﻿372 
  Lea 
  — 
  Color 
  Relations 
  of 
  Atoms* 
  Ions 
  and 
  Molecules. 
  

  

  A 
  Periodic 
  Law 
  of 
  Color. 
  

  

  It 
  was 
  necessary 
  first 
  to 
  consider 
  the 
  elements 
  in 
  the 
  great 
  

   divisions 
  into 
  which 
  they 
  fall 
  by 
  reason 
  of 
  the 
  color 
  of 
  their 
  

   ions. 
  

  

  It 
  now 
  remains 
  to 
  consider 
  the 
  whole 
  range 
  of 
  elements 
  in 
  

   one 
  continued 
  series 
  from 
  hydrogen 
  to 
  uranium. 
  

  

  Commencing 
  with 
  hydrogen 
  (see 
  Plate 
  No. 
  IV*) 
  we 
  have 
  a 
  

   double 
  series 
  of 
  18 
  elements 
  with 
  colorless 
  ions 
  only. 
  Ap- 
  

   proaching 
  one 
  of 
  the 
  great 
  colored 
  groups 
  which 
  may 
  be 
  called 
  

   the 
  iron 
  group 
  we 
  find 
  two 
  intermediate 
  elements, 
  titanium 
  and 
  

   vanadium 
  which 
  have 
  both 
  colored 
  and 
  colorless 
  ions. 
  

  

  By 
  their 
  colorless 
  ions 
  they 
  are 
  united 
  to 
  the 
  series 
  which 
  

   immediately 
  precedes 
  them 
  in 
  the 
  order 
  of 
  numbers 
  and 
  by 
  

   their 
  colored 
  ions 
  they 
  are 
  united 
  with 
  the 
  iron 
  group 
  which 
  

   immediately 
  follows. 
  This 
  iron 
  group 
  commences 
  with 
  the 
  

   element 
  chromium 
  which 
  in 
  the 
  numerical 
  series 
  immediately 
  

   follows 
  vanadium, 
  so 
  that 
  after 
  the 
  transitionals 
  titanium 
  and 
  

   vanadium 
  each 
  of 
  which 
  has 
  at 
  least 
  one 
  colorless 
  ion, 
  comes 
  

   the 
  group 
  consisting 
  of 
  chromium, 
  manganese, 
  iron, 
  cobalt 
  and 
  

   nickel 
  ; 
  metals 
  which 
  have 
  colored 
  ions 
  only. 
  

  

  Approaching 
  the 
  next 
  colorless 
  series 
  we 
  find 
  interposed 
  the 
  

   transitional 
  element 
  copper, 
  a 
  metal 
  having 
  the 
  colorless 
  

   cuprous 
  and 
  the 
  blue 
  cupric 
  ions. 
  

  

  From 
  this 
  we 
  pass 
  to 
  a 
  colorless 
  series 
  commencing 
  with 
  

   zinc 
  and 
  continuing 
  with 
  gallium, 
  germanium, 
  arsenic, 
  selen- 
  

   ium 
  bromine, 
  rubidium, 
  strontium 
  and 
  concluding 
  with 
  yttrium. 
  

   The 
  ions 
  of 
  none 
  of 
  these 
  elements 
  show 
  any 
  tendency 
  to 
  

   color. 
  

  

  Continuing 
  in 
  numerical 
  order 
  the 
  next 
  colored 
  group 
  will 
  

   consist 
  of 
  the 
  metals 
  ruthenium, 
  rhodium 
  and 
  palladium. 
  But 
  

   in 
  approaching 
  these 
  we 
  find 
  precisely 
  as 
  in 
  the 
  previous 
  case 
  

   two 
  transitionals, 
  molybdenum 
  and 
  niobium. 
  

  

  These 
  are 
  connected 
  with 
  the 
  previous 
  colorless 
  group 
  by 
  

   their 
  colorless 
  ions 
  and 
  with 
  the 
  colored 
  group 
  next 
  following 
  

   by 
  their 
  colored 
  ions. 
  This 
  colored 
  group 
  (Pu, 
  Bh, 
  Pd,) 
  has 
  

   colored 
  ions 
  only. 
  

  

  Continuing 
  in 
  numerical 
  order 
  we 
  approach 
  the 
  next 
  color- 
  

   less 
  group. 
  But 
  as 
  we 
  pass 
  from 
  the 
  colored 
  to 
  the 
  colorless 
  

   we 
  find 
  as 
  before, 
  a 
  transitional, 
  in 
  this 
  case, 
  silver, 
  which 
  is 
  

   connected 
  with 
  the 
  previous 
  colored 
  group 
  by 
  its 
  colored 
  ions 
  

   corresponding 
  with 
  Ag 
  4 
  and 
  Ag 
  2 
  2 
  f 
  and 
  to 
  the 
  following 
  

   colorless 
  group 
  by 
  its 
  ion 
  corresponding 
  to 
  Ag 
  2 
  0. 
  

  

  * 
  In 
  the 
  plate 
  the 
  third 
  and 
  fourth 
  colored 
  groups 
  should 
  have 
  been 
  on 
  the 
  

   same 
  horizontal 
  line 
  as 
  the 
  first 
  and 
  second. 
  

  

  f 
  The 
  first 
  of 
  these 
  colored 
  ions 
  is 
  seen 
  in 
  the 
  deeply 
  colored 
  hemi-salts 
  of 
  

   silver. 
  Another 
  may 
  exist 
  in 
  the 
  peroxide 
  which 
  dissolves 
  in 
  snlphuric 
  acid 
  

   with 
  a 
  dark 
  green 
  color. 
  

  

  