646 Lm-d Kelvin [May 21, 



sphere, in which even the most oxidisable metals do not change, than 

 in a moist one, in which many metals undergo oxidation." 



§ 34. It is curious to find, thirty or forty years later, De la Rive 

 explaining away Volta's second discovery by moisture in the atmo- 

 sphere ! Fifty-one years ago, when I first learned Volta's second dis- 

 covery, by buying, in Paris, apparatus by which it has ever since been 

 shown in the ordinary lectures of my class in the University of 

 Glasgow, I was warned that De la Rive had found it wrong, and had 

 proved it to be due to oxidation of the zinc by 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, I cannot remember which), a varnished zinc 

 disc, scratched in places and moistened, sometimes 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 connection when the 

 discs were at their nearest, 



[§§ 35-40 are added in Feb. 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 hypo- 

 thesis, 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 ' Philo- 

 sophical Magazine ' a year later,* we find the following with reference 

 to writings of Helmholtz and myself on the contact-electricity of 

 metals : " Both these contact theories, in explaining the Volta effect, 

 ignore the existence of the oxidising 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 statement in the printed syllabus of his ' Six Lectures 

 adapted to a Juvenile Auditory, on the Principles of the Electric 

 Telegraph ' (Royal Institution, Dec. 28, 1897, Jan. 8, 1898). 



" Chemical method of producing a current — Voltaic cell — Two 

 " differently oxidisable metals immersed in an oxidising 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 oxidising gas and con- 

 " nected by a wire, can maintain an electric force or voltaic difference 

 " of potential in the space between them.] 



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

 brackets," 



And lastly, after some correspondence which passed between us 



t Prof. O. Lodge ' On the Seat of the Electromotive Force in a Voltaic Cell,* 

 Phil. Mag. Oct. 1885, p. 383. 



