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within the closed circuit, and, it is true, in the cells where the excited plates are placed, there is a state of chemical activity. This chemical activity is not, however, limited to this particular part of the circuit, but allows of being transferred to any other, wherever we place an intermediate cell, filled with a conducting liquid, through which, by means of homogeneous metallic plates, the galvanic current can be transmitted. It appears from this, that nothing more is necessary to shew this fact, than that the metallic conductor, which goes from one pole of the battery to the other, should be somewhere interrupted by a liquid conductor. If we allow that the metallic plates, which are immersed in this liquid, are perfectly homogeneous, so that they cannot possibly excite galvanic current, the following law invariably takes place :-The hydrogen of the liquid, or the element which represents it, is developed at that plate which is connected with the zinc pole; and the oxygen, or its representative, goes to the other metallic plate, or that connected with the copper pole of the battery.

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The celebrated English philosopher, Faraday, has given to those plates, which are placed in the liquid to be decomposed, the denomination of electrodes; and, indeed, he names that plate which is connected with the zinc, the cathode, and the other, which is connected with the copper, the anode. In consequence of these abbreviations, we shall employ them in the remaining part of this work. Fig. 25: A B represents a galvanic battery; C D is a vessel nearly filled with diluted sulphuric acid, in which are immersed the two platinum electrodes, a and c. Of these, a is the anode, and c the cathode. If the current be sufficiently powerful, c will liberate hydrogen, and a will liberate oxygen. instead of a solution of sulphuric acid, we place a solution of sulphate of copper in the decomposing cell C D, the laws of action will be such, that the plate a will, it is true, still liberate oxygen; but c will now become covered with a film of metallic copper; whilst the hydrogen, as has before been stated, will return to be employed in the reduction of the copper. If instead of the platinum electrodes, we place two copper ones, with diluted sulphuric acid, in the cell C D, the anode will develope either none, or but very little, oxygen; because, under these circumstances, it a has very great affinity for the copper, which it oxydizes, and then becomes dissolved in the sulphuric acid. The cathode will, at first, liberate hydrogen gas as usual, but the developement of this element will become gradually feebler, and shortly it will disappear, and in its stead the surface of the cathode will become covered with

reduced copper in a loose brown-red condition. The fluid itself, which at first was clear as water, becomes gradually of the blue colour of the sulphate of copper. Although I carried on this experiment for several successive days, a slight developement of gas at he cathode was still discernable; which demonstrated that, by this apparatus, the reduction of the copper goes on only in an incom

plete manner; even the precipitate does not become coherent, but granulous, which is probably due to the bubbles of hydrogen preventing the intimate connection of the cupreous particles. It is otherwise, however, when instead of diluted sulphuric acid, we employ an almost saturated solution of the sulphate of copper. In this case, however, the anode also dissolves, whilst the cathode bedecks itself with completely coherent clear red copper, as complete even as by proceeding according to the first method. The copper solution, by this means, continues in a state of saturation always proportional to the progress of reduction, by a supply of copper from the dissolving anode. A comparison of the weights of the plates shows, that the cathode obtains nearly the same quantity of copper as that which the anode loses. The small perceptible difference is too insignificant ever to become a consideration in a practical point of view.

Since by this arrangement of apparatus we are enabled to form copper plates in a vessel entirely separate from the battery, and entirely dispense with the large porous diaphragm used by the first described method, we arrive at an almost unlimited freedom in reference to the size and shape of the original from which we wish to take copies. Nor is the advantage less in reference to the time necessary for the operation, which by this means is much shortened; so that daily a film can be deposited of nearly a line or of an inch thick, and in many cases it will reach the thickness of a common card. To accelerate the reduction still more than this rate, appears not to be prudent, because of its affecting the firmness of the copper. When the current is sufficiently powerful to reduce two solotnik of one square inch in twenty-four hours, we obtain a loose incoherent product. It is possible, however, that by the employment of other salts of copper, we might obtain very different results.

