809 



photographic reproductions flig. 1) show, \vc have complete control 

 over the phenomenon '). 



For a definite Cl-ion concentration the density of current is to be 

 chosen so that the time during which tlie most passive state prevails, 

 is about equally long as that during which the most active slate 

 continues to exist. See tig. la. 



If the density of current is made slightly greater, the passive state 

 continues to exist longer Ihau the active stale, which afipears very 

 clearly from fig. 16. In the maxima the iron is passive, and in the 

 minima active. The oscillations in the potential whicii we have 

 observed here, are very great; the maxima and liie minima lie 1,74 

 Volts apart, the duration of tiie periods amounting to (J. 54 and 5.8 

 seconds. It is self-evident (luit the periodic phenomenon described 

 here only manifests itself clearly when the iron-surface is small. The 

 larger the iron-surface the smaller the chance that the iron is passive 

 or active all over its surface at the same moment ; hence this [)heno- 

 menon is less regular as the surface is taken larger, and vaiushes 

 entirely for a large surface. 



We will point out here that |)eriodic phenomena during the elec- 

 trolytic solution of passifiahle electrodes liave already been observed 

 by different investigators, but none of these investigators has suc- 

 ceeded in calling forth a periodic |)henomenon of such large regidar 

 oscillations as here. 



Adlek '), who has perhaps done the most meritorious work in 

 this region, obtained periodic oscillations in the polciilial dilference 

 anode-electrolyte among others for iron l)y generating hydrogen at 

 the backside of the iron wall, part of which acted as anode. The 

 explanation is as follows : The hydrogen diffuses through the iron, 

 and arriving at the anodeside it can then exert its catalylic action. 

 Though Adi.kk's \ iew was different from ours we may say that here 

 too the periodic phenomenon was oblained by the use of a catalyst, 

 but here the catalyst enters thnniyh. the metal, hence the phc'no- 

 menon is brought about in another way than ours. 



Further the iron in Adi.kk's experiments remained a lung time 

 passive and a very short time active, and the potential difference 

 only amounted to 180 millivolls, the phenomenon being besides less 

 regular than ours. The curve found by him is lepresented to scale 



') The iron rod was not suspended on a platinum wire, but was cemented 

 with shellac in the short leg of a U-sbaped tube. Alter it had appeared that 

 electrolytic iron behaves perfectly uniformly to iron with a small carbontHiiilenl, 

 we have afterwards always used the latlei-. 



-) Z f. phys. Ghem. 80, 38.5 (1912). 



52* 



