﻿Botallon 
  ill 
  Iron 
  Cathode 
  Films. 
  101 
  

  

  difFereut 
  types 
  of 
  films 
  which 
  may 
  be 
  produced 
  by 
  the 
  cathode 
  

   deposit 
  : 
  the 
  writer, 
  however, 
  is 
  obliged 
  to 
  ditter 
  from 
  them 
  

   in 
  some 
  of 
  their 
  conclusions. 
  

  

  A 
  comparison 
  of 
  their 
  work 
  with 
  the 
  present 
  shows 
  at 
  once 
  

   that 
  what 
  are 
  here 
  referred 
  to 
  as 
  " 
  oxidized 
  " 
  fihns 
  would 
  

   seem 
  to 
  be 
  identical 
  with 
  their 
  " 
  dark 
  " 
  films, 
  and 
  the 
  " 
  pure 
  

   iron^^ 
  films 
  of 
  the 
  present 
  work, 
  while 
  not 
  produced 
  in 
  just 
  

   the 
  same 
  way, 
  have 
  the 
  general 
  appearance 
  and 
  non-selective 
  

   reflecting 
  and 
  absorbing 
  powers 
  wh-'ch 
  they 
  describe 
  for 
  their 
  

   ''metallic" 
  type. 
  The 
  chief 
  objection 
  to 
  calling 
  the 
  latter 
  

   two 
  %'irtually 
  identical 
  is 
  the 
  abnormally 
  high 
  rotating 
  power 
  

   (per 
  unit 
  thickness) 
  and 
  extinction 
  coefficients 
  which 
  they 
  

   assign 
  to 
  these 
  " 
  metallic 
  ^' 
  films. 
  These 
  are 
  some 
  three 
  times 
  

   larger 
  than 
  for 
  electrolytic 
  iron, 
  while 
  their 
  determinations 
  

   of 
  these 
  quantities 
  for 
  electrolytic 
  films 
  are 
  considerably 
  higher 
  

   than 
  those 
  of 
  other 
  observers. 
  

  

  A 
  possible 
  explanation 
  of 
  this 
  lies 
  in 
  their 
  thickness 
  

   measurements 
  ; 
  for 
  their 
  method, 
  it 
  is 
  believed, 
  results 
  in 
  a 
  

   considerable 
  underestimate 
  of 
  the 
  thickness 
  of 
  the 
  film 
  and 
  

   hence 
  overestimate 
  of 
  its 
  rotating 
  power 
  and 
  extinction 
  co- 
  

   efficient. 
  Their 
  method 
  — 
  an 
  interference 
  one 
  — 
  involves 
  the 
  

   assumption 
  that 
  the 
  phase 
  changes 
  on 
  reflexion 
  at 
  glass 
  and 
  

   iron 
  surfaces 
  are 
  the 
  same, 
  and 
  this 
  they 
  were 
  able 
  to 
  justify 
  

   for 
  very 
  thin 
  films. 
  But 
  Koenigsberger 
  and 
  Bender 
  {Ann. 
  d. 
  

   Phys. 
  xxvi. 
  p. 
  763, 
  1908) 
  find 
  for 
  the 
  cases 
  of 
  gold 
  and 
  

   platinum 
  at 
  least, 
  that 
  while 
  this 
  is 
  true 
  for 
  extremely 
  thin 
  

   films 
  it 
  is 
  by 
  no 
  means 
  the 
  case 
  for 
  thicker 
  ones. 
  

  

  To 
  test 
  this 
  for 
  iron, 
  a 
  number 
  of 
  films 
  were 
  deposited 
  on 
  

   optical 
  glass 
  and 
  their 
  thicknesses 
  measured 
  for 
  sodium 
  light 
  

   after 
  the 
  manner 
  of 
  Skinner 
  and 
  Tool. 
  The 
  thicknesses 
  were 
  

   then 
  measured 
  by 
  the 
  method 
  of 
  Patterson 
  (Phil. 
  Mag. 
  ii. 
  

   p. 
  652, 
  1902), 
  whereby 
  the 
  effect 
  of 
  any 
  possible 
  phase 
  

   change 
  is 
  eliminated 
  by 
  a 
  second 
  cathode 
  deposit, 
  and 
  in 
  

   every 
  case 
  this 
  gave 
  considerably 
  larger 
  results. 
  The 
  true 
  

   thickness 
  was 
  from 
  a 
  fraction 
  larger, 
  to 
  more 
  than 
  twice 
  as 
  

   great 
  as 
  that 
  measured 
  in 
  the 
  first 
  way, 
  and 
  this 
  is 
  more 
  

   than 
  sufficient 
  to 
  explain 
  the 
  abnormal 
  values 
  above 
  

   mentioned. 
  

  

  Since 
  the 
  magnetic 
  rotation 
  for 
  sodium 
  light 
  as 
  well 
  as 
  the 
  

   thickness 
  was 
  measured 
  for 
  these 
  latter 
  films, 
  data 
  were 
  

   obtained 
  for 
  checking 
  the 
  estimates 
  of 
  the 
  thickness 
  of 
  the 
  

   films 
  used 
  in 
  Part 
  II. 
  of 
  the 
  present 
  work. 
  These 
  were 
  

   checked 
  to 
  within 
  a 
  few 
  per 
  cent., 
  or 
  within 
  the 
  error 
  of 
  

   measurement. 
  

  

  In 
  regard 
  to 
  the 
  yellow-brown 
  or 
  " 
  dark 
  '^ 
  films, 
  Skinner 
  

   and 
  Tool 
  consider 
  that 
  the 
  metal 
  is 
  here 
  in 
  a 
  different 
  state 
  

  

  