The  outer  wall  is  double,  about  3  feet  thick,  the  two  portions
being  separated  by  a  space  filled  with  sand,  in  order  to  close
up  any  cracks  produced  by  heat  in  the  brickwork.
The  cold  air,  as  before  mentioned,  passes  into  the  calciner
at  Nos.  1  and  2,  then  circulates  through  3,  4,  and  5,  con-
taining ore  already  calcined,  the  air  being  heated  at  its
expense.  The  air  then  arrives  at  No.  6,  where  the  ore  is
red-hot.  The  fire  commences  here  and  occupies  No.  7  and
part  of  No.  8.  This  is  the  zone  of  greatest  heat  The  gases
then  traverse  the  remaining  chambers,  giving  up  their  heat
to  the  matter  being  calcined.
The  fuel  is  charged  in  small  quantities  through  openings
in  the  roof,  when  it  inflames  immediately,  and  burns  without
smoke,  because  of  the  high  temperature  of  the  air  employed
for  its  combustion,  and  the  incandescent  space  into  which  it
arrives.  The  same  openings  are  only  used  for  charging  every
twenty-four  hours,  a  fresh  range  being  opened  every  four
hours,  working  in  the  direction  of  the  general  current.
Every  twenty-four  hours  the  operation  in  one  chamber  is
completed.  Then  the  iron  partition  (p)  is  transferred  from
No.  1  to  No.  2,  the  doorway  of  No.  1  closed  and  that  of
No.  3  opened.  The  damper  of  No.  16  is  then  closed  and  that
of  No.  1  opened.  Now  the  contents  of  No.  3  are  discharged
and  the  chamber  No.  2  re-filled.  The  operation  is  thus  con-
tinuous, and  only  stopped  in  the  case  of  serious  repairs.
This  furnace  is  chiefly  used  for  baking  bricks  and  burning
limestone.  An  oval  form  is  also  employed  in  some  works.
136  PRINCIPLES  OF  METALLURGY.
CHAPTER  IX.
INDIRECT  METHOD  OF  EXTRACTION.
