﻿948 
  Messrs. 
  Marsden 
  and 
  Wood 
  on 
  the 
  Determination 
  of 
  

  

  The 
  other 
  more 
  penetrating 
  types 
  of 
  radiation 
  from 
  the 
  

   actinium 
  and 
  thorium 
  products 
  do 
  not 
  appear 
  to 
  be 
  directly 
  

   comparable. 
  For 
  example, 
  the 
  hard 
  radiations 
  from 
  thorium 
  

   D 
  and 
  actinium 
  D 
  show 
  wide 
  differences 
  in 
  penetrating 
  

   power. 
  

  

  When 
  the 
  types 
  of 
  7 
  radiation 
  emitted 
  not 
  only 
  by 
  all 
  

   /3-ray 
  products 
  but 
  also 
  by 
  all 
  a-ray 
  products 
  are 
  known, 
  

   very 
  valuable 
  data 
  will 
  have 
  been 
  obtained 
  for 
  throwing 
  

   light 
  not 
  only 
  on 
  the 
  modes 
  of 
  vibration 
  of 
  the 
  radioactive 
  

   elements 
  but 
  also 
  on 
  their 
  atomic 
  constitution. 
  

  

  University 
  of 
  Manchester, 
  

   October, 
  1913. 
  

  

  LXXXIII. 
  A 
  Method 
  for 
  the 
  Determination 
  of 
  the 
  Molecular 
  

   Weights 
  of 
  the 
  Radioactive 
  Emanations 
  with 
  Application 
  to 
  

   Actinium 
  Emanation. 
  By 
  E. 
  Maksden, 
  M.Sc.^ 
  and 
  A. 
  B. 
  

   WOOD, 
  M.Sc, 
  University 
  of 
  Manchester 
  *. 
  

  

  IN 
  all 
  the 
  methods 
  previously 
  employed 
  to 
  determine 
  the 
  

   molecular 
  weights 
  of 
  the 
  emanations 
  from 
  their 
  rates 
  of 
  

   diffusion 
  or 
  effusion 
  f 
  , 
  the 
  results 
  have 
  been 
  dependent 
  on 
  a 
  

   comparison 
  of 
  the 
  corresponding 
  rate 
  for 
  some 
  other 
  gas. 
  

   Further, 
  the 
  measurements 
  in 
  most 
  cases 
  are 
  undoubtedly 
  

   affected 
  by 
  the 
  fact 
  that 
  it 
  is 
  impossible 
  to 
  deal 
  in 
  this 
  way 
  

   with 
  the 
  emanations 
  except 
  when 
  mixed 
  in 
  extremely 
  small 
  

   proportion 
  with 
  other 
  gases. 
  In 
  the 
  method 
  to 
  be 
  described 
  

   both 
  these 
  objections 
  are 
  avoided 
  by 
  working 
  at 
  such 
  low 
  

   pressures 
  that 
  the 
  molecules 
  have 
  a 
  mean 
  free 
  path 
  large 
  in 
  

   comparison 
  with 
  the 
  size 
  of 
  the 
  containing 
  vessel, 
  and 
  by 
  

   using 
  definite 
  kinetic 
  theory 
  data 
  not 
  involving 
  comparison 
  

   with 
  other 
  gases. 
  

  

  Suppose 
  we 
  have 
  two 
  vessels 
  I. 
  and 
  II. 
  of 
  volumes 
  V 
  1 
  and 
  V 
  2 
  

   separated 
  by 
  a 
  hole 
  of 
  area 
  A 
  and 
  containing 
  N 
  x 
  and 
  N 
  2 
  

   molecules 
  per 
  c.c. 
  respectively. 
  Then 
  if 
  fl 
  is 
  the 
  mean 
  

   molecular 
  velocity, 
  it 
  can 
  be 
  calculated 
  from 
  the 
  kinetic 
  

   theory 
  that 
  the 
  number 
  of 
  molecules 
  from 
  I. 
  passing 
  through 
  

   the 
  'opening 
  will 
  be 
  J 
  NxOAJ, 
  and 
  the 
  number 
  returning 
  

   from 
  IT. 
  will 
  be 
  ^N 
  2 
  12A. 
  Consequently, 
  on 
  the 
  whole 
  

   JflA(N 
  1 
  — 
  X 
  2 
  ) 
  molecules 
  will 
  cross 
  from 
  I. 
  to 
  II. 
  per 
  second. 
  

  

  Suppose 
  that 
  molecules 
  of 
  actinium 
  emanation 
  are 
  supplied 
  

   at 
  a 
  constant 
  rate 
  in 
  I. 
  and 
  allowed 
  to 
  pass 
  into 
  II. 
  Then 
  a 
  

  

  * 
  Communicated 
  by 
  Prof. 
  E. 
  Rutherford, 
  E.R.S. 
  

  

  t 
  E. 
  Rutherford, 
  " 
  Radioactive 
  Substances," 
  p. 
  379 
  et 
  seq. 
  

  

  X 
  M. 
  Knudsen, 
  Ann. 
  d. 
  Physik, 
  xxviii. 
  p. 
  999 
  (1909). 
  

  

  