393 



SCIENTIFIC RECREATIONS. 



Fig. 406 Rollers. 



time being wound round the cylinder over a small 

 fire. Some metals can be drawn much finer than 

 others. Gold wire can be obtained of a " thick- 

 ness " (or thinness) of only the 5,oooth part of an 

 inch, 550 feet weighing one grain ! But platinum 

 has exceeded this marvellous thinness, and wire 

 the 3O,oooth part of an inch has been produced. 

 Ductility and malleability are not always found 

 in the same metal in proportion. The sizes of 



wires are gauged by the i 



instrument shown in the L"N'i''*H t ) H > > i|g| 

 margin. The farther the f 

 wire will go into the groove 

 the smaller its " size." 



M i i i i n IT 



Fig. 407. Wire size. 



Steel contains a certain amount of carbon, generally about i to 2 

 per cent. Cast steel is prepared from cast iron. Steel from bar-iron has 



Fig. 408. Coarse wire-drawing. 



carbon added, and is termed bar-steel. The process is called " cementation," 

 and is carried on by packing the bars of iron in brick-work boxes, with a 

 mixture of salt and soot, or with charcoal, which is termed " cement." 

 Steel is really a carbide of iron, and Mr. Bessemer founded his process of 

 making steel by blowing out the excess of carbon from the iron, so that the 

 proper amount 1-5 per cent. should remain. 



A brief summary of the Bessemer process may be interesting. If a 

 bar of steel as soft as iron be made red-hot and plunged into cold water, it 

 will become very hard. If it be then gently heated it will become less hard, 

 and is then fitted for surgical instruments. The various shades of steel are 



