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  XLYI. 
  Attempts 
  to 
  detect 
  the 
  Production 
  of 
  Helium 
  from 
  the 
  

   Primary 
  Radio- 
  Elements. 
  By 
  Frederick 
  Soddy, 
  M. 
  A* 
  

  

  [Plate 
  XX.] 
  

  

  THE 
  question 
  whether 
  helium 
  is 
  a 
  product 
  of 
  the 
  radio- 
  

   active 
  changes 
  of 
  uranium 
  and 
  thorium 
  has 
  proved 
  an 
  

   extremely 
  difficult 
  one 
  to 
  settle 
  experimentally. 
  The 
  work 
  

   here 
  described 
  was 
  commenced 
  in 
  1905, 
  and 
  the 
  results 
  ob- 
  

   tained, 
  though 
  by 
  themselves 
  of 
  no 
  significance 
  as 
  yet, 
  appear 
  

   to 
  warrant 
  the 
  hope 
  that 
  all 
  difficulties 
  have 
  been 
  overcome, 
  

   and 
  now 
  only 
  a 
  sufficiently 
  long 
  period 
  of 
  time 
  and 
  periodic 
  

   testing 
  are 
  required 
  to 
  solve 
  the 
  problem. 
  The 
  quantities 
  of 
  

   helium 
  that 
  have 
  been 
  looked 
  for 
  throughout 
  are 
  of 
  the 
  order 
  

   of 
  a 
  millionth 
  part 
  of 
  a 
  cubic 
  centimetre, 
  which 
  is 
  the 
  amount 
  

   present 
  in 
  only 
  a 
  fraction 
  of 
  a 
  cubic 
  centimetre 
  of 
  atmospheric 
  

   air. 
  From 
  the 
  first 
  it 
  was 
  recognized 
  that 
  the 
  difficulties 
  in 
  

   the 
  way 
  of 
  ensuring 
  that 
  the 
  observations 
  had 
  a 
  real 
  sig- 
  

   nificance 
  were 
  so 
  formidable, 
  that 
  mere 
  qualitative 
  detection 
  

   or 
  non-detection 
  of 
  helium, 
  however 
  definite 
  or 
  frequently 
  

   repeated, 
  would 
  not 
  of 
  itself 
  suffice 
  to 
  settle 
  the 
  question 
  

   beyond 
  all 
  doubt. 
  The 
  objects 
  of 
  the 
  experiments 
  have 
  been 
  

   to 
  obtain 
  a 
  quantitative 
  estimate 
  by 
  the 
  spectroscope 
  of 
  the 
  

   amount 
  of 
  helium 
  produced 
  from 
  uranium 
  and 
  thorium, 
  or 
  in 
  

   the 
  event 
  of 
  negative 
  results 
  to 
  fix 
  a 
  definite 
  limit 
  to 
  the 
  

   quantity 
  of 
  helium 
  that 
  could 
  not 
  fail 
  to 
  have 
  been 
  detected 
  

   under 
  the 
  conditions. 
  To 
  be 
  quite 
  sure, 
  it 
  is 
  necessary 
  to 
  

   carry 
  out 
  a 
  long 
  series 
  of 
  consistent 
  quantitative 
  experiments, 
  

   in 
  which 
  helium 
  is 
  detected 
  with 
  regularity 
  when 
  the 
  time 
  

   of 
  accumulation 
  is 
  above, 
  and 
  not 
  detected 
  when 
  the 
  time 
  of 
  

   accumulation 
  is 
  below 
  a 
  certain 
  period. 
  These 
  are 
  only 
  

   now 
  being 
  commenced. 
  Whether 
  the 
  necessary 
  certainty 
  

   and 
  perfection 
  have 
  been 
  achieved 
  for 
  the 
  results 
  to 
  bear 
  

   a 
  significant 
  interpretation, 
  only 
  the 
  results 
  of 
  future 
  tests 
  

   will 
  disclose. 
  All 
  that 
  can 
  now 
  be 
  claimed 
  is 
  that 
  a 
  

   beginning 
  has 
  been 
  made. 
  

  

  Theoretical 
  Pate 
  of 
  Production 
  of 
  Helium. 
  

  

  At 
  the 
  commencement 
  of 
  the 
  work 
  radioactive 
  theory 
  

   afforded 
  only 
  a 
  halting 
  and 
  as 
  it 
  now 
  seems 
  deceptive 
  guide 
  

   to 
  the 
  order 
  of 
  the 
  amount 
  of 
  helium 
  to 
  be 
  looked 
  for 
  in 
  

   experiments 
  of 
  this 
  character. 
  The 
  calculations 
  were 
  so 
  

   indirect 
  and 
  so 
  imperfect 
  that 
  it 
  was 
  hardly 
  possible 
  to 
  say 
  

   as 
  the 
  result 
  of 
  any 
  given 
  experiment 
  that 
  it 
  definitely 
  

   conflicted 
  with 
  what 
  is 
  theoretically 
  to 
  be 
  expected* 
  The 
  

  

  * 
  Communicated 
  by 
  the 
  Author. 
  

   Phil 
  Mag. 
  S. 
  6. 
  Vol. 
  16. 
  No. 
  94. 
  Oct. 
  1908. 
  2 
  M 
  

  

  