The sheets of iron are pickled, scoured, and cleaned just the same as for ordinary tinning. A large wooden bath is then half filled with a dilute solution of muriate of tin, prepared by dissolving metallic tin in concentrated muriatic acid, which requires a period of about two or three days, and two quarts of the saturated solution are added to 300 or 400 gallons of the water contained in the bath. Over the bottom of the bath is first spread a thin layer of finely granulated zinc, then a cleaned iron plate, and so on, a layer of finely granulated zinc and a cleaned iron plate alternately, until the bath is full; the zinc and iron together with the fluid, constitute a weak galvanic battery, and the tin is deposited from the solution, so as to coat the iron with a dull, uniform layer of metallic tin in about two hours.

Whilst the above process is in operation a wrought-iron bath containing fluid zinc is prepared, the melted metal is covered with sal-ammoniac mixed with earthy matter, to lessen the volatilization of the sal-ammoniac, which becomes about as fluid as treacle. Two iron rollers immersed below the surface of the zinc are fixed to the bath, and are driven by machinery to carry the plates through the fluid metal at any velocity previously determined. The plates are now received one by one from the tinning bath, drained for a short time, and passed at once, whilst still wet, through the melted zinc by means of rollers; the plates thus take up a very regular and smooth layer of zinc, which owing to the presence of the tin beneath, assumes its natural crystalline character, giving the plates an appearance resembling that known as the moiree metallique.

When the sheet of metal is dipped vertically into the zinc, the lower edge is much longer in contact with the zinc, than the upper, and from the violent action of melted zinc or iron, this makes the bottom edges of the sheets sensibly more brittle than the upper; whereas the rollers cause every part of the sheet to bo acted upon in the same degree, and which degree may be exactly determined by the velocity given to the rollers. Consequently by the roller process thinner iron may be zinced than by dipping edgeways and vertically, as no part of the iron need to be immersed longer in the metallic bath than is absolutely necessary for its properly taking the coating of sine.

In addition to Mr. More wood's patent dated 1841 for the general proeess by ordinary tinning and ziucing, Messrs. Morewood and Rogers have patents dated respectively 1843 for the rollers, and the electro mode of tinning, in fact for the mode of covering metals by the conjoined processes, first of voltaic deposition, and subsequently of immersion in another fluid metal. - 1844 for new fluxes and details of management; - and 1845 for the manufacture of the galvanised tinned-iron plate into tiles and ridge pieces for roofing and other works, by various processes of stamping.

Craufurd's Patent (worked by the Galvanized Iron Company), and Morewood and Rogers's combined patents, have obtained very extensive employment for a great variety of purposes, and both methods are well supported by testimonials. But so far as the author can learn, the galvanized iron covered with pure zinc, is much more suitable to the sheathing of ships, for which it is highly economical, as it is proved to be much cheaper, and is expected to prove more durable than copper; - the galvanized tinned-iron plate is more malleable and may be used for thinner iron, and is therefore more suitable to being wrought, as by the tin-smith, with the hammer, and it is also found to answer thoroughly for roofing; as it can be bent and soldered with facility. Galvanized iron is now largely used by Government and by public companies for this purpose.

The author is informed that both kinds are open to two curious facts, the first that the chains of tillers and cranes, and objects exposed to much friction, do not lose their coating of zinc; this is accounted for by the smooth un-oxidized zinc surfaces of the chain moving freely on one another, whereas unprotected iron, when covered with rust (the peroxide used in polishing), is subject to continued wear; and it has also been imagined the zinc becomes as it were burnished into the surface of the iron. But it may happen, that when moisture is occasionally present, that the worn parts are then continually re-zinced from the neighbouring parts, as explained by the curious fact now to be noticed, and which on its discovery excited great surprise.

The edges of some galvanized iron plates cut with shears so as to expose the central iron, when attached to the piles of the Bell Rock Light House, for the purpose of experiment, became zinced around the cut edges, and at the holes where the nails were driven, and it was also observed that even the nails and fastenings made of un-galvanized iron became zinced from their proximity to the galvanized sheets. By the same action, the holes perforated through the sheathing for nailing it to the ship's sides become coated; and the zinced wires of the Electric Telegraph, where cut through, become coated by the action of the rain water on the galvanised portion of the surface.

Note Z, page 308. - To precede Section II. (Portable brat* furnace.)

Since the foregoing pages were printed, Holtxapfiel & Co. have constructed portable brass furnaces, made of the hexagonal form in sheet iron, lined with Stourbridge clay, and fitted with cast-iron pedestals, tiles, and stout sheet iron pipes complete, so as to be erected on any level spot of ground, and if near a dead wall so much the better.

The smaller-sised of these sir furnaces serve for about 10 pounds weight of brass or copper, and a large furnace on the same model will melt 20 pounds; when favourably managed they have been made hot enough to melt cast-iron. These furnaces have entirely superseded the little blast furnaces formerly made for the portable forge, shown on page 203 of Vol. I.