﻿Rhythmic 
  Precipitation. 
  3 
  

  

  break 
  up, 
  say 
  after 
  about 
  four 
  in 
  series 
  have 
  been 
  formed. 
  

   "When 
  the 
  experiment 
  is 
  performed 
  in 
  gelatine 
  they 
  are 
  held 
  

   in 
  place 
  and 
  do 
  not 
  break 
  up. 
  

  

  In 
  1901 
  Hausmann 
  (1) 
  carried 
  out 
  experiments 
  with 
  capillary 
  

   tubes 
  containing 
  gelatine 
  solutions 
  of 
  silver 
  nitrate 
  dipping 
  

   into 
  solutions 
  of 
  sodium 
  chloride, 
  etc., 
  and 
  found 
  that 
  the 
  

   heights 
  to 
  which 
  the 
  precipitates 
  extend 
  in 
  any 
  given 
  time 
  

   depend 
  on 
  the 
  concentration 
  of 
  the 
  silver 
  nitrate 
  and 
  of 
  the 
  

   sodium 
  chloride, 
  etc., 
  also. 
  Liesegang 
  had 
  claimed 
  that 
  equiva- 
  

   lent 
  weights 
  of 
  sodium 
  chloride, 
  strontium 
  chloride, 
  potassium 
  

   bromide, 
  and 
  potassium 
  iodide 
  diffuse 
  to 
  equal 
  heights 
  in 
  equal 
  

   times. 
  Hausmann 
  found 
  that 
  the 
  distances 
  varied 
  with 
  the 
  

   reagents, 
  with 
  their 
  concentrations, 
  and 
  with 
  the 
  concentra- 
  

   tions 
  of 
  the 
  gelatine. 
  In 
  some 
  of 
  the 
  reactions 
  banded 
  pre- 
  

   cipitates 
  were 
  formed. 
  

  

  Mention 
  is 
  made 
  of 
  little 
  brown 
  bands 
  between 
  the 
  main 
  

   ones 
  and 
  also 
  to 
  secondary 
  rings 
  in 
  the 
  upper 
  part 
  of 
  the 
  tube, 
  

   which 
  are 
  broader 
  than 
  those 
  in 
  the 
  lower 
  part 
  and 
  increase 
  in 
  

   breadth 
  upwards. 
  With 
  increase 
  of 
  gelatine 
  concentration 
  the 
  

   bands 
  were 
  found 
  to 
  be 
  brought 
  closer 
  together. 
  (Curves 
  

   given 
  pp. 
  117, 
  118, 
  loc. 
  cit.) 
  

  

  The 
  precipitation 
  of 
  metallic 
  sulphides 
  is 
  discussed. 
  Certain 
  

   compounds 
  are 
  formed 
  in 
  the 
  colloidal 
  state, 
  e. 
  g., 
  silver 
  iodide, 
  

   mercuric 
  oxide, 
  ferric 
  hydroxide, 
  copper 
  hydroxide, 
  and 
  copper 
  

   ferro-cyanide. 
  The 
  following 
  substances 
  were 
  formed 
  in 
  non- 
  

   banded 
  crystalline 
  precipitates: 
  — 
  barium 
  sulphate, 
  strontium 
  

   sulphate, 
  calcium 
  sulphate, 
  barium 
  oxalate, 
  silver 
  oxalate, 
  thal- 
  

   lium 
  chloride, 
  bromide 
  and 
  iodide. 
  In 
  certain 
  reactions 
  com- 
  

   pounds 
  separate 
  out 
  which 
  are 
  not 
  stable 
  at 
  atmospheric 
  

   temperatures 
  under 
  normal 
  conditions, 
  e. 
  g. 
  the 
  yellow 
  form 
  

   of 
  mercuric 
  iodide, 
  which 
  is 
  stable 
  above 
  126° 
  C, 
  and 
  certain 
  

   thallium 
  salts. 
  

  

  Attention 
  is 
  called 
  to 
  the 
  results 
  obtained 
  by 
  Larsen 
  on 
  

   cooling 
  a 
  salt 
  solution 
  whose 
  concentration 
  decreases 
  at 
  higher 
  

   temperatures. 
  Regular 
  layers 
  are 
  formed 
  with 
  increasing 
  dis- 
  

   tances 
  between 
  them, 
  and 
  having 
  decreasing 
  concentrations, 
  

   upward 
  (5). 
  

  

  With 
  regard 
  to 
  the 
  banded 
  precipitates 
  Hausmann 
  suggests 
  

   that 
  the 
  compounds 
  are 
  present 
  in 
  the 
  colloidal 
  state 
  before 
  

   precipitation 
  which 
  only 
  ensues 
  upon 
  super-saturation. 
  

  

  By 
  diffusing 
  different 
  chlorides 
  against 
  a 
  constant 
  strength 
  

   of 
  silver 
  nitrate 
  he 
  showed 
  that 
  the 
  distances 
  diffused 
  were 
  

   independent 
  of 
  the 
  metal 
  combined 
  with 
  the 
  chlorine, 
  depend- 
  

   ing 
  only 
  on 
  the 
  silver 
  and 
  chlor-ions, 
  their 
  speeds 
  of 
  diffusion 
  

   and 
  concentrations. 
  As 
  the 
  concentration 
  of 
  the 
  chloride 
  

   decreases 
  it 
  is 
  able 
  to 
  penetrate 
  less 
  and 
  less 
  into 
  the 
  (slightly 
  

  

  less 
  than 
  ) 
  silver 
  nitrate. 
  Weaker 
  solutions 
  of 
  copper 
  and 
  

   400/ 
  ' 
  l 
  

  

  