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  ANNUAL, 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  geological 
  periods 
  of 
  time 
  that 
  have 
  been 
  made 
  the 
  " 
  clocks 
  " 
  em- 
  

   ployed 
  do 
  not 
  fulfill 
  these 
  conditions. 
  Estimates 
  based 
  on 
  the 
  tem- 
  

   perature 
  of 
  the 
  earth 
  or 
  of 
  the 
  sun, 
  for 
  instance, 
  can 
  not 
  be 
  reliable, 
  

   for 
  the 
  temperature 
  of 
  a 
  body 
  may 
  fall 
  or 
  it 
  may 
  rise. 
  Further, 
  the 
  

   rate 
  of 
  change 
  of 
  the 
  temperature 
  depends 
  upon 
  a 
  variety 
  of 
  condi- 
  

   tions, 
  such 
  as 
  the 
  amount 
  of 
  energy 
  radiated, 
  the 
  supply 
  of 
  energy 
  

   to 
  it, 
  etc. 
  

  

  Attempts 
  have 
  been 
  made 
  to 
  deduce 
  the 
  age 
  of 
  certain 
  minerals 
  

   from 
  the 
  appearance 
  of 
  little 
  round 
  marks 
  in 
  them, 
  called 
  halos. 
  

   These 
  halos 
  are 
  supposed 
  to 
  be 
  due 
  to 
  radiation 
  from 
  minute 
  specks 
  

   of 
  radioactive 
  matter 
  at 
  their 
  centers. 
  The 
  colors 
  produced 
  by 
  

   radiation 
  in 
  transparent 
  substances 
  depend 
  to 
  a 
  considerable 
  extent 
  

   upon 
  the 
  temperature, 
  so 
  that 
  no 
  very 
  great 
  weight 
  can 
  be 
  put 
  upon 
  

   geological 
  periods 
  of 
  time 
  estimated 
  by 
  means 
  of 
  halos. 
  

  

  There 
  are, 
  however, 
  other 
  radioactive 
  processes 
  the 
  rates 
  of 
  which 
  

   do 
  not, 
  so 
  far 
  as 
  we 
  know, 
  depend 
  on 
  the 
  temperature 
  or 
  the 
  pressure 
  

   nor 
  upon 
  any 
  other 
  physical 
  or 
  chemical 
  state. 
  

  

  During 
  the 
  last 
  25 
  years 
  a 
  large 
  number 
  of 
  radioactive 
  trans- 
  

   formations 
  of 
  one 
  chemical 
  element 
  into 
  another 
  have 
  been 
  discov- 
  

   ered. 
  Students 
  of 
  the 
  subject 
  agree 
  that 
  these 
  transformations 
  take 
  

   place 
  in 
  one 
  direction 
  only, 
  i. 
  e., 
  from 
  an 
  element 
  of 
  higher 
  atomic 
  

   weight 
  to 
  an 
  element 
  of 
  lower 
  atomic 
  weight. 
  Further, 
  nobody 
  has 
  

   been 
  able 
  to 
  alter 
  the 
  rate 
  of 
  a 
  radioactive 
  transformation 
  by 
  any 
  

   process 
  whatsoever, 
  although 
  numerous 
  attempts 
  have 
  been 
  made 
  to 
  

   do 
  so. 
  These 
  radioactive 
  changes, 
  therefore, 
  seem 
  to 
  offer 
  a 
  reliable 
  

   means 
  of 
  estimating 
  certain 
  periods 
  of 
  time. 
  

  

  Among 
  the 
  radio-active 
  changes 
  appear 
  processes 
  in 
  which 
  the 
  

   metal 
  uranium 
  transforms 
  itself 
  through 
  successions 
  of 
  intermediate 
  

   stages 
  into 
  the 
  metal 
  lead 
  and 
  into 
  the 
  gas 
  helium. 
  It 
  does 
  not 
  seem 
  

   necessary 
  to 
  describe 
  in 
  detail 
  these 
  series 
  of 
  transformations 
  at 
  this 
  

   time. 
  Descriptions 
  of 
  them 
  may 
  be 
  found 
  in 
  the 
  literature 
  on 
  radio- 
  

   activity. 
  It 
  suffices 
  for 
  our 
  purposes 
  to 
  say 
  that 
  the 
  rate 
  of 
  trans- 
  

   formation 
  is 
  such 
  that 
  5 
  per 
  cent 
  of 
  a 
  quantity 
  of 
  uranium 
  changes 
  

   into 
  lead 
  and 
  helium 
  in 
  about 
  370 
  millions 
  of 
  years. 
  

  

  We 
  find, 
  uranium, 
  lead, 
  and 
  helium 
  associated 
  together 
  in 
  a 
  great 
  

   many 
  minerals 
  and 
  it 
  is 
  natural 
  to 
  suppose 
  that 
  the 
  helium 
  and 
  the 
  

   lead 
  were 
  produced 
  by 
  the 
  disintegration 
  of 
  the 
  uranium 
  during 
  the 
  

   past 
  ages. 
  Further, 
  if 
  we 
  determine 
  the 
  relative 
  amounts 
  of 
  urani- 
  

   um, 
  lead, 
  and 
  helium 
  in 
  a 
  mineral 
  we 
  can 
  form 
  an 
  estimate 
  as 
  to 
  how 
  

   long 
  these 
  chemical 
  elements 
  have 
  been 
  in 
  contact 
  with 
  each 
  other. 
  

   Estimates 
  of 
  this 
  kind 
  that 
  have 
  been 
  made 
  from 
  the 
  quantities 
  of 
  

   helium 
  in 
  uranium 
  ores 
  vary 
  between 
  8 
  and 
  700 
  millions 
  of 
  years 
  

   according 
  to 
  the 
  locality 
  from 
  which 
  the 
  ore 
  came. 
  Since 
  some 
  of 
  

   the 
  helium 
  (it 
  being 
  a 
  gas) 
  may 
  have 
  leaked 
  out 
  of 
  the 
  ores 
  these 
  

  

  