130  ON  CRYSTALLIZED  HYDRATE  OF  BARYTES. 
centimetres  of  the  solution  of  oxalic  acid  is  the  hundredth  part 
of  a  litre,  and  one  litre  of  the  acid  will  saturate  an  atom  of  caustic 
soda.  Therefore  the  number  of  cubic  centimetres  of  the  solution 
of  soda  required  to  saturate  ten  cubic  centimetres  of  the  solution 
of  oxalic  acid,  mast  be  multiplied  by  100  to  find  the  quantity 
required  for  one  atom  of  a  barytic  salt.  800  cubic  centimetres 
of  this  caustic  soda  would  decompose  122  grammes  of  crystallized 
chloride  of  barium,  or  180 J  grammes  of  nitrate  of  barytes.  The 
first  named  salt  is  the  cheaper  and  easier  to  be  prepared  ;  but 
when  any  trace  of  chlorine  is  to  be  avoided  in  an  investigation, 
the  nitrate  of  barytes  should  be  preferred. 
The  solution  of  caustic  soda  is  boiled  in  a  cast-iron  pan,  and 
the  weighed  quantity  of  the  barytic  salt  added  to  it.  A  pre- 
cipitate will  be  produced  in  all  cases,  because  the  caustic  soda 
is  hardly  free  from  carbonic  or  sulphuric  acid. 
The  decomposition  will  be  complete  as  soon  as  the  barytic  salt 
has  been  dissolved,  and  the  insoluble  product  may  be  easily 
separated,  in  consequence  of  the  great  solubility  of  caustic  barytes 
in  boiling  water.  It  may  be  effected  by  filtration,  but  the  pro- 
cess of  sedimentation  will  be  found  to  be  far  more  expeditious. 
With  this  view  the  liquid  is  left  covered  for  sometime  in  a  warm 
stove,  not  hot  enough  to  cause  ebullition  or  the  disturbance  of 
the  sediment,  but  sufficiently  not  to  prevent  crystallization. 
The  precipitate  of  carbonate  and  sulphate  of  barytes  is  de- 
posited very  quickly,  and  after  half  an  hour  the  liquid  will  be 
clear  enough  to  be  taken  off  by  a  syphon.  It  is  put  into  suitable 
vessels  of  iron,  china,  or  glass,  from  which  free  access  of  air  is 
excluded,  and  set  aside  in  a  cool  place  to  crystallize.  The  whole 
liquid  often  crystallizes  into  a  solid  mass  of  acicular  crystals  of 
caustic  barytes,  the  mother-liquor  being  entirely  absorbed  by  the 
mass.  The  crystals  are  put  into  a  funnel,  and  when  the  greater 
part  of  the  liquid  has  drained  away,  the  funnel  is  placed  in  an 
oblique  direction  in  a  centrifugal  engine,  and  by  the  rapid  motion 
of  the  engine  much  of  the  remaining  mother-liquor  is  thrown  off. 
The  crystals  are  thus  got  in  a  state  in  which  they  will  hardly 
wet  filtering  paper  when  placed  upon  it.  By  re-dissolving  in  a 
small  quantity  of  boiling  distilled  water,  and  crystallizing  a  second 
time  as  before,  the  crystals  are  obtained  in  a  sufficient  state  of 
purity. 