It is a fortunate circumstance, that for the decomposition of the copper solution between the copper electrodes, a battery of one pair only need be used. We can, it is true, employ more pairs in series for the purpose of accelerating the action; in the meantime, it is to be understood these laws will take place, viz.,-in proportion as the pairs of plates in series are augmented, more zinc becomes consumed: not only in one cell, but in every cell in the series does this augmented action take place.

The copper solution which is placed between the copper electrodes must, to a certain degree, be diluted, which is best ascertained by experience. If, at the commencement, we employ a completely saturated solution, we shall observe, sooner or later, that the needle of the galvanometer indicates a tendency to return to the meridian, or at least it begins to be very unsteady. When this is the case, and the unsteadiness of the needle is not occasioned by any irregularity in the battery, we must dilute the solution with water until the deflection of the needle becomes more steady. It is

sometimes useful to add to it a little sulphuric or nitric acid. The anode or the copper-plate that dissolves, generally becomes covered with a blackish or dark brown powder, which, if it falls down on the cathode, occasions the reduced copper to be brittle. This circumstance is easily avoided by placing between them a partition of flannel, linen cloth, &c. &c.

Notwithstanding the arrangement of this decomposing apparatus, no great difficulty is presented, since it can be disposed in many different ways; and as the most usual way is to place, in connection with the battery, the two copper plates, of any chosen form, opposite one another, in the decomposing cell, nearly filled with a solution of sulphate of copper, I shall, in fig. 26, shew the arrangement of the apparatus with those plates, arranged in horizontal positions. In this figure, a, b, c, d, represents a vessel or reservoir holding the is not seen in the figure. This vessel may be of glass, porcelain, or even of wood, if made water-tight by a lining of asphaltum or resinous cement. We may also use lead or copper for this purpose, taking care to line such vessels with plates of glass or asphaltum cement; e, f, represents the cathode, which serves as an original to be copied, and whose surface is intended to be covered with the reduced copper. This plate, by the conductor f, g, h, is connected with the zinc pole of the battery, which, as already stated, may consist of a pair of plates, of small or large dimensions, according to circumstances; i, k, represents a flannel or linen partition, stretched on a wooden frame, on which the 'copper plate, l, m, is placed: this latter plate is connected with the copper side of the battery by the conducting wire, n, o. Instead of a whole plate, we may employ even single fragments, which ought to be so placed as to touch one another. In the circuit is placed the magnetic compass, y. Let me remark, in this place, that the process is not so satisfactory when the distance between the plates is less than about an inch and a half or two inches. When the dimensions of the plates are pretty considerable, for instance, a foot and a half or two feet square, the distance between them may be allowed to be three or four inches, which would not be too much.

The use of the compass is not sufficiently valuable to recommend in all cases; it will become directly useful, probably indespensable, when the galvanoplastic art has become a profitable source of employment. When this period arrives, it will be necessary, at least advisable, to prepare a number of these simple instruments at the commencement of any extensive process. If we combine several decomposing apparatus, of equal or nearly equal dimensions, and the same number of single batteries in series, in the galvanic circle, we have no need of more than one compass; as may be understood by inspection of fig. 27. By this arrangement, nearly the same weight of reduced copper will be found on each plate, at one and the same time. This circumstance also takes place even when the plates are of unequal dimensions; so that the thickness of the films obtained

are inversely as the surfaces of the plates. From the laws exhibited in a previous part of this work, it is obvious that the smaller the decomposing apparatus, the greater the resistance to transmission. We also enfeeble the current, and check the reduction of the copper, by combining together a great number of plates in the decomposing cells. Nor is it advisable to combine many decomposing apparatus together with one battery, as represented by fig. 28, because the distribution of the action is not proportionate to the whole force employed; and they are also liable to many accidental circumstances not easily avoided.

