The zinc electrode combines with oxygen; the oxide thus formed combines with sulphuric acid and forms sulphate of zinc. Oxide of copper is separate from the sulphate; and the copper in this oxide is separated from the oxygen. The oxygen of the water is separated at the zinc electrode from the hydrogen, and at the other electrode this hydrogen recombines with the oxygen from the oxide of copper. This alternate decomposition and recombination of the elements of water can neither increase nor decrease the E.M.F. of the cell, the actions being equal and opposite. The result of the series of actions above described is that the sulphuric acid and oxygen of the sulphate of zinc are transmitted to the zinc, combine with it, and form fresh sulphate of zinc; the sulphuric acid and oxygen of the sulphate of copper are transmitted to the zinc set free by the above process, and reconvert it into sulphate of zinc; the copper of the sulphate of copper is transmitted to the copper electrode, and remains adhering to it. The whole result is therefore the substitution of a certain quantity of sulphate of zinc for an equivalent quantity of sulphate of copper, together with a deposition of copper on the copper or negative electrode.[X] The following is a plan of the process:-- Zinc............} } .{ Sulphate of Zinc found } Oxide of Zinc..} . { at positive plate. Water { Oxygen..} }. { Hydrogen.................}......} { Sulphuric Acid.....} } Water. Sulphate of { { Oxygen..} Copper { Oxide of Copper { { Copper at negative { Copper....{ plate. _Description of the "Callaud" and "Marié-Davy" Batteries._--The Voltaic batteries in general use for the different purposes of torpedo warfare have been fully described in Chapter IV., and therefore it will be only necessary here to explain the construction of the "Callaud" and "Marié-Davy" batteries, these being much used abroad in connection with telegraphy. The _Callaud_ cell, named from the inventor, is a modification of the Daniell cell, and is also called a _gravity_ battery, the liquids being simply prevented from mixing by the law of gravity forbidding the heavier of the two from rising through the lighter. It consists of a thin plate of copper, which is laid on the bottom of a good _insulating_ jar having an _insulated_ wire leading up the side, and on this plate are placed crystals of sulphate of copper. A solution of sulphate of zinc is then poured in, and on the top is fitted a zinc plate, which forms the positive element. The vessel must not be shaken, or the sulphate of copper when dissolving will mix with the solution above it. The _Marié-Davy_ cell consists of a carbon electrode in a paste of proto-sulphate of mercury and water contained in a porous pot, and a zinc electrode in dilute sulphuric acid, or in sulphate of zinc.