Electromotive Forces in the Voltaic Cell. 183 
it to be first exposed to. If the experiment is regarded as 
sufficiently direct and simple, this fact lends support to the 
view that Volta forces depend on the medium surrounding the 
metals. 
The second modification is to bring an earth-connected iron 
bar close to the drops, and to show that it reduces the deflec- 
tion. Mr. Hart thinks it reduces the oxidation by proximity; 
and certainly, provided the obvious action of a mere electro- 
static screen has been considered and provided against, the 
action by proximity is very remarkable. A similar effect has 
been observed and more fully worked out by Pellat, in a paper 
published in 1882*. Pellat says that if he places two metallic 
surfaces parallel to one another and very close together (say 
half a millimetre more or less, variations from 12 to ‘1), each 
metal undergoes a slight alteration of the properties of its 
superficial coat. The alteration takes some minutes to pro- 
duce, increases with time, but tends to a limit. When the 
influencing metal is removed, the other returns gradually to 
its primitive state. Lead and iron produce the largest influ- 
ence effects; copper, gold, and platinum give smaller but 
distinct effects ; zinc produces hardly any, unless it be put 
within a hundredth of a millimetre or so. Pellat does not 
attempt to account for this interesting phenomenon further 
than by suggesting some possible connection with the smell 
of metals. 
Mr. Hart’s theoretical views are at first sight somewhat 
analogous to my own, though they are by no means the same. 
He considers the case of two metals immersed in liquid elec- 
trolytes, and dismisses air by calling it a gaseous electrolyte. 
He believes zinc and copper in contact to be at the same 
potential, and throws the variation of potential on the air 
between them}. He considers the electrical effect brought 
about by the electro-negative ion oxygen combining with the 
zinc and charging it negatively, while some electro-positive 
ion combines with the copper and charges it positively, “though 
not unless the two metals are in sufficient proximity to over- 
come electrolytic diffusion”? (whatever that may mean as 
applied to this case). He thinks his mercury-dropping expe- 
riment in coal-gas is conclusive as to the equality of potential 
of metals in contact. This, I fear, is rather rapid induction. 
* Comptes Rendus, xciv. (1882) p. 1247: “Influence of Metals on one 
another at a Distance.” 
+ The diagrams of potential which Mr. Hart gives of cells were given . 
more fully by Prof. Exner in his paper on the Theory of Galvanic 
Elements (1880). Mr. Hart’s views are, in fact, rather similar to some 
of the more reasonable ones of Prof. Exner. 
