Manchester Memoirs, Vol. Ixi. (1917) No. 9 17 



By careful search through the data obtained from the 

 12,000 measurements made, the author has been unable to find 

 any point which is inconsistent with this theory. 



Further evidence of the existence of metallic hydrides, etc. 



Most of the metallic hydrides formed on the cathodes appear 

 to exist only under the influence of the high pressures prevailing 

 at the time. Their rapid and ready decomposition accounts for 

 their high solution potentials, and the fact that they are good 

 electrical conductors is e\adence in favour of the idea that they 

 ionise like a metal. 



In all cases where a metallic hydride is capable of inde- 

 pendent existence in presence of water, that hydride is formed 

 at a cathode of the given metal in an iaqueous electrolyte. Thus 

 the formation of arsine on a zinc cathode containing arsenic ha? 

 long been utilised in the detection and estimation of arsenic 

 Similarly, an antimony cathode liberates stibine, and a carbon 

 cathode gives hydrocarbon mixed with the hydrogen in an acid 

 electrolyte. A copper cathodq,, after use for a few seconds, 

 acquires a difference of potential from an unused plate of the 

 same metal which persists for many hours, and this is certainly 

 due to the formation of copper hydride, which is but slowly 

 decomposed by the acid present. Nickel behaves similarly. 



These hydrides also exist at high temperatures, a fact which 

 gives additional support to the suggestion that they are endo- 

 thermic compounds. Fowler (Trans. Roy. Soc, 1909, 2og, 447) 

 proved the existence of magnesium hydride in an arc of mag- 

 nesium in hydrogen, and Evans (Phil. Mag., 191 6 [VI.] j/, 55) 

 showed the presence of cadmium hydride in a hydrogen-filled 

 tube containing cadmium heated to 800° — 1,000° C. 



The higher oxides are frequently produced in quantity on 

 certain anodes; for example, lead dioxide on a lead anode, and 

 chromium trioxide on a chromium anode, and the ionisation of 

 lead dioxide giving the anion PbOg" ;is proved by the existence of 

 plumbites such as NagPbOa. The actual composition of these 

 ■hydrides^ etc., is to some extent a matter of speculation at 

 present, but suggestions have been made having a certain 

 degree of probability ,(J. Chem. Soc, 191 6, lOQ, 1363). 



Passivity and valve action. 



The phenomena fof passivity find a jteady and complete 

 explanation with Ithe aidi of the overvoltage theory. If the higher 

 oxides formed fon an anode are good electrical conductors, and 

 are also insoluble, ot nearly so, in the electrolyte, the metal itself 

 will be protected from the action of the deposited anions, and 

 plassivity will fee produced. At the same time the electrode 

 acquires the liigh positive potential of these higher oxides. 

 Passivity is more readily produced in alkali than in acid, partly 



