﻿The 
  fy-Iiays 
  of 
  Uranium 
  and 
  Radium. 
  621 
  

  

  of 
  from 
  0*26 
  to 
  0'94 
  cm. 
  of 
  lead 
  he 
  found 
  that 
  the 
  absorption 
  

   proceeded 
  according 
  to 
  an 
  exponential 
  law, 
  the 
  value 
  of 
  the 
  

   absorption 
  coefficient, 
  X(cm.)"^, 
  being 
  1*4. 
  For 
  radium 
  he 
  

   giyes 
  the 
  values 
  from 
  0*57 
  to 
  0-46 
  over 
  a 
  range 
  of 
  from 
  0*64 
  cm. 
  

   to 
  3*0 
  cm. 
  of 
  lead. 
  

  

  With 
  our 
  intensely 
  active 
  preparations 
  of 
  uranium 
  X 
  we 
  

   have 
  fully 
  confirmed 
  and 
  extended 
  Eve's 
  statement 
  with 
  

   regard 
  to 
  the 
  relative 
  poverty 
  of 
  uranium 
  in 
  7-rays 
  as 
  com- 
  

   pared 
  with 
  the 
  y8-rays, 
  and 
  an 
  accurate 
  comparison 
  of 
  the 
  

   ratio 
  of 
  the 
  yS- 
  to 
  7-rajs 
  for 
  uranium 
  X 
  and 
  for 
  radium 
  C 
  is 
  

   detailed 
  in 
  this 
  paper. 
  We 
  may 
  state 
  the 
  result 
  here 
  that 
  

   radium 
  gives 
  50 
  times 
  more 
  7-rays 
  in 
  proportion 
  to 
  /3-rays 
  

   than 
  is 
  the 
  case 
  with 
  uranium. 
  We 
  have 
  found, 
  however, 
  

   that 
  the 
  7-rays 
  of 
  uranium 
  are 
  not 
  absorbed 
  exponentially 
  

   until 
  after 
  about 
  1 
  cm. 
  of 
  lead, 
  or 
  an 
  equivalent 
  thickness 
  of 
  

   other 
  substances, 
  has 
  been 
  penetrated^ 
  and 
  then 
  they 
  are 
  

   similar 
  in 
  character, 
  and 
  only 
  slightly 
  inferior 
  in 
  penetrating 
  

   power 
  to 
  the 
  y-rays 
  of 
  radium, 
  the 
  absorption 
  coefficient 
  

   being 
  for 
  most 
  substances 
  about 
  1*2 
  times 
  greater 
  over 
  a 
  

   range 
  equivalent 
  to 
  from 
  1 
  to 
  5 
  cm. 
  of 
  lead. 
  For 
  thick- 
  

   nesses 
  less 
  than 
  1 
  cm. 
  of 
  lead 
  the 
  absorption 
  curve 
  is 
  not 
  

   exponential. 
  The 
  average 
  slope 
  of 
  the 
  curve, 
  however, 
  in 
  a 
  

   set 
  of 
  measurements 
  with 
  lead 
  agreed 
  approximately 
  with 
  

   Eve's 
  value 
  over 
  a 
  large 
  part 
  of 
  the 
  range 
  he 
  w^orked 
  over. 
  

   It 
  is 
  possible 
  that 
  a 
  soft 
  type 
  of 
  primary 
  y-rays 
  is 
  also 
  present, 
  

   but 
  so 
  far 
  w^e 
  have 
  not 
  been 
  able 
  to 
  be 
  sure 
  of 
  thisj 
  and 
  this 
  

   part 
  of 
  the 
  subject 
  is 
  reserved 
  for 
  a 
  future 
  communication. 
  

   In 
  any 
  case 
  the 
  7-rays 
  of 
  uranium 
  and 
  radium 
  examined 
  over 
  

   the 
  same 
  range 
  are 
  extremely 
  similar 
  in 
  penetrating 
  power 
  

   and 
  general 
  character, 
  and 
  if 
  a 
  soft 
  type 
  of 
  7-rays 
  from 
  

   uranium 
  exists, 
  not 
  represented 
  in 
  the 
  7-rays 
  from 
  radium, 
  

   it 
  is 
  relatively 
  feeble 
  in 
  character, 
  and 
  its 
  existence 
  is 
  difficult 
  

   to 
  establish. 
  

  

  [The 
  statement 
  we 
  made 
  in 
  preliminary 
  communications 
  

   that 
  the 
  absorption 
  coefficient 
  of 
  the 
  y-rays 
  of 
  uranium 
  was 
  

   two 
  and 
  a 
  half 
  times 
  greater 
  than 
  that 
  of 
  radium, 
  which 
  has 
  

   unfortunately 
  been 
  quoted 
  in 
  abstracts, 
  was 
  based 
  upon 
  a 
  

   published 
  value 
  for 
  the 
  radium 
  rays 
  which 
  we 
  have 
  since 
  

   found 
  is 
  altogether 
  in 
  error 
  (Section 
  8). 
  The 
  ratios 
  we 
  

   ourselves 
  have 
  found 
  for 
  13 
  substances 
  are 
  given 
  in 
  the 
  last 
  

   column 
  of 
  Table 
  III. 
  (p. 
  646).] 
  

  

  This 
  being 
  the 
  case 
  the 
  great 
  relative 
  poverty 
  in 
  7-rays 
  of 
  

   uranium 
  compared 
  to 
  radium 
  is 
  a 
  very 
  remarkable 
  fact. 
  

   The 
  /3-rays 
  of 
  uranium 
  are, 
  like 
  the 
  uranium 
  7-rays, 
  only 
  

   slightly 
  less 
  penetrating 
  than 
  the 
  corresponding 
  radium 
  rays. 
  

   It 
  is 
  hardly 
  possible 
  to 
  continue 
  to 
  regard 
  the 
  7- 
  ray 
  as 
  an 
  

  

  