﻿Helium 
  from 
  the 
  Primary 
  Radio-Elements. 
  517 
  

  

  there 
  are 
  four 
  a-ray 
  changes 
  known 
  in 
  the 
  actinium 
  series, 
  

   these 
  results 
  indicate 
  that 
  out 
  of 
  eight 
  atoms 
  of 
  uranium 
  

   disintegrating, 
  seven 
  go 
  to 
  produce 
  radium 
  and 
  one 
  to 
  

   produce 
  actinium. 
  With 
  these 
  assumptions 
  the 
  direct 
  and 
  

   indirect 
  values 
  for 
  the 
  rate 
  o£ 
  disintegration 
  o£ 
  uranium 
  

   become 
  consistent 
  and 
  give 
  the 
  same 
  value 
  2*7 
  X 
  10~ 
  10 
  (year) 
  -1 
  . 
  

   Thus 
  4*5 
  x 
  10" 
  12 
  (year) 
  -1 
  may 
  be 
  taken 
  as 
  the 
  more 
  probable 
  

   value 
  in 
  both 
  cases, 
  while 
  for 
  pitchblende, 
  assuming 
  that 
  all 
  

   the 
  helium 
  formed 
  comes 
  into 
  evidence 
  as 
  «-rays, 
  the 
  more 
  

   probable 
  value 
  is 
  four 
  times 
  that 
  of 
  the 
  uranium 
  it 
  contains. 
  

   The 
  smallest 
  possible 
  quantity 
  of 
  helium 
  that 
  can 
  be 
  detected 
  

   under 
  present 
  conditions 
  will 
  be 
  shown 
  to 
  be 
  about 
  a 
  millionth 
  

   of 
  a 
  cubic 
  centimetre, 
  or 
  1*8 
  x 
  10 
  -10 
  gram. 
  That 
  is 
  to 
  say, 
  

   a 
  gram 
  of 
  uranium 
  or 
  thorium 
  should 
  produce 
  a 
  just 
  de- 
  

   tectable 
  amount 
  of 
  helium 
  in 
  a 
  period 
  of 
  40 
  years, 
  while 
  

   for 
  a 
  quantity 
  of 
  pitchblende 
  containing 
  a 
  gram 
  of 
  uranium 
  

   and 
  all 
  its 
  products 
  the 
  same 
  should 
  occur 
  in 
  10 
  years. 
  

   Even 
  with 
  considerable 
  quantities 
  of 
  material 
  the 
  accumu- 
  

   lation 
  of 
  results 
  must 
  therefore 
  be 
  necessarily 
  a 
  slow 
  process. 
  

   So 
  far 
  it 
  has 
  not 
  been 
  practicable 
  to 
  deal 
  with 
  quantities 
  of 
  

   material 
  containing 
  more 
  than 
  a 
  third 
  of 
  a 
  kilogram 
  of 
  the 
  

   element, 
  so 
  that 
  on 
  the 
  new 
  data 
  it 
  is 
  to 
  be 
  expected 
  that 
  the 
  

   minimum 
  period 
  of 
  accumulation 
  to 
  give 
  a 
  positive 
  result 
  

   with 
  uranium 
  or 
  thorium 
  must 
  be 
  considerably 
  over 
  a 
  

   month, 
  on 
  the 
  indirect 
  value, 
  or 
  three 
  months 
  on 
  the 
  direct 
  

   value. 
  

  

  In 
  absence 
  of 
  direct 
  experiments 
  the 
  question 
  has 
  been 
  

   attacked 
  indirectly, 
  with 
  conflicting 
  conclusions 
  by 
  different 
  

   observers, 
  from 
  the 
  association 
  of 
  helium 
  with 
  uranium 
  and 
  

   thorium 
  in 
  minerals. 
  Boltwood 
  (Am. 
  Journ. 
  Sci. 
  1907, 
  

   xxiii. 
  p. 
  77) 
  considers 
  that 
  helium 
  is 
  probably 
  not 
  a 
  product 
  

   of 
  thorium 
  disintegration, 
  and 
  that, 
  even 
  in 
  thorianite, 
  the 
  

   helium 
  can 
  be 
  accounted 
  for 
  by 
  the 
  disintegration 
  of 
  the 
  

   uranium 
  series 
  alone. 
  This 
  would 
  of 
  course 
  involve 
  the 
  

   abandonment 
  of 
  the 
  view 
  that 
  the 
  a-particle 
  is 
  an 
  atom 
  of 
  

   helium 
  and 
  of 
  almost 
  all 
  the 
  preceding 
  theoretical 
  calculations. 
  

   The 
  only 
  datum 
  that 
  would 
  remain 
  would 
  be 
  that 
  from 
  the 
  

   " 
  direct 
  value 
  " 
  of 
  the 
  rate 
  of 
  disintegration 
  of 
  uranium, 
  

   namely, 
  that 
  the 
  helium 
  production 
  is 
  N 
  x 
  2.10 
  -12 
  (year) 
  -1 
  , 
  

   where 
  N 
  is 
  the 
  number 
  of 
  atoms 
  of 
  helium 
  formed 
  from 
  one 
  

   atom 
  of 
  uranium, 
  and 
  this 
  would 
  be 
  subject 
  to 
  an 
  increase 
  

   should 
  branch 
  series, 
  such 
  as 
  actinium, 
  occur. 
  Strutt 
  (com- 
  

   pare 
  Proc. 
  Roy. 
  Soc. 
  1908, 
  80 
  A, 
  p. 
  572), 
  on 
  the 
  other 
  hand, 
  

   has 
  advocated 
  the 
  other 
  view. 
  He 
  regards 
  the 
  existence 
  of 
  

   helium 
  in 
  a 
  specimen 
  of 
  fluor 
  from 
  Ivitgut, 
  Greenland, 
  

   examined 
  by 
  Prof. 
  Julius 
  Thomson, 
  which 
  contains 
  thorium 
  

  

  