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the wire, and entirely altered its condition, the electro process rather added to its tenacity.

The above is the mean of the results given by experiments upon six pieces of each. Another peculiar property of zinc when hot is the fact that zinc heated to 300° becomes tough and ductile, and is at that heat rolled into sheets; but above 400° it becomes brittle, and just below the melting point is so brittle that it may be pounded in a mortar like marble. Now zinc put upon iron at such a temperature has these evils: that portion which combines with the iron, for which it has a very great allinity, forms a very brittle alloy; and also that portion of zinc which adheres to this alloy, or coats it, does not possess the tenacity of ordinary zinc; besides, the contraction of hot zinc and iron being nearly as 3 to 1, the zinc being 3, the iron resists its contraction, so that the molecules of the metal have not the freedom of arranging themselves in that position necessary to ductility; for which reasons, zinc put upon iron by the hot process is brittle, and breaks by bending. Another objection is, that the zinc used in this process is not pure, not only from the presence of foreign matters required to keep the zinc in fusion, but from the impossibility, except at an enormous cost, of obtaining pure zinc; but, supposing these objections overcome, the alloy which the zinc forms with the iron at a melting heat destroys its protecting qualities. Impurities in ordinary metals tend, to a certain extent, to deteriorate their value; but impurities in zinc are its destruction, and this arises, as we have before stated, from the electro-positive qualities of zinc, and by which it protects other metals at the expense of itself.

Thus, supposing an atom of zinc to be surrounded with atoms of impurities, these, when brought into connexion and exposed to a fluid or to moisture, form a galvanic battery, the zinc being destroyed by this galvanic action, the time such destruction takes to be effected depending altogether upon the circumstances in which it is placed, whether in contact with moist air, water, or acid. This is the reason why the ordinary zinc of commerce does not last so long as might be reasonably expected from the known properties of that metal.

The hot process of zincing is also ill suited to the coating of articles of large dimensions, and all articles of minute workmanship must be necessarily injured by it, the melted zinc entering and filling up the outlines of the work. Before speaking of the electro process of zincing, we may say a few words upon the properties of zinc when in a state of purity. Dr. Kane says, "Zinc preserves the other metals, even if it be of iron, from oxidation; and again, zinc, when exposed to the air, even in presence of water, becomes covered with a varnish of a gray substance, probably a definite suboxide which is not further altered by exposure." Professor Graham, alluding to iron in water, says, "Articles of iron may be completely defended from the injury occasioned in this way by the more positive metal zinc, while the protecting metal itself wastes away slowly." And further speaking of zinc, "When exposed to air, or placed in water, its surface becomes covered with a gray film of suboxide, which does not increase; and this film is better calculated to resist both the mechanical and VOL. IX, 3RD SERIES-NO. 6.-JUNE, 1845.

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chemical effects of other bodies than the metal itself, and preserves it." And Professor Daniell in his last work says, "that a plate of pure zinc, when immersed in water, speedily becomes dulled by the formation of a thin coat of oxide, but the oxidation proceeds no further, because the adhesion of the metal prevents a renewed contact of the metal and the water." We therefore see that, not only has zine the property of protecting metals galvanically negative to it, but, unlike many other metals, becomes itself protected by its own oxide. We have before alluded to the difficulty of obtaining zinc in a state of purity except at a great cost, as well as the great difficulties, when so obtained of applying it to iron; the electro process fortunately saves us all trouble on these accounts, as impurities, though they may exist in the solution used, are thrown to the 'bottom, the pure metal only being deposited upon the iron. In the report to the French Academy we have before noticed, M. Dumas, in speaking of the electro process for the deposition of zinc, says, "Manufacturers, and those concerned in military affairs and the fine arts, will learn with interest that these processes enable us to zinc, in an economical manner, iron, steel, and cast-iron, by means of the pile, or battery, with the solution of zinc, by operating without heat, and consequently not interfering with the tenacity of the metal, by applying it in thin layers, and by thus preserving the general forms of the pieces, and even the appearance of their minutest details. The thinnest plate may receive this preparation without becoming brittle, and may be turned to account in roofing buildings."

