﻿164 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1929 
  

  

  it 
  receives 
  from 
  the 
  sun, 
  and 
  its 
  mass 
  continually 
  diminishes. 
  Accord- 
  

   ing 
  to 
  Jeffreys 
  ^ 
  the 
  outward 
  flow 
  of 
  radiation 
  just 
  inside 
  the 
  earth's 
  

   surface 
  is 
  about 
  1.9 
  X 
  10-^ 
  calorie 
  per 
  sq. 
  cm. 
  per 
  second, 
  all 
  but 
  about 
  

   13 
  per 
  cent 
  of 
  which 
  arises 
  from 
  radioactive 
  disintegration 
  of 
  the 
  sub- 
  

   stance 
  of 
  the 
  earth. 
  We 
  can 
  calculate 
  from 
  this 
  that 
  radioactive 
  dis- 
  

   integration 
  causes 
  the 
  earth's 
  mass 
  to 
  diminish 
  at 
  the 
  rate 
  of 
  rather 
  

   less 
  than 
  an 
  ounce 
  a 
  minute; 
  at 
  this 
  rate, 
  terrestrial 
  atoms 
  are 
  unbot- 
  

   tling 
  their 
  energy 
  and 
  pouring 
  it 
  into 
  space 
  in 
  the 
  form 
  of 
  radiation. 
  

   On 
  earth 
  at 
  least 
  the 
  stream 
  flows 
  ever 
  in 
  the 
  same 
  direction; 
  complex 
  

   atoms 
  giving 
  place 
  to 
  simple, 
  and 
  mass 
  changing 
  into 
  radiation. 
  It 
  

   is 
  natural 
  to 
  ask 
  whether 
  a 
  study 
  of 
  the 
  physics 
  of 
  the 
  universe 
  reveals 
  

   these 
  processes 
  as 
  part 
  only 
  of 
  a 
  closed 
  cycle, 
  so 
  that 
  the 
  wastage 
  

   which 
  we 
  see 
  in 
  progress 
  on 
  earth 
  is 
  made 
  good 
  elsewhere. 
  We 
  stand 
  

   on 
  the 
  banks 
  of 
  a 
  river 
  and 
  watch 
  its 
  current 
  ever 
  carrying 
  water 
  out 
  

   to 
  sea, 
  but 
  we 
  know 
  that 
  this 
  water 
  is 
  in 
  due 
  course 
  transformed 
  into 
  

   clouds 
  and 
  rain 
  which 
  replenish 
  the 
  river. 
  Is 
  the 
  physical 
  universe 
  a 
  

   similar 
  cyclic 
  system, 
  or 
  ought 
  we 
  rather 
  to 
  compare 
  it 
  to 
  a 
  stream 
  

   which, 
  having 
  no 
  source 
  of 
  replenishment, 
  must 
  cease 
  flowing 
  after 
  it 
  

   has 
  spent 
  itself? 
  To 
  answer 
  these 
  questions 
  we 
  must 
  attempt 
  first 
  to 
  

   trace 
  our 
  terrestrial 
  stream 
  back 
  to 
  its 
  source. 
  

  

  THE 
  ORIGIN 
  OF 
  TERRESTRIAL 
  RADIUM 
  AND 
  URANIUM 
  

  

  Radioactive 
  atoms 
  are 
  of 
  many 
  kinds, 
  but 
  all 
  have 
  in 
  common 
  the 
  

   property 
  of 
  spontaneous 
  disintegration. 
  The 
  period 
  of 
  time 
  required 
  

   for 
  this 
  disintegration 
  to 
  occur 
  varies 
  enormously, 
  some 
  types 
  of 
  atoms 
  

   having 
  long 
  lives 
  of 
  thousands 
  of 
  miUions 
  of 
  years, 
  while 
  others 
  have 
  

   short 
  lives 
  of 
  years, 
  days, 
  hours, 
  or 
  seconds, 
  the 
  most 
  ephemeral 
  of 
  all 
  

   being 
  actinium-A, 
  with 
  an 
  average 
  life 
  of 
  only 
  0.002 
  second. 
  Let 
  us 
  

   take 
  uranium 
  and 
  radium 
  as 
  being 
  typical 
  of 
  the 
  two 
  classes. 
  

  

  Spontaneous 
  disintegration 
  reduces 
  any 
  store 
  of 
  radium 
  to 
  half 
  in 
  

   1,580 
  years, 
  so 
  that 
  if 
  a 
  whole 
  earth 
  were 
  built 
  of 
  pure 
  radium 
  only 
  

   a 
  single 
  atom 
  would 
  be 
  left 
  after 
  a 
  quarter 
  of 
  a 
  million 
  years. 
  Since 
  

   the 
  earth 
  is 
  many 
  millions 
  of 
  years 
  old, 
  we 
  may 
  be 
  confident 
  that 
  every 
  

   atom 
  of 
  radium 
  now 
  on 
  earth 
  was 
  born 
  on 
  earth. 
  Soddy, 
  Boltwood, 
  

   and 
  others 
  have 
  investigated 
  the 
  ancestry 
  of 
  radium. 
  Its 
  direct 
  par- 
  

   ent 
  is 
  found 
  to 
  be 
  ionium, 
  and 
  it 
  traces 
  its 
  descent 
  back 
  through 
  uran- 
  

   ium-X 
  to 
  uranium 
  itself. 
  

  

  On 
  the 
  other 
  hand, 
  it 
  takes 
  5,000,000,000 
  jesLTs 
  for 
  a 
  store 
  of 
  

   uranium 
  to 
  diminish 
  to 
  half. 
  As 
  the 
  earth 
  was 
  born 
  out 
  of 
  the 
  sun 
  some 
  

   2,000,000,000 
  year5 
  ago, 
  the 
  greater 
  part 
  of 
  any 
  uranium 
  it 
  may 
  have 
  

   brought 
  with 
  it 
  out 
  of 
  the 
  sun 
  would 
  still 
  be 
  in 
  existence. 
  As 
  we 
  have 
  

   no 
  evidence 
  of 
  any 
  uranium 
  being 
  born 
  on 
  earth, 
  and 
  as 
  no 
  parent 
  

   substance 
  is 
  known 
  out 
  of 
  which 
  uranium 
  could 
  be 
  born, 
  it 
  is 
  reason- 
  

  

  2 
  "The 
  Earth," 
  p. 
  83. 
  

  

  