of a cell, and the ingenuity of inventors has been and is even at the present day exercised in finding means to avoid or at least reduce the effect of polarisation.
The first and completely successful attempt in this direction has been made by Daniell, in 1836. He recognised that the cure for polarisation lay in preventing any hydrogen being liberated and carried to the copper plate. If the liquid in the immediate vicinity of the copper plate contained a copper salt, it would not be hydrogen molecules, but copper molecules that are precipitated on the copper plate, and this could, of course, not alter the original condition of the cell. He used, therefore, a solution of sulphate of copper as the liquid into which the copper plate is immersed. But now arises another difficulty. We must not let the copper sulphate come into contact with the zinc, for this would not give the desired e.m.f., and it would also, by reason of the dissolution of the zinc, very quickly spoil the solution. It is thus necessary to still use dilute sulphuric acid as the liquid into which the zinc is immersed, and at the same time anything like a mixing of the two liquids must be avoided. This object is attained by the employment of a porous pot