This process is very simple. We take ordinary crystalised sulphate of zinc, dissolving it in water, in the proportion of one pound of the sulphate to one gallon of water, which forms our zincing solution. The iron to be zinced is first thoroughly cleaned by allowing it to remain for a short time in dilute sulphuric acid, and afterwards well scoured with sand: it is then placed in the zinc solution, attached to the negative pole of the galvanic battery (plates of zinc being attached to the opposite pole which face the articles in the solution) and the deposit takes place. After being a short time in the solution, the article should be taken out and scoured or brushed all over, so that any portion which may not have been very properly cleaned, and to which the zinc has not perfectly adhered in consequence, may be discovered. It is then returned to the solution and allowed to remain therein until a covering of the required substance is obtained. The advantages of this process are its simplicity, the absence of all injuri ous effect upon the iron, and the securing a coating of pure zinc. The largest articles can with facility be zinced, and articles of the most minute and elaborate workmanship are uninjured by the process. The operation may be intrusted to any one of ordinary capacity, and, what is of some importance in large operations, may be carried on any where.

It is not improbable that the practical details and manipulations of this process may be improved, but, in principle, we aver it is perfect. We need not particularize the uses to which iron so protected may be applied: to all the purposes for which iron is used it would be of ad

vantage, particularly where it is exposed out of doors. The iron-work of all agricultural implements, harness and cart-fittings, fencing and hurdling, the iron-work of hot and green-houses, latches, bolts, and hinges, brestsummers of buildings, &c., &c.; in fact, its uses are infinite, and the application of the process only requires to be known to secure its general adoption.

We shall now say a few words upon copper as a coating for iron. Copper is another metal which may be used with great advantage to protect iron, especially for ornamental purposes. It was probably the deposit of this metal from the sulphate of copper, in Daniell's battery, which first suggested to Spencer the application of the decomposition of metallic salts by the agency of the battery. Since that period, the science of electro-deposition has been gradually developing itself, year after year adding to our knowledge, and, at the same time, shewing us how little we know of the subject in comparison with what yet remains to be discovered. Long before the discovery of deposition by an electric current, it was well known that a piece of iron or steel placed in a solution of sulphate of copper became completely covered with metallic copper; and iron has been long used for collecting the copper from water impregnated with it, both in Wales, Cornwall, and Ireland. Following up this practical result without being acquainted with the cause of copper being deposited upon iron, many were induced to follow out the process of coating iron by electricity with the sulphate of copper, and, finding that the coating thus obtained had no adhesion to the iron, considered their experience a rule, and thus originated the prejudice against coppered iron. The coating of copper received upon iron, when placed in the sulphate (or other acid solution) of copper, is not from any galvanic action, as many have supposed, but from a chemical substitution only; the iron having a greater affinity for the sulphuric acid of the solution, it leaves the copper for the iron, and the portion of copper thus thrown out of solution becomes loosely attached to the iron. And even the addition of a galvanic battery effects no change in the condition of the metals, the chemical action preceding the galvanic; and, although a coating may be forced on by the power of the battery, it is never in direct contact with the iron, but there is interposed a portion of oxide, and the copper thus deposited may be removed with the slightest friction; so that the trials made with iron so coated have failed. So strongly has the prejudice thus created operated, that we have heard it asserted, within these few days, that it was impossible to give to iron a permanent coating of copper. In March, 1840, a patent was obtained for securing perfect and permanent coatings of copper upon iron, alkaline instead of acid solutions being used. Those we have found less suited to the purpose are the cyanides and ferro-cyanides, and what we have lately adopted is the ferro-cyanide of copper dissolved in the cyanide of potassium.

After the iron has been properly cleaned it is placed in this solution, heated to about 120°, in connexion with the battery: in from two to five minutes it will be found completely coated. The iron should

then be scoured with sand, and placed in an acid solution (by preference for cheapness the sulphate); if any portion of the iron should not have been coated in the alkaline solution, such part will immediately turn black, in which case it should be cleaned and returned to the alkaline solution for one or two minutes.

By this process any article of iron-work, whether cast or wrought, may be firmly coated with copper. To test the adhesion of the metals, we have had many iron bolts, of thirty inches long, driven through African oak of twenty-four inches thick, and with a very tight drift, without in the least disturbing the coating of copper; we have also heated them to a heat far above redness, and then plunged them into cold water, without any injury arising from the difference of expansion and contraction of the metals.

