Daniell's Cell, in Electricity, was the earliest of two-fluid batteries, where the deposition of hydrogen on the positive plate was avoided by its transfer to a solution of copper sulphate, and a deposition of copper effected instead. In its simple form it consists of a copper plate dipped into a saturated solution of copper sulphate. A porous pot, containing a stick of zinc in dilute sulphuric acid, is placed in the copper solution. When the zinc and copper are joined by a conductor the zinc begins to dissolve in the acid, hydrogen is evolved by this action, it is handed through the porous pot to the copper sulphate, and by combining therewith forms sulphuric acid, and sets free copper which is deposited on the copper plate. Thus the one solution becomes less acid and the other one more so; the stick of zinc diminishes in size and the copper plate increases.