EXTRACTION  OF  THALLIUM. 
43 
is,  I  regret  to  say,  of  but  very  limited  use,  and  has  caused  me 
many  disappointments  before  I  finally  determined  to  abandon  it, 
except  by  way  of  confirmation  in  subsequent  experiments.  The 
spectrum  by  itself  gives  no  indication  of  quantity.  The  green 
line  produced  by  a  residue  containing  but  one  part  of  thallium 
in  a  thousand  is  as  vivid  and  distinct  as  the  line  given  by  the 
pure  metal,  and  therefore,  before  I  could  decide  whether  a  de- 
posit contained  sufficient  thallium  to  repay  for  its  extraction,  it 
was  necessary  to  make  an  estimation  in  the  moist  way  by  exhaust- 
ing a  weighed  quantity  of  the  dust  with  water,  and  adding  hydro- 
chloric acid  to  the  solution.  Associated  with  thallium  in  these 
deposits  is  unfortunately  a  variety  of  other  metals,  which  ren- 
der the  separation  of  the  thallium  in  a  pure  state  a  rather  diffi- 
cult matter.  Amongst  these  metals  I  have  found  mercury,  cop- 
per, arsenic,  antimony,  iron,  zinc,  cadmium,  lime,  and  selenium, 
together  with  ammonia,  sulphuric,  hydrochloric,  and  nitric  acids. 
In  a  lecture  delivered  at  the  Royal  Institution  of  Great  Brit- 
ain on  Friday  evening,  March  27,  1863,  I  referred  to  a  curious 
parallelism  between  the  discovery  of  selenium  and  that  of  thal- 
lium. The  great  Swedish  chemist,  Berzelius,  was  engaged  in 
the  examination  of  a  residue  for  tellurium,  when  he  found  a 
hitherto  unknown  metalloid,  to  which  he  gave  the  name  of  selen- 
ium. In  my  own  case  a  very  similar  residue  was  also  being  ex- 
amined for  tellurium  by  means  of  the  spectroscope,  when  I 
first  noticed  the  green  line  which  led  me  to  the  discovery  of 
thallium.  Soon  after  the  publication  of  that  lecture,  Dr.  Alfred 
Swaine  Taylor  was  kind  enough  to  send  me  a  powder  which  is 
beyond  doubt  a  portion  of  the  identical  residue  in  which  Berze- 
lius found  selenium.  The  original  specimen  is  preserved  in  the 
laboratory  of  Guy's  Hospital,  in  a  bottle  labelled  as  follows  :— 
"  Mixture  containing  selenium  from  the  Gripsholm  Works  in 
Sweden.  Sent  by  Professor  Berzelius  to  William  Allen  in 
1820." 
William  Allen  was  formerly  lecturer  on  chemistry  in  Guy's 
Hospital.  The  powder  was  given  to  Dr.  Taylor  by  Mr.  Daniel 
Hanbury  some  years  since.  The  authenticity  of  the  specimen 
is  thus  placed  beyond  a  doubt.  It  is  of  especial  interest  to  be 
enabled  to  decide  positively  whether  thallium  was  present  or 
absent  in  the  mixture  in  which  Berzelius  discovered  selenium. 
This  I  am  now  able  to  do.    I  have  examined  the  specimen  sent 
