If, instead of nearly plane surfaces, it is required to operate upon objects in high relief, he has recourse to the circular depositing vat (Fig. 142), in which the cells form a circle, and the mould to be covered is in the centre.

Whatever be the shape of the mould, its position should now and then be changed, that is to say, the upper portions should be made the lower ones, and conversely. This is necessary, because the lower layers of the bath give the more abundant deposits, which is explained by the difference of specific gravity of the layers more or less charged with sulphate. In fact, a solution may contain very little sulphate at the surface, and be saturated with it at the bottom. This explains the practice of keeping the tray of sulphate at the top of the bath, instead of allowing the crystals to lie at the bottom.

Formerly, a certain apparatus, quite inconvenient, and called an electrotype, was used for the reproduction of

medals. It was composed of a cylindrical glass vase, with a wooden cover (Fig. 143), to which was fixed a support

binding screws.

The wooden cover had a circular open

ing through which passed a truncated cone of glass (Fig. 144), open above and closed below with a piece of bladder. For working this apparatus, the mould

was fixed upon a metallic disk, dipping into the solution of sulphate of copper, and connected by a bent wire with the top support. Another wire, bent twice at right angles, passed through the second

Fig. 144.

hole of the support, and carried a disk of zinc dipping into acidulated or salted water contained in the truncated cone of glass. By this disposition the mould and the zinc plate were kept horizontal and opposite each other, but separated by the bladder.

This apparatus, the manipulation of which was tedious and difficult, has been simply mentioned, because it was the first in use. It has been superseded by apparatus in which the cell and the moulds are placed vertically, which allows of the rapid removal and examination of the objects. It should, however, be mentioned that this latter disposition favors the production of striæ upon the deposits, should the operator neglect to change, now and then, the position of the articles. These striæ are mainly due to convection currents formed by liquid layers of unequal saturation which traverse the surface of the object, and they are the more numerous as the intensity of the current is greater. They may be avoided by frequently agitating the bath, or the objects.

As far as practicable, the liquids in the bath and in the cells should be maintained at the same level; and it is even more prudent to have that of the bath slightly above that of the cell, in order to prevent the solution of zinc from mingling by exosmose with the copper solution. The converse is preferable, although the zines become blackened by a pulverulent deposit of copper.

CHAPTER LII.

DIAPHRAGMS OR POROUS CELLS-KEEPING THE BATTERY IN ORDER-AMALGAMATING SALT-ACID BATHS.

Diaphragms or Porous Cells.

By this name we designate vessels made of materials capable of containing liquids, and, at the same time, of allowing them to ooze out slowly through their pores, especially under the influence of electricity. Pipe clay, pasteboard, bladder, gold-beater's skin, and parchment, certain tissues, especially sail-cloth, and certain kinds of woods, may be employed for diaphragms. But nothing answers the purpose so well as porcelain clay, or kaolin, submitted to a certain heat, which hardens the paste without destroying its porosity. Vases made with this material are just porous enough, and resist the action of the most corrosive acids. It is this latter quality which renders the kaolin cells so much superior to all the other substances indicated above.

It is often quite difficult to obtain cells of dimensions and shape suitable to our wants. The largest generally found in the trade do not exceed 4 inches in diameter, and 14 inches in height. And, as it often happens that we are required to effect deposits upon surfaces, the height of which is at least double that stated, we shall indicate the manner of remedying this inconvenience. Diaphragms of about 28 inches in length may be made as follows: Saw off carefully the bottom of a large cell, and grind well to match the top of another. Then heat the

two ground ends, place them one on the other with an interposed film of varnish, and wrap the joint with a ribbon of gutta percha, which is kept firmly in place by a screwed collar of copper. Heat the collar again slightly in order to soften the gutta percha, and screw up the collar to make the joint tighter. Lastly, cover the copper collar entirely with varnish, or, preferably, melted gutta percha (Fig. 145).

Fig. 145.

When the dimensions of the diaphragms are too great to allow of the employment of kaolin, we must have recourse to sail cloth, fixed with copper nails upon a frame of oak. The seams are covered with coal tar, and the cloth lasts longer than would be supposed. Diaphragms are also made of well-joined boxes of thin and porous woods. But there is nothing that answers so

well as the kaolin cell.

Keeping the Battery in order.

It has already been explained that the electric current was produced by the action of certain liquid excitants which slowly dissolve zinc, and that this liquid was commonly water with two or three per cent. of sulphuric or hydrochloric acid, or simply a solution of common salt. It is evident that, after a greater or less length of time, the action will be arrested by the saturation of the acid or of the salt; we must therefore maintain, or revive, the action by additions of the exciting liquid, for which purpose the procedure is as follows:

The battery, charged as described, will work well for twenty-four hours; and, for four consecutive days, it will be sufficient to add small quantities of acid and amalga