48 Messrs. Gladstone and Tribe on the [Nov. 25, 



nected with platinum in dilute hydrochloric acid, the more powerful or 

 electro-positive metal zinc displaces the hydrogen that is in combination 

 with chlorine, and the hydrogen makes its appearance against the less 

 powerful or electro-negative metal platinum. The chemical theory of 

 galvanism supposes that the force originates in the chemical action which 

 takes place between the ziuc and the acid ; the contact theory supposes 

 that it originates in some unexplained manner in the opposite electrical 

 condition of the two metals induced by their contact. If the chemical 

 theory be the true one, it is evident that a zinc-platinum cell can only be- 

 come active when the binary liquid contains hydrogen or some metal 

 which is less powerful than zinc. If, for instance, we were to employ a 

 potassium salt instead of a hydrogen compound, it is inconceivable, on the 

 pure chemical theory, that there should be any action at all. 



Such an action, however, does take place if we substitute an aqueous 

 solution of the chloride of potassium for the hydrochloric acid ; the zinc 

 combines with the chlorine, and the potassium is set free in some form, 

 against the platinum, manifesting itself by the presence of free alkali and 

 hydrogen gas. The same holds good with aqueous solutions of chloride 

 of sodium or ammonium, or barium, strontium, calcium, or magnesium. 



This action is slow ; but if magnesium be used instead of zinc, it takes 

 place sufficiently rapidly to be easily observed, and we have therefore 

 studied the action of platinum and magnesium in connexion. 



Experiments with Potassium Chloride. 



The solution employed was neutral to test-paper, and contained 5 grms. 

 of salt in 100 cub. centims. of water. A strip of magnesium connected with 

 one of platinum was immersed in such a way that the two metals remained 

 about j- inch apart. Bubbles of gas immediately made their appearance 

 on the platinum ; and a strip of red litmus-paper being placed in various 

 parts of the solution, became strongly blue in the vicinity of the platinum 

 plate, whereas but the faintest trace of alkaline reaction was observable 

 near the magnesium plate ; magnesium was found in solution. 



A similar experiment was made in a cell which was divided in the 

 middle by means of parchment-paper. The strip of platinum in one divi- 

 sion was connected with the strip of magnesium in the other by means of a 

 wire, and allowed to remain for an hour, when they were removed. The 

 liquid in the division which had contained the magnesium was slightly 

 turbid and faintly alkaline, but did not give the slightest precipitate when 

 poured into a solution of magnesium sulphate. The liquid in the division 

 that had contained the platinum plate was, on the contrary, clear and 

 strongly alkaline ; and it precipitated magnesium hydrate from the neutral 

 solution of that metal. This conclusively showed the presence of some 

 alkaline hydrate other than magnesium hydrate, and which could not have 

 been any thing else than potassium hydrate. The liquid in the second 

 division was about eight times as alkaline as that in the first. 



