﻿Helium 
  from 
  the 
  Primary 
  Radio-Elements. 
  529 
  

  

  of 
  helium 
  required 
  to 
  show 
  the 
  different 
  lines. 
  It 
  may 
  be 
  

   inferred 
  that, 
  so 
  long 
  as 
  a 
  discharge 
  will 
  pass 
  at 
  all, 
  D 
  3 
  will 
  

   be 
  seen 
  if 
  the 
  minimum 
  quantity 
  of 
  helium 
  is 
  present, 
  however 
  

   little 
  other 
  gas 
  is 
  present. 
  

  

  It 
  is 
  of 
  interest 
  to 
  compare 
  this 
  quantity 
  of 
  two 
  tenth-grams 
  

   of 
  helium 
  found 
  to 
  be 
  the 
  minimum 
  quantity 
  detectable 
  by 
  

   the 
  ( 
  spectroscope, 
  with 
  the 
  minimum 
  quantity 
  of 
  radium, 
  for 
  

   example, 
  detectable 
  by 
  radioactive 
  methods. 
  Using 
  the 
  ema- 
  

   nation 
  of 
  radium 
  for 
  the 
  test, 
  about 
  10 
  -11 
  gram 
  is 
  about 
  the 
  

   smallest 
  detectable 
  quantity, 
  though 
  no 
  doubt 
  with 
  some 
  

   refinements 
  this 
  could 
  be 
  with 
  certainty 
  reduced. 
  The 
  radio- 
  

   active 
  method 
  in 
  the 
  case 
  of 
  radium 
  is 
  thus 
  about 
  20 
  times 
  

   more 
  sensitive 
  than 
  the 
  spectroscopic 
  in 
  the 
  case 
  of 
  helium, 
  a 
  

   comparison 
  much 
  more 
  in 
  favour 
  of 
  the 
  older 
  method 
  than 
  

   might 
  at 
  first 
  have 
  been 
  anticipated. 
  

  

  Results 
  with 
  Thorium. 
  

  

  In 
  addition 
  to 
  the 
  calibration 
  tests 
  already 
  described, 
  in 
  all 
  

   about 
  30 
  tests, 
  involving 
  the 
  filling 
  of 
  the 
  apparatus 
  with 
  

   mercury 
  have 
  been 
  carried 
  out, 
  but 
  only 
  a 
  few 
  of 
  these 
  have 
  

   been 
  successful, 
  and 
  the 
  results 
  so 
  far 
  obtained 
  are 
  regarded 
  

   merely 
  as 
  preliminary. 
  Now, 
  however, 
  with 
  greater 
  expe- 
  

   rience, 
  comparative 
  certainty 
  and 
  immunity 
  from 
  failure 
  have 
  

   been 
  secured. 
  In 
  the 
  third 
  series 
  of 
  experiments, 
  which 
  

   remain 
  to 
  be 
  considered, 
  four 
  flasks 
  have 
  been 
  set 
  up, 
  con- 
  

   taining 
  thorium 
  nitrate, 
  uranium 
  nitrate, 
  pitchblende 
  in 
  nitric 
  

   acid 
  solution, 
  and 
  sodium 
  sulphate. 
  The 
  latter 
  is 
  intended 
  

   partly 
  as 
  a 
  blank 
  experiment. 
  Only 
  the 
  thorium 
  experiment 
  

   will 
  be 
  detailed 
  in 
  this 
  paper, 
  as 
  the 
  others 
  have 
  not 
  yet 
  

   been 
  under 
  observation 
  a 
  sufficient 
  period 
  of 
  time. 
  

  

  The 
  flask 
  contains 
  a 
  solution 
  of 
  ordinary 
  commercial 
  thorium 
  

   nitrate, 
  several 
  years 
  old, 
  corresponding 
  to 
  about 
  350 
  grams 
  

   of 
  the 
  element 
  thorium. 
  The 
  first 
  test 
  was 
  a 
  failure, 
  and 
  

   the 
  second 
  was 
  rendered 
  ambiguous 
  owing 
  to 
  a 
  trace 
  of 
  

   mercury 
  entering 
  the 
  spectrum-tube. 
  The 
  third 
  test 
  was 
  

   carried 
  out 
  successfully 
  after 
  a 
  period 
  of 
  accumulation 
  of 
  

   35 
  days. 
  The 
  absorption 
  of 
  the 
  gas 
  was 
  only 
  moderately 
  

   perfect, 
  but 
  no 
  indication 
  of 
  D 
  3 
  was 
  observed 
  at 
  any 
  stage 
  

   of 
  the 
  experiment. 
  The 
  subsidiary 
  volume 
  J 
  before 
  referred 
  

   to 
  was 
  not 
  used 
  in 
  this 
  experiment, 
  so 
  that 
  only 
  about 
  one 
  half 
  

   of 
  the 
  gas 
  would 
  have 
  been 
  obtained 
  for 
  the 
  test. 
  The 
  test 
  

   therefore 
  indicates 
  that 
  the 
  rate 
  of 
  production 
  of 
  helium 
  from 
  

   thorium 
  is 
  not 
  greater 
  than 
  10" 
  11 
  (year) 
  -1 
  , 
  which 
  is 
  in 
  agree- 
  

   ment 
  with 
  the 
  theory 
  given. 
  

  

  In 
  the 
  next 
  test 
  the 
  period 
  of 
  accumulation 
  was 
  203 
  days, 
  or 
  

   about 
  six 
  times 
  the 
  preceding. 
  As 
  before, 
  only 
  about 
  one-half 
  

  

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

  

  