403 



metals and the electromotive beliaveoiii' of Mg and Al now aliows 

 that tlieir oxids and hydroxides may exeil a similiar influence. 



(n a solution of MgSO^, to which no acid is added, some Mg(OH), 

 is ill sohition. If we dip a Mg-electrode into this solution then, 

 hesitles other parts, present in the electrolyte, also Mg(OH), will 

 solve iu the metal bounding layer. This does not mean to say, that 

 the Mg will lose any of its characterisiic properties such as the 

 power to precipitate mercury from a solution but this small quantity 

 of Mg(üH), seems to exert a retarding influence on the velocity 

 with which the internal metallic equilibrium is established. 



A Mg-electrode under the above conditions dissolves slowly, evolii- 

 ting hydrogen, and shows too low a potential owing to the disturbance 

 of the inner e(|uilibrium. Addition of sulphuric acid however decreases 

 hydrolysis, and with this the Mg(()H)j concentration in the metallic 

 surface and induces a change in the direction of the inner equili- 

 biium of the metal, such that the potential becomes more strongly 

 negative. 



This effect of adding acid is however twofold. Oji the one hand 

 the concentration of the negative catalyst in (he metal Mg(OH), is 

 decreased, on the other hand direct attack at the metal is increased. 

 This attack in cases where it is rapid, such as the one under con- 

 siderations, always gives rise to disturbances and it might be expected, 

 that the poteidial would first become Luore negative and finally 

 would fall a little. 



This was found to be the case by the author and the Gruyter 

 and also by Beck. 



Beck's table XII p. 42 shows this quite plainly. 



This table shows in addition that the differences between the Mg 

 and the hydrogen potentials are not constant and that, whilst the 

 hydrogen potential is becoming decreasingly negative, the magnesium 

 potential changes in the opposite direction. 



This means that Mg does not behave as a hydrogen electrode, 

 which woidd be the case if the magnesium surface was unchanged 

 and moreover was surrounded by a liquid layer saturated with 

 respect to the Mg(OH),. 



It is probable that this was the case with some solutions when 

 the H-ion concentration was veiy low, merelj' with the vigorous 

 stirring employed in these experiments. Tiie certain conclusion from 

 Beck's experiments is that, whether the surrounding liquid layer 

 was saturated with respect to Mg(OH), or not, the state of the Mg 

 bounding layer was changing with the hydrogen concentration. 



By increasing the hydrogen concentration the magnesium bounding 



