~Occurrence.~ The element iron has long been known, since its ores are very abundant and it is not difficult to prepare the metal from them in fairly pure condition. It occurs in nature in many forms of combination,--in large deposits as oxides, sulphides, and carbonates, and in smaller quantities in a great variety of minerals. Indeed, very few rocks or soils are free from small amounts of iron, and it is assimilated by plants and animals playing an important part in life processes. ~Metallurgy.~ It will be convenient to treat of the metallurgy of iron under two heads,--Materials Used and Process. ~Materials used.~ Four distinct materials are used in the metallurgy of iron: 1. _Iron ore._ The ores most frequently used in the metallurgy of iron are the following: Hematite Fe_{2}O_{3}. Magnetite Fe_{3}O_{4}. Siderite FeCO_{3}. Limonite 2Fe_{2}O_{2}.3H_{2}O. These ores always contain impurities, such as silica, sulphides, and earthy materials. All ores, with the exception of the oxides, are first roasted to expel any water and carbon dioxide present and to convert any sulphide into oxide. 2. _Carbon._ Carbon in some form is necessary both as a fuel and as a reducing agent. In former times wood charcoal was used to supply the carbon, but now anthracite coal or coke is almost universally used. 3. _Hot air._ To maintain the high temperature required for the reduction of iron a very active combustion of fuel is necessary. This is secured by forcing a strong blast of hot air into the lower part of the furnace during the reduction process. 4. _Flux._ (a) _Purpose of the flux._ All the materials which enter the furnace must leave it again either in the form of gases or as liquids. The iron is drawn off as the liquid metal after its reduction. To secure the removal of the earthy matter charged into the furnace along with the ore, materials are added to the charge which will, at the high temperature of the furnace, combine with the impurities in the ore, forming a liquid. The material added for this purpose is called the _flux_; the liquid produced from the flux and the ore is called _slag_. (b) _Function of the slag._ While the main purpose of adding flux to the charge is to remove from the furnace in the form of liquid slag the impurities originally present in the ore, the slag thus produced serves several other functions. It keeps the contents of the furnace in a state of fusion, thus preventing clogging, and makes it possible for the small globules of iron to run together with greater ease into one large liquid mass.