CRYOLITE. 
197 
this  character  can  he  prepared  only  from  alumina  from  cryolite, 
as  that  prepared  from  alum,  clays,  or  other  sources  of  alumina, 
cannot  be  free  from  a  great  excess  of  acid,  which  undoubtedly 
destroys  its  usefulness  for  many  purposes,  and  injures  it  for  art. 
The  solution  of  carbonate  of  soda,  before  described,  can  of  course 
be  applied  to  the  manufacture  of  caustic  soda,  by  heating  it  with 
freshly  slacked  lime,  in  the  ordinary  manner  practiced  with  ash 
and  other  commercial  forms  of  carbonate,  but  it  is  advantageous 
to  use  the  following  modification  of  the  process  known  as  the  wet 
process. 
In  this  mode  of  manufacture  the  cryolite  is  added  gradually 
to  a  boiling  milk  of  lime,  contained  in  an  upright  cylinder  or 
agitator,  provided  with  a  stirrer  attached  to  a  vertical  axis.  Now 
in  this  process,  thus  employed,  about  15,000  lbs.  of  quicklime 
are  slacked  in  a  large  wooden  vessel,  and  thoroughly  mixed  with 
enough  water  to  render  the  mixture  fluid.  It  is  then  run  into 
the  agitator,  and  boiled  by  injecting  steam.  Having  come  to 
the  boiling  point,  the  requisite  proportion  of  cryolite,  in  a  very 
fine  powder,  is  added,  and  the  boiling  continued  until,  by  testing, 
the  complete  decomposition  of  the  cryolite,  and  the  absence  of 
the  alumina  in  solution,  is  ascertained.  The  contents  of  the 
agitator  are  then  disposed  of  upon  filters  of  fire  brick,  through 
which  the  caustic  solution  is  filtered,  leaving  the  fluoride  of  cal- 
cium in  a  pasty  slop  upon  the  filter.  This  is  then  treated  with 
hot  water,  which  extracts  the  soda  liquor  by  displacement.  The 
caustic  soda  liquor  is  now  evaporated  in  iron  pans  up  to  a  density 
of  28°  B.,  when  the  concentration  is  completed  in  cast-iron 
kettles,  until  finally  the  caustic  is  fused  in  quantities  of  8000 
lbs.  each,  at  a  full  red  heat,  and  poured  into  iron  moulds  to  cool 
and  harden.  I  have  learned  from  parties  who  are  connected 
with  the  works  of  the  Pennsylvania  Salt  Manufacturing  Compa- 
ny, at  Natrona,  near  Pittsburg,  which  company  has  the  exclusive 
right  to  import  from  Greenland  this  mineral  called  cryolite,  that 
at  their  works  from  30,000  to  40,000  lbs.  of  cryolite  are  thus 
treated  every  day.  But  further,  now,  the  resulting  soda  is  of 
nearly  chemical  purity,  and  I  think  superior  to  any  otherwise 
made,  hydrate  of  soda,  containing  of  84  per  cent,  actual  hydrate, 
being  produced  without  difficulty  ;  the  impossibility  of  obtaining 
