﻿Homogeneous 
  Secondary 
  Rontgen 
  Radiations. 
  551 
  

  

  which 
  are 
  continually 
  being 
  tested 
  by 
  further 
  experiments 
  

   on 
  a 
  variety 
  of 
  substances 
  and 
  under 
  a 
  variety 
  of 
  conditions. 
  

  

  As 
  has 
  been 
  shown 
  in 
  previous 
  papers 
  *, 
  the 
  behaviour 
  of 
  

   substances 
  subject 
  to 
  X-rays 
  varies 
  enormously 
  with 
  the 
  

   atomic 
  weight 
  of 
  the 
  substance 
  exposed, 
  and 
  generalizations 
  

   cannot 
  safely 
  be 
  arrived 
  at 
  except 
  after 
  an 
  extensive 
  series 
  

   of 
  experiments 
  on 
  a 
  large 
  number 
  of 
  elements. 
  

  

  The 
  results 
  which 
  have 
  so 
  far 
  been 
  found 
  to 
  be 
  perfectly 
  

   general 
  will 
  be 
  briefly 
  stated 
  f 
  : 
  — 
  

  

  All 
  substances 
  subject 
  to 
  X-rays 
  are 
  a 
  source 
  of 
  secondary 
  

   X-rays. 
  

  

  The 
  radiation 
  from 
  a 
  given 
  element 
  is 
  independent 
  of 
  the 
  

   physical 
  state 
  of 
  the 
  substance 
  and 
  of 
  its 
  mixture 
  or 
  even 
  

   chemical 
  combination 
  with 
  other 
  elements. 
  

  

  The 
  character 
  of 
  the 
  secondary 
  radiation 
  from 
  an 
  element 
  

   is 
  independent 
  of 
  the 
  intensity 
  of 
  the 
  primary 
  radiation 
  

   producing 
  it. 
  

  

  The 
  intensity 
  of 
  secondary 
  radiation 
  from 
  an 
  element 
  is 
  

   proportional 
  to 
  the 
  intensity 
  of 
  the 
  primary 
  radiation 
  of 
  

   definite 
  character 
  producing 
  it. 
  

  

  The 
  absorption 
  by 
  a 
  thin 
  sheet 
  of 
  any 
  substance 
  of 
  the 
  

   secondary 
  rays 
  from 
  various 
  elements 
  subject 
  to 
  the 
  same 
  

   primary 
  beam 
  is 
  a 
  periodic 
  function 
  of 
  the 
  atomic 
  weight 
  of 
  

   the 
  radiating 
  substance. 
  

  

  There 
  are, 
  however, 
  groups 
  of 
  elements 
  of 
  neighbouring- 
  

   atomic 
  weight 
  into 
  which 
  substances 
  may 
  conveniently 
  be 
  

   divided 
  ; 
  for 
  when 
  a 
  primary 
  beam 
  of 
  ordinary 
  penetrating 
  

   power 
  is 
  used, 
  the 
  radiations 
  from 
  the 
  various 
  elements 
  in 
  

   one 
  group 
  are 
  very 
  similar 
  in 
  properties, 
  while 
  those 
  from 
  

   elements 
  in 
  different 
  groups 
  differ 
  considerably. 
  But 
  it 
  

   should 
  be 
  understood 
  that 
  this 
  grouping 
  is 
  somewhat 
  

  

  * 
  As 
  frequent 
  references 
  are 
  made 
  to 
  the 
  following 
  papers, 
  they 
  are 
  

   denoted 
  by 
  the 
  letters 
  a-g 
  : 
  — 
  

  

  Barkla 
  : 
  a. 
  Phil. 
  Mag. 
  June 
  1903. 
  pp. 
  685-698. 
  

  

  b. 
  Phil. 
  Mag. 
  May 
  1904,' 
  pp. 
  543-560. 
  

  

  c. 
  Phil. 
  Trans. 
  A, 
  vol. 
  204, 
  1905, 
  pp. 
  467-479. 
  

  

  d. 
  Boy. 
  Soc. 
  Proc. 
  A, 
  vol. 
  77, 
  1906. 
  pp. 
  247-255. 
  

  

  e. 
  Phil. 
  Mag. 
  June 
  1906, 
  pp. 
  812-828. 
  

   g. 
  Phil. 
  Mag. 
  Feb. 
  1908, 
  pp. 
  288-296. 
  

  

  Baekla 
  & 
  Sadler 
  : 
  f. 
  Phil. 
  Mag. 
  Sept. 
  1907, 
  pp. 
  408-422. 
  

   t 
  These 
  results 
  were 
  given 
  in 
  the 
  papers 
  to 
  which 
  reference 
  has 
  

   already 
  been 
  made 
  ; 
  later 
  experiments 
  have 
  not 
  revealed 
  any 
  exceptions. 
  

   It 
  appears 
  quite 
  possible, 
  however, 
  that 
  under 
  certain 
  conditions 
  ex- 
  

   ceptions 
  will 
  be 
  found. 
  Crowther, 
  who 
  by 
  careful 
  experiment 
  has 
  

   further 
  verified 
  some 
  of 
  these 
  results 
  (Phil. 
  Mag. 
  Nov. 
  1907), 
  finds 
  much 
  

   less 
  intense 
  radiation 
  from 
  nickel 
  when 
  in 
  combination 
  as 
  nickel 
  carbonyl 
  

   than 
  we 
  have 
  obtained 
  from 
  the 
  pure 
  element 
  in 
  the 
  solid 
  state. 
  As 
  this 
  

   cannot 
  be 
  accounted 
  for 
  by 
  variation 
  in 
  the 
  primary 
  beams 
  used, 
  it 
  is 
  

   perhaps 
  worthy 
  of 
  further 
  investigation. 
  

  

  