﻿the 
  y 
  Rays 
  of 
  the 
  Thorium 
  and 
  Actinium 
  Products, 
  947 
  

  

  pointed 
  out, 
  there 
  is 
  an 
  undoubtedly 
  close 
  analogy 
  between 
  

   the 
  B 
  products, 
  not 
  only 
  in 
  the 
  types 
  of 
  radiation 
  but 
  also 
  

   in 
  the 
  relative 
  penetrating 
  power, 
  the 
  differences 
  in 
  their 
  

   penetrating 
  power 
  are 
  sufficiently 
  marked 
  to 
  indicate 
  a 
  real 
  

   difference 
  in 
  the 
  wave-length 
  of 
  the 
  radiation 
  emitted 
  and 
  

   one 
  which 
  cannot 
  be 
  attributed 
  to 
  experimental 
  error. 
  

  

  It 
  is 
  of 
  interest 
  to 
  examine 
  whether 
  a 
  comparison 
  of 
  the 
  

   types 
  and 
  penetrating 
  power 
  of 
  the 
  radiations 
  from 
  the 
  

   thorium 
  and 
  actinium 
  products 
  gives 
  any 
  indication 
  as 
  to 
  

   the 
  atomic 
  weight 
  of 
  the 
  latter. 
  It 
  has 
  already 
  been 
  

   mentioned 
  that 
  the 
  " 
  L 
  " 
  type 
  of 
  radiation 
  seems 
  to 
  be 
  pre- 
  

   dominant 
  for 
  most 
  of 
  the 
  products, 
  and 
  for 
  this 
  radiation 
  

   there 
  is 
  a 
  close 
  agreement 
  between 
  the 
  penetrating 
  powers 
  

   of 
  the 
  radiations 
  emitted 
  by 
  analogous 
  products. 
  For 
  

   example, 
  actinium 
  B 
  emits 
  a 
  radiation 
  for 
  which 
  /z 
  = 
  31_, 
  and 
  

   a 
  corresponding 
  radiation 
  yn=32 
  is 
  given 
  out 
  by 
  thorium 
  B. 
  

   The 
  radiation 
  ^ 
  = 
  25 
  from 
  radioactinium 
  also 
  resembles 
  

   closely 
  the 
  radiation 
  from 
  mesothorium 
  2, 
  yu, 
  = 
  26. 
  Con- 
  

   sidering 
  the 
  very 
  close 
  analogy 
  in 
  chemical 
  and 
  radioactive 
  

   properties 
  of 
  the 
  thorium 
  and 
  actinium 
  series, 
  it 
  is 
  reasonable 
  

   to 
  suppose 
  that 
  those 
  bodies 
  which 
  emit 
  similar 
  types 
  of 
  

   radiation 
  of 
  nearly 
  equal 
  penetrating 
  power 
  have 
  nearly 
  

   the 
  same 
  atomic 
  weight. 
  For 
  example, 
  by 
  considering 
  the 
  

   number 
  of 
  a-ray 
  products 
  it 
  can 
  be 
  estimated 
  that 
  thorium 
  B 
  

   has 
  an 
  atomic 
  weight 
  212. 
  We 
  should 
  expect 
  actinium 
  B 
  

   to 
  have 
  an 
  atomic 
  weight 
  nearly 
  the 
  same. 
  In 
  the 
  same 
  way 
  

   we 
  should 
  expect 
  radioactinium 
  and 
  mesothorium 
  2 
  to 
  have 
  

   nearly 
  equal 
  atomic 
  weights. 
  Calculating 
  backwards 
  by 
  a 
  

   consideration 
  of 
  the 
  number 
  of 
  a-ray 
  products, 
  it 
  can 
  be 
  

   deduced 
  that 
  the 
  atomic 
  weight 
  of 
  actinium 
  should 
  be 
  

   (1) 
  228, 
  from 
  comparison 
  of 
  the 
  B 
  products 
  ; 
  (2) 
  228 
  from 
  

   comparison 
  of 
  radioactinium 
  with 
  mesothorium 
  2. 
  

  

  It 
  is 
  quite 
  probable 
  from 
  other 
  analogies, 
  that 
  the 
  actual 
  

   atomic 
  weight 
  might 
  be 
  two 
  units 
  greater 
  or 
  less 
  than 
  the 
  

   above 
  value. 
  In 
  the 
  one 
  case, 
  the 
  atomic 
  weight 
  230 
  would 
  

   indicate 
  that 
  actinium 
  was 
  derived 
  from 
  uranium 
  after 
  the 
  

   expulsion 
  of 
  two 
  « 
  particles, 
  and 
  the 
  other 
  that 
  it 
  arose 
  from 
  

   ionium. 
  On 
  this 
  point 
  of 
  view 
  radium 
  itself 
  does 
  not 
  seem 
  

   admissible 
  as 
  a 
  possible 
  origin 
  of 
  actinium, 
  and 
  this 
  is 
  

   supported 
  by 
  the 
  recent 
  experiments 
  of 
  Soddy*". 
  The 
  de- 
  

   duction 
  that 
  actinium 
  is 
  derived 
  from 
  uranium 
  seems 
  the 
  

   more 
  probable, 
  and 
  several 
  suggestions 
  of 
  this 
  possibility 
  

   have 
  already 
  been 
  made 
  -f. 
  

  

  * 
  Soddy, 
  Nature, 
  xci. 
  p. 
  634 
  (1913). 
  

  

  t 
  Boltwood, 
  Amer. 
  Journ. 
  Sci. 
  xxv. 
  p. 
  269 
  (1908). 
  Hahn 
  & 
  Meitner, 
  

   Phys. 
  Zeit. 
  xvi. 
  p. 
  752 
  (1913). 
  Antonoff, 
  Phil. 
  Mag. 
  xxii. 
  p. 
  419 
  (1911). 
  

  

  