By this second method, we must employ galvanic batteries that will afford a constant action, such as those described in section 6; or at least formed upon the same principles, having a diaphragm for the separation of the liquids with which it is charged. The manifold and various kinds of arrangements that I have hit upon, whilst pursuing other applications of galvanism to a certain extent, I shall describe on another occasion; nevertheless, I shall give a representation of one battery in this place. In fig. 29, a, b, c, d, represents a rectangular copper or leaden box; e, f, g, h, is a cylinder, also of lead or thin sheet copper, which touches three of the sides of the box. It is better to leave the cylinder open at the side, than to solder or rivet the edges of the sheet together, in order that it may be easily opened and bent, so as to remove the reduced copper with which it soon becomes covered; i, k, represents a cylinder of potter's clay, and l, m, a cylinder of zinc, which may be either solid or hollow; n, o, is a small box of either lead or copper, perforated at the sides and bottom like a sieve. It is for the purpose of holding bruised crystals of sulphate of copper. When the liquid is wanted to be let off from the apparatus, we can use a flexible tube such as has already been described. Usually, however, I employ another contrivance for this purpose, which operates upon the principle of the syphon, and which I will here explain. It may be employed for many other purposes, and, I believe, in this form it is quite new. In fig. 30, a, b, c, d, is a vessel, with its draining tube e, f, which passes through the bottom of the vessel, and whose upper orifice is on the same level as the surface of the liquid; g, h, is a tube, closed at the upper and open at the lower end. This I call the inverted tube, (Stulprohr).* The inner tube will discharge the most diluted part of the cupreous solution, by its overflowing at the top, when the outer tube is not in its place. If we use an apparatus to which no perforated magazine of crystals is attached, it will be necessary to employ a funnel with a long neck, when a new supply of the saturated solution of sulphate of copper is wanted, for the purpose of placing it at the lowest part of the apparatus, and thus lifting the old liquid so as to run off. If the whole of the liquid is to be discharged, the inverted tube, after being filled

* This word might be translated capped tube, but I have chosen inverted tube, as more likely to be understood.-TRANS.

with liquor, is to be placed over the inner one, as seen in the figure, and thus the syphon is formed, through which the whole of the liquid which stands above the lower end of the inverted tube will become discharged. It is scarcely worth mentioning, that the space between the top of the inner tube and the outer one must not be too small for the free flow of the liquid between them. When every part of the apparatus have the proper proportions, the discharge of the liquid need not depend upon its surface being above the level of the upper surface of the inner tube, because, by a motion up and down of the inverted tube g, h, the tube e, f, is easily filled, and the syphon thus brought into full play. This contrivance is very convenient for the purpose of charging several vessels at the same time: the inverted tubes are all arranged and fastened in a frame, in order that they may be let down at once. The discharging tube, e, f, must not be too wide, so that it may easily get filled. The solution of zinc will gradually become more and more saturated, if care be not taken to remove it and add water in its place, which must always be attended to. It may very easily be diluted, by pouring fresh water to the bottom of the vessel through a long-necked funnel, by which means the newly-added water gets beneath the saturated solution and lifts it up, mixing, at the same time, to a certain extent, with the lower strata. If it be thought proper to remove the whole of the zinc solution, the porous vessel must be taken out, and its contents poured off, which is but seldom necessary, as may easily be shewn by fig. 31, in which is seen a syphon, similar to the former, for removing the solution of zinc when required. The bottom of the copper vessel, a, b, c, d, is furnished with a small attached tube, in which another tube, passing through the bottom of the porous cylinder, e, f, g, h, is placed and cemented. Into the upper orifice of the latter is cemented a glass tube, i, k, l, m, whose upper end reaches to the level of the surface of the liquid. Both this and the outer tube, n, o, p, q, are best made of glass, and fastened, by some means, so as to prevent derangement by agitation; also constantly left open, so as to be ready, on all occasions, to let the old liquid run off in the direction of the arrows, when fresh supplies of saturated solution are introduced. Should it be desirable to remove the whole of the liquid, the last-named tube may be taken off, and replaced by another, similar to the inverted one g, h, in fig. 30, by which means the discharge will be as complete as by the former method. If there be sufficient room in the porous cylinder, e, f, g, h, the inverted tube may be sufficiently large to be placed over the outer glass tube, without removing it from its place. At an early period I employed an arrangement which I shall describe at another convenience; I may here state,

*This contrivance, although new to our author, is very old in this country. The syphon in Tantalus's Cup is very often made in the same manner.

We are of opinion that our author has made his apparatus rather too complex to become of general use.-TRANS.

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