 



and after opening and blowing in there was little appreciable change in 

 this value. If anything, it seemed rather lower; though the rapid 

 tarnishing of the potassium-sodium alloy, when a new surface of it was 

 exposed, indicated the presence of an ample amount of oxidising 

 medium. The decrease of potential difference, in this case also, was 

 therefore probably due to tarnishing of the zinc surface. The zinc 

 was almost as bright as when enclosed seven and a half years before; 

 but polishing a small portion of its surface with glass paper showed 

 that a slight film had formed. 



Of the three experiments, the first is the only one that lends any 

 support to the hypothesis they were designed to illustrate. The 

 laboratory notes show no difference between the first experiment and 

 the other two, beyond what may be gathered from the foregoing- 

 account. It seems unlikely that the three days of accidental metallic 

 contact in the first experiment or the distance apart of the plates in 

 intervals between observations can have affected the result ; and I am 

 unable to suggest any other explanation except the possibility that the 

 phosphorus pentoxide was left in the tube as well as the potassium 

 and sodium. I found on a previous occasion,* that when this substance 

 was enclosed with a copper-zinc pair, so as to dry the air surrounding 

 the pair, the difference of potential fell in 134 days by one-sixth of its 

 first value, and rose to its original amount immediately on admission of 

 the ordinary atmosphere. In No. Ill certainly no phosphorus pent- 

 oxide was enclosed, and there is no mention of it in my notes of 

 No. II. 



* ' Boy. Soc. Proc.,' vol. 41 (1886), p. 305. 



