culiar texture, are of an inferior quality, and should never be used for purposes requiring great strength: the filaments of bad varieties are short, and the fracture is of a deep color, between lead gray and dark gray.

271. The best wrought iron presents two varieties; the Hard and Soft. The hard variety is very strong and ductile. It preserves its granular texture a long time under the action of the hammer, and only develops the fibrous texture when beaten, or drawn out into small rods its filaments then present a silver-white appearance.

272. The soft variety is weaker than the hard; it yields easily to the hammer; and it commences to exhibit, under its action, the fibrous texture in tolerably large bars. The color of the fibres is between a silver white and light gray.

273. Iron may be naturally of a good quality, and still, from being badly refined, not present the appearances which are regarded as sure indications of its excellence. Among the defects arising from this cause are blisters, flaws, and cinder-holes. Generally, however, if the surface of fracture presents a texture partly crystalline and partly fibrous, or a fine granular texture, in which some of the grains seem pointed and crooked at the points, together with a light gray color without lustre, it will indicate natural good qualities, which require only careful refining to be fully developed.

274. The strength of wrought iron is very variable, as it depends not only on the natural qualities of the metal, but also upon the care bestowed in forging, and the greater or less compression of its fibres, when drawn or hammered into bars of different sizes.

275. In the Report made by the sub-committee, Messrs. Johnson and Reeves, on the strength of Boiler Iron (Journal of Franklin Institute, vol. 20, New Series), it is stated that the following order of superiority obtains among the different kinds of pig metal, with respect to the malleable iron which they furnish:-1 Lively gray; 2 White; 3 Mottled gray; 4 Dead gray; 5 Mixed metals.

The Report states, "So far as these experiments may be considered decisive of the question, they favor the lighter complexion of the cast metal, in preference to the darker and mottled varieties; and they place the mixture of different sorts among the worst modifications of the material to be used, where the object is mere tenacity."

276. These experiments also show that piling iron of different degrees of fineness in the same plate is injurious to its quality, owing to the consequent inequality of the welding.

277. From these experiments, the mean specific gravity of boiler iron is 7.7344, and of bar iron, 7.7254.

278. Durability of Iron. The durability of iron, under the different circumstances of exposure to which it may submitted, depends on the manner in which the casting may have been made; the bulk of the piece employed; the more or less homogeneousness of the mass; its density and hard

ness.

279. Among the most recent and able researches upon the action of the ordinary corrosive agents on iron, and the preservative means to be employed against them, those of Mr. Mallet, given in the Report already mentioned, hold the first rank. A brief recapitulation of the most prominent conclusions at which he has arrived, is all that can be attempted in this place.

280. When iron is only partly immersed in water, or wholly immersed in water composed of strata of different densities, like that of tidal rivers, a voltaic pile of one solid and two fluid bodies is formed, which causes a more rapid corrosion than when the liquid is of uniform density.

281. The corrosive action of the foul sea water of docks and harbors is far more powerful than that of clear sea or fresh water, owing to the action of the hydro-sulphuric acid which, being disengaged from the mud, impregnates the water, and acts on the iron.

282. In clear fresh river water, the corrosive action is less than under any other circumstances of immersion; owing to the absence of corrosive agents, and the firm adherence of the oxide formed, which presents a hard coat that is not washed. off as in sea water.

283. In clear sea water, the rate of corrosion of iron bars, one inch thick, is from 3 to 4 tenths of an inch for cast iron in a century, and about 6 tenths of an inch for wrought iron.

284. Wrought iron corrodes more rapidly in hot sea water than under any other circumstances of immersion.

285. The same iron when chill cast corrodes more rapidly than when cast in green sand; this arises from the chilled surface being less uniform, and therefore forming voltaic couples of iron of different densities, by which the rapidity of corrosion is increased.

286. Castings made in dry sand and loam are more durable under water than those made in green sand.

287. Thin bars of iron corrode more rapidly than those of more bulk. This difference in the rate of corrosion is more

striking in the soft, or graphitic specimens of cast iron, than in the hard and silvery. It is caused by the more rapid rate of cooling in thin than in thick bars, by which the density of the surface of the former becomes less uniform. These causes of destructibility may, in some degree, be obviated in castings composed of ribbed pieces, by making the ribs of equal thickness with the main pieces, and causing them to be cooled in the sand, before stripping the moulds.

288. The hard crust of cast iron promotes its durability; when this is removed to the depth of one-fourth of an inch, the iron corrodes more rapidly in both air and water.

289. Corrosion takes place the less rapidly in any variety of iron, in proportion as it is more homogeneous, denser, harder, and closer grained, and the less graphitic.

290. Preservatives of Iron.-The more ordinary means used to protect iron against the action of corrosive agents, consist of paints and varnishes. These, under the usual circumstances of atmospheric exposure are of but slight efficacy, and require to be frequently renewed. In water, they are all rapidly destroyed, with the exception of boiled coal-tar, which when laid on hot iron, leaves a bright and solid varnish of considerable durability and protective power.

