trated aqua fortis), 10 parts, in which they should remain for a greater or less length of time, according to the thickness of the coating of silver to be dissolved.

This liquid possesses the remarkable property, when not diluted with water, of dissolving the silver without sensibly affecting copper and its alloys, such as bronze, brass, German-silver, packfong, similor, etc. The articles introduced into it should therefore be dry and when not in use the liquid should be kept in a hermetically closed vessel, as it eagerly absorbs moisture from the air.

As far as practicable, the articles, a number of which are usually "stripped" together, should be placed in the liquid without touching each other, and in a vertical position, as shown in Fig. 130.

In proportion as the action of the liquid diminishes, small and successive additions of nitric acid are given to it. While the process of dissolving silver in the cold is the more regular and certain, its action is considered too slow by some operators, especially when the proportion of silver is great. Another, and more rapid, process is then resorted to, viz:

A shallow pan of enamelled cast-iron (Fig. 131) is nearly filled with concentrated sulphuric acid and the whole brought up to a temperature of from 300° to 400° F. Then, at the moment of using it, pinches of

Fig. 131.

dry and pulverized nitrate of potassium (saltpetre) are thrown into it, and the article, held with copper tongs, is plunged into the liquid. The silver is rapidly dissolved, while the copper, or its alloys, is but slightly corroded.

It is scarcely necessary to add the precaution that the articles should not be allowed to remain in the "stripping" liquids any longer than is actually necessary for the removal of the silver, in order to avoid the corrosion of the underlying metal.

According to the rapidity of the solution fresh additions of saltpetre are made. All the silver has been dissolved, when, after rinsing in water and dipping the articles into the cleansing acids, they present no brown or black spots, that is to say, when they behave like new.

These two methods are not suitable for removing the silver from wrought and cast-iron, zinc, or lead, for which purpose it is preferable to employ a cyanide bath with the electric connections arranged in the inverted order; in other words, to make the articles the anodes; or, to use mechanical processes.

These desilvering (stripping) liquids become green after a certain amount of use, and, in order to recover the

silver, they are first diluted with four or five times their volume of water, and then treated with hydrochloric acid, or common salt. The precipitation is complete when the supernatant liquid does not become turbid by a fresh addition of common salt, or of hydrochloric acid.

The resulting chloride of silver is separated from the liquid either by decantation or filtration, and is afterwards reduced to the metallic state by one of the methods to be hereafter indicated.

Where gold is to be removed from silver, which will sometimes be necessary, the methods described in Chapter XXVII. (page 260) should be followed. Occasionally also it becomes necessary to remove copper from silveras where, for example, a thin copper mould must be dissolved from a solid electro-deposit of silver. In such case it is recommended to immerse the article in boiling diluted hydrochloric acid, or in a hot solution of perchloride of iron (ferric chloride), made by dissolving, with the aid of gentle heat, the peroxide of iron (jeweller's rouge) in hydrochloric acid, to complete saturation. Some authorities suggest the removal of copper from silver, by making the article the anode in a sulphate of copper solution, and passing the current until the copper has disappeared; but there is a risk of losing some of the silver by solution, especially if the current employed is strong.

As above remarked, worn and defectively coated articles. that are to be replated should under no circumstances be replated without having first been " stripped." Some operators replate without this precaution, but the work thus performed is usually inferior, because of the difficulty of securing the perfect adhesion of the second coating of silver, on the portions of the original coating that remain, from which portions the newly-laid silver will often show a tendency to separate or "strip." A few unpleasant

experiences of this kind will soon impress on the mind of the operator more forcibly than precept, that in the long run it is safer to thoroughly remove an old coating before attempting to replate.

CHAPTER XLIII.

ELECTRO-SILVERING-CONCLUDING REMARKS.

IN preparing the cyanide plating solution by the chemical method described in this work (Chapter XXXIV., p. 288 et seq.), care should be taken by the operator, in dissolving his silver in nitric acid as directed, to use the pure acid, free from chlorine, with which the common. commercial nitric acid is always more or less contaminated. Otherwise, a variable quantity of chloride of silver will be formed, which will render it impossible to obtain a clear solution of the nitrate in water, and which will interfere with the nicety of the subsequent operations. To test the quality of the acid, it will be sufficient to pour a little of it into a test-tube with some distilled water, and to add to it a drop of nitrate of silver solution, when the liquid should remain clear. If a milky turbidity or precipitate occurs, the acid should not be used. The same precaution should be observed with respect to the water in which the nitrate is dissolved previous to its precipita tion with cyanide of potassium (or with hydrocyanic acid). For this purpose it is preferable to use distilled water, as ordinary water supplied to cities and towns invariably contains more or less of chlorides in solution. If, however, impure acid or water is used for the purpose, the insoluble matter should be allowed to subside, and, after the clear

liquid has been drawn off, should be reserved to recover the silver it contains.

In precipitating the nitrate of silver solution with cyanide of potassium special care must be taken not to add too much of this reagent, which would redissolve some of the precipitated cyanide of silver. To avoid this it is a good plan to conduct the operation in a glass vessel where it can be well observed, and towards the end of the operation, which can be told by the formation of a less voluminous precipitate upon addition of cyanide, to allow the precipitate to subside, and to add at intervals small quantities of cyanide at a time, stirring after each addition. By conducting this operation with the proper care, the point of perfect neutralization may be very closely arrived at. If by accident or carelessness too much cyanide has been added, the excess must be neutralized by the addition of a fresh supply of nitrate of silver, with due precaution, until the further addition of a drop causes no turbidity.

The cyanide plating solution may also be prepared with the aid of the electrical current, that is, by passing a strong current through a solution of cyanide of potassium (1 to 3 ounces of cyanide to the gallon according to purity) with a large sheet of silver as the anode, and a small strip of silver or of platinum as the cathode, and passing the current until enough silver has gone into. solution to give the bath the standard strength. The amount of silver dissolved can readily be told by weighing the anode, and comparing the weight with that before the operation; and the proper saturation of the solution may be told by suspending an article at the cathode, and observing how readily it takes a silver deposit.

Napier's plan (Electro-Plating, 135 et seq.) for making "silver solution with the battery" is to employ a solution