upon, that hitherto explained (405, 406, &c.) When a mixture of two or more compounds in solution, is traversed by an electric current of sufficient energy to accomplish any, even the least, decomposition, that compound in the mixture, most susceptible of decomposition, will yield first, and, under all circumstances, be the most copiously decomposed; and the other compounds will yield in succession to the assailing electric forces, and in the same proportion as they are susceptible of decomposition. 419. For the sake of clearness of reasoning, and perspicuity in distinguishing the principal phenomenon under contemplation, I have stated (415), that the particles of copper alone would, under certain circumstances there mentioned, be the only bodies employed in transporting the electric fluid to the receiving metal. This is absolutely the case when a single pair of low tension alone is employed, as represented by Fig. 8, page 88, vol. viii: and it is also the case within all compound batteries in which sulphate of copper is employed, when not sufficiently powerful to decompose the water of solution. In these cases I allude to the action within the battery only. But when the current from any voltaic battery, whether simple or compound, traverses a solution of any exterior compound liquid whatever, which is a good conductor, the whole force of the current is seldom, if ever, employed in the process of decomposition in that liquid; for a portion traverses the liquid as a conductor only. Hence, a current traversing a solution of the sulphate of copper, exterior to the latter, would not be wholly employed in decomposing the salt; but a portion would flow through the liquid totally unemployed in the decomposing process. 420. Many, if not all liquid conductors, may be traversed by an electric current without suffering any decomposition whatever. Such is the case with water, acid solutions, &c. Experiments of this kind give the most decisive results when large terminal metallic surfaces, such as broad platina plates, are exposed to the liquid operated on. A galvanometer needle placed in the circuit indicates the flow of the current: but no indications of decomposition are observable. If, however, the terminal metals were small, especially the receiving metal, decomposition might take place, but the magnetic deflections. would diminish. 421. If a feeble electric current, indicatable by the needle, traverse a very dilute solution of sulphate of copper between large terminal metals, no observable decomposition takes place, nor is any decomposition seen by withdrawing the delivering terminal only, so as to leave a mere corner in the liquid. But if the whole of the delivering terminal be permitted to remain immersed, and the receiving terminal be withdrawn till a corner only be left in the liquid, decomposition immediately takes place, and the immersed corner of the metal not only receives the electric fluid, but also a coating of copper, from the traversed liquor. 422. The electro-chemical action, exterior to the battery, appears, from the foregoing experiments (421), to require a certain degree of density of the current, below which the electric forces are too feeble to accomplish decomposition. 423. Electro-chemical phenomena are subject to one and the same code of laws, whatever may be the source of electric action. Electro-magnetic phenomena are also subject to one distinct code of laws, from whatever source they may emanate; but with respect to the distribution of the electric fluid constituting electric currents, the laws for chemical and magnetic action are different from each other. Many other particulars in the display of electro-dynamic phenomena might here be pointed out, were this memoir intended to give a general theory of the whole system, but as it is limited, or nearly so, to the voltaic battery action alone, they will be pursued no further at present. 423. The same laws which govern the action of a single pair of voltaic metals (405, 406), are applicable to the explanation of those decompositions of acid, and other liquors, which take place on the surfaces of single pieces of metal: and the results are modified according to the electric forces, whether those forces be varied by the character of the metal or by that of the liquid employed (356). And with respect to the decomposition of metallic salts, especially where a fine compact surface of the deposited metal is required, as in the processes of gilding, silvering, &c., an attention to these laws is of the highest consequence (415, 416). 424. In the usual process of silvering barometer scales, and other similar articles, the muriate of silver is employed, usually in a semiliquid state but since the voltaic electric forces on the surface of the copper to be silvered, are too feeble to accomplish copious decomposition of the argentine salt, recourse is had to bitartrate of potassa, which solution enhances the electric action, and produces an abundant precipitation of silver, finely deposited on the copper surface. 