We are aware that an objection may be made to the use of copper as a preserving coating to iron, viz: that, the copper being the negative metal, the galvanic action is against the iron. We admit this; but, if the coating of copper is perfect, this cannot be the case, as no galvanic action can possibly take place unless the metals are both together exposed to the fluid. The resisting qualities of copper to oxidation, when exposed to water and vegetable acids, render it a valuable protection to iron under all ordinary circumstances.

For ornamental work copper is best adapted, as it takes a beautiful bronze, either by exposure to the atmosphere or by artificial means. A great saving might also be effected by casting statues and other ornamental work in iron, and afterwards coppering it. The iron castings of the Colebrook Dale Works cannot be surpassed for sharpness and effect.

Works of art in iron, plasters, terra cotta, wood, and other substances, may be thus made to resemble antique bronzes, and the process may be advantageously adapted to machinery, especially that which is exposed in damp situations.

We may add that iron either zinced or coppered solders with great facility.

Trans. Soc. Arts.

Advance of Iron Shipbuilding-Iron Collier.

A great deal of curiosity has been excited by the arrival in the Thames of a Newcastle collier; she is a fine-looking vessel, built of iron, and fitted with a screw propeller worked by an engine of 20horse power; which is said to combine the advantages of several recent patents, and still different to all; it consists of four flies or flaps. of peculiar construction, bending over an angle of 45 degrees from a line with the axle; she is 130 ft. long, 27 ft. 6 in. wide, and, when loaded with 340 tons of coals, her draught is only 11 ft. 9 ins. aft, and 10 ft. 3 ins. forward; her hold is divided into separate chambers, after the principle of C. W. Williams's water-tight bulkheads, and each has a hermetically closed false door, into which water can be admitted by large taps, for ballast, and when not required is pumped out

again by the engine. Her bows are sharp and wedge-like, and her stern unusually projecting; though low, her counters are so flat that she lifts with every swell, and it is nearly impossible for a wave to break over her taffrail. She is called the Q. E. D., and on her stern is an armorial bearing, with the motto-"Spes mea Christus." From the step of the mainmast to the cap is 65 ft. and from her keel to the royal truck about 130 ft.; her mizenmast is of iron, and hollow, forming the engine chimney, and the emission of the smoke from such a situation has a very singular appearance. We understand she both sails and steers well, and the propeller has answered every expectation. She was built by her owner, Mr. Cootes, of Walker's Quay, Newcastle, and from the growing evidence of the superiority of iron for shipbuilding in lightness, strength, and economy, it is probable in a few years few will be constructed of wood, and that scarce a sailing vessel will be without a screw propeller, to have recourse to in time of need. Mining Journ.

Machinery for Carving.

A patent having been taken out some time since for a method of carving wood and stone by machinery, a large manufactory has been established by Mr. Pratt, of Bond Street, for the purpose of working it. The uses to which it is at present principally applied are to carve elaborate foliated tracery, crockets, finials, &c., required for external and internal paneling in wood and stone for churches. The rapidity and consequent economy with which the work is effected are astonishing. We saw, on a recent visit, a small piece of trefoil paneling in stone finished in 15 minutes, which by the ordinary processes would employ a skilful workman a whole day. Nor is the rapidity of execution detrimental to the accuracy and finish of the workmanship. We carefully examined a great number of the specimens, and were satisfied that the materials were not susceptible of more perfect finish. In the crockets, finials, bosses, and other solid work, indeed, much has to be done by hand after the mechanical work is finished. The mechanism here only cuts away the larger parts, and prepares a ground for the hand of the carver. But in paneling, little or nothing is left to manual skill; all that is requisite is to retouch some of the acuter angles of the tracery.

The machinery by which these wonderful processes are effected is, like all valuable machinery, very simple. Many of our readers have probably seen the experiment, which used to be performed in the Adelaide Gallery, of cutting through the hardest steel files by a circular disk of soft iron which revolved with very great velocity. This principle of increasing the efficiency of cutting tools by making them revolve with great rapidity is utilized in the present patent. Drills of various shapes are caused by steam power to revolve rapidly in a vertical position. The extremities of the drills are serrated, and it is therefore obvious that they will rapidly cut through wood or stone

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