PREPARATION  OF  PURE  CAUSTIC  SODA. 
431 
unchanged,  while  a  solution  of  gelatine  cannot  always  be  de- 
pended upon  even  the  day  after  it  is  made. 
I  observe  that  Fownes  estimates  the  tannic  acid  in  galls  at 
from  30  to  40  per  cent.,  whilst  Royle  puts  60  per  cent,  as  the 
maximum.  My  own  experiments  lead  me  to  be  dissatisfied  with 
blue  galls  containing  less  than  about  65  per  cent.  A  sample  of 
Chinese  galls  contained  52  per  cent. — Pharm.  Journal,  April, 
1862.  # 
ON  THE  PREPARATION  OF  PURE  CAUSTIC  SODA  ON  THE  LARGE 
SCALE. 
By  Dr.  Ph.  Pauli,  of  the  Union  Alkali  Works,  St.  Helens,  Lancashire. 
Three  tons  of  commercial  caustic  soda,  containing  excess 
of  water,  alumina,  and  all  the  impurities  which  commonly  occur 
in  this  substance,  are  fused  in  a*  cast-iron  pot.    During  the 
evaporation  nearly  all  the  carbonate,  and  by  far  the  larger 
quantity  of  the  other  salts,  separate  out  on  the  surface  as  a 
scum,  and  can  be  easily  removed.    The  liquid  mass  is  then 
heated  to  dull  redness,  and  kept  at  that  temperature  during  the 
night.    In  the  morning  the  mass  appears  perfectly  transparent, 
the  sides  and  bottom  of  the  vessel  being  coated  with  cauliflower- 
shaped  masses  of  crystals  consisting  of  silicate  of  alumina,  with 
chloride  and  sulphate  of  sodium  and  a  little  lime.    The  clear 
fused  liquid  is  ladled  off  from  these  crystals,  and  when  cooled  is 
ready  for  use. 
The  soda  thus  obtained  is  perfectly  free  from  alumina;  a 
small  quantity  of  the  soda  was  fused  in  a  platinum  crucible,  and 
some  pure  alumina  added.  This  remained  undissolved  in  the 
fused  mass,  swimming  about  like  a  precipitate  in  the  red-hot 
liquid.  On  cooling,  water  was  added  to  this  fused  mass,  and 
the  alumina  was  found  to  dissolve  completely.  If  the  commer- 
cial soda  contains  oxide  of  iron,  this  also  separates  out  com- 
pletely during  the  process  of  fusion.  Lime,  on  the  other  hand, 
is  dissolved  in  large  quantities  by  the  caustic  soda,  but  it  is  com- 
pletely separated  by  solution  in  water.  The  caustic  soda  pre- 
pared in  this  way  is  hard  and  brittle,  and  can  be  easily  obtained 
as  a  fine  powder  by  attrition  in  an  iron  mortar.  It  contains 
only  a  trace  of  carbonate  of  soda.    This  product,  which  will 
