MANUFACTURE  OF   AMMONIA,  ETC. 
131 
the  retort.  The  first  portion  of  this  second  distillation  is  a  solu- 
tion of  ammonia,  sufficiently  pure  for  common  purposes  ;  but  it 
may  be  still  further  purified  by  distilling  it  a  third  time  in  the 
same  manner  as  before,  preserving  that  portion  only  which  is 
made  by  the  absorption  of  the  ammoniacal  gas  in  water,  and  re- 
turning to  the  retort  the  latter  products  of  the  process,  which  con- 
sist of  ammonia  and  water  mixed  with  impurities."  Mr.  Croll's 
process  (patented  July  29th,  1840)  for  obtaining  the  salts  of  am- 
monia, are  of  two  kinds :  in  one  of  these  dilute  sulphuric  or  muri- 
atic acid  is  employed  to  abstract  the  ammonia  fforn  the  gas,  and 
in  the  other  chloride  and  sulphate  of  manganese  and  muriate  of 
iron  are  employed  for  the  same  purpose.  In  the  latter  case  a  ves- 
sel used  in  the  manufacture  of  gas  holding  wet  lime  for  the  pur- 
poses of  gas  purifying,  is  filled  with  a  solution  composed  of  1  cwt. 
of  chloride  of  manganese  to  forty  gallons  of  water,  and  the  gas  is 
forced  through  this  solution  in  the  usual  way  by  the  pressure  o 
the  retorts,  by  which  means  the  ammonia  is  absorbed.  As  soon 
as  this  solution  is  saturated  with  ammonia,  it  is  drawn  off,  and  the 
vessel  fresh  charged.  Sulphate  of  manganese  and  muriate  of  iron 
may  also  be  employed  in  the  same  way  to  absorb  the  ammonia 
produced  in  the  manufacture  of  coal-gas. 
In  the  case  of  using  sulphuric  acid,  a  vessel  more  commonly  em- 
ployed for  washing  gas  is  filled  with  a  solution  composed  of  100 
gallons  of  water  to  two  pounds  and  a  half  of  sulphuric  acid,  spec, 
grav.  1.845,  and  the  gas  is  passed  through  it  as  usual  until  it  has 
attained  the  spec.  grav.  of  1.170,  and  is  saturated  with  ammonia; 
it  is  then  drawn  off  and  the  vessel  charged  anew. 
When  muriatic  acfd  is  used  it  is  applied  in  the  same  manner, 
as  regards  proportion,  the  acid  being  of  spec.  grav.  1.165  before 
it  is  mixed  with  the  water  :  the  solution  of  muriate  of  ammonia  is 
to  be  drawn  off  when  it  has  acquired  a  density  of  1.176.  In  order 
to  obtain  the  ammoniacal  salts  (when  a  salt  has  been  used  for  pu- 
rification,) the  insoluble  part  of  the  mixture  is  allowed  to  settle, 
and  the  clear  liquor,  which  contains  muriate  of  ammonia  and  sul- 
phate of  soda,  is  drawn  off.  These  must  be  separated  from  each 
other,  either  by  crystallizing  the  salts  of  ammonia  from  that  ot  so- 
da, or  by  evaporating  both  to  dryness,  and  subliming  the  ammoni- 
acal salt.  If  an  acid  be  employed  if  is  only  necessary  to  evapo- 
rate the  ammoniacal  solution.    The  salts  formed  by  the  use  of  the 
