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  XV. 
  0)1 
  the 
  Distribution 
  of 
  Thorium 
  in 
  the 
  Earth^s 
  Surface 
  

   Materials. 
  By 
  G. 
  A. 
  Blanc, 
  Ph.D. 
  {Rome) 
  *. 
  

  

  ^I^HE 
  very 
  interesting 
  results 
  communicated 
  by 
  Professor 
  

   X 
  J. 
  Joly 
  in 
  the 
  May 
  number 
  of 
  this 
  Magazine 
  induce 
  

   me 
  to 
  recall 
  the 
  results 
  of 
  some 
  researches 
  which 
  I 
  have 
  

   made 
  on 
  the 
  same 
  subject. 
  

  

  In 
  a 
  first 
  note 
  published 
  in 
  February 
  1908 
  (JRendic 
  R, 
  

   Accad. 
  d. 
  Lincei, 
  xvii. 
  1st 
  sem. 
  p. 
  101, 
  1908, 
  and 
  also 
  Phys. 
  

   Zeit. 
  ix. 
  p. 
  294, 
  1908) 
  I 
  gave 
  the 
  results 
  of 
  an 
  investigation 
  

   made 
  by 
  me 
  in 
  order 
  to 
  determine 
  the 
  amount 
  of 
  thorium 
  

   contained 
  in 
  unit 
  mass 
  of 
  the 
  soil 
  in 
  Rome. 
  The 
  method 
  

   consisted 
  in 
  comparing 
  the 
  amounts 
  of 
  excited 
  activity 
  of 
  the 
  

   thorium 
  type 
  which 
  is 
  produced 
  on 
  a 
  negatively 
  charged 
  

   w^ire 
  placed 
  under 
  a 
  vessel 
  covering 
  a 
  given 
  area 
  of 
  soil 
  with 
  

   that 
  produced 
  when 
  the 
  vessel 
  is 
  placed 
  over 
  a 
  layer 
  of 
  the 
  

   same 
  earth 
  with 
  w^hich 
  a 
  known 
  quantity 
  of 
  thorium 
  hydroxide 
  

   in 
  radioactive 
  equilibrium 
  has 
  been 
  intimately 
  mixed. 
  

  

  The 
  result 
  w^as 
  that 
  the 
  Roman 
  soil 
  must 
  contain 
  

   ca. 
  1*45 
  X 
  10~^ 
  gr. 
  of 
  thorium 
  per 
  gram. 
  It 
  is 
  evident 
  that 
  

   this 
  value 
  must 
  be 
  considered 
  as 
  a 
  miuimnm, 
  the 
  experiments 
  

   having 
  been 
  made 
  wdth 
  thorium 
  hydroxide, 
  i. 
  e. 
  with 
  the 
  

   compound 
  which 
  has 
  the 
  strongest 
  emanating 
  powder, 
  while 
  

   we 
  do 
  not 
  know 
  the 
  real 
  state 
  of 
  combination 
  in 
  which 
  

   thorium 
  is 
  contained 
  in 
  the 
  soil. 
  

  

  In 
  this 
  same 
  note 
  I 
  announced 
  that 
  I 
  had 
  undertaken 
  

   an 
  investigation 
  with 
  the 
  object 
  of 
  determining, 
  in 
  a 
  more 
  

   ])recise 
  and 
  satisfactory 
  manner, 
  the 
  proportion 
  of 
  thorium 
  

   contained 
  in 
  a 
  certain 
  number 
  of 
  rocks 
  of 
  different 
  nature 
  

   and 
  origin. 
  

  

  The 
  method 
  foUow^ed 
  in 
  this 
  second 
  investigation 
  (Rendic. 
  

   R. 
  Accad. 
  d^ 
  Lincei, 
  xviii. 
  1st 
  sem. 
  p. 
  241, 
  1909) 
  consisted 
  in 
  

   dissolving, 
  after 
  fusion 
  with 
  alkaline 
  carbonates, 
  about 
  200 
  gr. 
  

   of 
  each 
  sam[)le 
  of 
  rock, 
  and 
  collecting 
  the 
  insoluble 
  hydroxides 
  

   (after 
  having 
  freed 
  the 
  solutions 
  from 
  radium 
  by 
  separating 
  

   the 
  insoluble 
  sulphates). 
  The 
  hydroxides 
  so 
  obtained 
  showed 
  

   a 
  noticeable 
  activity 
  when 
  tested 
  by 
  the 
  electroscopic 
  method; 
  

   this 
  activity 
  increased 
  for 
  a 
  few 
  weeks, 
  until 
  a 
  maximum 
  was 
  

   reached. 
  

  

  This 
  activity 
  could 
  not 
  be 
  due 
  to 
  radium, 
  for 
  the 
  solutions 
  

   from 
  which 
  the 
  hydroxides 
  were 
  obtained 
  no 
  longer 
  gave 
  

   any 
  appreciable 
  trace 
  of 
  radium 
  emanation 
  after 
  the 
  insoluble 
  

   sulphates 
  had 
  been 
  separated 
  ; 
  the 
  test 
  was 
  made 
  according 
  

  

  * 
  Communicated 
  bv 
  J. 
  Joly, 
  F.R.S. 
  

  

  