as desired, and in the direct ratio to the duration of the immersion.*

*This last assertion is in direct contradiction to the general theory of deposition by simple immersion, which holds that the metal deposited is replaced in the solution by an equivalent of the immersed metal, which continues to dissolve as long as it is not entirely covered by the deposited metal.

To account for this apparent conflict of theory and practice, Roseleur offers the following ingenious and rational explanation of this phenomenon. We have seen that when an article of iron is plunged into a solution of sulphate of copper, part of the iron is dissolved as sulphate of iron, while an equivalent proportion of copper is separated from the solution and coats the iron. In the same manner when we plunge an object of copper into a bath of double cyanide of silver and potassium, part of the copper is dissolved and forms a double cyanide of copper and potassium, while an equivalent proportion of silver is deposited upon the remaining copper. In either case, the exchange is arrested as soon as the copper is coated with a continuous layer of silver, which prevents the solution of the underlying metal.

The same phenomenon takes place at first, during the operation of cold silvering by the double sulphite of sodium and silver, and an equivalent proportion of double sulphite of sodium and copper is formed. But, afterwards, another and a different reaction sets in, by which the silver deposit continues to increase in thickness, without any further solution of copper, and this is due to certain chemical changes of the component parts of the bath.

To understand these changes it will be necessary to explain that silver has very little affinity for either sulphurous acid or oxygen, and, on the other hand, that sulphurous acid possesses a great tendency to become sulphuric acid by taking up more oxygen. In such a bath, therefore, silver will continue to be deposited, being separated in the metallic state by the reducing action of the sulphurous acid, without the solution of any more copper to take its place in the bath. This is proved by the fact that if a bath of this kind be allowed to remain in a glass vessel, it will be slowly and spontaneously decomposed, and will silver the glass itself. The sulphuric acid formed reacts upon a portion of the undecomposed sulphite of sodium, sets free an equivalent of sulphurous acid, which maintains the bath in the state of bisulphite, while, at the same time, an equivalent of sulphate of sodium is formed, which does not interfere with the silvering process.

This bath is formed of bisulphite of sodium, to which is added a salt of silver, preferably the nitrate, until it begins to be dissolved with difficulty. It is therefore with a double sulphite of sodium and silver that the cold silvering by immersion is effected. It is evident that the bisulphites of potassium, ammonium, and other alkalies, may be substituted for the bisulphite of sodium, but the latter is to be preferred, because its preparation is cheaper, easier, and better known.

Roseleur recommends the following method for the

PREPARATION OF BISULPHITE OF SODIUM FOR COLD SILVERING.

Into a tall vessel of glass or porcelain (Fig. 111) introduce

and pour mercury, about an inch or so deep, into the bottom of the vessel, so that the glass tube conveying the sulphurous acid gas dips into it. The object of this artifice is to prevent the clogging of the delivery tube by the crystals formed during the operation. Then rig up an apparatus for the production of sulphurous acid, as indicated in the chapter on CHEMICAL PRODUCTS, and pass the washed gas through the vessel containing the carbonate of sodium. A part of this salt becomes transformed into sulphite of sodium, which dissolves, and a part is precipitated as bicarbonate. The latter, however, is transformed into sulphite of sodium, by the continued action of sulphurous acid, and carbonic acid gas escapes with effer

vescence.

When all has become dissolved, the passage of sulphurous acid should be continued until the liquid slightly reddens blue litmus paper, and then allowed to stand aside

for twenty-four hours. At the end of that time, a certain quantity of crystals will be found upon the mercury, and the liquid above, more or less colored, constitutes the bisulphite of sodium of the silvering bath. The crystals

are separated from the mercury, drained, and kept for gilding baths. They are not suitable for silvering.

The liquid bisulphite of sodium thus prepared, should be stirred with a glass rod, in order to eliminate the carbonic acid which may still remain in it.

The liquid should then be again tested with litmus paper; and if the blue color is strongly reddened, carbonate of sodium is cautiously added little by little in order to neutralize the excess of sulphurous acid. On the other hand, if red litmus paper becomes blue, too much alkali is present, and more sulphurous acid gas must be passed

through the liquid, which is in the best condition for our work, when it turns litmus paper violet or slightly red. This solution should mark from 22° to 26° Baumé, and should not come in contact with iron, zinc, tin, or lead.

PREPARATION OF THE (COLD) SIMPLE-IMMERSION
SILVERING BATH.

A vessel of stoneware, glass, or porcelain is about threefourths filled with the liquid bisulphite of sodium, and a moderately concentrated solution of nitrate of silver in distilled water is gradually added, while the bath is continually stirred with a glass rod.

By the contact of the two liquids, a white, flocculent precipitate of sulphite of silver is produced, which, with the aid of stirring, is dissolved by the bisulphite of sodium, with the formation of the double sulphite of sodium and silver. The silver solution is added as long as the precipitate readily disappears, and is stopped when it begins to dissolve with difficulty.

Roseleur is authority for the declaration that in this state the bath is always ready to work, and produces, almost instantaneously, a magnificent silvering upon copper, bronze, or brass articles which have been thoroughly cleansed and "quicked," although this last operation is not absolutely necessary.

He affirms that we may obtain with this bath, and according to the length of time of the immersion

1. A very fine whitening by silver, which is as cheap as any of the previously described processes;

2. A bright silvering sufficiently durable, and especially adapted for jewellers' uses;

3. A dead silvering still more durable, for a multitude of objects, without the aid of electricity, and in the cold. In the measure that the silver of the bath becomes

impoverished, it is made good by additions of nitrate of silver; but when, as will happen in the course of time, the proportion of bisulphite present is no longer sufficient to dissolve the silver salt, it becomes necessary to add some bisulphite of sodium to restore the bath to its primitive state. Roseleur has used the same bath. now and then reinvigorated by additions of bisulphite of sodium, or of nitrate of silver, for five consecutive years, for silvering trinkets, etc.; and in this same bath as many articles were daily silvered as a man could conveniently carry, and at prices varying from five cents to one dollar per pound.

The silver, which is slowly deposited upon the sides of the vessel, may be easily dissolved in nitric acid for future

use.

Roseleur lays special stress upon the process, and recommends it to his confréres with the conviction that it will probably eventually replace all the other known methods.

ADDITIONAL FORMULA FOR DIRECT- OR IMMERSION-SILVERING.

The foregoing formulæ for silvering without the aid of the battery, may be varied considerably without noticeably affecting the quality of the work. Thus, for a thin silvering adapted for such small articles as pins, buttons, buckles, hooks-and-eyes, clock faces, barometer and thermometer plates, and many other articles of copper, brass, etc., Gore (Electro-Metallurgy, 152 et seq.) gives the following formula:—

1. Chloride of silver and bitartrate of potassium, equal parts.

2. Chloride of silver, 1 part; alum, 2 parts; common salt, 8 parts; and cream of tartar, 8 parts.