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 cov erings, 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 un changeable 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 aiready 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.

The proportions of the above compositions are given for 100 parts, by weight, with the exception of lacker 2.

The beautiful black polish on the Berlin castings for ornamental purposes, is said to be produced by laying the following composition on the hot iron, and then baking it.

Enough oil of turpentine is to be added to this mixture to make it spread.

266. From experiments made by Mr. Mallet, on the preservative properties of paints and varnishes for iron immersed in water, it appears that caoutchouc varnish is the best for iron in hot water, and asphaltum varnish under all other circumstances; but that boiled coal-tar, laid on hot iron, forms a superior coating to either of the foregoing.

267. Mr. Mallet recommends the following compositions for a paint, termed by him zoofagous paint, and a varnish to be used to preserve zincked iron both from corrosion and from fouling in

sea water.

Varnish for zincked Iron.

To 50 lbs. of foreign asphaltum, melted and boiled in an iron. vessel for three or four hours, add 16 lbs. of red lead and litharge ground to a fine powder, in equal proportions, with 10 gals. of