284 THE AMERICAN MONTHLY [Oct 



Pure iron, being an elementary body, is made up of one 

 substance only, to which the name "ferrite" has been 

 given. It is composed of a number of interlocking crystals. 



Steel is composed of iron containing approximately 

 from 0*1 to 2'0 per cent of carbon, and it has the peculiar 

 property of becoming much harder when it is made red hot 

 and quenched. The carbon in steel which has been slowly 

 cooled is combined with a portion of the iron, forming iron 

 carbide, which is known as "cementite." This constitu- 

 ent contains about 6-6 per cent of carbon, and remains 

 bright after a polished section of the steel is attacked by 

 an infusion of liquorice, or a solution of nitric acid. Ce- 

 mentite is very hard, and stands in relief when the steel 

 is polished on the finest rouge on wet cloth. A good spe- 

 cimen of cementite is shown when magnified 850 dia- 

 meters. Free cementite, however, does not often occur in 

 low carbon steel, but usually assumes the form of "pearl- 

 ite," which is an intimate mixture of cementite and fer- 

 rite arranged in laminae, which are alternately hard and 

 soft. These laminae are very minute, and it is necessary 

 to use a magnification of at least 300 diameters for their 

 identification. The laminae of pearlite often assume a 

 more or less granular form. Pearlite is so called on ac- 

 count of its resemblance to mother-of-pearl. When pearl- 

 ite is attacked with either an infusion of liquorice or a 

 solution of nitric acid, a voltaic action is set up which 

 causes the ferrite to become dark in color. A steel con- 

 taining 0-9 per cent of carbon will consist entirely of 

 pearlite ; if the carbon be less than this amount the mass 

 will be composed of pearlite and ferrite. If the carbon ex- 

 ceeds 0.9 per cent it will consist of pearlite and cementite. 



Cementite may be distinguished from ferrite by its 

 greater hardness. The cementite appears to stand in re- 

 lief. Ferrite is easily scratched by an ordinary sewing 

 needle, while cementite is not. 



Cast iron contains more carbon than steel, the amount 



