(_d_) Special tanks are required, as the protecting lead liner cannot be employed, since the danger of current-leakage through it is increased, owing to the higher voltages required. Hence special acid-resisting material, such as slate, is necessary, the expense of which is considerable. (_e_) The cost of stripping the cathodes is high, and the operation is often difficult. (_f_) The cost of maintaining the plant is greater. The special advantages of the multiple system are that— (1) It is applicable to all grades of metal; (2) It permits of either high or low current-density being employed; (3) It permits the use of cheaper lead-lined wooden tanks for working; (4) Anode plates may be used without previous refining, if so desired. It is, however, general to carry out this preliminary refining, which yields sounder anodes, keeps the electrolyte purer, and promotes the more regular working of the electrodes and electrolyte—although some smelters still cast the anodes direct from converter metal. Summarising, it is found generally that— (1) A marked saving is effected in operating the multiple system (this has been estimated by Barnett, at as much as $2 per ton of refined metal—8s. 4d.); (2) A greater efficiency is obtained, the tank efficiency of the multiple system being 95 per cent. compared with 90 per cent. for the series-system; (3) Less copper is held up in the multiple system, since less anode copper is required, under like conditions as regards cathode surface and current density. OUTLINE OF THE PROCESS.—The electro-refining industry is a highly specialised one, and the methods of putting the comparatively simple underlying principles into practical operation have assumed great complexity and diversity in detail, concerning which Ulke has collected and published much valuable information. The following details have more particular reference to the multiple system of working, as being the most representative of the electro-refining methods in general use. The outlines of the process constitute the passing of direct electric current through tanks containing acidified solutions of copper sulphate, employing plates of crude copper as anodes, and depositing pure metal upon cathode-plates of specially-refined thin sheets of pure copper. The precious metals and most of the impurities of the anode metal are liberated as small insoluble particles which gradually settle to the bottom of the tanks in the form of mud, soluble constituents, such as iron and zinc, first passing into solution. _General Conditions—Anodes._—The usual dimensions of the anode-plates are 3 feet high by 2 feet 6 inches wide and about 2 inches thick; they are generally cast with lugs, so as to allow of suspension in the tanks. The anode metal is usually brought by a preparatory operation, to as high a state of purity as is economically practicable—