1900] MICROSCOPICAL JOURNAL. 285 



varying from two to live per cent. It practically consists 

 of three varieties — white, mottled and grey. The first va- 

 riety is composed of pearlite and cementite, the second 

 of pearlite, cementite and a little graphite, and the third 

 of pearlite, together with either cementite or ferrite and 

 graphite. In both cast iron and steel, it very seldom hap- 

 pens that free cementite and ferrite exist in the same 

 specimen. 



As it has been before stated, steel differs from cast iron 

 by being capable of acquiring various degrees of hard- 

 ness, and it is upon this special property that the great 

 value of steel depends. Although there is no well-defin- 

 ed line of demarcation between high carbon steel and white 

 iron, yet the former has a much wider range of hardness 

 than the latter, when submitted to suitable thermal treat- 

 ment. 



The changes in the hardness of a steel are accompanied 

 by a corresponding change of structure. Take, for ex- 

 ample, the ordinary process of tempering. Steel is tem- 

 pered by two processes — (1) hardening by quenching in 

 water, oil or mercury, and (2) re-heating the hardened 

 steel to a given temperature and plunging in water. The 

 quenched structure of steel is composed of a system of 

 interlacing crystalline fibres, and is known as "marten- 

 site," after Prof. Martens, of Berlin. The structure of 

 martinsite is developed by prolonged etching with an in- 

 fusion of liquorice, or by an attack of a solution of nitric 

 acid. When a quenched steel is tempered, the interlac- 

 ing crystalline fibres disappear, and the structure becomes 

 granular. The character of the tempered structure varies 

 greatly with the temperature and the time the steel is re- 

 heated. No name has yet been assigned to the temper- 

 ed structure. 



In practice, the workman tempers steel by watching the 

 various colors assumed by the surface of the metal dur- 

 ing the progress of the operation, and when the proper 



