occur, the original electrodes now constituting a battery in relation to this second decomposition cell: the current, however, would gradually weaken until it ceased. The cause of all these actions and reactions is this: The article being plated with silver in connection with the battery, exhausts the solution of silver around it. leaving free cyanide of potassium in solution, while around the sheet of silver which is serving as the positive electrode, the solution is on the contrary becoming saturated with silver, so that we have all the conditions necessary to constitute a battery, having silver in two kinds of solution-the one capable of dissolving silver, the other not. In these conditions lies the source of the annoyances described above. From the moment the deposition of metal begins, there also arises an opposite current of electricity, tending to neutralize the effects of the battery, which current goes on increasing in quantity until the two currents neutralize each other, or it may be until the current from the trough overpowers that from the battery. In the latter case, as we have said, there may, at the termination of the ordinary period, be little or no silver deposited on the articles intended to be plated. Motion in the silver or depositing solution will prevent all these annoyances; and this being now generally adopted, these phenomena are not now observed, but the effects take place less or more in every solution.

Fig. 588.

TEST FOR THE QUANTITY OF FREE CYANIDE OF POTASSIUM IN SOLUTIONS.-It has been already mentioned that the cyanide of silver, as it forms upon the surface of the silver plate, is dissolved by the cyanide of potassium. This renders it necessary to have always in the solution free cyanide of potassium. Were we to use the pure crystalline salt of cyanide of potassium and silver, dissolved in water, without any free cyanide of potassium, we should not obtain a deposit beyond a momentary blush; as the silver plate or electrode would get an instantaneous coating of cyanide of silver, and this not being dissolved the current would stop. The quantity of free cyanide of potassium required in the solution varies according to the amount of silver that is present, and the rapidity of the deposition. If there be too little of it, the deposit will go on slowly; if there be too much, the silver plate will be dissolved in greater proportion than the quantity deposited, and the solution will consequently get stronger. The proportion we have found best is about half by weight of free cyanide of potassium to the quantity of silver in solution; thus, if the solution contains two ounces of silver to the gallon, it should have one ounce of free cyanide of potassium per gallon. This is known

by taking some nitrate of silver, dissolving it in distilled water

and placing it in a common alkalimeter, graduated into 100 parts, Fig. 588. The proportion of the nitrate of silver in the solution is that every two graduations of the solution should contain 1 grain. A given quantity of the plating solution is now taken-say 1 ounce by measure, and the test solution of nitrate of silver is added to it by degrees, so long as the precipitate formed is redissolved. When this ceases the number of graduations is then noted, and the following equation gives the quantity of free cyanide. Every 175 nitrate of silver are equal to 130 cyanide of potassium in solution. Suppose 20 graduations were taken, equal to 10 grains nitrate of silver, then 175: 130: 10: 7.4 grains free cyanide of potassium. This, multiplied by 160, the number of fluid ounces per gallon, will make about 2 ounces. We have taken 2 graduations to one grain of nitrate of silver, that the solution may be considerably dilute and less liable to error. The following table is calculated at a half grain nitrate of silver to the graduation, and will be a guide to the student or workman. The quantity of solution tested is one ounce by measure.

Another method may be adopted. If, for instance, we dissolve a small quantity of sulphate of copper and add to it an excess of ammonia, there is produced a deep blue color. Cyanide of po tassium will destroy the blue color, in a fixed chemical proportion. To obtain this proportion, take ten grains of pure cyanide of po tassium and dissolve in water; then take a certain quantity, say 100 grains, of sulphate of copper and convert it into ammoniuret, the whole measuring a given quantity, and pour from an alkalime

ter this blue liquor into the cyanide of potassium till it ceases to destroy the color, then mark the number of graduations required, and that amount of copper solution will represent 10 grains cyanide of potassium-a quantitative test will thus be got for the full cyanide of potassium in the solution, and should be used as follows: Say that the color of 60 graduations of the blue solution was destroyed by the 10 grains of cyanide of potassium, then to test the quantity of free cyanide of potassium in the plating solution, take 60 graduations of the blue liquor in any convenient vessel, and add to it from an alkalimeter the plating solution till the color of the blue liquor is destroyed, then note the quantity which contains 10 grains free cyanide, from which the quantity in the whole solution may be calculated.

RATE OF DEPOSITING SILVER.-When articles are taken out of the solution they are swilled in water, and then put into boiling water. They are afterwards put into hot sawdust, which dries them perfectly. Their color is chalk-white. They are generally weighed before being scratch-brushed; that is, brushed with the fine wire brushes and stale beer as already described. Although this operation does not displace any of the silver, still, in taking off the chalky appearance, there is a slight loss of weight. The appearance after scratching is that of bright metallic silver. Any thickness of silver may be given to a plate by continuing the operation a proper length of time. One ounce and a quarter to one ounce and a half of silver to the square foot of surface, will give an excellent plate about the thickness of ordinary writing paper. BRIGHT DEPOSIT.-A little sulphuret of carbon added to the plating solution prevents the chalky appearance, and gives the deposit the appearance of metallic silver; the reaction which takes place in this mixture is not yet understood. The best method of applying the sulphuret of carbon is to put one or two ounces into a large bottle, then fill it with strong silver solution, having an excess of cyanide of potassium, and let it repose for several days, shaking it occasionally. A little of this silver solution is added, as required, to this plating solution, which will give the articles plated the same appearance as if scratched. It is also found that the presence of sulphuret of carbon prevents the solution from going out of order; indeed we have seen a solution that has been constantly working from two to three years, while, generally, they were subject to go out of order for a time, in less than one yearalthough, after standing a time, they would recover-but these are curious reactions not yet investigated.

