BESSEMER STEEL. 



399 



carefully watched, the change of colour being due to the varying thick- 

 ness of the oxide; for we know that when light falls upon very thin films of 

 a substance, soap-bubbles, for instance, the light reflected from the under 



Fig. 409. Fine wire-drawing. 



and upper surfaces interfere, and cause colour, which varies with the thickness 

 of the film. These colours in steel correspond to different temperatures, and 

 the " temper " of the 'steel depends upon the temperature it has reached. 

 The following table extracted from Haydn's " Dictionary of Science " gives 

 the " temper," the colour, and the uses of the various kinds of steel. 



The Bessemer process transfers the metal into a vessel in which there 

 are tubes, through which air is forced, which produces a much greater heat 

 than a bellows does. Thus in the process the carbon of the iron acts as fuel 

 to maintain the fusion, . and at the same time by the bubbling of the 

 carbonic acid mixes the molten iron thoroughly. 



During the bubbling up of the whole mass of iron, and the extreme 

 elevation of temperature caused by the union of the carbon of the impure 

 iron with the oxygen of the air, the oxide of iron is formed, and as fast as 

 it forms fuses into a sort of glass ; this unites with the earthy matters of the 

 " impure" iron, and floats on the upper part as a flux, thus ridding the " cast 

 iron " of all its impurities, with no other fuel than that contained in the 



