Electromotive Forces in the Voltaic Cell. by 
and because of the vague and unsatisfactory views of phy- 
sicists in general on the matter (e. g. of Hart); but I am bound 
to say that, so far as I can judge, Dr. von Zahn’s own ex- 
periments are not of that conclusive and decided character 
that one had hoped for from his start, and he signally fails to 
sum up the facts in a neat and crisp manner. He adheres to 
the contact view, but his adhesion scarcely seems to me to be 
based on strong evidence ; and in fact his theoretical views 
seem a little superficial considering the date at which he 
writes ; so that one may admit pretty well all he says about 
contact and not chemical action as the cause of the Volta 
effect without being deeply committed to any specially true 
or specially false position *. His best experiment, and a very 
crucial one if only it could be perfectly performed, is the 
attempt to measure the Volta effect in an absolute vacuum. 
A pair of zinc and platinum plates are sealed up in a brass 
and glass vessel in such a way that one of them is capable 
of moving up and down, and thus of varying its distance from 
the other. Gravity is employed to separate the plates, the 
whole vessel being inverted. The vessel is filled with dry 
nitrogen and exhausted for some days, occasional heat, P,O;, 
and melted sodium, being employed to improve the vacuum. 
The result is that the Volta effect is very decidedly “‘ too 
small,’ going down to half a Daniell, so far as the measure- 
ments made by his not entirely unobjectionable method can 
be trusted ; but he does not seem to think that this is much 
of an argument either way, and, not finding any further change 
after some days, he did not pursue the investigation further 
by letting in some air and seeing whether the old value is 
restored, though he perceives clearly that this would be a 
crucial experiment. This abstinence is so remarkable that it 
antimony is destroyed by immersing the pile in pure nitrogen ; and Young 
of Princetown takes the trouble to examine whether it is so experimentally 
(see Phil. Mag. x. 1880, p. 450), and finds the thermoelectric power of 
metals the same at one millionth atmosphere as at 1. This, however 
would be altogether inconclusive if experiment were needed to settle it. 
* T am afraid this is not peculiar to Dr. von Zann. It may be that the 
German writers on the subject are too busy accumulating facts to care 
much about their precise theoretical bearing; but I notice a very loose 
and unsatisfactory way of putting forward secondary matters as 1f they 
were the real points at issue, and of never really getting to the heart of 
the matter. It is singular that the four questions or heads under which 
that eminent writer, Professor Wiedemann, proceeds to discuss the attempts 
which have been made to settle the question of the seat of E.M.F. are 
such that if a categorical answer to each were, by supernatural means, 
-vouchsafed to us, we should be, I believe, none the wiser (Wiedemann, 
Elek, vol. ii. p. 985, new edition). 
