﻿276 
  Dr. 
  A. 
  S. 
  Eve 
  on 
  Primary 
  

  

  have 
  proved 
  the 
  existence 
  o£ 
  secondary 
  radiations 
  of 
  the 
  

   7-ray 
  type, 
  less 
  penetrating 
  than 
  the 
  primary 
  7 
  radiations 
  to 
  

   which 
  such 
  secondary 
  rays 
  are 
  due. 
  

  

  The 
  transmitted 
  or 
  emergent 
  corpuscular 
  secondary 
  ra- 
  

   diations 
  at 
  first 
  received 
  less 
  attention^ 
  although 
  it 
  was 
  

   shown 
  * 
  in 
  1904 
  that 
  for 
  varying 
  thickness 
  of 
  the 
  secondary 
  

   radiator 
  the 
  intensity 
  of 
  the 
  emergent 
  radiation 
  followed 
  a 
  

   ourve 
  of 
  the 
  well-known 
  type 
  whose 
  equation 
  involves 
  the 
  

   difference 
  of 
  two 
  simple 
  exponentials. 
  A. 
  S. 
  Mackenzie 
  f 
  

   has 
  called 
  attention 
  to 
  the 
  remarkable 
  fact 
  that 
  there 
  travels 
  

   in 
  the 
  direction 
  of 
  the 
  7 
  rays, 
  when 
  absorbed, 
  a 
  stronger 
  

   stream 
  of 
  negative 
  secondary 
  rays 
  than 
  in 
  the 
  reverse 
  

   direction. 
  This 
  fact, 
  substantiated 
  by 
  Bragg, 
  is 
  one 
  of 
  the 
  

   arguments 
  in 
  the 
  chain 
  of 
  reasoning 
  which 
  led 
  him, 
  in 
  spite 
  

   of 
  able 
  opposition 
  and 
  many 
  difficulties 
  of 
  experiment 
  and 
  

   theory, 
  to 
  controvert 
  the 
  sether-pulse 
  theory. 
  

  

  Bragg 
  1 
  has 
  proved 
  that 
  the 
  emergent 
  secondary 
  cor- 
  

   puscular 
  radiation 
  from 
  various 
  elements 
  does 
  not 
  follow 
  the 
  

   order 
  of 
  atomic 
  weights, 
  but 
  that 
  the 
  curve, 
  plotted 
  with 
  

   atomic 
  weights 
  as 
  abscissse 
  and 
  intensities 
  of 
  secondary 
  

   radiation 
  as 
  ordinates, 
  roughly 
  resembles 
  a 
  parabola 
  with 
  

   vertex 
  downwards, 
  such 
  as 
  may 
  be 
  seen 
  in 
  fig. 
  3 
  of 
  this 
  paper. 
  

   Again, 
  there 
  is 
  some 
  divergence 
  in 
  the 
  views 
  and 
  results 
  of 
  

   Bragg 
  and 
  Kleeman, 
  for 
  the 
  latter 
  found 
  evidence 
  of 
  selective 
  

   absorption, 
  while 
  Bragg 
  and 
  Madsen 
  attributed 
  the 
  observed 
  

   effects 
  to 
  the 
  hardening 
  of 
  the 
  rays. 
  Also 
  Kleeman, 
  using 
  

   both 
  primary 
  and 
  secondary 
  7 
  rays, 
  divided 
  the 
  metals 
  

   examined 
  into 
  three 
  well-marked 
  groups 
  ; 
  but 
  Bragg's 
  work, 
  

   and 
  Madsen's, 
  indicates 
  no 
  sharp 
  line 
  of 
  demarcation 
  into 
  

   groups. 
  The 
  present 
  work 
  was 
  undertaken 
  to 
  investigate 
  

   this 
  point 
  also 
  ; 
  and 
  it 
  will 
  be 
  seen 
  that 
  the 
  results 
  favour 
  the 
  

   conclusions 
  of 
  Bragg 
  and 
  Madsen. 
  

  

  Apparatus, 
  

  

  A 
  rectangular 
  brass 
  electroscope 
  (fig. 
  1), 
  18 
  X 
  16 
  X 
  12 
  cm., 
  

   was 
  used, 
  but 
  both 
  ends 
  were 
  removed 
  and 
  covered 
  with 
  thin 
  

   aluminium 
  foil. 
  It 
  requires 
  about 
  12 
  layers 
  of 
  this 
  foil 
  to 
  

   stop 
  the 
  a 
  rays 
  of 
  radium 
  ; 
  so 
  that 
  it 
  may 
  be 
  expected, 
  and 
  it 
  

   was 
  found, 
  that 
  in 
  the 
  experiments 
  described 
  the 
  foil 
  pro- 
  

   duced 
  a 
  negligible 
  effect 
  on 
  the 
  secondary 
  rays 
  investigated, 
  

   when 
  proceeding 
  from 
  the 
  surfaces 
  of 
  plates 
  close 
  to 
  the 
  

   ■aluminium. 
  The 
  radium, 
  14 
  mg. 
  pure 
  RaBrg, 
  was 
  placed 
  

  

  * 
  Phil. 
  Mag. 
  Dec. 
  1904. 
  

   t 
  Phil. 
  Mag. 
  .Tulv 
  1907. 
  

   X 
  Phil. 
  Mag. 
  Dec. 
  1908. 
  

  

  