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Another criticism advanced against the theory of electrolytic dissociation was that, if the ions are free in the solution, it should be possible to separate them. The answer is that such a separation can, in fact, occur. If a layer of pure water is poured over a solution of hydrochloric acid, the hydrogen ions, which move more rapidly than the chloride ions, as we know from direct measurements of the speeds of ions in a potential gradient, diffuse into the water. Since, however, they carry positive charges, they will charge the water layer positively, and leave the negative chloride ions in the layer of acid, which becomes charged negatively. By reason of the great electrostatic forces soon set up, the hydrogen ions tend to be dragged back into the acid, and the chloride ions to be pulled out, so that in a short time both ions migrate together with equal speeds, and the acid appears to diffuse as a whole. The existence of the electrical charges may easily be seen by placing platinum wires in the water and in the acid, and connecting these with a galvanometer. A current flows from the water to the acid. If a non-electrolyte, such as sugar or alcohol, is used no trace of current can be detected.
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