46 
EXTRACTION  OF  THALLIUM. 
tallic  state  from  the  sulphate,  it  is  now  necessary  to  again  con- 
vert the  chloride  into  sulphate.  For  this  purpose  we  add  the 
dry  chloride  gradually  to  hot  sulphuric  acid,  using  four  parts 
by  weight  of  strong  acid  to  six  parts  of  the  chloride.  The  mix- 
ture so  obtained  is  heated  strongly  until  all  the  hydrochloric  acid 
is  expelled,  and  the  residue  assumes  the  form  of  a  dense  liquid. 
This,  being  set  aside,  solidifies  on  cooling  to  white  crystalline 
mass.  When  this  is  dissolved  in  water  an  immense  amount  of 
heat  is  evolved,  and  great  care  must  be  taken  to  avoid  breakage 
of  the  vessels.  The  best  way  of  dissolving  it  is  to  add  it  slowly 
to  ten  times  its  weight  of  hot  water.  A  solution  is  thus  obtained, 
which  must  be  filtered,  and  on  being  concentrated  and  set  aside 
to  cool,  crystals  of  sulphate  of  thallium  will  be  obtained,  which 
may  be  rendered  quite  pure  by  re-crystallisation,  a  little  hydro- 
sulphuric  acid  being  previously  added,  if  necessary,  to  separate 
arsenic,  mercury,  &c. 
The  final  step  in  the  process  is  the  reduction  of  the  metal  from 
this  sulphate.  Many  ways  can  be  adopted  for  the  reduction 
when  only  a  few  ounces  are  under  experiment ;  but  when  the 
quantity  of  crystallised  salt  is  from  a  quarter  to  half  a  hundred- 
weight, a  process  must  be  devised  which  will  admit  of  considera- 
ble quantities  being  reduced  without  too  much  expenditure  of 
time  or  trouble.  Experiments  in  the  dry  way  were  not  very  suc- 
cessful. No  difficulty  was  experienced  in  partially  reducing  the 
metal  by  igniting  the  sulphate  with  black  flux,  or  with  cyanide 
of  potassium,  in  a  clay  crucible ;  but  I  could  not  remove  all  the 
sulphur  in  this  way.  When  sulphate  of  thallium  is  projected 
into  a  crucible  containing  fused  cyanide  of  potassium  there 
is  an  immediate  reduction  to  the  state  of  protosulphide,  which 
forms  a  brittle,  metallic-looking  mass,  of  the  lustre  of  plumbago, 
and  fusing  more  readily  than  the  metal. 
A  few  cells  of  Groves'  batteries  offer  a  most  ready  means  of 
reducing  the  sulphate,  and  in  quantities  less  than  half  a  pound 
nothing  can  be  simplier  than  the  electrolytic  process. 
For  reducing  the  metal  in  quantity  I  found  no  plan  so  good  as 
its  precipitation  with  metallic  zinc.  Plates  of  pure  zinc  (which 
must  leave  no  residue  whatever  when  dissolved  in  sulphuric  acid) 
are  arranged  vertically  round  the  sides  of  a  deep  porcelain  dish 
holding  a  gallon.    About  seven  pounds  of  crystallised  sulphate 
