called ferrite—soft, weak and very duc-tile. * * * *

"2. A definite iron carbide called cementite, which is harder than glass and nearly as brittle. * * * *

"Besides these two constituents of prime importance, there are three others of moment:

"1. Graphite—unimportant in steel, but important in gray cast iron.

"2. Slag—present in wrought iron.

"3. Austenite—hardened steel: steel hardened by sudden cooling from a red heat consists essentially of austenite, a solid solution of carbon in iron of varying degrees of concentration."

Steel is usually divided into four general classes; namely, Converted or Cemented Steel, Crucible Cast Steel, Bessemer (acid or basic)  Cast  Steel,  and  Open-Hearth  (acid