﻿358 
  Lea 
  — 
  Color 
  Relations 
  of 
  Atoms, 
  Ions 
  and 
  Molecules. 
  

  

  dissimilar 
  atom 
  or 
  atoms 
  to 
  form 
  a 
  molecule. 
  That 
  is 
  to 
  say 
  it 
  

   does 
  not 
  belong 
  to 
  the 
  ion 
  with 
  exclusion 
  of 
  the 
  molecule 
  as 
  

   some 
  have 
  held, 
  nor 
  to 
  the 
  molecule 
  with 
  exclusion 
  of 
  the 
  ion 
  

   as 
  has 
  been 
  held 
  by 
  others. 
  Color 
  when 
  it 
  appears, 
  is 
  the 
  essen- 
  

   tial 
  property 
  of 
  the 
  atom, 
  possessed 
  by 
  it 
  in 
  the 
  free 
  state 
  and 
  

   carried 
  by 
  it 
  into 
  any 
  electrolyte 
  which 
  it 
  forms. 
  

  

  This 
  fact 
  can 
  be 
  conclusively 
  proved 
  from 
  the 
  researches 
  of 
  

   Glan 
  and 
  of 
  Ewan. 
  Each 
  of 
  these 
  chemists 
  studied 
  the 
  action 
  

   of 
  copper 
  salts 
  and 
  more 
  particularly 
  of 
  the 
  sulphate. 
  Both 
  

   observers 
  used 
  virtually 
  the 
  same 
  method. 
  A 
  ray 
  of 
  light 
  was 
  

   passed 
  through 
  a 
  small 
  stratum 
  of 
  strong 
  solution 
  and 
  then 
  

   through 
  a 
  larger 
  quantity 
  of 
  distilled 
  water 
  and 
  the 
  amount 
  of 
  

   absorption 
  was 
  noted. 
  In 
  another 
  parallel 
  tube 
  the 
  two 
  were 
  

   mixed 
  and 
  the 
  amount 
  of 
  absorption 
  was 
  compared, 
  in 
  each 
  

   case 
  for 
  particular 
  wave 
  lengths. 
  Ewan 
  used 
  an 
  eight-fold 
  

   dilution. 
  Glan 
  usually 
  a 
  seven-fold 
  but 
  sometimes 
  a 
  three 
  or 
  

   five. 
  In 
  all 
  cases 
  the 
  difference 
  found 
  was 
  extremely 
  small, 
  

   scarcely 
  if 
  at 
  all 
  exceeding 
  the 
  magnitude 
  of 
  experimental 
  

   error.* 
  Had 
  the 
  color 
  been 
  a 
  property 
  of 
  the 
  ion 
  only, 
  or 
  of 
  

   the 
  molecule 
  only, 
  the 
  differences 
  found 
  would 
  have 
  been 
  very 
  

   great, 
  in 
  opposite 
  directions. 
  

  

  Ewan 
  calculates 
  from 
  the 
  numbers 
  found 
  by 
  Kohlrausch 
  

   that 
  in 
  a 
  solution 
  of 
  cupric 
  sulphate 
  containing 
  2*38 
  gram 
  

   equivalents 
  to 
  the 
  liter, 
  the 
  dissociation 
  amounts 
  to 
  15*3 
  per 
  

   cent. 
  A 
  dilution 
  reducing 
  the 
  proportion 
  to 
  0*2856 
  equiva- 
  

   lents 
  to 
  the 
  liter 
  increases 
  the 
  dissociation 
  to 
  31*7 
  per 
  cent. 
  

   Therefore, 
  if 
  the 
  color 
  depended 
  upon 
  the 
  ions 
  only, 
  the 
  total 
  

   absorption 
  would 
  be 
  more 
  than 
  doubled. 
  On 
  the 
  other 
  hand 
  

   if 
  the 
  color 
  depended 
  upon 
  the 
  molecules 
  only, 
  the 
  absorption 
  

   would 
  be 
  materially 
  diminished. 
  Now 
  the 
  work 
  of 
  Ewan, 
  of 
  

   Glan, 
  and 
  of 
  previous 
  observers 
  shows 
  that 
  neither 
  of 
  these 
  

   large 
  changes 
  takes 
  place 
  but 
  that 
  the 
  absorption 
  varies 
  between 
  

   narrow 
  limits. 
  Therefore 
  it 
  is 
  certain 
  that 
  the 
  color 
  belongs 
  

   to 
  the 
  atom, 
  whether 
  it 
  exists 
  as 
  an 
  ion 
  or 
  whether 
  by 
  union 
  

   with 
  a 
  dissimilar 
  ion, 
  it 
  forms 
  part 
  of 
  an 
  electrolyte. 
  

  

  These 
  proofs 
  would 
  seem 
  to 
  be 
  sufficient 
  but 
  others 
  can 
  be 
  

   had 
  in 
  abundance. 
  In 
  dilute 
  solutions 
  of 
  cobaltous 
  salts 
  the 
  

   ions 
  exhibit 
  the 
  color 
  characteristic 
  of 
  cobalt. 
  But 
  cobalt 
  

   cyanide 
  also 
  exhibits 
  that 
  color 
  ; 
  it 
  is 
  quite 
  anhydrous, 
  no 
  ions 
  

   can 
  be 
  present 
  and 
  therefore 
  the 
  color 
  must 
  be 
  due 
  to 
  the 
  

   atoms. 
  Cobalt 
  carbonate 
  exists 
  in 
  two 
  forms, 
  one 
  freely 
  

   hydrated, 
  the 
  other 
  anhydrous, 
  both 
  show 
  the 
  characteristic 
  

   color. 
  In 
  the 
  hydrated 
  salt 
  the 
  ions 
  may 
  possibly 
  be 
  dissoci- 
  

   ated, 
  in 
  the 
  other 
  they 
  cannot 
  be. 
  Nickel 
  cyanide 
  is 
  also 
  

   anhydrous 
  and 
  yet 
  shows 
  the 
  characteristic 
  nickel 
  color. 
  

  

  * 
  Both 
  Ewan's 
  and 
  Glan's 
  results 
  will 
  be 
  found 
  tabulated 
  in 
  Dr. 
  Ewan's 
  paper 
  

   in 
  the 
  Philosophical 
  Magazine, 
  vol. 
  xxxiii 
  at 
  p. 
  336. 
  (1892.) 
  

  

  