﻿THE 
  

  

  \ 
  

  

  AMERICAN 
  JOURNAL 
  OF 
  SCIENCE 
  

  

  [FOURTH 
  SERIES.] 
  

  

  Art. 
  I. 
  — 
  Retarded 
  Diffusion 
  and 
  Rhythmic 
  Precipitation 
  ; 
  

   by 
  J. 
  Stansfield, 
  Geological 
  Dept., 
  McGill 
  University, 
  

   Montreal. 
  

  

  Historical. 
  

  

  Owing 
  to 
  the 
  fact 
  that 
  the 
  greater 
  part 
  of 
  the 
  literature 
  of 
  

   this 
  subject 
  is 
  in 
  the 
  German 
  language, 
  it 
  has 
  been 
  considered 
  

   advisable 
  to 
  give 
  the 
  following 
  somewhat 
  detailed 
  account 
  of 
  it. 
  

  

  The 
  rhythmic 
  precipitates 
  formed 
  by 
  diffusion 
  of 
  reagents 
  

   in 
  gelatine 
  or 
  other 
  media, 
  now 
  generally 
  known 
  as 
  Liesegang 
  

   rings, 
  were 
  first 
  described 
  in 
  a 
  publication 
  which 
  is 
  not 
  generally 
  

   available* 
  (1). 
  The 
  precipitates 
  appear 
  to 
  have 
  been 
  shown 
  to 
  

   "W. 
  Ostwald 
  by 
  Liesegang, 
  as 
  a 
  result 
  of 
  which 
  Ostwald 
  pub- 
  

   lished 
  a 
  short 
  note 
  regarding 
  them 
  (2). 
  The 
  ring-formed 
  pre- 
  

   cipitates 
  were 
  formed 
  by 
  diffusion 
  of 
  silver 
  nitrate 
  from 
  a 
  drop 
  

   placed 
  upon 
  a 
  gelatine 
  containing 
  potassium 
  chromate. 
  Ost- 
  

   wald 
  considered 
  that 
  the 
  diffusion 
  outward 
  of 
  the 
  silver 
  nitrate 
  

   and 
  the 
  inward 
  diffusion 
  of 
  the 
  chromate 
  resulted 
  in 
  the 
  

   formation 
  of 
  silver 
  chromate, 
  which 
  was 
  present, 
  at 
  first, 
  as 
  a 
  

   super-saturated 
  solution 
  in 
  the 
  meta-stable 
  state. 
  Consequent 
  

   upon 
  the 
  setting 
  up 
  of 
  the 
  labile 
  state 
  a 
  precipitate 
  formed 
  

   removing 
  the 
  silver 
  chromate 
  in 
  excess 
  of 
  saturation 
  and 
  the 
  

   diffusion 
  continued 
  with 
  similar 
  results. 
  As 
  the 
  removal 
  of 
  

   silver 
  chromate 
  continued 
  the 
  solutions 
  became 
  continuously 
  

   more 
  dilute, 
  so 
  that 
  the 
  precipitates 
  were 
  formed 
  at 
  continu- 
  

   ously 
  greater 
  distances 
  apart. 
  

  

  In 
  1903 
  Morse 
  and 
  Pierce 
  (3) 
  regarded 
  the 
  precipitation 
  as 
  

   certainly 
  due 
  to 
  a 
  super-saturation 
  effect. 
  They 
  produced 
  by 
  

   diffusion 
  ring 
  precipitates 
  of 
  mercurous 
  chromate, 
  lead 
  chrom- 
  

   ate, 
  and 
  Berlin 
  blue, 
  which 
  had 
  been 
  obtained 
  previously 
  by 
  

   Liesegang, 
  and 
  also 
  ring 
  precipitates 
  of 
  lead 
  sulphate, 
  silver 
  

  

  * 
  See 
  literature 
  references 
  at 
  the 
  end 
  of 
  this 
  paper. 
  

   Am. 
  Jour. 
  Sci.— 
  Fourth 
  Series, 
  Vol. 
  XLIII, 
  No. 
  253.— 
  January, 
  1917. 
  

  

  