484 



i;i;roin — 1884. 



best t'xporiinent, inul a very criiciiil ono if only it could !)(• perfectly por- 

 formc'd. is tlio attempt to lueasnro tlio Volta eili'ct in an absolute vaeuura. 

 A pair of zinc and platinun. plates are soaled up in a glass vesRci 

 in HUcV a way that one of them is capable of movinj^ up ami down, and 

 thus (if varyirif^ its distance from tlie other. (Gravity is em|)loyed to 

 separate the plates, the whole vessel being inverted. The ves.sel is tilled 

 with dry nitrogen and e.\hauHted for some <lays, occasional lieat, I'.^O,,, 

 and melted BO(lium being employed to improve the vacuum. 



The result is that the \'oUa effect is very decidedly ' too SJuall,' going 

 down to half a Daniell, so far as the measurement s made by his not 

 entirely unobjectionable nielhod can bo trusted; but he does not seem to 

 thini; that this is much of an argumint either way, and, not linding any 

 further change after some days, hedid not pursue the investigation further 

 by letting in son\e air and seeing whether the oKl value is restoivd, 

 though he perceives clearly that this would be a crucial experiment. 

 This abstinence is so renuirkablc that it seems necessary to quote his 

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



Zahn goes on to describe an experiment with bright sodium iti variin 

 instead of zinc, the sodium having been long kept melted in a laterally 

 cf)nnccted bulb before being introduced into position. Ho tinds the sodium 

 strongly positive to copper, but there can l)0 not hing crucial about this 

 experiment, I imagine, for metal in contact with glass may so easily give 

 rise to disturbing electriflcations. 



I believe ho nmst have employed the best vacuum of any experi- 

 menter on this subject, and that he has therefore gone most near in 

 the proof of what 1 cannot help believing will bo found to bo the trutli, 

 viz., that the Volta elFect in an absolute vacuum or perfectly inert 

 gas Cold air sheets et hoc ijenn.'i o)ii)i(' having been thoroughly removed) is 

 very small. But if it be the case, as I believe it is, that the elTect is 

 almost independent of the qiiantitii of oxygon present, so long as it is 

 present, the difficulty of making the experiment so as to be sure of tlie 

 absence of even the last few thousand or million oxygen molecules is 

 almost overwhelming. The question of the dependence of Volta fore- on 

 atmosphere remains thus undecided, and all the evidence which 1 can 

 adduce in favour of such dependence is this incipient decrease observed by 



•j;eltingf to tbc heart of tlie matter. It i.s singular that tlio four questions or licaJs 

 umterwhichtliatcmini'nt writer, I'rofessorWiodeniann.procceils to (liscus.s the ;itt(-iiii)t> 

 winch liavo boon made to settle the question of the .seat ot K.M.F. are such that il 

 a categorical answer to each were, l)y supernatural means, vouchsafed to us, we .slioiiM 

 lie, I believe, none the wiser. Wiedemann, J'Jlek., vol. ii. p. 1)85, new edition. 



' ' Andcre etwas bessero 13cobachtun};sreihen gaben iihnliche AV'ertlic. 15ci alien 

 war die Kleinbeit von C. (the apparent Volta cfFect Zn/l't) auffallend. Nun .stcllt jn 

 diesor Worth, wie oben besprochen, nicht die Potentialditferenz Zink-Platin dur, fiir 

 ."^oine Kloinheit muss aber eino anderweitige Ursache \ orliogcn. Diesulbo kann ich 

 ziiuiichst nur darin finden, dass die Zinkplatte bci Anfertigung und weitorer 

 liehandlung des Apparates sichtbar angclaufen i.st (auf dor einen Hiilfte sogar 

 biaue Fiirbung angenommon hat). 



' Ks lioaso sich allerdings vcrmuthcu, dass diesc oflfenbar zu gcringc DitTcrcnz in 

 ih.T wirklich wescntlichen Verminderung von Feuehtigkeit und SauerstotT gesucht 

 worden miisste, so dass dor Apparat nach dem Oelf nen eine starkere Spannung zoigi-n 

 wiirde. Dies wiire dann wirklich ein expcHmentum criinx zu Gunsten dor chemischcn 

 Theorie. Dicse Entscheidung vorzunehmon wird aber erst dann nothwendig sein, 

 wenn nach liingercr Zeit, wo das eingeschmolzeno Natrium noch melir alle Reste von 

 Sauerstoff beseitigt haben wird, eventuell noch Wiederhitzen und dergl. dor jetzigf 

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

 Memoir, p. 48. 



vie; 



