Mr. F. S. Spiers on Contact Electricity. 89 



the P.D. was exactly zero, and on taking out the iron plate 

 and examining it I found it completely covered with a thin 

 black scale of iron oxide. There is no doubt, then, as to the 

 effect of surface oxidation ; and it is in agreement with other 

 experimenters, who find that oxidation always causes the 

 contact activity of metals to disappear. 



§ 15. The net result of the experiments described above in 

 § 14 is strongly in favour of that theory of the Volta effect 

 which supposes it to be due to some electrolytic action of the 

 medium or media surrounding the dissimilar metals *. We 

 have seen how, by as far as possible removing the oxygen 

 from the surfaces of an iron-platinum couple and replacing it 

 by hydrogen, there resulted an enormous total change in 

 the contact E.M.F., namely from +0*37 volt to — O60 volt; 

 how on again admitting air, so that a mixture of air and 

 hydrogen surrounded the plates, the P.D. slowly moved in 

 the positive direction, reaching after 16 hours a value of 

 — 0*17 volt ; and how, after heating the plates so as to 

 artificially hasten the removal of hydrogen from the surfaces 

 of the metals and its replacement by air, the value of the P.D. 

 became still more positive, namely +0*22 volt. There is a 

 strong presumption of truth in the assertion that these curious 

 variations in the contact force under varying conditions of 

 atmosphere, which become so intelligible when looked at in 

 the light of the electrolytic theory, appear impossible and 

 incomprehensible when considered from a standpoint like that 

 taken up by the pure contact theorists. 



§ 16. This investigation does not in any way pretend to be 

 final: it is hardly more than preliminary; but I venture to think 

 that I have accomplished, or at least got near accomplishing, 

 . what has never before been attained in the history of contact 

 electricity; namely, the entire removal (by chemical means) 

 of the " ancient air-sheets " from the surfaces of the metals 

 and making measurements in an entirely different medium, 

 without at the same time permanently altering the surfaces 

 of the metals by causing them to form salts (as happened, for 

 instance, in the valuable experiments of Mr. J. Brown |). 

 Unfortunately, circumstances compelled me to abandon this 

 work for the time being, and thus prevented me from con- 

 tinuing experimental investigations in the same direction. 



* The supposed mechanism of this action 1 need not discuss here. 

 Valuable and ingenious suggestions on this head have been made by 

 Prof. Oliver Lodge in his Beport to the British Association in 1884, 

 quoted above, and they have been further elaborated in an interesting 

 series of letters that passed between Prof. Lodge and Prof. Burnside in 

 the columns of ' Nature,' vol. xliii. 



t Phil, Mag. Aug. 1878, and Feb. 1879. 



