We have thus obtained the fused nitrate of silver (lunar caustic), which is more or less white, or gray, according to the purity of the silver employed.

When the whole is quite cool, the dish is turned upside down upon a clear sheet of paper, and by a gentle tap on the sides, the mass is detached.

2. The nitrate of silver is dissolved in ten or fifteen times its weight of distilled water, and treated with a ten per cent. solution of pure cyanide of potassium (Roseleur suggests the use of hydrocyanic (prussic) acid), which immediately produces an abundant white precipitate of cyanide of silver. A sufficiency of the reagent has been employed, when, after allowing the precipitate to subside, the addition of another drop to the clear supernatant liquid produces neither turbidity nor precipitate.

The whole is then thrown upon a linen filter stretched upon a wooden frame, and the cyanide of silver remains on the cloth, while the nitric acid and the excess of cyanide pass through. The precipitate left upon the filter is washed two or three times with pure water.

3. The cyanide of silver thus prepared is placed in the vessel intended for the bath, and stirred into the 1000 parts of water prescribed in the formula. The cyanide of potassium is then added, which, after solution, also dissolves the cyanide of silver. The silvering bath is therefore a solution of the double cyanide of potassium and silver.

Referring to the proper mode of preparing the electrosilvering bath, Roseleur remarks that operators on a small scale, in order to render the operation more rapid, frequently substitute the nitrate, or the chloride of silver, for the cyanide. This mode of operation is not economical in the long run, since, after having maintained the strength of their baths with such materials, these baths

will become loaded with chloride of potassium, or nitrate of potassium, and, becoming too dense, will not be readily traversed by the electric current. Moreover, in cold weather, these salts will crystallize upon the immersed articles, and, as silver will not be deposited upon these crystals, the silver coating will be marred by spots which cannot be removed by burnishing it, and the operation will have to be begun anew.

In this connection, we will recall the fact that Roseleur, in the preparation of his double cyanide electrogilding solution, uses the same method, to which he very properly here objects, in the preparation of the electrogilding solution (see pages 204-205 of this work, and our remarks on page 209).

We should say, nevertheless, that amateurs or operators who employ small baths, often revivified, may substi tute for the cyanide of silver the chloride, or, better still, the nitrate of this metal. But, in the latter case, the quantity of cyanide of potassium should be increased. Such baths will be prepared as follows:

1. The nitrate of silver is prepared in the manner indicated above, and 15 parts of it (nearly equal to 10 parts of pure silver) are dissolved in 1000 parts of water.

2. The cyanide of potassium (pure), about 25 parts, is then added to the above solution.

After stirring to facilitate complete solution, the liquid is filtered, in order to separate the iron contained in the cyanide. This operation, however, is not absolutely necessary, as the iron rapidly falls to the bottom of the bath, and the solution becomes limpid.

In this case, it will be noticed, we have only 10 parts of silver per 1000 of water; but this is enough for ordinary purposes. On the other hand, some silver electroplaters employ much richer baths, holding up to 100

parts of metal per 1000 of water, but an average of from 20 to 25 parts furnishes very good and uniform results.

The proportion of cyanide of potassium employed is much more than is required for dissolving the indicated quantity of silver, since 1 parts of good cyanide is rather more than sufficient for 1 part of silver; but experience demonstrates that, unless there be an excess of cyanide of potassium, the baths do not conduct electricity well, and the deposit of silver is granulated, striated, and irregular. The necessity of the presence of free cyanide in electrogilding and silvering baths has been fully explained in Chapter XXVII., page 256 et seq., to which the reader is referred.

The operation is effected with the current, and with baths, either warm or cold. The latter method is generally adopted for articles which require great solidity. The hot process is employed for small articles, although it is preferable also for steel, iron, zinc, lead, and tin, which have been previously electro-coppered.

The hot electro-silvering baths are generally kept in enamelled cast-iron kettles, and the articles are either suspended, or moved constantly about in them. The pre

liminary operations of cleansing in acids and passing through "quicking" solution are necessary. A somewhat energetic current is needed, especially when the articles. are moved about, in order to operate rapidly. The cur rent is too strong, when the articles connected with the negative electrode become gray or black, or produce an abundant disengagement of gas bubbles.

In the hot silvering baths, the separate battery is often replaced by the use of a zinc wire wrapped around the articles. The points of contact of the two metals are often black or gray, but the strain rapidly disappears by

plunging the object into the bath for a few moments, after it has been separated from the zinc, and carefully scratch-brushed.

A few gold and silver electroplaters employ, instead of separate batteries, the simple apparatus (Fig. 112), which

Fig. 112.

consists of a glass, porcelain, or stoneware vessel for containing the bath, in the centre of which is a porous cell filled with a solution containing 10 per cent. of cyanide of potassium, or of common salt. The cylinder of zinc, immersed in this exciting liquid, carries a circle of brass wire, the cross wires of which intersecting at the centre are soldered to the zinc. It is then sufficient to suspend the well-cleansed articles to the brass circle by means of wires. At the beginning, the operation goes on rapidly, and the deposit is good; but, after a certain length of time, the solution of zinc traverses the porous cell by exosmose, and the purity of the bath is impaired.

When the bath becomes impoverished it is revivified by additions of equal parts of cyanide of potassium and of silver salt. It is also necessary to replace the water in proportion as it evaporates.

Roseleur, whose usually eminently practical instructions we have largely followed in this work, affirms that the previous coating with copper, of articles of steel, as well as those of Britannia-metal, iron, lead, tin, and zinc, is indispensable as a preparatory step to securing an adherent coating of silver. So far as this statement relates to steel and Britannia-metal, this usually correct authority is altogether wrong.

In the United States, which has some of the largest silver-plating establishments in the world, notably the Meriden Britannia Company at Meriden, Conn., the Wm. Rogers Manufacturing Co., of Hartford, Conn., and others, the practice of previous coppering is not adopted either with Britannia-metal (white-metal, as it is sometimes called) or steel. Indeed, we may affirm on the best authority that this practice has not been in vogue in any establishment of importance for many years-steel tableknives being plated directly on the steel, and table-ware, such as cake-baskets, ice-water pitchers, coolers, etc., receiving their silver deposits directly on the Britannia. It is doubtful in fact if coppered wares, such as Roseleur describes, would be marketable. It would It would appear, therefore, that this practice, which appears to be obsolete in England also, is confined to the French platers.

The practice of different establishments, of cleansing their work, differs somewhat, but all aim at the same result, viz., to secure a smooth, adherent coating of metal upon an inferior base.

The practice in the Meriden Britannia Co.'s works, at Meriden, Conn, as observed by the author, is substantially as follows: With "white-metal:" The article is first cleansed of all grease by immersion in boiling alkali; then into dilute muriatic acid; then into a "striking" solution, viz., a weak cyanide of silver solution with a large propor