﻿514 
  Mr. 
  F. 
  Soddy 
  on 
  the 
  Production 
  oj 
  

  

  theoretical 
  estimates 
  formed 
  from 
  now 
  available 
  data 
  are 
  

   quite 
  different 
  from 
  what 
  were 
  formed 
  when 
  the 
  work 
  was 
  

   commenced, 
  and 
  the 
  difference 
  between 
  the 
  older 
  and 
  the 
  

   newer 
  estimates 
  indicated 
  by 
  theory 
  have 
  steadily 
  widened 
  

   as 
  the 
  new 
  data 
  have 
  been 
  accumulated, 
  Had 
  the 
  present 
  

   data 
  been 
  available, 
  the 
  task 
  would 
  have 
  appeared 
  even 
  more 
  

   hopeless 
  to 
  attempt 
  than 
  it 
  did 
  at 
  first, 
  for 
  the 
  theoretical 
  

   estimates 
  of 
  the 
  rate 
  of 
  disintegration 
  of 
  the 
  primary 
  radio- 
  

   elements 
  have 
  steadily 
  decreased. 
  Thus 
  Professor 
  Rutherford 
  

   gave 
  for 
  the 
  radioactive 
  constant 
  of 
  uranium, 
  in 
  1905, 
  

   I0 
  -9 
  (year) 
  -1 
  (' 
  Radioactivity,' 
  2nd 
  edition, 
  p. 
  458), 
  and 
  in 
  

   1906 
  2 
  X 
  10- 
  10 
  (year)" 
  1 
  (' 
  Radioactive 
  Transformations,' 
  

   p. 
  158), 
  while 
  the 
  present 
  value, 
  calculated 
  by 
  the 
  same 
  

   method 
  as 
  the 
  last, 
  from 
  more 
  recent 
  data, 
  has 
  again 
  been 
  

   lowered 
  to 
  l*2xl0~ 
  10 
  . 
  Considering 
  the 
  nature 
  of 
  the 
  

   quantity, 
  the 
  alteration 
  does 
  not 
  appear 
  very 
  vital, 
  but 
  for 
  

   the 
  purposes 
  of 
  the 
  present 
  work 
  the 
  difference 
  between 
  the 
  

   first 
  and 
  last 
  value 
  is 
  just 
  the 
  difference 
  between 
  waiting 
  

   six 
  weeks 
  and 
  waiting 
  a 
  year 
  between 
  tests 
  before 
  sufficient 
  

   helium 
  to 
  be 
  detected 
  accumulates. 
  The 
  first 
  is 
  practicable, 
  

   the 
  second 
  is 
  hardly 
  so. 
  

  

  While, 
  however, 
  the 
  quantitative 
  estimates 
  of 
  the 
  rate 
  of 
  

   production 
  of 
  helium 
  have 
  decreased, 
  the 
  'general 
  progress 
  

   of 
  radioactivity 
  since 
  the 
  commencement 
  of 
  the 
  work 
  has 
  

   increased 
  the 
  probability 
  that 
  helium 
  is 
  a 
  product 
  of 
  uranium 
  

   and 
  thorium, 
  and 
  indeed 
  of 
  all 
  radio-elements 
  producing 
  

   a-rays, 
  to 
  such 
  an 
  extent 
  that 
  it 
  is 
  difficult 
  at 
  the 
  present 
  

   time 
  to 
  maintain 
  the 
  opposite 
  view. 
  The 
  chief 
  evidence, 
  of 
  

   course, 
  is 
  derived 
  from 
  the 
  elucidation 
  of 
  the 
  nature 
  of 
  the 
  

   a-rays 
  by 
  Rutherford 
  ; 
  but 
  strong 
  support 
  is 
  also 
  given 
  by 
  

   the 
  way 
  in 
  which 
  the 
  atomic 
  weights 
  of 
  uranium, 
  radium, 
  and 
  

   polonium, 
  the 
  latter 
  estimated 
  from 
  its 
  probable 
  position 
  in 
  

   the 
  periodic 
  table, 
  conform 
  to 
  the 
  simple 
  supposition 
  that 
  

   each 
  a-ray-producing 
  change 
  results 
  in 
  a 
  reduction 
  of 
  the 
  

   atomic 
  weight 
  by 
  four 
  units. 
  

  

  It 
  would 
  scarcely 
  be 
  profitable 
  to 
  discuss 
  all 
  the 
  possi- 
  

   bilities 
  that 
  have 
  to 
  be 
  taken 
  into 
  consideration 
  in 
  attempting 
  

   to 
  arrive 
  at 
  the 
  upper 
  and 
  lower 
  limits 
  of 
  the 
  rate 
  of 
  pro- 
  

   duction 
  of 
  helium 
  indicated 
  by 
  theory. 
  It 
  is 
  better 
  to 
  make 
  

   the 
  most 
  probable 
  assumptions 
  and 
  to 
  consider 
  in 
  which 
  

   direction 
  the 
  rate 
  would 
  be 
  affected 
  if 
  these 
  assumptions 
  are 
  

   altered. 
  

  

  The 
  value 
  1*2 
  x 
  10 
  -10 
  (year) 
  -1 
  , 
  just 
  referred 
  to, 
  is 
  arrived 
  

   at 
  by 
  the 
  application 
  of 
  the 
  disintegration 
  theory 
  in 
  its 
  

   simplest 
  form, 
  from 
  the 
  ratio 
  of 
  the 
  quantities 
  of 
  uranium 
  to 
  

   radium 
  in 
  minerals 
  and 
  the 
  rate 
  of 
  disintegration 
  of 
  radium. 
  

  

  