DIFFERENT METALS FOR PLATING.-Silver may be deposited upon any metal, but not upon all with equal facility. Copper, brass, and German silver, are the best metals to plate; iron, zinc, tin, pewter, and Britannia metal, are much more difficult; lead is easier, but it is not a good metal, because of the rapidity with which it tarnishes, and, from its softness, easily yields to the pressure of the burnisher: nevertheless all these metals and alloys may be,

and are, plated, but cannot give the satisfaction which brass, copper, or German silver afford. In plating upon alloys having tin in them, such as Britannia metal, they must not be dipped into nitric acid previous to plating; but into a hot and strong solution of caustic potash or soda for five minutes, and put directly into the plating solution (which should have an excess of cyanide of potas sium, and the battery be as strong as the liquor will admit of with out gas being evolved), until covered, when the silver may be thickened by an ordinary solution and battery.

ELECTRICITY GIVEN OFF FROM SANDY DEPOSITS.-We may mention that when depositing silver upon a large surface, and the solution or battery being in the condition to give the sandy deposit, or rather when the deposit has gone on for a long time and the solution not been agitated, so that it has become very much exhausted of silver round the article, the deposit towards the end of the time has been almost impalpable to the touch, like flour: sometimes the grains were a little coarser. The practice, in such cases, is to lift the articles from the solution, and to place them in boiling water, and after steeping there some time, to take them out, when the heat of the metal soon causes it to dry. Under these circumstances, when the deposit was of the sort stated, we have seen on a large waiter or tray, when the hand was rapidly drawn over the surface, after it was dried in the manner described, the same effect produced as when the hand is drawn over an electrified handkerchief, or sheet of paper, accompanied with a crackling noise and pricking sensation. We have repeatedly observed these phenomena, but never having chanced to be in the dark, no light was visible from the surface rubbed. Although these are the conditions under which the observations were made, the phenomena were not produced every time these conditions were found. It is probably caused by the fact that this kind of deposit, which is of a chalky appearance, is a bad conductor of electricity, and as the boiling water was often very impure, holding salts in solution, the rapid evaporation of the water from the surface of this sort of deposit might leave it excited for a short time, and the hand being drawn across at the time of excitation, the electricity was liberated.

THE OLD METHOD OF PLATING.-Many objections have been urged against the application of electro-deposition to the purposes of plating, as a branch of manufacture, either as a competitor or substitute for the old method, technically called Sheffield plate-so called because Sheffield is a principal seat of that manufacture. To enable our readers to form a proper estimate of the objections urged, by enabling them to judge of the relative importance and value of the two processes, we shall add a brief description of the old method.

An ingot of copper being cast, was filed square and smooth, and a piece of silver was placed upon it, the two surfaces being perfectly clean: a little borax having been introduced between the two metals, they were bound together with iron wire, and then

neated in a furnace nearly to the melting point; the small quantity of borax increased the fusibility of the two metals at their surface, and thus they were fused together. When fusion was effected the metals were subjected to the dilating process of heavy rollers, the dimensions in length and width being regulated according to the articles to be made. This sheet formed the base or foundation of every article, of whatever shape or form, and however it was to be ornamented when finished.

To produce ornaments, leaf silver was stamped in iron dies representing the ornaments required, which, when removed from the dies, were filled with an alloy of lead and tin. These were then soldered upon the flat or shaped plain surface with soft solder, which melts at a very low temperature: thus were produced the silver edges, or mounts.

The quality of the ornament depended entirely upon the price of the article; but whatever the quality, all ornaments in the old mode of plating were thus made, the only difference being the thickness of the silver leaf used:-Ornamental feet, handles, knobs, etc., were made in the same manner, being struck up in two parts, filled with lead and tin, soldered together with soft solder, and afterwards soldered to the main body. Articles (such as table candlesticks) which would be too heavy if filled with lead, were filled with rosin, pitch or any other similar substance, for the purpose of preventing the article being flattened by pressure. Hence it is evident that no solid article could be made by the old mode of plating, the only way of producing articles being to work them up by the hammer, or to strike them in dies from a flat surface: and being restricted to the use of soft solder, on account of the plated metal and the shells of silver, forming the edges, not supporting the required heat to melt silver solder, it is equally evident that the joinings so constructed would be easily removed either by force or heat.

The nearest approach to solid articles made by the old method of plating, were forks and spoons: these were generally made of iron, thin silver being soldered upon the surface, which was afterwards dressed smooth, and polished.

The heat used in this operation was merely that of an ordinary soldering iron; because, were a greater heat applied, the silver would form an alloy with the tin and lead of the solder, and melt: the same heat that cemented the metals in the first instance would be sufficient to disunite them; and thus, when these forks were exposed in hot gravy, the solder was liable to become soft, and the silver covering, yielding readily to the knife, to peel up or become abraided, in consequence of the soft intervening metal.

ADVANTAGES OF ELECTRO-PLATING. The advantages offered to the plater by the electro-process are many, arising from the fact of the articles being plated after, instead of before, being manufac tured. This at once entirely removed all those restrictions on taste and design, under which the plater was forced, by the nature of his process, to labor.