Contact Electricity of Metals, 111 



to oxidation of the zinc hy moisture from the air. I soon 

 tested the value of this warning by the experiments of § 5 

 above, and a considerable variety of equivalent experiments, 

 in one of which (real or ideal, 1 cannot remember which), a 

 varnished zinc disc, scratched in places and moistened, some- 

 times on the scratched parts and sometimes where the varnish 

 was complete, was tested in the usual manner by separating 

 from contact with an unvarnished or varnished copper disc, 

 with or without metallic connexion when the discs were at 

 their nearest. 



[§§ 35-40 are added in February 1898.] 



§ 35. Within the last eighteen or twenty years there has 

 been a tendency among some writers to fall back upon De la 

 Rive's old hypothesis, of which there are signs in expressions 

 quoted by Professor Oliver Lodge in his great and valuable 

 report of 1884, and in some statements also of Professor 

 Lodge's own views. 



In what is virtually a continuation of this report in the 

 Phil. Mag. a year later*, we find the following with 

 reference to writings of Helmholtz and myself on the eon- 

 tact-electricity of metals : — " Both these contact theories, in 

 explaining the Yolta effect, ignore the existence of the 

 oxidizing medium surrounding the metals. My view explains 

 the whole effect as the result of this oxygen bath, and of the 

 chemical strain by it set up." With views seemingly un- 

 changed, he returned to the subject at the end of 1897 with 

 the following slateraent in the printed syllabus of his " Six 

 Lectures adapted to a Juvenile Auditory, on the Principles of 

 the Electric Telegraph" (Royal Institution, December 28, 

 1897, January 8, 1898) :— 



" Chemical method of producing a current — Voltaic cell — 

 Two differently oxidizable metals immersed in an oxidizing 

 liquid and connected by a wire can maintain an electric 

 current, through the liquid and through the wire, so long 

 as the circuit is closed. [The same two metals immersed 

 in a potentially oxidizing gas and connected by a wire, can 

 maintain an electric force or voltaic difference of potential 

 in the space between them.] 



ci N.B. — No one need try too hard to understand sentences 

 in brackets." 



And lastly, after some correspondence which passed between 

 us in December, I have to-day (Feb. 14) received from him 



* Prof. 0. Lodge " On the Seat of the Electromotive Force in a Voltaic 

 Cell," Phil. Mag. October 1885, p. 383. 



