﻿Dispersion 
  hij 
  Metallic 
  Vapours. 
  409 
  

  

  of 
  the 
  photographic 
  plates 
  are 
  necessary 
  as 
  the 
  light 
  intensity- 
  

   is 
  small 
  and 
  the 
  plates 
  sensitive 
  in 
  this 
  region 
  are 
  not 
  rapid. 
  

  

  In 
  the 
  case 
  of 
  potassium 
  the 
  temperature 
  required 
  is 
  not 
  

   so 
  high 
  as 
  in 
  the 
  case 
  o£ 
  sodium, 
  and 
  a 
  burner 
  consisting 
  

   of 
  a 
  brass 
  tube 
  with 
  small 
  holes 
  at 
  intervals 
  of 
  about 
  a 
  

   centimetre 
  was 
  used. 
  This 
  heated 
  twelve 
  to 
  fifteen 
  centi- 
  

   metres 
  of 
  the 
  tube 
  and 
  was 
  found 
  satisfactory 
  when 
  quite 
  

   small 
  flames 
  about 
  1 
  cm. 
  high 
  just 
  reached 
  the 
  tube. 
  The 
  

   chief 
  region 
  of 
  anomalous 
  dispersion 
  for 
  potassium 
  is 
  about 
  

   the 
  first 
  pair 
  of 
  the 
  primary 
  series 
  (w.-ls. 
  7699, 
  7665). 
  

   Figure 
  2 
  is 
  a 
  photograph 
  of 
  the 
  effect 
  when 
  small 
  dispersion 
  

   is 
  used. 
  Here 
  the 
  anomalous 
  dispersion 
  appears 
  as 
  if 
  only 
  

   one 
  absorption 
  line 
  were 
  concerned. 
  If 
  a 
  larger 
  dispersion 
  

   in 
  the 
  spectroscope 
  be 
  employed, 
  then 
  the 
  double 
  effect, 
  as 
  in 
  

   fig. 
  3, 
  appears. 
  It 
  is 
  difficult 
  to 
  photograph 
  this 
  condition, 
  

   as 
  the 
  absorption 
  rapidly 
  increases 
  with 
  the 
  density 
  of 
  the 
  

   vapour 
  and 
  the 
  light 
  is 
  not 
  intense 
  enough 
  to 
  produce 
  a 
  

   good 
  negative. 
  Figure 
  3 
  was 
  taken 
  with 
  a 
  pair 
  of 
  heavy 
  glass 
  

   prisms 
  in 
  the 
  spectroscope. 
  With 
  the 
  only 
  grating 
  available 
  

   the 
  dispersion 
  was 
  too 
  great 
  and 
  the 
  loss 
  of 
  light 
  too 
  much 
  

   for 
  a 
  satisfactory 
  negative 
  to 
  be 
  obtained, 
  the 
  region 
  between 
  

   the 
  two 
  Jines 
  being 
  specially 
  difficult 
  to 
  photograph. 
  If 
  the 
  

   temperature 
  be 
  too 
  high 
  and 
  the 
  potassium 
  vapour 
  in 
  conse- 
  

   quence 
  too 
  dense, 
  absorption 
  takes 
  place, 
  the 
  region 
  with 
  

   longer 
  wave-lengths 
  disappearing 
  firsts 
  the 
  dispersion 
  in 
  the 
  

   meantime 
  increasing, 
  so 
  that 
  dispersion 
  may 
  be 
  observed 
  

   extending 
  almost 
  down 
  to 
  the 
  D 
  lines. 
  At 
  the 
  same 
  time 
  

   some 
  beautiful 
  absorption 
  bands 
  appear 
  w^hich 
  seem 
  to 
  possess 
  

   some 
  regularity 
  (fig. 
  4). 
  These 
  the 
  author 
  hopes 
  to 
  be 
  able 
  

   to 
  investigate 
  later, 
  but 
  there 
  appears 
  to 
  be 
  no 
  relation 
  

   between 
  them 
  and 
  the 
  tabulated 
  lines 
  of 
  the 
  emission 
  spec- 
  

   trum 
  of 
  potassium. 
  And 
  further, 
  there 
  appears 
  to 
  be 
  no 
  

   anomalous 
  dispersion 
  exhibited 
  at 
  these 
  bands. 
  Anomalous 
  

   dispersion 
  also 
  appears 
  at 
  the 
  pair 
  of 
  lines 
  4047, 
  4044 
  with 
  

   potassium. 
  These 
  are 
  the 
  second 
  pair 
  of 
  the 
  primary 
  series. 
  

   Again, 
  the 
  ultra-violet 
  region 
  contains 
  the 
  other 
  primary 
  

   series 
  lines 
  for 
  potassium 
  and 
  as 
  yet 
  these 
  have 
  not 
  been 
  

   investigated. 
  

  

  The 
  conditions 
  in 
  the 
  tube 
  as 
  to 
  the 
  amount 
  of 
  gas 
  present 
  

   do 
  not 
  seem 
  to 
  matter 
  very 
  much 
  as 
  regards 
  the 
  anomalous 
  

   dispersion 
  due 
  to 
  the 
  metallic 
  vapour. 
  In 
  the 
  case 
  of 
  lithium 
  

   and 
  potassium 
  the 
  effect 
  shows 
  very 
  clearly 
  both 
  at 
  a 
  low 
  

   pressure 
  and 
  at 
  a 
  pressure 
  somewhat 
  greater 
  than 
  an 
  

   atmosphere. 
  In 
  each 
  case 
  the 
  gas 
  in 
  the 
  tube 
  was 
  hydrogen. 
  

   In 
  the 
  case 
  of 
  lithium, 
  hydrogen 
  is 
  rapidly 
  absorbed 
  when 
  

   the 
  metal 
  is 
  heated 
  and 
  the 
  pressure 
  becomes 
  low. 
  In 
  any 
  

  

  