291. The rapidly increasing purposes to which iron has been applied, within the last few years, has led to researches upon the agency of electro-chemical action, as a means of protecting it from corrosion, both in air and water. Among the processes resorted to for this purpose, that of zinking, or as it is more commonly known, galvanizing iron has been most generally introduced. The experiments of Mr. Mallet, on this process, are decidedly against zinc as a permanent electro-chemical protector. Mr. Mallet states, as the result of his observations, that zinc applied in fusion, in the ordinary manner, over the whole surface of iron, will not preserve it longer than about two years; and that, so soon as oxidation commences at any point of the iron, the protective power of the zinc becomes considerably diminished, or even entirely null. Mr. Mallet concludes: "On the whole, it may be affirmed that, under all circumstances, zinc has not yet been so applied to iron, as to rank as an electro-chemical protector towards it in the strict sense; hitherto it has not become a preventive, but merely a more or less effective palliative to destruction."

292. In extending his researches on this subject, with alloys of copper and zinc, and copper and tin, Mr. Mallet found that the alloys of the last metals accelerate the corro

sion of iron, when voltaically associated with it in sea water; and that an alloy of the two first, represented by 23Zn+ 8Cu, in contact with iron, protects it as fully as zinc alone, and suffers but little loss from the electro-chemical action; thus presenting a protective energy more permanent and invariable than that of zinc, and giving a nearer approximation to the solution of the problem, "to obtain a mode of electro-chemical protection such, that while the iron shall be preserved the protector shall not be acted on, and whose protection shall be invariable."

293. In the course of his experiments, Mr. Mallet ascertained that the softest gray cast iron bears such a voltaic relation to hard bright cast iron, when immersed in sea water and voltaically associated with it, that although oxidation. will not be prevented on either, it will still be greatly retarded on the hard, at the expense of the soft iron.

294. In concluding the details of his important researches on this subject, Mr. Mallet makes the following judicious remarks: "The engineer of observant habit will soon have perceived, that in exposed works in iron, equality of section or scantling, in all parts sustaining equal strain, is far from insuring equal passive power of permanent resistance, unless, in addition to a general allowance for loss of substance by corrosion, this latter element be so provided for, that it shall be equally balanced over the whole structure; or, if not, shall be compelled to confine itself to portions of the general structure, which may lose substance with injuring its stability."

"The principles we have already established sufficiently guide us in the modes of effecting this; regard must not only be had to the contact of dissimilar metals, or of the same in dissimilar fluids, but to the scantling of the casting and of its parts, and to the contact of cast iron with wrought iron or steel, or of one sort of cast iron with another. Thus, in a suspension bridge, if the links of the chains be hammered, and the pins rolled, the latter, where equally exposed, will be eaten away long before the former. În marine steam-boilers, the rivets are hardened by hammering until cold; the plates, therefore, are corroded through round the rivets before these have suffered sensibly; and in the air-pumps and condensers of engines working with sea water, or in pit work, and pumps lifting mineralized or 'bad' water from mines, the cast iron perishes first round the holes through which wrought iron bolts, &c., are inserted. And abundant other instances might be given, showing that the effects here spoken of are in prac

tical operation to an extent that should press the means of counteracting them on the attention of the engineer."

295. Since Mr. Mallet's Report to the British Association, he has invented two processes for the protection of iron from the action of the atmosphere and of water; the one by means of a coating formed of a triple alloy of zinc, mercury, and sodium, or potassium; the other by an amalgam of palladium and mercury.

296. The first process consists of forming an alloy of the metals used, in the following manner. To 1,292 parts of zinc by weight, in a state of fusion, 202 parts of mercury are added, and the metals are well mixed, by stirring with a rod of dry wood, or one of iron coated with clay; sodium, or potassium is next added, in small quantities at a time, in the proportion of one pound to every ton by weight of the other two metals. The iron to be coated with this alloy is first cleared of all adhering oxide, by immersing it in a warm dilute solution of sulphuric, or of hydrochloric acid, washing it in clear cold water, and detaching all scales, by striking it with a hammer; it is then scoured clean by the hand with sand, or emery, under a small stream of water, until a bright metallic lustre is obtained; while still wet, it is immersed in a bath formed of equal parts of the cold saturated solutions of chloride of zinc and sal-ammoniac, to which as much more solid sal ammoniac is added as the solution will take up. The iron is allowed to remain in this bath until it is covered by minute bubbles of gas; it is then taken out, allowed to drain a few seconds, and plunged into the fused alloy, from which it is withdrawn so soon as it has acquired the same temperature. When taken from the metallic bath, the iron should be plung

ed in cold water and well washed.

297. Care must be taken that the iron be not kept too long in the metallic bath, otherwise it may be fused, owing to the great affinity of the alloy for iron. At the proper fusing temperature of the alloy, about 680° Fahr., it will dissolve plates of iron one-eighth of an inch thick in a few seconds; on this account, whenever small articles of iron have to be protected, the affinity of the alloy for iron should be satisfied, by fusing some iron in it before immersing that to be coated.

298. The other process, which has been termed palladiumizing, consists in coating the iron, prepared as in the first process for the reception of the metallic coat, with an amalgam of palladium and mercury.

299. Corrugated Iron. This term is applied to sheet iron prepared by being moulded, and having the plane surface