When crude iron comes from the smelting furnace it is brittle; and when it is white within, it is extremely hard; but when it has a black grain, owing to its having more phlogiston, it is softer, and may be filed and bored. Cast iron becomes _malleable_ by being exposed to a blast of air when nearly melting; the consequence of which is a discharge of inflammable air, and the separation of a liquid substance, which, when concreted, is called _finery cinder_. The iron generally loses one fourth of its weight in the process. Crude iron contains much _plumbago_, and the access of pure air probably assists in discharging it, by converting it into air, chiefly inflammable. Malleable iron, exposed to a red heat in contact with charcoal, called _cementation_, converts it into _steel_, which has the properties of becoming much harder than iron, and very elastic, by being first made very hot, and then suddenly cooled, by plunging it in cold water. By first making it very hard, and then giving different degrees of heat, and cooling it in those different degrees, it is capable of a great variety of _tempers_, proper for different uses. Of the degrees of heat workmen judge by the change of colour on its surface. Steel, like crude iron, is capable of being melted without losing its properties. It is then called _cast steel_, and is of a more uniform texture. Iron acquires some little weight by being converted into steel; and when dissolved in acid, it yields more plumbago. Steel has something less specific gravity than iron. If the cementation be continued too long, the steel acquires a darkish fracture, it is more fusible, and incapable of welding. Steel heated in contact with earthy matters, is reduced to iron. Iron is the only substance capable of _magnetism_; and hardened steel alone is capable of retaining magnetism. The loadstone is an ore of iron. LECTURE XXVI. _Of Tin._ Tin is a metal of a slightly yellowish cast, though harder than lead, very malleable, but of no great tenacity; so that wires cannot be made of it. It easily extends under the hammer, and plates of it, called _tinfoil_, are made only one thousandth part of an inch thick, and might be made as thin again. Tin has less specific gravity than any other metal. It melts long before ignition, at 410 of Fahrenheit, and by the continuance of heat is slowly converted into a white powder, which is the chief ingredient in _putty_, used in polishing, &c. Like lead, it is brittle when heated little short of fusion, and may be reduced into grains by agitation as it passes from a fluid to a solid state. The calx of tin resists fusion more than that of any other metal, which makes it useful in making an opaque white enamel. Tin loses its bright surface when exposed to the air, but is not properly subject to rust; so that it is useful in protecting iron and other metals from the effects of the atmosphere.