Mr. J. Brown on the Theory of Voltaic Action. 213 



of potential diminishes, and finally disajjpears if the experiment 

 be continued sufficiently long. This diminution is also seen 

 in the copper-nickel experiment in hydrochloric acid gas. If 

 the potential-difference is due to " contact action " between the 

 metals, their sulphides, and the surrounding gas, why does it 

 disappear while all these conditions are still present ? Its dis- 

 appearance seems to me to be obviously due to the cessation 

 of chemical action; and even its diminution under these cir- 

 cumstances appears to completely dispose of the assumption 

 that the effect is due to such " contact action." From the 

 statement that my zinc plates " were coated with zinc sul- 

 phide or zinc chloride," one is with regret led to 



infer that Professors Ayrton and Perry have not paid much 

 attention to the work they criticise. No zinc was used in the 

 experiments with sulphur and chlorine compounds. 



It is needless to occupy space by repeating my communica- 

 tion to the i Electrician ' respecting Dr. Exner's view of the 

 voltaic theory and my own; but I am not conscious of having 

 made the mistake attributed to me at p. 46. I said nothing 

 about the " actual electric charge " in the oxide film, gas film, 

 or platinum, and was quite aware it is not measured by Kohl- 

 rausch's method. 



Respecting PfafF's work, it is well to remember that, accord- 

 ing to De la Rive*, the electricity produced by the oxidation 

 of '00000000002 gramme of zinc is sufficient, with the help 

 of a condenser, to cause divergence of the gold leaves of an 

 electroscope. Did Pfaff succeed in keeping his apparatus free 

 from quantities of oxygen of this order ? Even the Crookes's 

 vacuum in Professors Ayrton and Perry's proposed experi- 

 ments may not give any but negative results. In such expe- 

 riments " the gas film previously condensed on the metal is 

 not removed, since it is held fast by a force far greater than 

 the pressure of an atmosphere "f. 



The extreme difficulty of getting rid of occluded or con- 

 densed gas from the metallic electrodes of highly exhausted 

 tubes is well knownj. 



Yours truly, 



J. Brown. 



Belfast, Feb. 12, 1881. 



* Traite d'Electricite, t. ii. p. 777. 



t Wiedemann, Galvanismus, i. p. 13. 



X See Hittorf, Wied. Ann. 1879, No. 8, p. 574. 



