﻿326 
  Dr. 
  T. 
  M. 
  Lowry 
  on 
  a 
  Method 
  of 
  

  

  was 
  difficult 
  to 
  avoid 
  a 
  displacement 
  of 
  the 
  readings 
  by 
  the 
  

   appearance 
  of 
  the 
  copper 
  line 
  Cu 
  510(5, 
  as 
  a 
  ghostly 
  partner 
  

   of 
  the 
  green 
  cadmium 
  line 
  Cd 
  5086. 
  After 
  many 
  unsuc- 
  

   cessful 
  attempts 
  a 
  workable 
  method 
  of 
  producing 
  the 
  cadmium 
  

   spectrum 
  was 
  found 
  in 
  the 
  use 
  of 
  an 
  alloy 
  of 
  silver 
  and 
  

   cadmium. 
  It 
  is 
  perhaps 
  not 
  very 
  widely 
  known 
  that 
  these 
  

   metals 
  are 
  isomorphous, 
  and 
  form 
  an 
  excellent 
  series 
  of 
  

   alloys. 
  These 
  have 
  the 
  advantage 
  that 
  no 
  eutectic 
  is 
  formed, 
  

   the 
  melting-points 
  throughout 
  the 
  series 
  lying 
  above 
  that 
  of 
  

   cadmium 
  and 
  over 
  a 
  considerable 
  range 
  approximating 
  some- 
  

   what 
  closely 
  to 
  the 
  melting-point 
  of 
  silver. 
  Thus 
  whilst 
  the 
  

   addition 
  of 
  2S 
  per 
  cent, 
  of 
  copper 
  (mp. 
  1082°) 
  lowers 
  the 
  

   melting-point 
  of 
  silver 
  from 
  960^ 
  to 
  780°, 
  the 
  addition 
  of 
  

   28 
  per 
  cent, 
  of 
  cadmium 
  (mp. 
  322°) 
  only 
  lowers 
  the 
  melting- 
  

   point 
  to 
  860°. 
  Nearly 
  50 
  per 
  cent, 
  of 
  cadmium 
  must 
  be 
  

   added 
  to 
  lower 
  the 
  melting-point 
  to 
  780°, 
  and 
  even 
  a 
  60 
  per 
  

   cent, 
  alloy 
  melts 
  as 
  high 
  as 
  700°. 
  These 
  alloys, 
  which 
  can 
  

   be 
  turned 
  up 
  like 
  pure 
  silver, 
  were 
  supplied 
  by 
  Messrs. 
  

   Johnson 
  and 
  Matthey 
  in-the 
  form 
  of 
  rods 
  ^ 
  inch 
  in 
  diameter 
  

   and 
  1;^ 
  inch 
  long. 
  

  

  For 
  spectroscopic 
  work 
  a 
  tiny 
  arc 
  can 
  be 
  burnt 
  quite 
  

   steadily 
  between 
  the 
  points 
  of 
  the 
  rods, 
  in 
  great 
  contrast 
  to 
  

   the 
  behaviour 
  of 
  pure 
  cadmium, 
  which 
  splutters 
  very 
  badly 
  

   and 
  gets 
  choked 
  up 
  with 
  oxide, 
  even 
  when 
  the 
  current 
  is 
  

   kept 
  so 
  small 
  as 
  not 
  to 
  melt 
  the 
  metal. 
  

  

  For 
  polarimetric 
  work 
  a 
  greater 
  intensity 
  of 
  light 
  is 
  

   desirable, 
  and 
  this 
  is 
  obtained 
  by 
  using 
  a 
  heavier 
  current, 
  

   and 
  rotating 
  the 
  electrodes 
  in 
  opposite 
  directions 
  (compare 
  

   Baly, 
  Spectroscopy^ 
  p. 
  370) 
  in 
  order 
  to 
  maintain 
  the 
  arc 
  in 
  

   a 
  central 
  position. 
  The 
  rods 
  of 
  alloy 
  were 
  screwed 
  for 
  half 
  

   their 
  length 
  into 
  copper 
  cylinders 
  f 
  inch 
  in 
  diameter, 
  which 
  

   served 
  the 
  double 
  purpose 
  of 
  cooling 
  the 
  electrodes 
  — 
  a 
  point 
  

   of 
  some 
  importance 
  — 
  and 
  connecting 
  them 
  with 
  the 
  iron 
  

   spindles 
  by 
  means 
  of 
  which 
  the 
  rotation 
  was 
  produced. 
  

   When 
  run 
  at 
  the 
  highest 
  intensity 
  both 
  rods 
  become 
  red 
  hot, 
  

   and 
  one 
  of 
  the 
  copper 
  cylinders 
  is 
  usually 
  luminous, 
  but 
  it 
  is 
  

   not 
  desirable 
  to 
  over-run 
  the 
  arc 
  since 
  even 
  if 
  the 
  electrodes 
  

   do 
  not 
  melt 
  the 
  cadmium 
  distils 
  out 
  irregularly 
  and 
  causes 
  a 
  

   certain 
  amount 
  of 
  spluttering. 
  

  

  The 
  electrodes 
  are 
  filed 
  up 
  before 
  the 
  arc 
  is 
  started, 
  and 
  are 
  

   carefully 
  adjusted 
  so 
  as 
  to 
  run 
  true 
  to 
  centre 
  ; 
  alternatively 
  

   they 
  may 
  be 
  allowed 
  to 
  burn 
  until 
  the 
  ends 
  are 
  flat 
  and 
  then 
  

   used 
  without 
  further 
  attention 
  except 
  to 
  adjust 
  the 
  length 
  of 
  

   the 
  arc 
  from 
  time 
  to 
  time. 
  The 
  cadmium 
  spectrum 
  thus 
  

   produced 
  is 
  of 
  great 
  brilliance 
  — 
  the 
  green 
  line 
  is 
  even 
  

   brighter 
  than 
  that 
  of 
  mercury, 
  and 
  can 
  be 
  read 
  with 
  a 
  half- 
  

   shadow 
  angle 
  of 
  3° 
  or 
  less. 
  From 
  some 
  points 
  of 
  view 
  

  

  