370 Prof. 0. Lodge on the Controversy 



The evidence that leads Lord Kelvin to the conclusion 

 that the aether or space or vacuum is just as effective (or 

 ineffective) as oxygen, is derived from such experiments as 

 those of Bottomley (see also von Zahn, &c.) to the effect that 

 the Volta force continues unchanged in a vacuum : that an 

 alteration of the metals affects its value largely, but that 

 a change of the surrounding medium does not matter, unless 

 it actually corrodes the metals and spoils them. 



Bat this evidence derived from experiments on vacua has 

 never seemed to me conclusive, because nothing like a real 

 vacuum is ever attained, and even if it were, it is known with 

 what surprising tenacity ancient air-films cling to solid sur- 

 faces ; the gaseous substance within molecular range of the 

 solid substance being almost a part of it and not readily 

 changed. It clings by cohesion, just as the parts of the solid 

 substance itself cling. It is subject to Laplace's K, enormous 

 molecular pressure. Mere exhaustion, lessening the pressure 

 by one atmosphere, hardly disturbs it. 



It was the extreme difficulty of making experiments in 

 vacuum and substituted-media crucial that long ago deterred 

 me from attempting them. Mr. J. Brown attempted some * ; 

 and now fortunately Mr. Spiers has made a brave and dis- 

 tinctly successful attempt in the same direction, an attempt 

 that will, I hope, be persisted in f . 



That the free or bounding surface of the metal has some- 

 thing to do with the Yolta effect, at least in modifying its 

 value, is proved not only by the long-known observations, 

 say, of Ayrton & Perry, of Pellat and others, on the effect of 

 corrosion and oxide films, but also by the interesting expe- 

 riments of Mr. Erskine Murray on burnishing \. Simple 

 burnishing of the parts not in contact can run the Volta force 

 up considerably, from *7 volt to 1*02 volt in a certain instance ; 

 and scratching drops it again §. But these surface-effects are 

 admitted by Lord Kelvin, and are not thought to be even 



* Proc. Roy. Soc. vol. lxiv. p. 369 ; see also vol. xli. p. 294, and Phil. 

 Mag. 1878, 1879, and 1881. 



t Phil. Mag. January 1900. 



X Proc. Roy. Soc. vol. lxiii. p. 113. 



§ To this I should like to add that the gigantic differences of potential 

 caused by the contact and separation of insulators, or of a metal and an 

 insulator, e. g. the contact and separation of mercury or tin amalgam and 

 glass, are very familiar; and the great change, sometimes even of sign, 

 caused in this potential-difference by a modification of surface, for 

 instance by using glass whose smooth or burnished surface has been 

 ground with sand-paper, is well known, and inevitably suggests some 

 cause, whatever it may be, akin to that which causes a less violent but 

 somewhat similar drop of position in a volta series when a burnished 

 surface of zinc is scratched with sand-paper. 



