178     Prof.  McCoy  on  tlie  Relation  between  Radioactivity 
The  details  of  the  process  were  as  follows  : — 29  g.  of  pitch- 
blende (from  the  Wood  Mine,  Colorado),  containing  60  per 
cent,  of  uranium j  and  being  about  2' 5  times  as  active  as  pure 
U308,  were  dissolved  in  nitric  acid  and  the  solution  evaporated 
to  dryness.  The  residue  was  taken  up  in  water  and  filtered, 
and  the  filtrate  added  to  an  excess  of  ammonium  carbonate 
solution.  The  solution  was  boiled  and  filtered  from  the  pre- 
cipitate containing  iron,  &c.  Copper,  lead,  &c.  were  removed 
from  the  filtrate  by  treatment  with  ammonium  sulphide. 
The  uranium  was  precipitated  from  a  small  portion  of  the 
filtrate  by  prolonged  boiling.  Ignition  of  the  precipitate  of 
ammonium  uranate  gave  U308  which  was  4*9  per  cent,  more 
active  than  the  standard  oxide.  The  treatment  up  to  this 
point  had  therefore  removed  all  but  about  2  per  cent,  of  the 
radioactive  impurities  originally  present  in  the  ore.  To  the 
main  portion  of  the  solution  was  added  a  solution  of  2  g.  of 
barium  chloride.  The  solution  was  heated  to  boiling  and  a 
slight  excess  of  ammonium  sulphate  added.  The  barium 
sulphate,  so  formed,  was  filtered  out.  It  was  freed  from 
traces  of  uranium  by  treatment  with  hydrochloric  acid.  It 
was  still  about  as  active  as  uranium  oxide.  That  the  activity 
of  the  barium  sulphate  was  due  chiefly  to  radium  and  not  to 
UX  was  shown  by  the  facts,  first,  that  the  rays  were  almost 
wholly  absorbed  by  a  thin  sheet  of  aluminium,  and  second 
that  the  activity  increased  in  intensity  with  time.  The  treat- 
ment of  the  solution  with  barium  chloride  and  ammonium 
sulphate  was  repeated  twice.  The  second  lot  of  barium 
sulphate  was  but  0*05  as  active  as  the  first,  while  the  third 
lot  was  practically  inactive.  The  U3Os  prepared  from  the 
solution  after  the  third  precipitation  differed  in  activity  from 
the  old  standard  U308  by  only  0' 15  per  cent.  This  difference 
is  just  about  the  limit  of  accuracy  of  the  method  of  com- 
parison. Further  treatments  with  barium  salts  did  not  lower 
the  activity  of  the  uranium  oxide.  These  results  show  that 
radium  and  other  active  bodies  occurring  in  uranium  ores  are 
easily  removed.  There  is  therefore  no  doubt  that  the 
uranium  oxide  finally  obtained  was  free  from  such  active 
impurities,  in  appreciable  quantities.  In  addition  it  is 
established  that  the  material  previously  used  *  in  deter- 
mining the  absorption  coefficients  and  activities  of  uranium 
compounds  was  also  of  the  same  high  degree   of  purity  t- 
*  McCoy,  J.  Amer.  Cliem.  Soc.  xxvii.  p.  391  (1905). 
t  The  work  on  absorption  coefficients  of  pure  uranium  compounds  is 
being  greatly  extended  by  Mr.  H.  M.  G-oettsch,  working  in  tins  laiDoratorv  : 
the  results  previously  announced  are  being  fully  confirmed.  Attempts 
by  Mr.  Goettsch  to  lower  the  activity  of  the  Kahlbaum  uranium  nitrate, 
used  in  the  preparation  of  the  standard  oxide,  by  treatment  with  barium 
salts,  were  without  offset. 
