Am'TA^;S^m'}'  Cryolite  and  its  Uses.  63 
smpurities  of  cryolite  generally  consist  of  carbonate  of  iron  and 
sulphides  of  copper,  iron  and  lead,  the  latter  in  very  pretty  crystals.  In 
some  specimens  traces  of  gold  and  several  rare  metals  have  been  found, 
•and  quartz  crystals  occur  often  in  connection  with  it. 
Cryolite,  chemically  considered,  is  a  double  salt  of  aluminum  and 
sodium  with  fluorine,  the  formula  being  3NaF.AlF3  (Bloxam's  chem- 
istry). It  can  be  artificially  prepared  by  mixing  calcined  alumina  and 
carbonate  of  soda  with  an  excess  of  hydrofluoric  acid. 
Cryolite  is  not  very  hard,  and  can  be  easily  reduced  to  a  fine  powder. 
In  this  condition,  mixed  with  sand  in  the  proportions  of  one  part  to 
three  or  four  of  sand,  it  has  come  into  use  in  the  manufacture  of  a 
beautiful  white  glass  or  porcelain  ware,  which  is  easily  moulded  and  cut 
and  is  remarkable  for  its  tenacity. 
It  could  be  used  for  many  purposes  if  the  advantages  were  suffi- 
cient to  pay  the  difference  in  cost  of  importation. 
As  a  source  of  soda,  it  is  very  profitable  on  account  of  the  large 
percentage  which  it  contains  (about  35  per  cent.)  and  the  ease  with 
which  it  is  separated.  The  alumina  present  in  it  is  no  small  item, 
and  is  now  extensively  used  in  the  manufacture  of  the  alum  salts, 
which,  as  prepared  from  cryolite,  are  quite  free  from  iron,  generally 
containing  but  a  trace.  In  the  manufacture  of  the  metal  aluminum, 
cryolite  has  been  used  to  a  certain  degree.  But  the  process  of  isolation 
is  not  perfect,  and  I  believe  does  not  pay  very  well.  Cryolite  is  inso- 
luble in  water,  but  when  long  boiled  with  lime,  decomposition  gradually 
takes  place.  It  is  fusible  at  a  red  heat,  and  on  cooling  forms  a  kind  of 
glass  which  is  slightly  soluble  in  water.  To  thoroughly  separate  the 
mineral  into  its  constituents  it  is  first  necessary  to  convert  it  into  a 
soluble  compound,  which  is  readily  accomplished,  in  a  large  way,  by 
first  bringing  it  to  a  very  fine  state  of  division  by  passing  it  through  a 
crusher,  then  through  several  mills  of  different  degrees  of  fineness, 
after  which  it  is  passed  through  sieves  and  bolting  cloth,  making  it  as 
fine  as  flour.  It  is  then  mixed  with  about  an  equal  weight  of  lime, 
and  calcined  at  a  dull  red  heat  in  a  reverberatory  furnace  for  several 
hours,  when  it  assumes  a  grayish  appearance,  being  decomposed  into 
insoluble  fluoride  of  calcium  and  soluble  aluminate  of  sodium,  besides 
a  small  percentage  of  carbonate  and  hydrate  of  sodium.  These  are 
then  separated  from  the  fluoride  of  calcium  by  lixiviation  with  hot 
water. 
