830 PRINCIPLES OF CHEMISTRY 



mass. This method, proposed by Bottcher and applied by Klein, gives, 

 as R. Lenz showed, iron containing occluded hydrogen, which is dis- 



process is that not only do the carbon, silicon, and manganese, but also a great part of 

 the sulphur and phosphorus of the cast iron burn out at the expense of the oxygen of the 

 iron oxides. During the last decade the manufacture of steel and its application for 

 rails, armour plate, guns, boilers, &c., has developed to an enormous extent, thanks to 

 the invention of cheap processes for the manufacture of large masses of homogeneous 

 cast steel. Wrought iron may also be melted, but the heat of a blast furnace is insufficient 

 for this. It easily melts in the oxyhydrogen flame. It may be obtained in a molten 

 state directly from cast iron, if the latter be melted with nitre and sufficiently stirred up. 

 Considerable oxidation then takes place inside the mass of cast iron, and the temperature 

 rises to such an extent that the wrought iron formed remains liquid. A method is also 

 known for obtaining wrought iron directly from rich iron ores by the action of carbonic 

 oxide : the wrought iron is then formed as a spongy mass (which forms an excellent 

 filter for purifying water), and may be worked up into wrought iron or steel either by 

 forging or by dissolving in molten cast iron. 



Everybody is more or less familiar with the difference in the properties of steel and, 

 wrought iron. Iron is remarkable for its softness, pliability, and small elasticity, whilst 

 steel may be characterised by its capability of attaining elasticity and hardness if it be 

 cooled suddenly after having been heated to a definite temperature, or, as it is termed, 

 tempered. But if tempered' steel be re-heated and slowly cooled, it becomes as soft as 

 wrought iron, and can then be cut with the file and forged, and in general can be made 

 to assume any shape, like wrought iron. In this soft condition it is called annealed steel. 

 The transition from tempered to annealed steel thus takes place in a similar way to the 

 transition from white to grey cast iron. Steel, when homogeneous, has considerable 

 lustre, and such a fine granular structure that it takes a very high polish. Its fracture 

 clearly shows the granular nature of its structure. The possibility of tempering steel 

 enables it to be used for making all kinds of cutting instruments, because annealed steel 

 can be forged, turned, drawn (under rollers, for instance, for making rails, bars, &c.), filed, 

 &c., and it may then be tempered, ground and polished. The method and temperature 

 of tempering and annealing steel determine its hardness and other qualities. Steel is 

 generally tempered to the required degree of hardness in the following manner : It is 

 first strongly heated (for instance, up to 600), and then plunged into water that is, 

 hardened by rapid cooling (it then becomes as brittle as glass). It is then heated until 

 the surface assumes a definite colour, and finally cooled either quickly or slowly. 

 When steel is heated up to 220, its surface acquires a yellow colour (surgical instru- 

 ments) ; it first of all becomes straw-coloured (razors, &c.), and then gold-coloured ; then 

 at a temperature of 250 it becomes brown (scissors), then red, then light blue at 285 

 (springs), then indigo at 300 (files), and finally sea-green at about 340. These colours 

 are only the tints of thin films, like the hues of soap bubbles, and appear on the steel 

 because a thin layer of oxides is formed over its surface. Steel rusts more slowly than 

 wrought iron, and is more soluble in acids than cast iron, but less so than wrought iron. 

 Its specific gravity is about 7'6 to 7'9. 



As regards the formation of steel, it was a long time before the process of cementation 

 was thoroughly understood, because in this case infusible charcoal permeates unfused 

 wrought iron. Caron showed that this permeation depends on the fact that the charcoal 

 used in the process contains alkalis, which, in the presence of the nitrogen of the 

 air,. form metallic cyanides; these being volatile and fusible, permeate the iron, and, 

 giving up their carbon to it, serve as the material for the formation of steel. Thia 

 explanation is confirmed by the fact that charcoal without alkalis or without nitrogen 

 will not cement iron. The charcoal used for cementation acts badly when used over 

 again, as it has lost alkali. The very volatile ammonium cyanide easily conduces to the 

 formation of steel. Although steel is also formed by the action of cyanogen compounds, 

 nevertheless it does not contain more nitrogen than cast or wrought iron (O'Ol p.c.), and 



