﻿76 
  Scientific 
  Intelligence. 
  

  

  8. 
  The 
  Influence 
  of 
  Dissolved 
  Salts 
  on 
  the 
  Absorption 
  Bands 
  

   of 
  Water. 
  — 
  Some 
  important 
  facts 
  bearing 
  on 
  the 
  theory 
  of 
  solu- 
  

   tions 
  have 
  been 
  discovered 
  by 
  H. 
  C. 
  Jones, 
  J. 
  S. 
  Guy 
  and 
  E. 
  J. 
  

   Shaeffer. 
  The 
  fundamental 
  piece 
  of 
  apparatus 
  used 
  was 
  a 
  

   specially 
  constructed 
  radiomicrometer 
  of 
  short 
  period. 
  A 
  Nernst 
  

   glower 
  produced 
  the 
  radiations. 
  The 
  region 
  of 
  wave-lengths 
  

   investigated 
  extended 
  from 
  0*7l0/x 
  to 
  l*445/x, 
  since 
  the 
  infra-red 
  

   absorption 
  bands 
  of 
  water 
  fall 
  within 
  this 
  interval. 
  The 
  experi- 
  

   mental 
  method 
  may 
  be 
  made 
  clear 
  by 
  the 
  following 
  concrete 
  

   example. 
  The 
  deflection 
  of 
  the 
  radiomicrometer 
  was 
  observed 
  

   when 
  a 
  chosen 
  wave-length 
  had 
  passed 
  through 
  a 
  cell 
  containing 
  

   a 
  layer 
  of 
  solution 
  l 
  mm 
  deep. 
  Then 
  the 
  deflection 
  was 
  read 
  when 
  

   the 
  same 
  radiation 
  had 
  passed 
  through 
  a 
  cell 
  containing 
  n 
  mm 
  of 
  

   the 
  same 
  solution. 
  The 
  ratio 
  of 
  these 
  two 
  deflections 
  measures, 
  

   in 
  a 
  certain 
  sense, 
  the 
  absorption 
  which 
  would 
  be 
  produced 
  by 
  a 
  

   layer 
  of 
  the 
  solution 
  10 
  mm 
  thick. 
  The 
  object 
  in 
  using 
  two 
  depths 
  

   was 
  to 
  eliminate 
  the 
  effects 
  of 
  the 
  ends 
  of 
  the 
  cells. 
  Next 
  the 
  

   percentage 
  by 
  volume 
  of 
  water 
  in 
  the 
  solution 
  was 
  calculated 
  

   from 
  the 
  specific 
  gravity 
  of 
  the 
  solution 
  and 
  from 
  its 
  concentra- 
  

   tion. 
  This 
  percentage 
  may 
  be 
  formulated 
  as 
  100s 
  — 
  O'lkm 
  

   where 
  s 
  denotes 
  specific 
  gravity, 
  k 
  means 
  the 
  multiple 
  of 
  normal, 
  

   and 
  m 
  stands 
  for 
  the 
  molecular 
  weight 
  of 
  the 
  dissolved 
  substance. 
  

   This 
  method 
  of 
  calculation 
  assumes 
  that 
  the 
  solvend 
  and 
  solvent 
  

   are 
  present 
  in 
  the 
  solution 
  quite 
  independently 
  of 
  each 
  other. 
  If 
  

   the 
  specific 
  gravity 
  of 
  a 
  6 
  X 
  normal 
  solution 
  of 
  potassium 
  chloride 
  

   is 
  1*236 
  the 
  per 
  cent 
  calculated 
  would 
  be 
  78*9, 
  (k 
  = 
  6, 
  m 
  = 
  74*6, 
  

   * 
  = 
  1-236). 
  Consequently 
  the 
  cell 
  was 
  set 
  at 
  a 
  depth 
  of 
  l 
  mm 
  , 
  

   filled 
  with 
  pure 
  water, 
  and 
  the 
  deflection 
  noted. 
  Then 
  the 
  

   micrometer 
  reading 
  corresponding 
  to 
  a 
  depth 
  of 
  8*89 
  mm 
  of 
  pure 
  

   water 
  was 
  taken. 
  As 
  before, 
  the 
  ratio 
  of 
  these 
  two 
  deflections 
  is 
  

   a 
  measure 
  of 
  the 
  absorption 
  caused 
  by 
  a 
  column 
  of 
  water 
  7*89 
  mm 
  

   deep, 
  that 
  is, 
  due 
  to 
  a 
  column 
  equivalent 
  (on 
  the 
  assumption 
  of 
  

   independence 
  of 
  solvent 
  and 
  dissolved 
  salt) 
  to 
  the 
  water 
  present 
  

   in 
  10 
  mra 
  of 
  solution. 
  For 
  each 
  salt 
  studied 
  two 
  curves 
  are 
  plotted 
  

   on 
  the 
  same 
  diagram. 
  The 
  abscissae 
  are 
  wave-lengths 
  and 
  the 
  

   ordinates 
  are 
  proportional 
  to 
  the 
  ratios 
  of 
  intensities 
  as 
  explained 
  

   above. 
  One 
  curve 
  pertains 
  to 
  pure 
  water 
  and 
  the 
  other 
  corre- 
  

   sponds 
  to 
  the 
  aqueous 
  solution. 
  The 
  dissolved 
  substances 
  were 
  

   chosen 
  so 
  as 
  to 
  have 
  no 
  appreciable 
  absorption 
  in 
  the 
  region 
  of 
  

   the 
  infra-red 
  absorption 
  bands 
  of 
  water. 
  For 
  the 
  chlorides 
  and 
  

   nitrates 
  of 
  ammonium 
  and 
  potassium 
  the 
  curves 
  for 
  the 
  solutions 
  

   and 
  for 
  the 
  equivalent 
  depths 
  of 
  water 
  were 
  practically 
  coinci- 
  

   dent. 
  This 
  means 
  that 
  the 
  dissolved 
  substance 
  and 
  the 
  solvent 
  

   are 
  effectively 
  independent 
  in 
  solution, 
  so 
  far 
  as 
  the 
  absorption 
  

   of 
  light 
  is 
  concerned. 
  On 
  the 
  other 
  hand, 
  when 
  the 
  salts 
  dis- 
  

   solved 
  were 
  calcium 
  chloride, 
  or 
  magnesium 
  chloride, 
  or 
  alumin- 
  

   ium 
  sulphate 
  the 
  solutions 
  were 
  far 
  more 
  transparent 
  than 
  the 
  

   equivalent 
  depths 
  of 
  pure 
  water. 
  This 
  experimental 
  fact 
  is 
  very 
  

   striking. 
  The 
  last 
  three 
  salts 
  have 
  been 
  shown 
  by 
  Jones 
  to 
  be 
  

   the 
  most 
  strongly 
  hydrated 
  substances 
  known, 
  hence 
  the 
  obvious 
  

  

  