METALS AND SOME OF THEIR COMPOUNDS 265 



absorption of heat at about, but not exactly, the same points. 

 These and other observations have led to the hypothesis that 

 iron is capable of existing in two or more allotropic states, that 

 is, conditions in which a molecule of the metal is composed of 

 different members of atoms. In the harder steels it must be 

 assumed that the carbon plays a very important part. It seems 

 to be capable of dissolving in molten iron and on cooling it enters 

 into chemical combination with the metal. One, and perhaps 

 the most important compound formed, is cementite, to which is 

 attributed the formula Fe 3 C. The different varieties of steel 

 when in the solid state may be supposed to be mixtures in various 

 proportions of this compound with one or other of the allotropic 

 forms of the metal, or of a solidified solution of carbon in the 

 metal. Many of the steels introduced into modern practice for 

 special purposes contain other ingredients. 



Manganese has been recognised as a necessary ingredient in 

 steel ever since the introduction of the Bessemer and open- 

 hearth processes for the manufacture of the metal. The amount 

 present does not usually exceed 1 per cent, but for special pur- 

 poses manganese steels are made containing much larger quan- 

 tities. 



Nickel is a familiar white metal, which is about as difficult to 

 melt as wrought-iron. Some thirty years ago, when it began to 

 be available on a large scale, various alloys of nickel with iron 

 were tried and since that time have rapidly extended in use. 

 Nickel added to iron has a toughening effect, and when added 

 in proportions from 12 to 20 per cent it increases greatly both 

 the tensile strength and elastic limit. Nickel steel has been largely 

 used for armour plate. The magnetic properties of this alloy 

 are remarkable ; when about 25 per cent of nickel is present in 

 steel it is almost non-magnetic unless exposed to a temperature 

 of -40 C. After cooling to this low temperature it remains 

 magnetisable at ordinary temperatures, but if heated to 600 C. 

 it recovers its original non-magnetisable condition. 1 



Chromium is a metal which was almost unknown till Moissan's 

 introduction of the electric furnace. A ferro-chromium alloy 

 was formerly made in the blast-furnace, but chromium and other 

 alloys for use in steel making are now manufactured in the 

 electric furnace. The metal can also be obtained by the thermit 

 process already explained. Pure chromium is hard enough to 

 1 Harbord's Steel, 2nd ed., 1905, p. 628. 



