136  MANUFACTURE  OF  COMMERCIAL  CARBONATE  OF  AMMONIA. 
common  chalk,  heated  in  retorts  and  sublimed.  The  decom- 
position of  the  chloride  of  ammonium  may  be  represented 
thus  : — 
(3H4N;Cl)+3(CaO;C02)=3CaC14-2H4NO;3C02+H3N+HO. 
In  the  manufacture  of  ammonia  alum,  the  ammonia  is  derived 
from  gas  liquor ;  the  liquor  is  heated,  and  nearly  all  the  volatile 
ammonia  driven  off.  The  residue  is  taken  out  of  the  boilers 
and  used  for  the  manufacture  of  carbonate  of  ammonia.  It  is 
treated  with  a  little  acid  till  it  is  neutral,  then  evaporated  by 
means  of  heat  in  large  hemispherical  iron  pans  set  in  brick- 
work. When  it  has  arrived  at  the  crystallizing  point,  it  is 
allowed  to  cool,  and  crystals  are  then  deposited;  or  the  hot 
liquor  may  be  run  into  -other  coolers  for  crystallization.  The 
mother  liquor  is  syphoned  off,  and  then  the  inside  of  the  pan 
is  seen  studded  with  intense  black  crystals,  of  the  prismatic 
form  when  sulphuric  acid  has  been  used  for  the  neutralization, 
and  cubical  with  hydrochloric  acid.  The  crystals  are  now 
shovelled  out  of  the  pan  and  washed  with  the  mother  liquor. 
They  are  then  re-dissolved,  the  liquor  run  into  coolers,  and  re- 
crystallized.  In  the  re-dissolving  a  great  deal  of  sediment  is 
deposited,  consisting  chiefly  of  the  matter  mechanically  locked 
up  in  the  crystals.  The  crystals  when  dry  are  of  a  dirty  white 
color ;  they  are  now  ready  for  the  next  operation  of  converting 
them  into  carbonate  of  ammonia.  For  this  purpose  cast-iron 
retorts,  the  shape  of  an  elongated  muffle,  are  used.  The  neck 
of  the  retort  is  round,  and  closed  with  an  iron  door,  kept  m  its 
place  by  means  of  a  screw.  The  retorts  are  about  seven  feet 
long  and  one  and  a-half  deep.  Three  are  set  in  brickwork  in 
the  form  of  a  triangle,  and  heated  by  one  fire.  They  communi- 
cate by  means  of  iron  pipes  with  a  leaden  chamber  which  is 
technically  called  a  balloon.  It  is  about  six  feet  high,  eight 
long,  and  two  and  a-half  wide.  These  balloons  are  supported 
upon  scaffolding  so  as  to  be  on  a  line  with  the  retorts,  and  are 
kept  in  their  places  by  means  of  iron  bands.  At  the  bottom 
of  each  balloon  is  a  small  pipe,  which  is  always  kept  open  to 
allow  for  the  escape  of  steam,  and  water  highly  charged  with 
carbonate  of  ammonia.  There  is  a  constant  dropping  from 
this  pipe,  which  is  collected  in  a  pail,  and  re- sublimed.    If  this 