425. When polished copper is immersed in a solution of the nitrate of silver, the voltaic forces brought into play are much too powerful to allow of a quiet deposition of silver for the acid also becomes decomposed, and copper is dissolved during the process. But if the action of the liquid be modified by the introduction of cyanuret of potassa, then the electric forces brought into play by the surface of copper and the liquor, become too feeble to produce the necessary deposition of silver though such a liquid is very suitable in the process of electro-silvering, when the electric forces of a few voltaic pairs in series are employed. A similar treatment of muriate of gold, produces a good liquor for the electro-gilding process; though the ferro-cyanuret of potassa mixed with the muriate of gold, produces a still better. : 426. In these processes (424, 425), however, we are not to lose sight of the influence of the metal potassium, which no doubt has a considerable share in bringing about the excellence of the deposited metallic surfaces, by operating as an electro-amalgamating medium between the two metals. 427. The view thus taken of the influence of the potassium (426), is highly favoured by some facts in the processes of soldering, especially in soldering cast iron. Until lately, the liquid employed in soldering iron, was a solution of muriate of ammonia; now dilute muriatic acid is the liquid generally used, and answers very well for wrought iron. Cast iron, however, can not so easily be made to take the soft solder by this process; but by first dissolving a small portion of zinc in the muriatic acid, the cast iron receives a full flow of the melted solder in the most perfect manner. 428. With a view of accomplishing a good deposition of silver on the surface of copper, independently of the employment of a battery, I mixed a portion of bitartrate of potassa with the solution of nitrate of silver and cyanurate of potassa (425); which produces a compound that yields silver in abundance on any piece of clean copper by mere immersion. The deposition of silver is enhanced by first dipping the clean copper in very dilute nitric acid, and then plunging it in the argentine liquid. The coating of silver thus obtained is quite as good as that by the old process of silvering (424), and, in point of time employed is much preferable. In my lectures at this Institution, I have silvered a great number of new farthings before my audience, in less than a minute of time. 429. I have applied the same solution for silvering zinc, German silver &c., with perfect success. Spoons, sugar-tongs, &c., of German silver, receive a beautiful coating of pure silver, by merely washing them in the above liquor. The deposition, however, is facilitated by previously washing the articles in cream of tartar liquor. The plate of silver thus given, will stand a high polish with soft leather. 430. The argentine liquor (428) would be exceedingly useful in families which are in the habit of using plated articles for the table, as any defect arising from long use might be repaired in a few minutes, by the application of the liquor to the exposed copper; and by this means the use of many articles might be continued a long time. 431. New articles of copper, brass, German silver, &c., which require a thicker coating of plate than can be produced by the simple immersion alone, may first receive that coating, and afterwards be subjected to the action of a small voltaic series, by which means the plating may be carried on to any required thickness. 432. As but few articles in common use are made of pure zinc, I turned the silvering process to account in covering one side of plates of zinc with silver, for the purpose of forming a voltaic battery. In this capacity the silvering process is exceedingly useful, and in the structure of the dry electric column, I anticipate an extensive use of the process. I have already silvered two square feet of the thinnest sheet zinc I could procure, and afterwards punched it into discs, with which a dry column is formed, and the result, so far, is admirable. 433. Besides the facility of applying silver in thinner laminæ than can possibly be procured by the hammer, we have the advan tage of the closest metallic contact that the particles of metal will allow of indeed, a perfect union of the silver and zinc. Hence the principal desideratum in perpetuating the action of the dry electric column is thus supplied; and when the surfaces are once properly protected from chemical action, a source of electric action will be established which thousands of years cannot exhaust; nay, it is highly probable, that such a source of electric action would be as durable as time itself. 