ASCERTAINING THE WEIGHT OF DEPOSITED SILVER. 319

rience, that it is nearly time to withdraw the spoons from the solution. He thereupon removes, dries, and weighs them and returns them again to the bath with the abovenamed precautions; and this operation must usually be repeated several times until the proper weight of silver has been deposited.

Others adopt this plan: they cleanse the spoons or forks and put them immediately into the bath, except one, which is treated as above and is used to control the operation. This piece is now and then removed from the bath to ascertain its increase of weight, and when it has acquired its correct proportion of silver as determined by previous calculation, it is assumed that the balance of the articles have also received their proper quantity and that the operation is complete.

Such methods are obviously defective and erroneous. In the first place, the operation is tedious, and it will be one chance out of a thousand if the correct result is arrived at. Again the quantity of silver deposited on the piece that serves to control the operation will be materially affected by the condition of the wire from which it is hung, the cleanliness of its contacts and its distance from the anodes, from which it will be apparent that the most erroneous results may be reached, if the quantity of silver supposed to be deposited upon the other articles be calculated from the increase of weight of the test piece.*

* Strongly amalgamated articles will become sensibly oxidized during the drying which precedes their weighing.

When, during the operation, the objects have been dried in order to ascertain the proportion of deposited silver, they should not be returned to the bath without having been cleaned in a hot solution of cyanide of potassium, which dissolves the grease from the handling, passed again through the "quicking" solution, and rinsed. Alcohol may be substituted for the hot solution of cyanide, but the results are not so sure, and the expense is greater.

This inconvenience is obviated, and the precise moment when the given proportion of silver has been deposited may be recognized, by the following very simple procedure.

After having removed one of the pans of an ordinary balance, substitute for it a metallic frame which supports the articles to be silvered, and which communicates through the beam and column of the balance, with the negative electrode of the battery. The anode is connected with the positive pole (Fig. 118).

Fig. 118.

When the articles, spoons and forks for instance, sus

pended from this frame, are placed in the silvering bath, the equilibrium of the balance is established by placing weights in the other pan. The quantity of silver that it is intended the articles shall receive being determined, the corresponding weight is placed in the scale-pan. It is scarcely necessary to add that the operation will be finished when the equilibrium of the balance is re-established.

This method is not mathematically accurate, since the

articles are more bulky after than before the silvering process; nevertheless it is sufficiently exact for all practical purposes.

A careful operator will employ a balance for each bath, especially when silvering spoons and forks. The supporting frames may be circular, in order to have the anode in the centre of the bath so that the articles shall be at equal distances from it in order that they shall all receive the same amount of deposit. The central anode may be reinforced by another circular anode laid on the sides of

Fig. 119.

the vessel, so that the articles are exposed to the action. of the current on both sides. Lastly, there is no difficulty in so disposing a signal bell in connection with the pointer of the balance that it will indicate the precise moment

when equilibrium is established and the operation com pleted (Fig. 119).

We see that, in this arrangement, the articles to be silvered are connected with the negative pole through the column, the knife-edges, and the beam of the balance. The anodes communicate with the positive pole by means of a conducting wire passing through the column of the balance, but insulated from it by a glass tube which surrounds it. This apparatus may be modified in many

ways.

An apparatus of the character of that just described. will obviously enable the operator to determine with great precision the weight of silver deposited; it will save much labor, it will economize battery power; and finally, where several of the above-described baths are employed, there will be no danger of a complete stoppage of work by reason of a faulty bath, which is frequently the case where a single depositing vat is used.

Mechanical devices for automatically breaking circuit, when the predetermined amount of silver has been laid on articles in the plating-bath, and for indicating the fact by signal, or otherwise, are numerous. Some of these have been made the subject of patents, and in various modifications such devices are in use in all the large establishments.

CHAPTER XXXVIII.

ROSELEUR'S PLATING-BALANCE.

A PLATING-BALANCE WHICH DEPOSITS WITHOUT SUPERVISION, AND WITH CONSTANT ACCURACY, A PREDETERMINED QUANTITY OF SILVER, AND WHICH AUTOMATICALLY BREAKS CIRCUIT WHEN THE OPERATION IS TERMINATED.

WE describe in this chapter an automatic platingbalance, designed by Roseleur, a very elaborate but perfect apparatus for its intended purpose. We give the description in the words of the inventor.

The several forms of plating-balance in general use, though very convenient, are still imperfect, for the reason that the operator must be present when the operation is finished, in order to withdraw the objects from the bath, or to arrest the action of the battery. The personal attention thus demanded is not always possible, and is frequently irksome and inconvenient, especially where baths are kept in operation day and night which will deposit a great excess of silver, if they are not arrested in time.

We have succeeded in avoiding all the inconveniences pointed out by means of the plating-balance about to be described.

Of the four sizes of this apparatus manufactured, the smallest is sufficient for a half-dozen of forks or spoons, and is intended for the use of amateurs, chemical laboratories, etc.

The other three sizes are designed for manufacturers,