488 



many years now as to 

 whether each of these 

 varieties is to be regarded 

 as an allotropic form of 

 the metal. Even now 

 this question is not yet 

 settled, chiefly because 

 there is as yet no con- 

 sensus of opinion as to 

 the definition of allo- 

 tropy. If, however, a 

 change in crystalline 

 symmetry be regarded 

 as the criterion of an 

 allotropic change, then 

 7 iron is certainly to be 

 regarded as an allotrope, 

 whereas ^ and a iron 

 are to be regarded as 

 the same variety, the 

 only difference being 

 that ^ iron is a iron 

 which owing to rise of 

 temperature has lost its 

 magnetic properties. 

 From the point of view 

 of the hardening of steel, 

 the important difference 

 between these varieties 

 of iron is that y iron 

 can dissolve substantial 

 amounts of carbon, 

 whereas ^ and a iron 

 cannot, and the unique 

 position occupied by car- 

 bon as a hardening re- 

 agent in steel is inti- 

 mately connected with 

 this difference in pro- 

 perties. 



By adding carbon to 

 iron and thus making 

 steel, a marked influence 

 on the critical points of 

 the metal is observed. 

 Taking, for example, a 

 steel with 0*15 per cent 



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