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40 vapours, and that it is by the intervention of this gaseous connecting link that the phenomenon called contact electricity takes place.

Whether the one set of theorists or the other have come near a true explanation, or whether both are mistaken, is not a matter which need concern us; the important fact is that electrification is produced by the contact between two metals, and that the intensity of their electrification, or the force by which the two electricities are separated across the boundary line of the two surfaces, does not depend on the extent of the surface of contact, but only on the quality of metals used in the combination. The force is greater with some combinations and smaller with others. By testing various combinations, it is thus possible to range all metals in a series, in which that metal which, combined with any other always shows a positive charge, stands at one end. This had already been done by Volta himself, who gave the series:. Zinc stands at the positive, and gold at the negative end of the series. This sequence has been verified by all later observers, who have also confirmed another