196 
CRYOLITE. 
a  continuous  stream  of  carbonic  acid  gas,  which  is  the  result  of 
the  combustion  of  the  coke  or  pure  lime  kiln,  in  which  coke  is 
used  as  fuel.  In  a  few  hours  the  absorption  of  carbonic  acid 
gas  is  complete,  and  the  alumina  separates  in  the  form  of  a  fine 
granular  powder,  which  is  a  terhydrate  of  alumina.  The  Contents 
of  the  agitator  are  now  put  into  suitable  vessels,  and  then  allowed 
to  settle.  The  clear  solution  of  carbonate  of  soda  having  a 
density  of  about  31°  Baume,  it  is  then  run  off  into  flat  pans,  and 
is  evaporated  more ;  when  about  37°  B.  it  is  removed  to  cast- 
iron  pans  to  crystallize  and  form  sal  soda,  or,  as  it  is  commonly 
known,  washing  soda.  Then  what  are  known  as  the  mother 
liquors  are  returned  again  to  the  pans,  and  remixed  with  a  new 
batch  of  liquor,  and  again  evaporated  and  crystallized. 
For  converting  the  alumina  into  a  sulphate,  it  is  added,  in 
small  portions  at  a  time,  to  a  boiling  solution  of  dilute  sulphuric 
acid  of  about  30°  B.,  until  rather  more  alumina  has  been  used  than 
is  sufficient  to  form  the  tersulphate  of  alumina,  the  chemical  for- 
mula of  which  is  A1203+3S03,  having  an  excess  of  alumina  always 
present.  After  boiling  for  a  few  hours  the  liquor  is  tested,  and  as 
it  is  prepared  in  the  arts  to  have  the  proportions  of  a  slightly  basic 
sulphate,  these  are  adjusted  so  that  for  each  equivalent  of  alu- 
mina present  there  should  be  2T8(j  eq.  of  sulphuric  acid,  or  t2q  of  an 
eq.  less  than  the  sulphate,  as  it  exists  in  common  alum.  The  solu- 
tion is  then  allowed  to  remain  at  rest  for  some  days,  until  per- 
fectly clear,  when  it  is  decanted  into  copper  vessels,  and  evapo- 
rated until  a  drop  of  it  solidifies  into  a  hard  brittle  mass.  It  is 
then  collected  into  leaden  moulds  to  cool,  when  it  is  packed  in 
boxes  or  barrels  for  market. 
Sulphate  of  alumina,  as  thus  prepared,  is  of  a  white  translu- 
cent color,  having  the  hardness  and  appearance  of  alabaster.  It 
contains  merely  traces  of  iron,  and,  as  before  said,  there  is  pres- 
ent in  combination  also  sulphuric  acid,  as  with  alumina  in  common 
alum.  It  also  contains  about  15  per  cent,  of  alumina,  or  about 
one-half  more  than  is  present  in  an  equal  weight  of  common  pot- 
ash alum.  It  is  on  account  of  its  basic  properties,  and  conse- 
quent freedom  from  acid,  its  small  amount  of  iron,  its  trifling 
cost,  that  by- calico  printers,  sugar  refiners,  paper  makers,  &c, 
it  is  consumed  in  immense  quantities.    Sulphate  of  alumina  of 
