ON  THE  MANUFACTURE  OF  TARTARIC  ACID,  ETC.  831 
a  tartrate  of  potash  and  soda,  and  sulphuretted  hydrogen  is 
liberated,  which  escapes  by  the  exit  tubes.  The  resulting  solution 
of  tartrate  of  potash  and  soda,  being  separated  either  by  decan- 
tation  or  filtration  from  the  insoluble  impurities  contained  in  the 
crude  argol,  crude  tartar  or  impure  bitartrate  of  potash,  is  con- 
centrated in  a  suitable  evaporating  vessel  to  about  sp.  gr.  1.425 
at  212°  F.,  and  allowed  to  cool  to  60°  F. ;  the  crystals  obtained 
are  separated  from  the  mother-liquors,  and  these  liquors  are  again 
concentrated  to  sp.  gr.  1 .425  at  212°  F.  and  allowed  to  crystallize ; 
the  mother-liquors  left  after  this  crystallization,  if  they  contain 
much  foreign  matter,  maybe  boiled  with  animal  charcoal,  filtered, 
evaporated,  and  set  aside  to  crystallize.    The  remaining  mother- 
liquors  may  be  similarly  treated,  or  mixed  with  liquors  resulting 
from  subsequent  operations.    The  crystals  of  tartrate  of  potash 
and  soda  thus  obtained,  if  they  be  not  sufficiently  pure,  are  to  be 
dissolved  in  water  and  re-crystallized.    Having  obtained  tartrate 
of  potash  and  soda  of  the  desired  purity,  the  crystals  are  dissolved 
either  in  hot  or  cold  water,  and  a  saturated  or  partially  saturated 
solution  obtained  ;  into  this  solution  sulphurous  acid  is  to  be 
passed,  which  unites  with  the  soda,  forming  soluble  sulphite  and 
sulphites  of  soda,  and  bitartrate  of  potash  is  precipitated.  Sul- 
phurous acid  is  to  be  added  until  the  double  tartrate  of  potash 
and  soda  is  decomposed  (which  may  be  known  when  no  further 
precipitation  of  bitartrate  of  potash  occurs  on  the  addition  of  a 
small  quantity  of  an  acid  stronger  than  tartaric  to  a  portion  of 
the  solution) ;  the  deposited  bitartrate  of  potash  is  .allowed  to 
settle,  and  the  solution  is  drawn  off,  and  entirely  or  partially  re- 
saturated  with  tartrate  of  potash  and  soda.    Sulphurous  acid  is 
again  passed  into  the  solution,  and  the  former  operations  repeated, 
the  supernatant  solution  may  then  be  separated,  and  will  contain 
sulphites  of  soda  and  bitartrate  of  potash.    The  solution  may 
be  neutralized  with  sulphide  of  sodium  or  carbonate  of  soda  and 
concentrated,  when  tartrate  of  potash  and  soda,  sulphite  of  soda, 
and  (if  sulphide  of  sodium  has  been  employed)  hyposulphite  of 
soda,  will  crystallize  out.    These  salts  may  be  separated  'by 
crystallization,  and  the  tartrate  of  potash  and  soda  may  be  de- 
composed with  sulphurous  acid  as  before  mentioned.    The  bitar- 
trate of  potash,  thus  obtained,  is  to  be  washed  with  cold  water, 
until  the  whole  of  the  sulphites  of  soda  are  removed,  and  then 
dried,  when  it  will  be  ready  for  use. 
