broken by longitudinal or sectional ridges, for the purpose of giving the sheet great stiffness and strength. Corrugated iron is used principally for roofing, and sometimes in place of brick for forming the arches between the iron beams in fire-proof structures. 300. Steel. The name steel is given to a compound of iron and carbon, in which the amount of iron is usually not less than 97 per cent. Where the amount of carbon is less than .0065, the compound is termed steely iron; when more than 1.8 the compound is cast iron. Steel, like iron, is seldom pure, containing other substances, of which sulphur and silicon are the most common. The different kinds of steel are named either from the modes of manufacture, or their appearance, or from some constituent, or from some inventor's process. Thus we have natural steels obtained directly from the ores and bearing mostly local names; blistered, shear, tilted and crucible or cast steel; Woolz or Damask steel; Bessemer's and Martin's steel; tungstein, chromium, and titanium steel. These varieties are obtained by various processes. Thus we have the puddling process by which the varieties of natural steel are made; the cementative process; the Martin-Siemems process; the Bessemer process, &c. The average specific gravity of natural steels is 7.5; of tilted steel 7.9; cast steel 7.8; Bessemer steel from 7.79 to 7.87; chromium steel from 7.81 to 7.85. The chromium steel is said to possess the greatest tensile strength; and among those more abundantly manufactured the Bessemer still ranks highest in this respect. COPPER 301. 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. 302. 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. 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. 303. 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. 304. 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. X. PAINTS AND VARNISHES. 305. 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. 306. 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. 307. 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. 308. 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. 309. 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. 310. The following are some of the more usual compositions of paints and varnishes. The proportions of the above compositions are given in 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. 311. 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. 312. Varnish for Zincked Iron. 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. 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 drying linseed oil, and bring the whole to a nearly boiling temperature. Melt, in a second vessel, 8 lbs. of gum-anime; to which add 2 gals. of drying linseed oil at a boiling heat, with 12 lbs. of caoutchouc partially dissolved in coal-tar naphtha. Pour the contents of the second vessel into the first, and boil the whole gently, until the varnish, when taken up between two spatulas, is found to be tough and ropy. This composition, when quite cold, is to be thinned down for use with from 30 to 35 gals. of spirits of turpentine, or of coal naphtha. 313. It is recommended that the iron should be heated before receiving this varnish, and that it should be applied with a spatula, or a flexible slip of horn, instead of the ordinary brush. When dry and hard, it is stated that this varnish is not acted upon by any moderately diluted acid or alkali; and, by long immersion in water, it does not form a partially soluble hydrate, as is the case with purely resinous varnishes and oil paints. It can with difficulty be removed by a sharppointed tool; and is so elastic, that a plate of iron covered |