137
IRON.
3.  Cart  iron,  is  iron  combined  with  a  stil  greater  Chaptu.
proportion  of  carbon  than  is  necessary  for  forming  steel.
The  quantity  has  not  yet  been  ascertained  with  precision: ¬
  Mr  Clouet  makes  it  amount  to  g  of  the  iron.
The  blackness  of  the  colour,  and  the  fusibility  of  cast
iron,  are  proportional  to  the  quantity  of  carbon  which
it  contains.  Cast  iron  is  almost  always  contaminated
with  foreign  ingredients  :  These  are  chiefly  oxide  of
iron,  phosphuret  of  iron,  and  silica".
It  is  easy  to  see  why  iron  is  obtained  from  its  ore  in
the  state  of  cast  iron.  The  quantity  of  charcoal,  along
with  which  the  ore  is  fused,  is  so  great,  that  the  iron
has  an  opportunity  of  saturating  itself  with  it.
The  conversion  of  cast  iron  into  wrought  iron  is  effected ¬
  by  burning  away  the  whole  of  the  charcoal,  and
depriving  the  iron  wholly  of  oxygen  :  this  is  accomplished ¬
  by  heating  it  violently  while  exposed  to  the
airf.  Mr  Clouet  has  found,  that  when  cast  iron  is  mixed ¬
  with  z  of  its  weight  of  black  oxide  of  iron,  and  heated ¬
  violently,  it  is  equally  converted  into  pure  iron.  The
oxygen  of  the  oxide,  and  the  carbon  of  the  cast  iron,
combine,  and  leave  the  iron  in  a  state  of  purity  f.
The  conversion  of  iron  into  steel  is  effected  by  combining ¬
  it  with  carbon.  This  combination  is  performed
in  the  large  way  by  three  different  processes,  and  the
products  are  distinguished  by  the  names  of  natural
steel,  steel  of  cementation,  and  cast  steel.
*  A  substance  which  shall  be  described  in  the  next  book.
A  detailed  account  of  the  processes  used  at  Sheffield  for  converting
cast  iron  into  pure  iron,  has  been  published  by  Mr  Collier  in  the  sth  volume ¬
  of  the  Mancbester  Memoirs,  p.  1II.
1  Jour,  de  Min.  An.  vii.  p.8.