﻿Rhythmic 
  Precipitation. 
  5 
  

  

  He 
  found 
  that 
  on 
  diffusing 
  ammonium 
  chromate 
  into 
  a 
  silver 
  

   solution 
  in 
  gelatine 
  no 
  precipitate 
  forms. 
  In 
  the 
  diffusion 
  of 
  

   silver 
  nitrate 
  by 
  increasing 
  the 
  ammonium 
  nitrate 
  thicker 
  rings 
  

   were 
  obtained. 
  

  

  In 
  1907 
  Liesegang 
  (7) 
  showed 
  that 
  a 
  later 
  set 
  of 
  rings 
  may 
  

   be 
  formed 
  cutting 
  across 
  an 
  earlier 
  set, 
  apparently 
  without 
  

   being 
  disturbed 
  or 
  influenced 
  by 
  the 
  rings 
  already 
  present. 
  

   Close 
  examination 
  shows 
  that 
  the 
  earlier 
  formed 
  rings 
  are 
  

   extended 
  in 
  comb-like 
  forms 
  to 
  a 
  very 
  small 
  distance 
  by 
  the 
  

   second 
  deposition. 
  But 
  the 
  "sowing" 
  action 
  of 
  the 
  first 
  

   formed 
  precipitates 
  is 
  almost 
  negligible. 
  

  

  In 
  1912 
  Hatschek 
  (8) 
  produced 
  by 
  diffusion 
  in 
  gelatine, 
  agar- 
  

   agar 
  or 
  in 
  silicic 
  acid 
  precipitates 
  of 
  calcium 
  sulphate, 
  and 
  car- 
  

   bonate, 
  barium 
  carbonate, 
  sulphate 
  and 
  chromate, 
  lead 
  chloride, 
  

   iodide, 
  bromide, 
  chromate, 
  ferro-cyanicle, 
  ferri-cyanide, 
  stron- 
  

   tium 
  carbonate, 
  sulphate, 
  phosphate, 
  oxalate 
  and 
  silicofluoride, 
  

   copper 
  phosphate, 
  cadmium 
  sulphide, 
  manganese 
  ferro-cyanide, 
  

   silver 
  bichromate, 
  and 
  silicofluorides 
  of 
  sodium 
  and 
  potassium. 
  

   Some 
  of 
  these 
  were 
  always 
  formed 
  in 
  layers 
  and 
  some 
  always 
  

   as 
  macroscopic 
  crystals. 
  According 
  to 
  Hatschek 
  the 
  results 
  

   are 
  always 
  the 
  same, 
  no 
  matter, 
  which 
  ion 
  diffuses 
  into 
  the 
  

   jelly 
  containing 
  the 
  other 
  ion. 
  

  

  With 
  a 
  view 
  to 
  testing 
  the 
  super-saturation 
  theory 
  of 
  Ost- 
  

   wald 
  he 
  impregnated 
  a 
  jelly 
  with 
  lead 
  iodide 
  and 
  potassium 
  

   iodide, 
  diffusing 
  a 
  solution 
  of 
  lead 
  into 
  it. 
  The 
  rings 
  of 
  lead 
  

   iodide 
  were 
  formed 
  just 
  as 
  though 
  the 
  medium 
  were 
  not 
  sown 
  

   with 
  lead 
  iodide. 
  He 
  holds 
  that 
  super-saturation 
  cannot 
  be 
  

   used 
  as 
  a 
  general 
  explanation 
  of 
  the 
  formation 
  of 
  precipitates 
  

   in 
  layers. 
  The 
  work 
  of 
  Liesegang 
  quoted 
  above 
  would 
  appear 
  

   to 
  invalidate 
  this 
  conclusion. 
  

  

  In 
  1914 
  Liesegang 
  (9) 
  claimed 
  that 
  the 
  presence 
  of 
  a 
  small 
  

   amount 
  of 
  acid 
  and 
  of 
  gelatose 
  is 
  necessary 
  for 
  the 
  formation 
  

   of 
  the 
  rings 
  in 
  gelatine, 
  none 
  being 
  formed 
  in 
  pure 
  gelatine. 
  

   If 
  the 
  amount 
  of 
  acid 
  is 
  increased 
  the 
  rings 
  do 
  not 
  appear, 
  so 
  

   that 
  a 
  definite 
  amount 
  is 
  essential. 
  By 
  increasing 
  the 
  amount 
  

   of 
  acid 
  present 
  the 
  silver 
  chromate, 
  which 
  is 
  soluble 
  in 
  acid, 
  is 
  

   enabled 
  to 
  diffuse 
  to 
  a 
  greater 
  distance 
  before 
  precipitation 
  

   takes 
  place. 
  The 
  result 
  is 
  the 
  bands 
  are 
  more 
  widely 
  spaced, 
  

   and 
  are 
  thicker. 
  Finally, 
  by 
  greater 
  increase 
  of 
  the 
  acid 
  the 
  

   silver 
  chromate 
  forms 
  as 
  a 
  continuous 
  mass. 
  Spiral 
  bands 
  were 
  

   produced, 
  and 
  an 
  examination 
  of 
  some 
  of 
  the 
  older 
  published 
  

   figures 
  reveals 
  the 
  fact 
  that 
  they 
  are 
  spiral 
  and 
  not 
  actually 
  

   rings 
  as 
  they 
  were 
  thought 
  to 
  be. 
  Liesegang 
  mentions 
  the 
  

   presence 
  of 
  spirals 
  on 
  the 
  retina 
  in 
  some 
  birds. 
  

  

  When 
  the 
  drops 
  of 
  silver 
  nitrate 
  from 
  which 
  the 
  diffusion 
  

   proceeds 
  are 
  not 
  circular, 
  cracks 
  appear, 
  cutting 
  the 
  bands, 
  and 
  

   are 
  free 
  from 
  precipitate. 
  This 
  shows 
  how 
  short 
  spaces 
  may 
  

  

  