[AS.] A hard metal made by heating iron with charcoal. Steel is the form of iron in which the amount of carbon is intermediate between that contained in cast iron and in wrought iron. In steel, the carbon is either chemically united with the metal or dissolved in it. It may contain silicon and manganese in small quantities, but sulphur or phosphorus is regarded as an impurity. It is malleable and ductile, fusible, and capable of acquiring, by being tempered, great hardness, which renders it suitable for cutlery and the different varieties of cutting tools. Steel was at one time always prepared from wrought iron by heating the bars for some time in contact with charcoal. This method of preparation is known as cementation. The process of manufacturing steel known as the Bessemer or pneumatic process is of very great industrial importance. It consists in blowing air through molten pig iron in a vessel called the converter. The carbon and the silicon which the pig iron contains are thus oxidized, and the iron is brought to the condition of wrought iron. After the iron is completely deprived of carbon, a certain quantity of pure cast iron is added in order to supply the carbon necessary to convert it into steel. The metal is then cast into ingots. This kind of steel is used for railway axles and rails, for boiler plates, and for ships. Large buildings, as churches, colleges, and schools, are now made with frame-work of steel. The walls are double, and the air between acts as a non-conductor of heat.