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.

247. 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 zincking, 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."

248. 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 corrosion 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."

249. 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.

250. 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 ensuring 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 without 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. In 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 practical operation to an extent that should press the means of counteracting them on the attention of the engineer."

251. 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.

252. The first process consists of forming an alloy of the metals used, in the following manner. To 1292 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 hydro-chloric 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 plunged in cold water and well washed.

253. 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.

254. 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.

COPPER.

255. The most ordinary and useful application of this metal in constructions, is that of sheet copper, which is used for roof coverings, and like purposes. Its durability under the ordinary changes of atmosphere is very great. Sheet copper, when quite thin, is apt to be defective, from cracks arising from the process of drawing it out. These may be remedied, when sheet copper is to be used for a water-tight sheathing, by tinning the sheets on one side. Sheets prepared in this way have been found to be very durable.

The alloys of copper and zinc, known under the name of brass, and those of copper and tin, known as bronze, gun-metal, and bell-metal, are, in some cases, substituted for iron, owing to their superior hardness to copper, and being less readily oxidized than iron.

ZINC.

256. This metal is used mostly in the form of sheets; and for water-tight sheathings it has nearly displaced every other kind of sheet metal. The pure metallic surface of zinc soon becomes covered with a very thin, hard, transparent oxide, which is unchangeable both in air and water, and preserves the metal beneath it from farther oxidation. It is this property of the oxide of zinc, which renders this metal so valuable for sheathing purposes; but its durability is dependent upon its not being brought into contact with iron in the presence of moisture, as the galvanic action which would then ensue, would soon destroy the zinc. On the same account zinc should be perfectly free from the presence of iron, as a very small quantity of the oxide of this last metal when contained in zinc, is found to occasion its rapid destruction.

257. Besides the alloys of zinc already mentioned, this metal alloyed with copper forms one of the most useful solders; and its alloy with lead has been proposed as a cramping metal for uniting the parts of iron work together, or iron work to other materials, in the place of lead, which is usually employed for this purpose, but which accelerates the destruction of iron in contact

with it.

TIN.

258. The most useful application of tin is as a coating for sheet iron, or sheet copper: the alloy which it forms, in this way, upon the surfaces of the metals in question, preserves them for some time from oxidation. Alloyed with lead it forms one of the most useful solders.

LEAD.

259. Lead in sheets forms a very good and durable roof covering, but it is inferior to both copper and zinc in tenacity and durability; and is very apt to tear asunder on inclined surfaces, particularly if covered with other materials, as in the case of the capping of water-tight arches.

PAINTS AND VARNISHES.

260. Paints are mixtures of certain fixed and volatile oils, chiefly those of linseed and turpentine, with several of the metallic salts and oxides, and other substances which are used either as pigments, or to give what is termed a body to the paint, and also to improve its drying properties.

261. Paints are mainly used as protective agents to secure wood and metals from the destructive action of air and water. This they but imperfectly effect, owing to the unstable nature of the oils that enter into their composition, which are not only destroyed by the very agents against which they are used as protectors, but by the chemical changes which result from the action of the elements of the oil upon the metallic salts and oxides.

262. Paints are more durable in air than in water. In the latter element, whether fresh or salt, particularly if foul, paints are soon destroyed by the chemical changes which take place, both from the action of the water upon the oils, and that of the hydrosulphuric acid contained in foul water upon the metallic salts and oxides.

263. However carefully made or applied, paints soon become permeable to water, owing to the very minute pores which arise from the chemical changes in their constituents. These changes will have but little influence upon the preservative action of paints upon wood exposed to the effects of the atmosphere, provided the wood be well seasoned before the paint is applied, and that the

latter be renewed at suitable intervals of time. On metals these changes have a very important bearing. The permeability of the paint to moisture causes the surface of the metal under it to rust, and this cause of destruction is, in most cases, promoted by the chemical changes which the paint undergoes.

264. Varnishes are solutions of various resinous substances in solvents which possess the property of drying rapidly. They are used for the same purposes as paints, and have generally the same defects.

265. The following are some of the more usual compositions of paints and varnishes.

White Paint, (for exposed wood.)

The white lead to be ground in the oil, and the spirits of tur

pentine added.