Electromotive Forces in the Voltaic Cell. 271 
are pure and perfectly clean—a most difficult condition to 
attain for even a few seconds. 
Before leaving this subject it may be well to point out that, 
whereas the calculation of a Volta effect depends on data 
obtained by allowing oxygen atoms to approach the metal so as 
completely and actually to combine, the experimental determi- 
nation by Kohlrausch’s and similar methods depends on letting 
the atoms approach somewhat nearer to one metal and recede 
somewhat further from another ; while the compensation form 
of the experiment employed by Pellat and others depends 
upon forcing back and restoring the atoms to their original or 
standard positions. Now if the views here just expressed 
have any sense or signification whatever in actual fact, it 
would be very natural to suppose that the numbers obtained 
in these three ways might be slightly different. But to 
specify the direction in which we should expect the differences, 
if any, to lie would require us to have made up our minds as 
to the probable variation of chemical attraction with distance. 
Assuming an inverse variation, the Pellat method should give 
the least, the Volta or Kohlrausch method the next, and the 
calculation method the greatest, value for the Volta effect. 
But all these ideas complicate the matter somewhat, and 
they are quite possibly unnecessary. If it be considered that 
we have no data at present, it may be permitted to work on 
the simplest hypothesis, viz. that the step v is independent of 
how nearly chemical action has occurred—that it is the same 
for atoms straining at one another at their normal distance as 
for atoms on the verge of combination. And it may be argued 
in favour of this view that we really have some data, viz. 
these. 
If it were not true, results obtained by Pellat’s method 
could not be expected to agree exactly with those obtained by 
Kohlrausch’s (of which Ayrton and Perry’s or Clifton’s may 
be taken as the best examples). Now results obtained by 
these different methods do agree very fairly well; exact 
agreement cannot be predicated, for the most trifling circum- 
stances cause large variations in the Volta effect, but no 
decided disagreement is observable. Again, if it were not 
true, the Volta effect observed when two metals far apart in 
the series (e.g. zine and platinum) were employed would be 
inconsistent with the results obtained by using metals nearer 
together, say zinc and tin, or tin and platinum ; and if this 
were so, the metals could not be arranged in the linear series 
which eighty years ago Volta showed they could be. These 
arguments throw no light on what may happen just before 
actual combination, still they a encouraging as far as they go. 
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