88 PHYSICAL SCIENCE 



to those we have studied In other cases, are 

 formed between iron and carbon, and the phe- 

 nomena of equilibrium between the Hquid and 

 solid phases, even when no other component is 

 present, are very complicated. 



Owing to their industrial importance, the 

 alloys of iron have been investigated more ex- 

 tensively than those of any other metal, and the 

 various compounds and solid solutions identified 

 have received definite names, which, in many 

 cases, were given long before the application 

 by Roozeboom of the theory of solid solutions 

 enabled the true phenomena of equilibrium to be 

 understood. Roozeboom's diagram for alloys of 

 iron and carbon, containing less than 7 per cent, 

 of carbon, is reproduced in Fig. 18. Its general 

 meaning will be clear in the light of what has 

 been said in the case of the bronzes. Here again 

 changes occur at definite temperatures, even in 

 alloys which are completely solid. The viscosity 

 of the material makes these changes very slow, 

 and very different proportions of the various 

 possible constituents will be found in alloys that 

 have been cooled quickly and slowly. The effects 

 of tempering steel and iron thus receive a physical 

 explanation. 



By heating iron above one of the transforma- 

 tion temperatures indicated in the diagram, and 

 maintaining it at a high temperature for some 

 time, it will obviously be possible to produce 

 extensive changes in the physical nature of the 

 metal. Experiment by Mr J. E. Stead has shown, 

 that when steel rails have become dangerously 

 brittle and crystalline by long use, they can be 

 reconverted into a toucrh, elastic, and therefore 