434. In the formation of voltaic batteries the silvering of zinc will be found of great advantage; for the deposited metal may be applied to any required thickness, and the tedious and disagreeable soldering process is entirely done away with. 435. It may possibly be supposed that, as the modern voltaic batteries are very differently formed to those first brought into use, the silvering of one side of plates of zine for voltaic batteries can now be of but little use; but I beg permission to observe that, although some of the recently contrived batteries are superior to the old forms, for certain voltaic processes, none of them are so well adapted as the old ones for the display of certain phenomena: and especially in the charging of coated glass. In this capacity no form of battery is qualified to cope with that of Cruickshanks, or the original pile of Volta. 436. In the original pile, and the Cruickshanks battery, the surfaces of the plates are opposite and parallel to each other: the best possible position for accomplishing electro-polarization of both the metals and the water employed. Hence the superiority of such arrangements over all others for giving pure electric tension at the poles. The arrangement of all dry electric columns is similar to that of the original voltaic pile. Hence, in the formation of those batteries, or piles, best adaped for displaying tension at the poles, and in which water alone is employed, there is no process hitherto resorted to, for preparing the compound plates, either se economical, elegant, or durable, as that which I have here described. 437. For by extensive voltaic arrangements (and nothing short of 500 pairs will give good results in charging coated glass), the silvering process would still be expensive: I was therefore desirous of coating zinc with copper, and thus reducing the expense to a mere trifle, even in the preparation of many thousands of pairs. 438. It is well known that iron can be coated with copper by merely immersing it in a solution of the sulphate of copper but perhaps it is not so well known that different kinds of iron produce very different effects. Or, in other words, the precipitated copper assumes very different appearances by the electric action of different kinds of iron. 439. Steel, and fine iron, when well polished, receive a deposit of copper from the sulphate solution of the finest description: the copper surface being perfect and as even as the ferruginous surface that receives it. But if the iron be coarse, and its surface rough, the liberated copper is in coarse grains, which do not wholly adhere to the other metal. 440. When zine is immersed in a solution of the sulphate of copper, the electric forces thus brought into play are still more powerful than those of the coarsest kind of iron and both the cupreous salt and the water suffer decomposition, and the result is a dirty mass of liberated copper and oxide of zinc. 441. In order to slacken the electric action of zinc and sulphate of copper; and, if possible to obtain a liquor from which a good copper surface might be deposited on the zinc, I prepared the well known purple ammoniacal solution; and, to my great satisfaction, the first piece of polished zinc that I immersed in the liquor, received the finest possible copper surface. This accomplished, I have since availed myself of that simple process for giving a copper surface to plates of zinc and form with them voltaic piles, without either sacrificing time in the tedious process of soldering, or incurring the great expense of purchasing the necessary sheet copper. 442. For the purpose of adapting the copper surfaces to those batteries in which acid matter is employed, I thicken the plate first laid on by the simple process above stated, by the ordinary electrotype process, by which means I obtain a copper plate, well secured to the zinc, as thick as I please. Some of the first pieces of zinc thus prepared, were shown on the lecture table of this Institution during the last session but I have not had time till lately, to turn the process to account in any branch of the arts. : 443. I now find that this simple process of covering zinc, and other cheap and easily fused metals, may be extensively used in the arts. Statuary, for instance, to any extent, may be cast of zinc, and of any required thickness, at a very trifling expense. It may afterwards be washed with the ammoniacal solution (442), which will give to it a beautiful copper surface of sufficient thickness to be bronzed for in-door figures, and well prepared to receive a thicker covering by the process of electrotyping. This latter process need not be carried on for more than a few hours, and the figure will then have received a beautiful fine granular covering, which gives it a still more pleasing effect than that derived from the smooth surface. 444. On many figures the artist will be enabled to produce a good effect by combining the smooth and the granulated surfaces; and especially in those of an equestrian character, where the steeds, equipments, &c., being granulated, would give a good contrast to the smooth parts of the bronze. 445. Medallions, struck out with zinc, or any alloy in which it forms a prominent part, are bronzed with great facility by this simple process. And the only thing to be feared is that our copper coinage will be still more liable to be counterfeited than hitherto, when once the process becomes generally known. 446. I have made some progress in depositing iron in compact. |