with a main pipe, running along the sides of the vats, and conveying the electrolyte solution. The supply of liquid to each rake is controlled by a screw clamp, q. The solution is run away from beneath the slime tray through the siphon, x, into the channel, z.

From Figs. 92, 93, and 94 shown on Plate II., will be seen both the arrangement of the vats and the method of circulating the solution. In order not to detract from the clearness of these illustrations the electric connections of the baths are omitted, but are shown separately in Fig. 95; this figure scarcely requires further explanation. It will be observed that the solution is distributed to the baths from the elevated reservoir, B, and returns through the channel, z, to the receivers, C and D. If all of the liquid is to be discharged, or if one of the receivers should leak, the solution is drawn off through a conduit beneath the floor of the room into the tank, E. By means of the pump, P, the circulation of the liquid can either be renewed by transferring it from C, D, or E to the reservoir, B, or, if too impure for further use, it may be removed to the regenerating or cementation plant. Regeneration consists generally in evaporation or crystallisation, if necessary with the aid of chemical precipita

Fig. 95.-Scheme of electrical connections in copper vats (Siemens). tion. Concerning the cementation process, metallurgical handbooks usually give sufficient information

The points to which special attention must be devoted during the process have been made clear by the work of Kiliani, which has already been described in full. Reference has also been made to the deterioration of the deposit caused by the diminution in the proportion of copper in the bath, which results, under the conditions explained, from an accumulation of non-precipitable metals. So also the importance of maintaining a brisk circulation of the electrolyte, of preserving a certain degree of acidity, and of using a small current density, were sufficiently emphasised by Kiliani.

Borchers' Improvements in the Siemens-Halske Plant.But the means which, until the last few years, had been commonly employed to fulfil the necessary conditions and to avoid the corresponding sources of failure were quite insufficient. The need of completely renewing the solutions when using the more impure (or even comparatively good) sorts of copper recurred more often than was to be desired in a continuous process; and those who have experienced the trouble of working up such waste liquors would welcome almost any means by which such a labour might be avoided. Many suggestions were, of course,

made with the object of surmounting the difficulty, and the most natural of these was that a current of air should be forced through the liquors in the vats during the electrolysis, in order to peroxidise the compounds of arsenic, cobalt, nickel, and iron in the bath, and so to induce their precipitation in the vats as arseniates. But the proposal itself was simpler than the method of carrying it into effect. The direct introduction of a blast of air into the solution was satisfactory only for a short time, because it soon led to the production of a turbid liquid, which might almost without exaggeration be compared with chocolate. The hope that this air-treatment would be successful

Fig. 96.-Siemens-Borchers' copper vats (longitudinal section). was therefore doomed to disappointment; and yet air was clearly the best agent for the purification of the liquors, if only it could be satisfactorily applied, and the idea of using it must undoubtedly be considered one of the most important discoveries in connection with the refining of copper. The merit of solving the problem belongs to Messrs. H. and K. Borchers of the above mentioned firm of Borchers Bros.; and this invention, it should be observed, is not patented. The method of carrying it into effect will be first described, and the advantages which it has entailed will be discussed afterwards. A bath fitted with Borchers' apparatus is shown in Figs. 96 and 97. On comparing these with the

previous figures, it will be seen that there has been practically no change in the arrangement of the electrodes. But there has been added a wide leaden pipe, b, which passes downwards from the upper surface of the liquid to the very centre of the space beneath the mud-depositing tray, s; and within this pipe is a glass tube, g, drawn out to a fine jet at its lower extremity. The glass tube is passed through a stopper in the leaden cap, d, by which the tube, b, is surmounted, and is so arranged that it

may easily be raised or lowered in position. A current of air is continuously forced through the glass tube into the column of liquid in the pipe, b, with the result that the air, dividing itself into small bubbles as it rises to the surface, lowers the specific gravity of the contents of the tube, b, and causes a portion of the liquid to overflow into the vat, where it mixes with the bulk of the electrolyte. Meanwhile, solution from the lowest level of the bath is constantly entering at the bottom of the tube, b. The aëration of the electrolyte is thus effected very