This is a double-liquid battery devised by Fitzgerald and Molloy; it is said to be as constant as the Bunsen, almost as effective, and much cheaper. The chief point is to secure a large negative surface, and, by a soft porous cell, to reduce the internal resistance. The best form is a carbon cylinder surrounding a large porous cell containing a zinc cylinder. The carbon for the cylinder must be ground to fine powder, mixed into a stiff dough with water and sugar syrup, baked until hard, plunged, while still hot, into a strong solution of sugar or tar, heated to whiteness, and cooled slowly. Another arrangement is as follows :-In a large soft porous cell is centrally placed a thin carbon or Bunsen rod, with a screw affixed. A quantity of broken carbon, in lumps as large as hazel-nuts, is packed around the rod. Melted pitch is run over the top, and a conical hole is left for the introduction of the liquid.

Fig. 38.

The outer cell contains a zinc cylinder of only just sufficient diameter to admit the porous cell freely, the object being to have the zinc near the negative element. To allow the outer liquid greater freedom of action, the zinc cylinder has a separation of about £ in. The elements are thus carbon and zinc. The exciting solutions are: - (a) For porous cell: 2 oz. lime chromate, 5 oz. warm water, 5 oz. sulphuric acid. (b) For the outer cell: 1 pint water, 3 oz. sulphuric acid. Little or no fumes are given off. The electro-motive force is slightly greater than that of the Bunsen; but the internal resistance is also greater. The same cell is available for use with another excitant, which works with even greater force, and gives no fumes for the first 2 hours:-For the porous cell: 2 oz. potash bichromate, 10 oz. nitric acid, 2 oz. sulphuric acid. The solution in the outer cell is the same as for the Bunsen. This works with greater power than the Bunsen, owing to the arrangement of the carbon; the internal resistance is less, but the cost of working is rather greater. After use, the porous cells should be emptied of their contents, and kept in water till again wanted. The same solution may be used 2 or 3 times, and on any appearance of a paucity of potash salt, more should be added.

The carbon cell demands a strongly acid mixture. The battery may be advantageously used as a Bunsen, and gives a better current than common forms at little extra cost of construction. (Urquhart.) Silver Chloride. - Skrivanow, of Paris, covers a carbon plate on both sides with silver chloride, and immerses it in a solution of potassium or sodium hydrate, the hydrate being in the proportion of 30 to 40 per cent. of the weight of the water. A zinc plate is immersed in the same solution close to the carbon, which may, however, be wrapped in asbestos cloth or placed in a porous cell. The battery may be revivified by plunging the carbon plate in a bath composed of nitric and hydrochloric acids, or sodium chloride dissolved in water, the object being to chloridize the metallic silver. The hydrates may be dissolved in glycerine if the battery is exposed to frost.

Amongst portable batteries may be noted one by Mackenzie, which is a modification of the silver chloride arrangement. A short piece of copper tube has one end stopped by a cap of the same metal, and, the interior being plated with silver, is subsequently covered with a layer of fused silver chloride; the other end of the tube is closed with a cap of some insulating materia], through which passes a zinc screw, holding in the interior of the tube a zinc cylinder or tube, extending nearly to the bottom of the cell thus formed. The exciting liquid is sodinm chloride (common salt) or sulphate, or zinc chloride. The wires are attached respectively to the zinc screw and to the exterior of the copper tube.