Under  this  head  comes  the  principal  item  of  heat  available
for  the  furnace.  In  reckoning  it  we  should  calculate  the  total
heat  which  could  be  generated  by  the  perfect  combustion  of
the  gas  used,  to  CO2,  N2  and  H2O  vapor.  If  there  is  in  reality
imperfect  combustion,  as  is  shown  by  analysis  of  the  chimney
gases,  that  is  a  defect  of  operation  of  the  furnace  which  should
be  written  down  against  it.  Problem  71  showed  an  actual  case
in  which  there  was  very  incomplete  combustion  in  the  body  of
the  furnace,  but  where  combustion  was  afterwards  completed
in  the  regenerators.  In  such  a  case  the  same  principle  applies;
the  furnace  must  be  charged  with  the  total  calorific  power  of
the  fuel  used,  and  incomplete  combustion  can  be  charged
against  the  furnace  as  a  whole  only  on  the  basis  of  uncon-
sumed  ingredients  in  the  chimney  gases — the  products  finally
rejected  by  the  furnace.  If  there  is  poor  combustion  in  the  body
of  the  furnace  and  combustion  is  only  completed  in  the  re-
generators, the  furnace  will  not  give  as  high  net  thermal  effi-
ciency as  if  combustion  were  complete  above  the  hearth.
(5)  OXIDATION  OF  THE  BATH.
The  oxidation  of  carbon,  iron,  silicon,  manganese  and  some-
times phosphorus  and  sulphur,  add  a  not  inconsiderable  amount
to  the  heat  resources  of  the  furnace.  Carbon  should  be  burnt
to  CO2,  iron  is  usually  oxidized  to  FeO,  manganese  to  MnO,
silicon  to  SiO2,  phosphorus  to  P2O5,  and  sulphur  to  SO2.  All
of  these  oxidations  generate  heat,  and,  moreover,  heat  which
should  be  very  efficiently  utilized,  being  produced  in  direct
contact  with  the  metallic  bath ;  it  should  all  be  charged  against
the  furnace  as  part  of  its  available  heat.
(6)  FORMATION  OF  SLAG.
