IRON 3C7 of iron and quinia; iron and strychnia, &c. The astringent preparations are the perchlo- ride, subsulphate, and ferric alum. The fresh- ly precipitated sesquioxide is the best antidote for arsenic. In cases of decided ansomia, the stronger preparations of iron are indicated, as the tincture of the chloride, the perchloride, and the sulphate. In other cases the milder salts are generally preferred. The iodide of iron is especially adapted to scrofulous affec- tions. The addition to iron of nux voniica or strychnia, or of bitter tonics like gentian, often enhances its therapeutic action. The adminis- tration of iron is contra-indicated by gastric or gastro-enteric inflammation, by plethora, fever, and febrile conditions generally. The dose of iron varies with the preparation used. It is a sort of food, and is best given with or near meals. During its use, the fseces are colored dark by it. In the arts, iron occurs in three forms, as wrought iron, cast iron, and steel. Wrought iron is nearly pure, and highly malleable, ductile, and weldable. It is fused with difficulty, and its finished forms are there- fore generally wrought at a welding heat. It contains invariably a small amount of chemi- cally combined carbon, 0'25 per cent, or less, and intermingled cinder. Its specific gravity varies from 7'3 to 7'8. Its temperature of fusion is about 1800 C. or 3240 F. Oast or pig iron is in most respects the opposite of wrought iron. It is not in the slightest degree malleable, ductile, or weldable. It is readily fusible, and is therefore always cast in moulds. It is much harder than wrought iron, and is relatively rigid and brittle. There are many varieties of cast iron, exhibiting great diversity of properties. In color, the extremes are white and black, with a number of intermediate shades of gray. The hardness and brittleness vary through wide limits. White cast iron is the hardest, most rigid, and most brittle; it resists the action of the file and drill, while many of the dark varieties can be tooled with ease. The fusibility of the different varieties of cast iron likewise differs greatly. The dark irons generally require a high heat for fusion and become thinly liquid ; they fill forms well, and, as they expand in cooling, make sharp castings, and are hence often called foundery irons. The lighter shades do not become so thinly liquid when fused, and as they contract on cooling are not adapted for castings ; they usually contain a smaller amount of foreign matters, and hence, being adapted to conversion into wrought iron, are called forge irons. The specific gravity of cast iron varies from 6'9 to 7'7; its fusing point is about 1500 0. or 2700 F. The difference between gray and white iron is strongly marked in the molten condition, as they flow from the furnace. Dark cast iron flows quickly and sets without any movement of the surface ; when hard, the upper surface is smooth and convex. White iron emits an abundance of brilliant sparks, and its surface is vigorously agitated by the formation of crys- 437 VOL. ix. 24 tals ; the forms of the crystals are characteris- tic of the grade of the iron ; when hard, the surface is honeycombed and depressed. There are two other varieties of cast iron : specu- lar iron, or Spiegeleiaen, and silvery or glazy iron. They are both white, but differ in char- acter and composition from each other and from ordinary white iron. Chemically, cast iron is further removed than wrought iron from the pure metal; it always contains from 2 to 5 per cent, of carbon. The union of tho carbon with the iron may be either chemical or mechanical, and usually both conditions are present in the same mass. The result of the chemical union of iron and carbon is white iron, while the mechanical mixture of iron with black scales of graphite is dark-colored; the preponderance of one or the other of these conditions gives the various shades from black to white. Sometimes cast iron is com- posed of a mixture of white and gray iron in patches; the iron is then called mottled. The different grades of pig iron are general- ly designated by numbers. No. 1 stands for highly graphitic open-grained iron, and Nos. 2, 3, and 4 for the lighter and more compact varieties, passing toward white. Mottled and white irons are generally designated by name. Malleable castings (see IKON 'MANUFACTURE) are cast iron which has been rendered partially malleable without alteration of form. Steel holds, both in physical properties and in chemical composition, an intermediate position between cast and wrought iron. It may be considered as a compound of iron with 0'25 to 2 per cent, of carbon. The limits given for carbon in wrought iron, steel, and cast iron, viz., 0'25 per cent, or less in wrought iron, 0'25 to 2 per cent, in steel, and from 2 to 5 per cent, in cast iron, are to be regarded as ap- proximate only. No sharp and accurate dis- tinction, based on chemical composition, can be drawn between these three varieties of iron. Steel with the minimum of carbon (mild or soft steel) can scarcely be distinguished from wrought iron ; it exhibits the properties of malleability, ductility, and weldability nearly to an equal degree with wrought iron ; while steel with the maximum of carbon (strong or hard steel) approximates so nearly to cast iron that the above properties are almost entirely wanting. The fusing point of steel is interme- diate between those of cast and wrought iron. The properties that preeminently characterize steel are connected with hardening and temper- ing. When heated to redness and sudden- ly cooled by plunging into water or other liquid, it becomes hard in proportion to the amount of carbon it contains, the temperature to which it has been raised, and the rate of cooling. Such hardened steel, being again heated and allowed to cool slowly in the air, loses its hardness to a degree proportional to the temperature to which it was reheated. This process is called tempering. Hardening and tempering are generally regarded as pe-
