The  hydrochloric  acid  gas  which  is  given
off  escapes  from  the  furnace  through  a  flue  with
the  products  of  combustion  into  higli  brick


towers  filled  with  coke  or  stones,  over  which
a  stream  of  water  trickles  down,  the  whole  of
the  acid  vapours  are  thus  condensed,  the  smoke
passing  off  by  a  chimney  connected  with  the
towers.  The  diluted  acid  solution  thus  formed
is  concentrated  by  the  aid  of  the  apparatus
shown  in  section  in  figs.  1,  2,  and  3.
This  apparatus  consists  of  several  cast-iron
cylinders,  57  feet  long  by  27  feet  in  diameter,
closed  in  the  same  manner  as  gas  retorts,  by
lids  luted  with  clay.  One  of  the  lids  has  an
opening  o,  into  which  is  fitted  the  stoneware
or  leaden  pipe  a,  conveying  the  bydrochloric
acid  to  the  condensing  apparatus.  The  other,
or  posterior  lid,  is  also  provided  with  an  open-
ing d,  through  which  is  passed  the  tube  of  a
leaden  funnel,  so  that  after  the  retort  is  filled
with  salt  sulphuric  acid  may  be  poured  in.
The  construction  of  the  furnace  in  which  two
retorts  are  usually  placed,  permits  the  flame


Fio.  1.
of  the  fire  at  O  to  play  round  the  cylinders
before  reaching  the  flue  leading  to  the  chiin-
ney  F.  B  is  an  arch  over  the  furnace.  The
first  stage  of  the  operation  consists  in  filling
each  cyUnder  with  330  lbs.  of  salt.  The  lids
or  covers  are  then  luted  on,  and  the  fire  is
kindled.  The  requisite  quantity  of  strong
sulphuric  acid  is  next  poured  into  the  retort,
and  the  funnel  having  been  withdrawn  from
D,  the  hole  is  covered  by  a  clay  plug.
As  soon  as  the  reaction  is  over,  the  396  His.
of  sulphate  of  soda  produced  are  removed,
and  the  operation  repeated.
