Electromotive Forces in the Voltaic Cell. 179 
Zahn goes on to describe an experiment with bright sodium 
in vacuo instead of zine, the sodium having been long kept 
melted in a laterally connected bulb before being introduced 
into position. He finds the sodium strongly positive to copper; 
but there can be nothing crucial about this experiment, I 
imagine, for metal in contact with glass may so easily give 
rise to disturbing electrifications. 
I believe he must have employed the best vacuum of any 
experimenter on this subject, and that he has therefore gone 
most near to the proof of what I cannot help believing will 
be found to be the truth, viz. that the Volta effect in an abso- 
lute vacuum or perfectly inert gas (old air-sheets et hoc genus 
omne having been thoroughly removed) is very small. But 
if it be the case, as I believe it is, that the effect is almost 
independent of the guantity of oxygen present, so long as it 
is present, the difficulty of making the experiment so as to be 
sure of the absence of even the last few thousand or million 
oxygen moiecules is almost overwhelming. The question of 
the dependence of Volta force on atmosphere remains thus 
undecided; and all the evidence which I can adduce in favour 
of such dependence is this incipient decrease observed by von 
Zahn, the little too-mixed-up observation of Mr. Hart (de- 
scribed later), the measurements of Schulze-Berge, and the 
more decided experiments of Brown. It may indeed be 
readily held that the weight of experimental evidence tends 
the other way, since most experimenters on the subject (Pellat, 
Schultze-Berge, von Zahn, and I may add Sir W. ee 
have left off just as pure contact theorists as they began. 
would attempt an experiment myself, save that I am so pro- 
foundly impressed with the difficulty of making one in which 
no fault or loophole can be found, and which will by every one 
be deemed satistactory and final ; so I prefer to base my views 
on a general survey and on fairly conclusive reasoning, rather 
than on a crucial but almost impossible experiment. 
5. Perhaps this is now the place to refer to the somewhat 
erratic series of papers by Professor Franz Exner, of Vienna*. 
Differenz in der wirklich wesentlichen Verminderung von Feuchtigkeit 
und Sauerstoff gesucht werden musste,so dass der Apparat nach dem 
Oeffnen eine starkere Spannung zeigen wiirde. Dies ware dann wirklich 
ein experimentum crucis zu Gunsten der chemischen Theorie. Diese 
Entscheidung vorzunehmen wird aber erst dann nothwendig sein, wenn 
nach laingerer Zeit, wo das eingeschmolzene Natrium noch mehr alle Reste 
von Sauerstoff beseitigt haben wird, eventuell noch Wiederhitzen und 
dergl. der jetzige Zustand des Apparates unverdndert wieder gefunden 
sein wird.”—Von Zahn’s Memorr, p. 48. 
* Exner, Sitzb. der Akad. der Wissensch. Wien: July 1878, “On the Nature 
of Galvanic Polarization;” July 1879, “On the Cause of the Production 
