MANGANESE, IRON, COBALT, NICKEL, CHROMIUM. 247 



the fibres of the iron intermingle. For this reason welding 

 adds to the strength of the material, and accordingly arti- 

 cles which require to be very strong, such as anchors, are 

 made not in a single piece, but by welding together a 

 bundle of bars of iron. 



343. Steel. Steel is a form of iron midway between 

 wrought and cast iron as to the quantity of carbon it con- 

 tains, which is from one to two per cent., while that of cast 

 iron is five per cent. It may be made from cast iron by 

 burning out half of its carbon, or from wrought iron by re- 

 storing half of the carbon of which it was deprived in its 

 preparation. The latter is the usual process, and consists 

 in heating the wrought iron in close iron boxes containing 

 charcoal for several days. Steel can be made to have dif- 

 ferent properties according to the uses to whicli we wish 

 to put it. If it be heated to redness, and then be quickly 

 quenched, it is rendered hard and brittle ; if cooled rather 

 gradually it becomes elastic ; and if cooled very slowly it 

 becomes soft, ductile, and malleable, like bar-iron. When 

 it is cooled slowly it is said to be annealed. 



344. Bessemer Process. This is a new and very rapid 

 method of preparing cast steel, of the greatest industrial 

 importance. It consists in burning out all the carbon and 

 silicon in cast iron by passing a blast of atmospheric air 

 through the molten metal, and then in adding such a quan- 

 tity of a pure cast iron as is necessary to give carbon 

 enough to convert the wrought iron into steel ; the melted 

 steel is then at once cast into ingots. In this way six tons 

 of cast iron can be converted into steel in one operation 

 lasting twenty minutes. This process has in large measure 

 revolutionized the old iron industry. 



345. Tempering. Steel when hardened, as mentioned 

 above, is not fit for use till it is tempered, as it is termed, 

 to the particular use for which it is designed. This tern- 



