The positive element is zinc, and the negative element black oxide of copper. The exciting fluid is a solution of caustic potash. The black oxide of copper plates are suspended from the cover of the jar by a light framework of copper, one end of which forms the positive pole of the battery. A zinc plate is suspended on each side of the copper oxide element and kept from coming in contact with the latter by means of vulcanite buttons. When the cell is in action, the water is decomposed, and the oxygen thus liberated combines with the zinc and forms oxide of zinc, which combines with the potash to form a double salt of zinc and potash. The last combination dissolves as rapidly as it is formed. The hydrogen liberated by the decomposition of the water reduces the copper oxide to pure metallic copper. It is highly important that the copper oxide plates be completely submerged in the solution of caustic potash, and that heavy paraffin oil be poured on top of the solution to the depth of about 1/4 of an inch to exclude the air. If oil be not used, the formation of creeping salts will reduce the life of the battery fully two-thirds. The battery has a low electromotive force, about 0.7 of a volt, but as the internal resistance is also very low, quite a large current can be drawn from the cell. The _Bunsen Cell_, shown in figs. 49 and 50, is a two fluid cell constructed with zinc and carbon electrodes. The negative plate is carbon, the positive plate amalgamated zinc. The excitant is a dilute solution of sulphuric acid. The top part of the carbon is sometimes impregnated with paraffin (to keep the acid from creeping up). The force of the Bunsen cell increases after setting up for about an hour, and the full effect is not attained until the acid soaks through the porous cell. Carbons are not affected and last any length of time. The zinc is slowly consumed through the mercury coating. =Grenet Bichromate Cell.=--In this cell, as shown in figs. 49 and 50, the positive element is zinc and the negative element carbon. The electrolyte is a solution of bichromate of potash in a mixture of sulphuric acid and water.