at the Contact of Solids and Gases. 3 



zation not merely portions of gas adhering to the surface, but 

 also portions which have penetrated deeper into the platinum 

 must play a part — the possibility of which had been already 

 indicated by Graham's experiments on palladium and plati- 

 num. In fact, Crova*, and after him Root alsof, succeeded 

 in proving that, with the electrolysis of dilute sulphuric acid, 

 hydrogen penetrates through a platinum plate, since the plate 

 not only exhibited polarization on the side where the electro- 

 lysis took place, but was protected on the opposite side from 

 all electrolytic action. 



But few investigators, in studying galvanic polarization, 

 have taken into consideration other gases than hydrogen and 

 oxygen ; hence the question arises whether the views which 

 hold good for these two gases, and preferably for hydrogen, 

 can be extended in their entirety to all cases of polarization. 

 A series of experiments which I have made with palladium 

 and carbon electrodes may contribute to the answering of this 

 question. 



To procure exact knowledge respecting the electromotive 

 position of palladium is a very difficult task. Palladium as 

 obtained in commerce has always been heated to incandes- 

 cence, and has taken in gases in the process, as Graham has 

 shown. The means usually employed to expel such occluded 

 gases, especially hydrogen, from the palladium are so far suffi- 

 cient that chemical analysis can show no traces remaining, 

 but do not suffice to destroy all change in the electromotive 

 state of the metal. This is especially true of the treatment 

 with the mercury air-pump ; I have never been able in this 

 way to bring back a palladium plate quite to its previous 

 electromotive position after evolution of hydrogen had taken 

 place on it. On the other hand, the last trace of hydrogen 

 can be completely removed by a long-continued evolution of 

 oxygen. But then the plate becomes covered with a coat of 

 brown oxide ; and if this be ever so carefully rubbed off, yet 

 the plate always takes a much more negative position in the 

 tension series than if it had been cleaned with hydrochloric 

 acid. For the determination of that position I made use of 

 my universal compensator J, with which also all the other 

 measurements of the differences of tension we shall have to 

 consider were made. The palladium plate to be tried dipped 

 into very dilute sulphuric acid (1 : 100), and thus formed the 

 negative constituent of an element, of which the positive con- 

 sisted of an amalgamated zinc cylinder standing in a concen- 



* Mondes T. V. p. 210 (1864) ; Wied. Galv. (2) i. § 498. 

 t Monatsber. d. Bcrl. Akad. d. Wiss. 1876, p. 217. 

 % Wied. Ann. iii. p. 1. 



B2 



