﻿70 
  Messrs. 
  McClung 
  and 
  Mcintosh 
  on 
  Absorption 
  of 
  

  

  If 
  two 
  metal 
  plates, 
  which 
  are 
  at 
  different 
  potentials, 
  are 
  

   arranged 
  parallel 
  to 
  each 
  other, 
  and 
  the 
  X-rays 
  allowed 
  to 
  

   pass 
  between 
  them, 
  a 
  current 
  will 
  pass 
  from 
  one 
  plate 
  to 
  the 
  

   other. 
  The 
  amount 
  of 
  this 
  current 
  will 
  be 
  proportional 
  to 
  the 
  

   intensity 
  of 
  the 
  rays, 
  provided 
  other 
  conditions 
  remain 
  the 
  

   same. 
  If 
  the 
  current, 
  which 
  passes 
  between 
  two 
  such 
  plates, 
  

   is 
  measured 
  for 
  different 
  intensities 
  of 
  rays, 
  a 
  direct 
  com- 
  

   parison 
  of 
  the 
  intensities 
  is 
  obtained. 
  Such 
  a 
  current 
  may 
  be 
  

   measured 
  accurately 
  by 
  means 
  of 
  an 
  electrometer, 
  and 
  thus 
  

   the 
  relative 
  amount 
  of 
  rays 
  which 
  pass 
  through 
  different 
  

   substances 
  can 
  be 
  ascertained. 
  

  

  The 
  bulb 
  used 
  to 
  generate 
  the 
  rays, 
  along 
  with 
  the 
  

   induction-coil 
  by 
  which 
  the 
  bulb 
  was 
  run, 
  was 
  contained 
  in 
  

   a 
  small 
  metal 
  room 
  connected 
  to 
  earth. 
  The 
  rays 
  from 
  the 
  

   bulb 
  passed 
  through 
  a 
  small 
  orifice 
  C 
  in 
  a 
  thick 
  lead 
  plate 
  

   AA'. 
  A 
  few 
  centimetres 
  in 
  front 
  of 
  this, 
  another 
  thick 
  lead 
  

   plate 
  BB 
  ; 
  was 
  placed, 
  and 
  the 
  rays 
  passed 
  through 
  a 
  rect- 
  

   angular 
  orifice 
  D, 
  6 
  cms. 
  by 
  0*6 
  cm., 
  in 
  it. 
  After 
  emerging 
  

   from 
  D, 
  the 
  cone 
  of 
  rays 
  then 
  passed 
  between 
  a 
  set 
  of 
  insulated, 
  

   parallel 
  zinc 
  plates, 
  arranged 
  in 
  the 
  form 
  of 
  a 
  guard-ring. 
  

   The 
  large 
  plate 
  S 
  was 
  charged 
  to 
  about 
  six 
  hundred 
  volts, 
  by 
  

   being 
  connected 
  to 
  one 
  pole 
  of 
  a 
  battery 
  of 
  small 
  accumu- 
  

   lators, 
  while 
  the 
  other 
  pole 
  was 
  to 
  earth. 
  The 
  central 
  plate 
  

   R 
  of 
  the 
  guard-ring 
  was 
  connected 
  through 
  suitable 
  keys 
  to 
  

   one 
  pair 
  of 
  quadrants 
  of 
  the 
  electrometer, 
  while 
  the 
  other 
  pair 
  

   was 
  connected 
  to 
  earth. 
  The 
  outside 
  plates 
  N 
  and 
  N 
  / 
  were 
  

   also 
  to 
  earth. 
  When 
  the 
  rays 
  passed 
  between 
  these 
  plates, 
  a 
  

   current 
  passed 
  from 
  S 
  to 
  R 
  proportional 
  to 
  the 
  intensity 
  of 
  

   the 
  rays, 
  provided 
  the 
  voltage 
  was 
  large 
  enough 
  to 
  produce 
  

   a 
  saturation 
  or 
  maximum 
  current. 
  The 
  potential 
  gradient 
  

   employed 
  was 
  about 
  seventy 
  volts 
  per 
  centimetre. 
  When 
  

   the 
  pair 
  of 
  quadrants 
  connected 
  to 
  R 
  is 
  insulated, 
  the 
  

   deflexion 
  of 
  the 
  electrometer-needle 
  per 
  second 
  is 
  proportional 
  

   to 
  the 
  current 
  passing 
  between 
  S 
  and 
  R 
  ; 
  and 
  since 
  the 
  current 
  

   is 
  proportional 
  to 
  the 
  intensity 
  of 
  the 
  rays, 
  the 
  intensity 
  of 
  the 
  

   rays 
  is 
  therefore 
  proportional 
  to 
  the 
  deflexion 
  of 
  the 
  electro- 
  

   meter-needle. 
  

  

  The 
  cell, 
  which 
  contained 
  the 
  liquid 
  whose 
  absorption 
  was 
  

   to 
  be 
  measured, 
  consisted 
  of 
  a 
  rectangular 
  box, 
  of 
  which 
  the 
  

   parallel 
  sides 
  through 
  which 
  the 
  rays 
  passed 
  were 
  of 
  thin 
  

   glass. 
  The 
  capacity 
  of 
  the 
  box 
  was 
  about 
  300 
  c. 
  c. 
  This 
  

   box, 
  containing 
  the 
  liquid, 
  was 
  placed 
  in 
  the 
  path 
  of 
  the 
  

   rays 
  at 
  M, 
  the 
  thickness 
  of 
  the 
  liquid 
  traversed 
  by 
  the 
  rays 
  

   being 
  2*6 
  cms. 
  

  

  In 
  this 
  investigation, 
  the 
  usual 
  difficulty 
  which 
  arises 
  

   in 
  making 
  a 
  series 
  of 
  measurements 
  on 
  X-rays, 
  namely, 
  the 
  

  

  