484 EEroRT— 1884. 



best experiment, and a very crucial one if only it could be perfectly per- 

 formed, is the attempt to measure the Volta effect in an absolute vacuum. 

 A pair of zinc and platinum plates ai'e sealed up in a glass vessel 

 iu suc!l 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 2 5 , 

 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 measurements 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 farther 

 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 seems necessary to quote his 

 own grounds for it, and I do so in a foot-note. 1 



Zahn goes on to describe an experiment with bright sodium in vacuo 

 instead of zinc, 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 experi- 

 menter 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 absolute vacuum or perfectly inert 

 gas Cold 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 quantity 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 molecules 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 



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. 



1 ' Andere etwas bessere Beobachtungsreihen gaben iihnliche Werthe. Bei alien 

 war die Kleinheit von C. (the apparent Volta effect Zn/Pt) auffallend. Nun stellt ja 

 diescr Worth, wie oben besprochen, nicht die Potentialdifferenz Zink-Platin dar, fur 

 seine Kleinheit muss aber eine anderweitige Ursache vorliegen. Dieselbe kann ich 

 zunachst nur darin finden, dass die Zinkplatte bei Anfertigung und weiterer 

 liehandlung des Apparates sichtbar angelaufen ist (auf der einen Hiilfte sogar 

 blaue Farbung angenommen hat). 



' Es liesse sich allerdings vermuthen, dass diese offenbar zu geringe Differenz in 

 dor wirklich wesentlichen Verminderung von Feuchtigkeit und Sauerstoff gesucht 

 werden musste, so dass der Apparat nach dem Oeff nen eine starkere Spannung zeigen 

 wiirde. Dies ware dann wirklich ein expervmentum cruris zu Gunsten der chemischen 

 Theorie. Diese Entscheidung vorzunehmen wird aber erst dann nothwendig sein, 

 wenn nach langerer Zeit, wo das eingeschmolzene Natrium noch mehr alle Reste von 

 Sauerstoff beseitigt haben wird, eventuell noch Wiederhitzen und dergl. der jetzige 

 Zustand des Apparates unveriindert wieder gefunden sein wird.' Von ZahrCs 

 Memoir, p. 48. 



