EXTRACTION  OF  THALLIUM. 
45 
broken  in  one  night.  Further,  the  choice  of  a  vessel  in  which 
the  material  could  be  boiled  was  a  matter  of  some  difficul- 
ty. Enamelled  iron  vessels,  which  seemed  to  offer  the  best  means, 
were  found  to  be  very  objectionable.  The  acid  solution  rapidly 
destroyed  the  enamel,  and  the  thallium  was  then  precipitated 
upon  the  exposed  iron.  These  are  a  few  of  the  difficulties  which 
we  had  to  encounter  in  the  early  stages  of  the  proceedings  ;  but 
they  have  happily  been  surmounted,  and  the  process  I  am  about 
to  detail  is  one  by  which  any  manufacturer,  bearing  in  mind  the 
facts  I  have  just  stated,  may  set  about  the  extraction  of  thallium 
on  as  large  a  scale  as  he  may  deem  desirable. 
The  process  which  is  at  present  adopted  at  Messrs.  Hopkin 
and  Williams's  laboratory  is  as  follows  : — The  thalliferous  dust  is 
first  treated  in  wooden  tubs  with  an  equal  weight  of  boiling  water, 
and  is  well  stirred ;  during  this  operation  a  considerable  quantity 
of  nitrous  acid  is  evolved ;  after  which  the  mixture  is  allowed  to 
rest  for  twenty-four  hours  for  the  undissolved  residue  to  deposit. 
The  liquid  is  then  syphoned  off,  the  residue  is  washed,  and  after- 
wards treated  with  a  fresh  quantity  of  boiling  water.  The  col- 
lected liquors,  which  have  been  syphoned  off  from  the  deposit, 
are  allowed  to  cool,  precipitated  by  the  addition  of  a  considera- 
ble excess  of  strong  hydrochloric  acid,  and  the  precipitate,  con- 
sisting of  very  impure  chloride  of  thallium,  is  allowed  to  subside. 
The  chloride  obtained  in  this  way  is  then  well  washed  on  a 
calico  filter,  and  afterwards  squeezed  dry.  I  may  mention  that  from 
three  tons  of  the  dust  I  obtained  68  pounds  of  this  rough  chloride. 
The  next  step  in  the  process  with  this  large  quantity  is  neces- 
sarily a  tedious  one.  It  consists  in  treating  the  crude  chloride 
in  a  platinum  dish  with  an  equal  weight  of  strong  sulphuric  acid, 
and  afterwards  heating  the  mixture  to  expel  the  whole  of  the 
hydrochloric  acid.  To  make  sure  of  this,  the  heat  must  be  con- 
tinued until  the  greater  part  of  the  excess  of  sulphuric  acid  is 
volatilised.  After  this  the  mass  of  sulphate  of  thallium  is  dis- 
solved in  about  twenty  times  its  weight  of  water,  and  the  solu- 
tion filtered.  On  the  addition  of  hydrochloric  acid  to  this  solu- 
tion, nearly  pure  chloride  of  thallium  is  thrown  down  ;  this  is 
collected  on  a  calico  filter,  well  washed,  and  then  squeezed  dry. 
The  object  of  the  process  so  far  has  been  to  obtain  a  tolerably 
pure  chloride  ;  but  as  thallium  is  most  easily  reduced  to  the  me- 
