six bearings where the knife-edges rest, until all the polished parts of the latter are covered. This mercury presents the following advantages:

1. It prevents the corrosive action of the acid vapors of the work-room upon the polished parts of the knifeedges and their bearings.

2. The friction is lessened, and the weighings are more

correct.

3. It considerably increases the surfaces of contact for the passage of the electric current.

4. It prevents the loss of temper which the knife-edges would otherwise suffer by the excessive heat to which they would be exposed.

Lastly, the insulated steel cup is filled with mercury, until the point of the platinum wire attached to the beam just touches it when the equilibrium is established, that is to say, when the index marks zero. The surface of the mercury should be, now and then, cleaned of any dust which may prevent the passage of the current. The small flexible pocket is used for raising or lowering the level of the mercury, and thus permitting of the exact adjustment of the balance.

USE OF THE BALANCE.

1. The depositing vat is filled up with the silver bath to within a few inches of the upper edge.

2. The anodes are hooked upon their respective rods, each of which rests upon the brass rectangle nailed upon the vat, forming a connected system, communicating by means of the binding screw with the positive electrode.

The silver anodes should dip entirely into the liquid, otherwise they will be rapidly cut at the water line, while, with complete immersion, the platinum supporting wires

are unacted upon.* The anodes are placed parallel to each other, one at each end of the trough, and the intermediate ones are about 8 to 10 inches apart in order to leave a sufficient space for two cross-rods, supporting the spoons and forks.

3. Two transverse wooden bars are placed under the suspending frame, thus insulating it from contact with the anodes, and the cross-rods supporting the objects are placed upon the frame in such a manner that there are two rods for each space between two silver anodes. We must be careful to preserve the same distance between the anodes and the spoons or forks, and, in order to avoid all uncertainty, the positions of the rods for the anodes and for those supporting the articles, are marked with a file. It is evident that if these equal distances be not preserved, the articles nearer the anodes will be charged more rapidly, and that the total amount of silver will not be equally divided. Furthermore, since two rows of objects are situated between two anodes, the surfaces facing the anodes will be coated more rapidly. It is necessary therefore, during the operation, to change the position of the supporting rods end for end. This manipulation is necessary if only a single row of articles is placed between, and at equal distances from, two silver anodes.

Before being introduced into the bath, the articles should be thoroughly cleansed and "quicked." For the rapid and easy handling of forks and spoons during these operations, a convenient holder (Fig. 126) is shown herewith,

*These anodes are plates of pure laminated silver, and should be at least as high as the articles to be silvered. Their width is from 1 to 11⁄2 inches less than that of the vat. As it often happens that the rolls have left upon the silver traces of grease and of oxide of iron, the plates should be heated to a dull red and cleansed before using, or a simple cleansing in boiling lye is sufficient.

made of gutta-percha, and composed of a handle attached to a plate having twelve notches sufficiently narrow at their entrance to allow of the introduction of the forks and spoons by their thin edges only. From the entrance

Fig. 126.

Fig. 127.

1010

the notches gradually widen, so that the forks and spoons when introduced will stand upright by their own weight. The edges of the notches are likewise bevelled, in order to diminish their surface of contact with the metal to be cleansed.

When twelve spoons or forks have been cleansed in boiling lye, and passed through the acid pickle, they are suspended to this support, and successively dipped into aqua fortis, the mixed acids, and the "quicking" solution. The holder and its contents are then plunged into a large tub filled with water, when, by giving it a sudden downand-up motion, the articles are released and fall from the notches.

This gutta-percha holder is scarcely attacked by the cleansing acids, and presents the advantage of giving more rapidity in the operation, great cleanliness, and a sensible economy. Two of these supports are necessary;

one of which is being filled while the other is being passed through the cleansing acids.

When all the articles for the same bath are thus prepared they are suspended from the rods. This labor is made easy by supporting each rod at its extremities upon two long hooks fixed to the wall, and then introducing each piece into the hook of the slinging wire.

When a rod is thus charged, it is taken by its extremities, and the suspended articles are plunged first into quicking solution, or very dilute sulphuric acid, then into a bath of fresh water, and lastly into the silvering bath. The operation is repeated with the other rods.

When the bath is filled with the articles, the wooden box is weighted until the equilibrium of the balance is established, when the index marks 0. The wooden bars which prevent the supporting frame from resting directly upon the trough, and thus making contact with the anodes, are raised up.

The equilibrium of the balance is now destroyed by placing upon the intermediate scale-pan a weight equal to that of the silver which we desire to deposit on all the articles. Following the inclination of the beam, the platinum wire attached to it penetrates the mercury contained in the iron cup; and it is then sufficient to connect the battery with the apparatus by the two conducting wires communicating respectively with the cup and the anodes, in order to commence plating.

It will be readily understood from the foregoing description that when the articles have been coated with a proportion of silver equal to the weight in the pan on the other side of the beam, the equilibrium of the balance will be re-established, and the platinum wire will be raised out of the mercury, thus breaking the electric cir-·

cuit, and stopping the deposition of silver, precisely as if one of the conducting wires had been cut.

The operation will, therefore, run its course without personal surveillance or attention; and, what is more, the results will not be modified, whatever be the length of time the articles remain in the solution. An excess of silver cannot be deposited, since there is no electric current; on the other hand, should the bath dissolve a certain proportion of the deposited silver, the articles become lighter, and the equilibrium of the balance is again broken, resulting in the platinum wire re-entering the mercury, and closing the circuit anew. We have therefore a series of oscillations, due, on the one hand, to a slight excess of the electro-deposit, and on the other to its partial solution in the cyanide, and the result is that the amount of the deposited silver remains within the proportions determined upon in advance.

GENERAL DIRECTIONS FOR KEEPING THE PLATING-BALANCE

IN ORDER.

When a silvering operation is complete, the weights should be carefully removed from the scale pan, in order that the supporting frame carrying the silvered objects shall come to rest gently and squarely upon the vat. Any violence in this operation may endanger the knifeedges. The rods with the articles may then be easily removed.

The parts of the apparatus which demand the greatest cleanliness are the two binding screws through which the current is transmitted; the points of contact of the rods carrying the articles, with the supporting frame; and lastly those of the anode rods with the rectangular brass rod fixed to the vat.

Acids should never be used for cleansing any part of