270 Prof. Oliver Lodge on the Seat of the 
S (1°8—:8) 
StH 
measures , where 2 is the capacity of the electro- 
meter used. 
In Pellat’s form the charges acquired by contact are 
neutralized, and the oxygen atoms driven back to their normal 
distance, by an extraneous Hi.M.F. carefully adjusted until 
the plates, after separation, exhibit no charge. This measures 
1:8—°8 directly. : 
16. So far I have spoken as if I were sure that (granting 
the hypothesis) the potential of clean zinc is 1°8 volt below 
the air; but I am not really sure that this is anything better 
than an approximation. The fact that no actual combination 
occurs makes the matter perhaps a little indefinite. If an 
oxygen atom unites with a zinc atom, one has a right to say 
distinctly that g has stepped down v; but suppose they are 
only facing one another, and wishing to combine, are we 
justified then in asserting that the step v is ready for ¢ to go 
down, and that it is the same v as before? It almost seems 
to depend on whether chemical attraction becomes greater as 
two atoms approach one another, or becomes less. 
Suppose, first, it becomes greater, which is the natural 
hypothesis, then the v calculated from data obtained by per- 
mitting the combination to occur will be too large for the step 
of potential caused by the attraction of metal for oxygen over 
a standard distance. On the other hand, the differential force 
urging electricity across a junction of two metals, which is 
observed in the Volta effect, may be somewhat greater than 
simply the differences in their pull reckoned at standard 
distances, because the approach of atoms to the zine will 
increase it on this side, and the recession of atoms from the 
copper will decrease it on that. Hence the Volta effect may 
perhaps be expected to agree better with calculation than the 
air /metal potential-difference does, if this latter could be 
experimentally observed, which it never yet has been. 
Next, suppose that chemical attraction becomes less as atoms 
approach: the step of potential between a metal and air will 
now be greater than that calculated from chemical data ; 
nevertheless the Volta effect will be somewhat less than that 
due to the differences of such steps for two metals, and may 
thus possibly agree pretty well with calculation. 
The agreement or non-agreement of Volta effects with 
calculation does not therefore quite establish the accuracy of 
our calculated metal-air contact-forces. But we have no right 
to assume that even Volta effects will agree with calculation 
particularly well so long as our data are so slender. They 
have no chance of accurately agreeing unless the metals used 
