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It is well known that one metal exerts a greater chemical force than another, and is capable of displacing it from its combinations. Among those metals with which we are familiar, potassium is looked upon as the most powerful; and it is a certain fact that calcium, barium, strontium, aluminium, and magnesium have been isolated by its agency. It could scarcely be expected, therefore, that any other metal could directly replace potassium. If such should happen, we would have an instance of rever­ sal, and should expect to find, on examination of the conditions, an agent capable of doing just the reverse work of what is usually assigned to affinity. It is also well known that in a simple voltaic cell, such as zinc connected with platinum in dilute hydrochloric acid, the more powerful or 1 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 zinc and the acid ; the contact theory supposes f 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 i 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.

I. On the replacement of electro-positive by electro-negative metals in a voltaic cell