﻿408 
  Dr. 
  H. 
  S. 
  Allen 
  on 
  Molecular 
  

  

  Thus 
  for 
  the 
  halogen 
  derivatives 
  of 
  thallium 
  the 
  interesting 
  

   result 
  is 
  found 
  that 
  the 
  "frequency 
  numbers" 
  (15, 
  14, 
  13) 
  

   diminish 
  by 
  unity 
  in 
  passing 
  from 
  chloride 
  to 
  bromide 
  and 
  

   from 
  bromide 
  to 
  iodide. 
  

  

  It 
  must 
  be 
  stated 
  that 
  the 
  suggestion 
  which 
  attributes 
  the 
  

   two 
  absorption 
  bands 
  of 
  NaCl, 
  KC1, 
  KBr 
  to 
  separate 
  atoms 
  

   is 
  not 
  a 
  new 
  one*. 
  Nernst 
  is 
  of 
  opinion 
  that 
  it 
  was 
  a 
  mere 
  

   coincidence 
  — 
  " 
  a 
  very 
  curious 
  and 
  misleading 
  one 
  indeed" 
  — 
  

   that 
  calculations 
  of 
  the 
  specific 
  heat 
  on 
  that 
  supposition 
  gave 
  

   quite 
  good 
  results. 
  Rubens 
  was 
  unable 
  to 
  find 
  two 
  bands 
  

   in 
  the 
  case 
  of 
  AgCl 
  and 
  PbCl 
  2 
  , 
  and 
  came 
  to 
  the 
  conclusion 
  

   that 
  the 
  two 
  apparent 
  bands 
  were 
  due 
  simply 
  to 
  water 
  vapour, 
  

   which 
  has 
  a 
  great 
  number 
  of 
  absorption 
  bands. 
  If 
  this 
  con- 
  

   clusion 
  be 
  accepted 
  the 
  results 
  quoted 
  above 
  will 
  require 
  

   modification 
  in 
  the 
  sense 
  that 
  the 
  " 
  molecular 
  number 
  " 
  must 
  

   be 
  employed 
  instead 
  of 
  the 
  atomic 
  number; 
  but 
  the 
  pro- 
  

   posed 
  relation 
  will 
  still 
  hold 
  good. 
  Thus 
  for 
  NaCl 
  we 
  find 
  

   Ni/ 
  = 
  8x20'6xl0 
  12 
  , 
  for 
  KOI 
  Nv 
  = 
  8 
  x2Olxl0 
  12 
  , 
  and 
  for 
  

   KBrNv 
  = 
  9x21-6xl0 
  12 
  . 
  

  

  It 
  would 
  appear 
  probable 
  that 
  the 
  relation 
  here 
  discussed 
  

   applies 
  only 
  in 
  the 
  case 
  of 
  the 
  longer 
  residual 
  rays, 
  having 
  a 
  

   wave-length 
  greater 
  than 
  say 
  20//,. 
  For 
  shorter 
  wave- 
  

   lengths, 
  corresponding 
  to 
  a 
  higher 
  frequency, 
  the 
  value 
  of 
  

   the 
  product, 
  Ni/, 
  is 
  so 
  large 
  that 
  no 
  real 
  test 
  of 
  the 
  proposed 
  

   relation 
  can 
  be 
  obtained. 
  It 
  is 
  unfortunate 
  for 
  our 
  present 
  

   purpose 
  that 
  although 
  the 
  residual 
  rays 
  from 
  quartz 
  in 
  the 
  

   region 
  of 
  9/xand 
  13//, 
  have 
  been 
  measured 
  with 
  considerable 
  

   accuracy, 
  the 
  longer 
  waves 
  have 
  not 
  as 
  yet 
  been 
  determined 
  

   accurately; 
  we 
  know 
  only 
  that 
  quartz 
  shows 
  strong 
  selective 
  

   absorption 
  for 
  the 
  region 
  between 
  60 
  and 
  80 
  \x. 
  

  

  It 
  may 
  be 
  worthy 
  of 
  mention 
  that 
  water 
  vapour 
  has 
  an 
  

   absorption 
  band 
  at 
  14'3yu,, 
  which 
  is 
  the 
  wave-length 
  corre- 
  

   sponding 
  to 
  a 
  frequency^ 
  210 
  x 
  10 
  12 
  sec. 
  -1 
  , 
  that 
  is 
  the 
  

   frequency 
  here 
  denoted 
  by 
  v 
  A 
  . 
  Further, 
  the 
  vapour 
  of 
  carbon 
  

   dioxide 
  has 
  an 
  absorption 
  band 
  at 
  14'1/x. 
  It 
  would 
  be 
  of 
  

   interest 
  to 
  know 
  whether 
  other 
  substances 
  show 
  absorption 
  

   in 
  the 
  same 
  region. 
  

  

  The 
  possibility 
  of 
  deducing 
  the 
  wave-length 
  of 
  the 
  infra-red 
  

   radiation 
  from 
  the 
  elastic 
  properties 
  of 
  the 
  solid 
  has 
  been 
  

   discussed 
  by 
  Madelung 
  and 
  by 
  Sutherland. 
  By 
  considering 
  

   a 
  cubical 
  space-lattice 
  the 
  former 
  obtained 
  for 
  the 
  wave-length 
  

   the 
  expression 
  

  

  * 
  Cf. 
  Nernst, 
  < 
  The 
  Theory 
  of 
  the 
  Solid 
  State/ 
  p. 
  80 
  (1914). 
  

  

  