﻿producing 
  an 
  intense 
  Cadmium 
  Spectrum. 
  325 
  

  

  that 
  tables 
  for 
  these 
  wave-lengths 
  shall 
  be 
  supplied 
  as 
  a 
  

   matter 
  of 
  course 
  with 
  instruments 
  of 
  the 
  Pulfrich 
  pattern. 
  

   It 
  may 
  be 
  noted 
  that 
  the 
  mercury 
  and 
  hydrogen 
  violet 
  lines 
  

   differ 
  bv 
  only 
  18 
  Angstrom 
  units, 
  the 
  mercury 
  line 
  having 
  

   the 
  longer 
  wave-len 
  g-th 
  : 
  in 
  a 
  Pulfrich 
  instrument 
  the 
  two 
  

   lines 
  are 
  indistinguishable, 
  but 
  the 
  edge 
  that 
  is 
  read 
  with 
  a 
  

   hydrogen-mercury 
  vacuum-tube 
  (such 
  as 
  is 
  .sometimes 
  sent 
  

   out 
  with 
  the 
  instrument) 
  is 
  due 
  to 
  mercury 
  and 
  not 
  hydrogen. 
  

   The 
  adaptation 
  of 
  a 
  polarimeter 
  for 
  use 
  with 
  mercury 
  light 
  

   costs 
  about 
  £2, 
  with 
  a 
  further 
  £3 
  for 
  the 
  lamp. 
  

  

  Cadmium. 
  

  

  The 
  cadmium 
  spectrum 
  is 
  much 
  less 
  easy 
  to 
  produce 
  than 
  

   that 
  of 
  mercury. 
  Michelson 
  made 
  use 
  of 
  a 
  strongly 
  heated 
  

   vacuum-tube 
  with 
  aluminium 
  electrodes 
  connected 
  to 
  platinum 
  

   wires 
  passing 
  through 
  the 
  glass. 
  This 
  was 
  improved 
  upon 
  by 
  

   Hamy 
  (^Comptes 
  Hendus, 
  1897, 
  cxxiv. 
  p. 
  749) 
  who 
  used 
  a 
  

   copper 
  heating-jacket 
  and 
  external 
  electrodes, 
  thus 
  avoiding 
  

   the 
  risk 
  of 
  cracking 
  the 
  hot 
  glass 
  by 
  wires 
  passing 
  through 
  

   it. 
  I 
  have 
  had 
  no 
  personal 
  experience 
  of 
  such 
  lamps, 
  but 
  

   am 
  doubtful 
  whether 
  they 
  would 
  give 
  a 
  sufficiently 
  intense 
  

   light 
  for 
  use 
  in 
  polarimetry. 
  The 
  amalgam 
  lamp 
  with 
  an 
  

   arc 
  enclosed 
  in 
  silica 
  can 
  be 
  made 
  to 
  give 
  a 
  splendid 
  series 
  

   of 
  lines 
  for 
  use 
  in 
  spectroscopy, 
  but 
  I 
  have 
  found 
  that 
  it 
  is 
  

   useless 
  for 
  polarimetric 
  work, 
  since 
  even 
  the 
  green 
  cadmium- 
  

   line 
  can 
  only 
  be 
  read 
  with 
  a 
  half-shadow 
  angle 
  of 
  nearly 
  20°. 
  

   Apparently 
  the 
  current 
  is 
  carried 
  mainly 
  by 
  the 
  mercury, 
  

   and 
  the 
  other 
  metals 
  show 
  only 
  weakly 
  in 
  the 
  spectrum. 
  I 
  

   have 
  not 
  been 
  able 
  as 
  yet 
  to 
  find 
  any 
  description 
  of 
  an 
  

   enclosed 
  cadmium 
  arc, 
  though 
  I 
  believe 
  they 
  have 
  been 
  tried 
  

   — 
  evidently 
  not 
  with 
  complete 
  success, 
  or 
  the 
  results 
  would 
  

   be 
  more 
  widely 
  known. 
  

  

  The 
  method 
  set 
  out 
  below 
  is 
  not 
  put 
  forward 
  as 
  the 
  ideal 
  

   way 
  of 
  producing 
  an 
  intense 
  cadmium 
  spectrum, 
  but 
  rather 
  

   as 
  an 
  intermediate 
  stage 
  in 
  the 
  development 
  of 
  the 
  perfect 
  

   cadmium 
  lamp 
  of 
  the 
  future. 
  It 
  was 
  found 
  that 
  brilliant 
  

   spectral 
  lines 
  could 
  be 
  sent 
  into 
  the 
  polarimeter 
  by 
  using 
  an 
  

   arc 
  burning 
  between 
  metallic 
  poles 
  rotating 
  in 
  opposite 
  

   directions. 
  Copper, 
  for 
  instance, 
  gave 
  a 
  valuable 
  series 
  of 
  

   lines, 
  and 
  brass 
  electrodes 
  were 
  found 
  to 
  be 
  very 
  efficient 
  for 
  

   developing 
  a 
  zinc 
  spectrum, 
  the 
  red 
  line 
  Zn 
  6364, 
  and 
  the- 
  

   three 
  blue 
  lines 
  Zn 
  4811, 
  Zn 
  4722, 
  and 
  Zn 
  4680, 
  standing 
  

   out 
  very 
  distinctly 
  from 
  the 
  copper 
  lines. 
  A 
  brilliant 
  cadmium 
  

   spectrum 
  could 
  be 
  produced 
  by 
  melting 
  the 
  metal 
  onto 
  

   copper 
  electrodes, 
  but 
  it 
  soon 
  burned 
  off, 
  and 
  in 
  any 
  case 
  it 
  

  

  P/iil. 
  Mag. 
  S. 
  6. 
  Vol. 
  18. 
  No. 
  104. 
  Aug. 
  1909. 
  " 
  Z 
  

  

  