nature  of  the  flux  depends  upon  the  ore.  If  this  con-
tains silicon  dioxide  or  clay,  lime  is  added ;  while,
if  it  contains  considerable  lime,  minerals  rich  in  silicic
acid  are  used,  such  as  feldspar,  clay-slate,  etc.  The
object  of  the  flux  is  to  form  a  slag  in  which  the  re-
duced iron  collects,  and  by  which  it  is  protected  from
oxidation.  When  the  fire  is  once  started  in  a  blast-
furnace the  operation  of  reduction  is  continuous  until
the  furnace  is  burned  out.  Alternate  layers  of  ore  and
flux  and  carbon  are  added,  and,  as  the  reduced  iron  col-
lects below,  it  is  from  time  to  time  drawn  off  and  allowed
to  solidify  in  moulds  of  sand.  The  operation  requires
close  attention.  The  ores  must  be  carefully  studied,  and
the  nature  and  amount  of  flux  regulated  according
to  the  character  of  the  ore  as  above  stated.  Then,
too,  the  temperature  of  the  furnace  is  a  matter  of  im-
portance, and  must  be  watched,  and  regulated  by  means
of  the  blast.  The  reduction  is  largely  accomplished
by  carbon  monoxide.  In  the  lower  part  of  the  furnace
the  fuel  burns  to  carbon  dioxide,  but  this  comes  in  con-
tact with  hot  carbon,  and  is  then  reduced  to  the  monox-
ide. The  hot  monoxide  in  contact  with  the  oxides  of
iron  reduces  these,  and  is  itself  converted  into  the  diox-
ide. A  large  proportion  of  the  carbon  monoxide,  how-
ever, escapes  oxidation,  and  this  is  carried  off  from  the
top  of  the  furnace  to  the  bottom  by  properly  arranged
pipes,  and  is  then  utilized  as  fuel.  A  furnace  lasts  from
two  to  twenty  years,  and  sometimes  longer.
