of working if from 2 to 3 per cent is present.

Phosphorus, which hardens and weakens the metal but makes it easier to
cast. Three-tenths per cent of phosphorus serves as a hardening agent and
may be present in good steel if the percentage of carbon is low. More
than this weakens the metal.

Sulphur, which tends to make the metal hard and filled with small holes.

Manganese, which makes the steel so hard and tough that it can with
difficulty be cut with steel tools. Its hardness is not lessened by
annealing, and it has great tensile strength.

Alloy steel has a varying but small percentage of other elements mixed with
it to give certain desired qualities. Silicon steel and manganese steel are
sometimes classed as alloy steels. This subject is taken up in the latter
part of this chapter under _Alloys_, where the various combinations
and their characteristics are given consideration.

Steel has a tensile strength varying from 50,000 to 300,000 pounds per
square inch, depending on the carbon percentage and the other alloys
present, as well as upon the texture of the grain. Steel is heavier than
cast iron and weighs about the same as wrought iron. It is about one-ninth
as good a conductor of electricity as copper.

Steel is made from cast iron by three principal processes: the crucible,
Bessemer and open hearth.

_Crucible steel_ is made by placing pieces of iron in a clay or
graphite crucible, mixed with charcoal and a small amount of any desired