﻿158 
  Prof. 
  Becquerel 
  on 
  Dispersion 
  of 
  Magnetic 
  Rotatory 
  

  

  positive 
  electrons, 
  of 
  various 
  degrees 
  of 
  responsiveness 
  and 
  

   whose 
  effects 
  are 
  superposed, 
  may 
  give 
  rise 
  to 
  phenomena 
  

   possessing 
  very 
  great 
  complications. 
  It 
  is 
  effects 
  of 
  this 
  

   nature, 
  I 
  think, 
  which 
  were 
  observed 
  in 
  nitrate 
  of 
  neodymium 
  

   by 
  Professor 
  Wood. 
  

  

  The 
  only 
  means 
  of 
  elucidating 
  this 
  question 
  consists 
  in 
  

   separating 
  complex 
  groups 
  into 
  their 
  components 
  : 
  and 
  I 
  

   have 
  shown 
  that 
  we 
  mav 
  in 
  a 
  number 
  of 
  cases 
  succeed 
  in 
  

   doing 
  this 
  by 
  studying 
  the 
  absorption 
  at 
  very 
  low 
  tempe- 
  

   ratures. 
  

  

  Having; 
  been 
  led 
  to 
  investigate 
  whether 
  or 
  not 
  changes 
  of 
  

   period 
  produced 
  by 
  a 
  magnetic 
  field 
  depend 
  upon 
  tempera- 
  

   ture, 
  I 
  observed 
  a 
  new 
  phenomenon*. 
  Under 
  the 
  influence 
  

   of 
  a 
  lowering 
  of 
  temperature 
  (in 
  the 
  absence 
  of 
  any 
  action 
  of 
  

   a 
  magnetic 
  field) 
  the 
  absorption 
  bands 
  of 
  solids 
  and 
  solutions 
  

   become 
  much 
  finer 
  ; 
  so 
  much 
  so 
  indeed 
  that 
  at 
  the 
  tempera- 
  

   ture 
  of 
  liquid 
  air 
  the 
  salts 
  of 
  rare 
  earths, 
  either 
  crystallized 
  

   or 
  in 
  frozen 
  solution, 
  exhibit 
  absorption 
  spectra 
  having 
  very 
  

   remarkable 
  sharpness. 
  

  

  Measurements 
  made 
  upon 
  several 
  bands 
  of 
  tysonite 
  showed 
  

   that, 
  down 
  to 
  the 
  temperature 
  of 
  liquid 
  air, 
  the 
  breadth 
  of 
  

   these 
  bands 
  varies 
  proportionately 
  to 
  the 
  square 
  root 
  of 
  the 
  

   absolute 
  temperature. 
  Besides 
  this 
  t, 
  at 
  —190° 
  C. 
  the 
  bands 
  

   have 
  an 
  intensity 
  which 
  is 
  in 
  general 
  much 
  greater 
  than 
  at 
  

   ordinary 
  temperatures 
  ; 
  not 
  only 
  because 
  the}' 
  are 
  more 
  con- 
  

   stricted, 
  but 
  also 
  as 
  I 
  have 
  established, 
  because 
  the 
  quantity 
  

   of 
  energy 
  absorbed 
  increases 
  in 
  the 
  cnse 
  of 
  nearly 
  all 
  bands. 
  

  

  These 
  researches 
  have 
  just 
  been 
  pushed 
  to 
  the 
  lowest 
  tem- 
  

   peratures 
  at 
  present 
  realizable. 
  Jn 
  the 
  cryogenic 
  laboratory 
  

   in 
  the 
  University 
  of 
  Leyden, 
  Professor 
  Kamerlingh 
  Onnes 
  

   and 
  myself 
  % 
  have 
  been 
  able 
  to 
  study 
  absorption 
  and 
  magneto- 
  

   optic 
  phenomena 
  in 
  crystals 
  at 
  temperatures 
  down 
  to 
  that 
  of 
  

   the 
  solidification 
  of 
  hydrogen 
  ( 
  — 
  '251)° 
  C). 
  

  

  We 
  observed 
  that 
  the 
  majority 
  of 
  bands 
  continue 
  to 
  narrow 
  

   down, 
  but 
  in 
  general 
  at 
  a 
  less 
  rate 
  than 
  according 
  to 
  the 
  law 
  

   which 
  was 
  proved 
  to 
  be 
  valid 
  down 
  to 
  the 
  temperature 
  of 
  

   liquid 
  air. 
  

  

  We 
  have 
  shown, 
  moreover, 
  that 
  for 
  each 
  band 
  a 
  tempe- 
  

   rature 
  exists 
  for 
  which 
  the 
  absorj>t/on 
  passes 
  through 
  a 
  maximum. 
  

   Certain 
  bands 
  which, 
  on 
  cooling 
  to 
  —190° 
  0., 
  became 
  ex- 
  

   tremely 
  intense, 
  diminished 
  afterwards 
  till 
  they 
  became 
  

  

  * 
  Comptes 
  J!n,<lu*. 
  25 
  p 
  e 
  b., 
  LS 
  May, 
  17 
  June, 
  1907. 
  

   t 
  Comptes 
  Rendus, 
  1 
  1 
  & 
  '2~> 
  Nov., 
  L907 
  ; 
  /.'' 
  Radium, 
  iv. 
  No. 
  11. 
  p. 
  385 
  

   (1907); 
  Physik. 
  Zeitschr. 
  ix. 
  No, 
  8. 
  p. 
  94 
  (1908). 
  

  

  | 
  K.Akad. 
  Amsterdam, 
  29 
  Feb., 
  L908; 
  Comptes 
  Mendtu, 
  23 
  March, 
  

  

  1908. 
  

  

  