264 CHEMICAL DISCOVERY AND INVENTION 



iron in union with 2 to 5 per cent of carbon, and small quantities 

 of sulphur, phosphorus, and silicon. Iron of this kind is brittle, 

 though hard and much more easily fusible than wrought-iron. 



Steel is made by several processes, all of which have for their 

 object the production of a carbide or mixture of carbides of iron 

 containing an amount of carbon which may range from -1 per 

 cent in mild steel up to about 2-0 per cent or a little more in 

 hard tool steel. The presence of sulphur and phosphorus in 

 steel is detrimental. Steel is in all varieties less brittle than 

 cast-iron and has a greater tensile strength than wrought-iron. 

 The milder varieties, that is those which contain the smallest 

 percentage of carbon, can be forged and welded. The character 

 which formerly was considered distinctive of steel is its property 

 of becoming hardened by quenching in water or oil when at a 

 high temperature. The degree of hardness to be given can be 

 regulated by the temperature to which it is heated and the rate 

 at which it is cooled down. This is called tempering and is some- 

 times regulated by observing the colour of the film of oxide 

 which is formed on the surface when the metal is heated. The 

 blue colour of a watch-spring is familiar and is indicative of 

 great elasticity : heated to redness and then plunged into cold 

 water it becomes brittle. But during the last thirty years great 

 advances have been accomplished in the knowledge of the internal 

 structure of the metal by the aid, not only of chemical analysis, 

 but the use of the microscope and a study of the peculiar phe- 

 nomena which iron exhibits in changing temperatures. The in- 

 troduction of electrical resistance thermometers now enables 

 the manufacturer to test and regulate the temperature of his 

 furnaces, a point of great importance which was beyond control 

 only a few years ago. The constituents of steel have been the 

 subject of very numerous researches, and even now authorities 

 differ in some points of detail. 



On heating a mass of steel to redness and then allowing it to 

 cool slowly it is observed that the temperature does not drop 

 regularly but at certain points the cooling seems to hesitate and 

 proceed more slowly. There are three of these critical points or 

 points of recalescence, namely at about 825, 735, and 660, 

 observed in very mild steel. In hard steel the critical tempera- 

 tures are both relatively and absolutely somewhat different, but 

 similar phenomena are noticed. On gradually heating up a 

 mass of steel the rise of temperature is retarded, showing an 



