﻿Molecular 
  Weights 
  of 
  Radioactive 
  Emanations. 
  951 
  

  

  •004075 
  sq. 
  cm. 
  area. 
  The 
  mean 
  molecular 
  weight 
  calcu- 
  

   lated 
  from 
  the 
  result 
  with 
  this 
  hole 
  was 
  228. 
  The 
  difference 
  

   between 
  this 
  and 
  the 
  former 
  value 
  is 
  well 
  within 
  the 
  experi- 
  

   mental 
  error, 
  considering 
  that 
  the 
  calculated 
  value 
  depends 
  

   on 
  the 
  squares 
  of 
  A 
  and 
  (N^^— 
  Ij. 
  

  

  The 
  chief 
  importance 
  of 
  a 
  measurement 
  of 
  the 
  atomic 
  

   weight 
  of 
  actinium 
  emanation 
  is 
  that 
  it 
  allows 
  the 
  atomic 
  

   weight 
  of 
  actinium 
  itself 
  to 
  be 
  estimated, 
  and 
  thus 
  should 
  

   throw 
  light 
  on 
  the 
  question 
  of 
  the 
  origin 
  of 
  actinium. 
  The 
  

   experiments 
  of 
  Boltwood 
  * 
  show 
  that 
  the 
  amount 
  of 
  actinium, 
  

   measured 
  by 
  its 
  activity, 
  present 
  in 
  uranium 
  minerals, 
  is 
  

   approximately 
  proportional 
  to 
  the 
  percentage 
  of 
  uranium. 
  

   This 
  indicates 
  that 
  actinium, 
  like 
  radium, 
  must 
  belong 
  to 
  the 
  

   uranium 
  series 
  and 
  stand 
  in 
  some 
  genetic 
  relation 
  with 
  it. 
  

   From 
  the 
  relative 
  activities 
  of 
  the 
  actinium 
  and 
  uranium 
  

   present, 
  however, 
  Rutherford 
  f 
  has 
  concluded 
  that 
  actinium 
  

   must 
  arise 
  as 
  a 
  branch 
  product 
  at 
  some 
  point 
  of 
  the 
  series. 
  

   and 
  calculates 
  the 
  ratio 
  of 
  the 
  number 
  of 
  atoms 
  on 
  the 
  

   branch 
  to 
  the 
  number 
  on 
  the 
  main 
  family 
  to 
  be 
  '08. 
  Three 
  

   possible 
  products 
  of 
  the 
  series 
  have 
  been 
  suggested 
  as 
  the 
  

   parent 
  of 
  actinium, 
  namely 
  uranium 
  X, 
  radium 
  itself, 
  and 
  

   radium 
  C 
  J. 
  The 
  evidence 
  with 
  regard 
  to 
  the 
  former 
  is 
  

   purely 
  theoretical 
  and 
  is 
  derived 
  from 
  considerations 
  of 
  the 
  

   valency 
  of 
  actinium. 
  The 
  evidence 
  as 
  to 
  a 
  branch 
  product 
  

   in 
  radium 
  is 
  obtained 
  from 
  the 
  presence 
  of 
  a 
  very 
  feeble 
  soft 
  

   /5 
  radiation 
  in 
  that 
  product, 
  which 
  may 
  be 
  presumed 
  to 
  come 
  

   from 
  the 
  atoms 
  giving 
  rise 
  to 
  the 
  branch. 
  With 
  regard 
  to 
  

   radium 
  C, 
  the 
  fraction 
  of 
  atoms 
  on 
  the 
  radium 
  C 
  2 
  branch 
  

   appears 
  to 
  be 
  only 
  of 
  the 
  order 
  3/10000, 
  and 
  further, 
  Fajans 
  

   failed 
  to 
  detect 
  actinium 
  in 
  the 
  active 
  deposit 
  of 
  radium. 
  A 
  

   further 
  possible 
  source 
  of 
  actinium 
  is 
  that 
  it 
  may 
  be 
  derived 
  

   from 
  the 
  branch 
  product 
  Ur 
  Y, 
  found 
  by 
  Antonoff 
  §. 
  The 
  

   existence 
  of 
  this 
  product 
  has, 
  however, 
  recently 
  been 
  ques- 
  

   tioned 
  by 
  Fleck 
  || 
  . 
  

  

  Taking 
  the 
  mean 
  value 
  232 
  obtained 
  above 
  as 
  the 
  atomic 
  

   weight 
  of 
  actinium 
  emanation 
  and 
  adding 
  8 
  units 
  for 
  the 
  

   a 
  particles 
  of 
  radio-actinium 
  and 
  actinium 
  X, 
  we 
  obtain 
  a 
  

   value 
  210 
  for 
  the 
  atomic 
  weight 
  of 
  actinium. 
  This 
  would 
  

   place 
  actinium 
  just 
  outside 
  the 
  uranium 
  series, 
  as 
  the 
  atomic 
  

   weight 
  of 
  uranium 
  is 
  only 
  238*5. 
  

  

  * 
  Boltwood, 
  Amer. 
  Journ. 
  Sci. 
  xxv. 
  p. 
  269 
  (1908). 
  

  

  t 
  Cf. 
  ' 
  Radioactive 
  Substances,' 
  p. 
  522. 
  

  

  % 
  Hahn 
  & 
  Meitner, 
  Phys. 
  Zeit. 
  xiv. 
  p. 
  758 
  (1913) 
  ; 
  Soddv, 
  Chera. 
  

   News, 
  cvii. 
  p. 
  97 
  (1913) 
  ; 
  Fleck, 
  Phil. 
  Mag. 
  xxvi. 
  p. 
  531 
  (1913)"; 
  Fajans 
  

   & 
  Gohring, 
  Phys. 
  Zeit 
  xiv. 
  p. 
  679 
  (1913). 
  

  

  § 
  Antonoff, 
  Phil. 
  Mag. 
  xxii. 
  p. 
  431 
  (1911) 
  ; 
  xxvi. 
  p. 
  332 
  (1913). 
  

  

  || 
  Fleck, 
  Phil. 
  Mag. 
  xxv. 
  p. 
  710 
  (1913). 
  

  

  