﻿2 
  J. 
  Stansfield 
  — 
  Retarded 
  Diffusion 
  and 
  

  

  carbonate, 
  silver 
  pyrophosphate, 
  silver 
  thiocyanate, 
  silver 
  

   bromide, 
  cobalt 
  hydroxide, 
  barium 
  chromate, 
  mercurous 
  brom- 
  

   ide, 
  and 
  carbon 
  di-oxide, 
  the 
  last-named 
  as 
  rings 
  of 
  bubbles. 
  

  

  They 
  performed 
  experiments 
  of 
  a 
  quantitative 
  character, 
  

   using 
  capillary 
  tubes 
  containing 
  a 
  weak 
  potassium 
  chromate 
  

   solution 
  in 
  gelatine, 
  which 
  were 
  dipped 
  into 
  silver 
  nitrate 
  solu- 
  

   tions 
  of 
  different 
  strengths. 
  The 
  precipitates 
  appeared 
  sud- 
  

   denly 
  as 
  bands 
  across 
  the 
  capillary 
  tubes, 
  and 
  were 
  observed 
  

   and 
  measured 
  by 
  means 
  of 
  a 
  cathetometer, 
  the 
  times 
  being- 
  

   observed 
  at 
  the 
  formation 
  of 
  the 
  precipitates. 
  Assuming 
  that 
  

   the 
  chromate 
  solution 
  is 
  completely 
  dissociated, 
  they 
  proved 
  

  

  , 
  • 
  n 
  i 
  i 
  distance 
  

  

  mathematically 
  that 
  the 
  - 
  — 
  — 
  - 
  tor 
  all 
  the 
  layers 
  m 
  all 
  tubes 
  

   J 
  ,y/time 
  d 
  

  

  with 
  the 
  same 
  initial 
  concentrations 
  is 
  constant. 
  They 
  gave 
  

   twenty-three 
  tables 
  of 
  observed 
  figures, 
  which 
  show 
  this 
  con- 
  

   stancy, 
  within 
  the 
  limits 
  of 
  experimental 
  error. 
  At 
  a 
  lower 
  

   temperature, 
  with 
  the 
  same 
  initial 
  concentrations, 
  the 
  distances 
  

   appear 
  to 
  be 
  greater, 
  i. 
  e. 
  the 
  diffusion 
  is 
  more 
  rapid 
  at 
  higher 
  

   temperatures. 
  

  

  In 
  a 
  saturated 
  solution, 
  for 
  equilibrium 
  Ag/ 
  X 
  CrO/' 
  = 
  Jc. 
  

   In 
  an 
  unsaturated 
  solution, 
  for 
  equilibrium 
  Ag/ 
  X 
  CrO/ 
  = 
  k. 
  

   AgCr0 
  4 
  . 
  Is 
  there 
  a 
  boundary 
  which 
  cannot 
  be 
  overstepped 
  

   in 
  a 
  super-saturated 
  solution 
  without 
  precipitation 
  following? 
  

  

  Ag 
  2 
  ' 
  X 
  00/ 
  = 
  H. 
  

  

  Morse 
  and 
  Pierce 
  found 
  that 
  for 
  a 
  given 
  concentration 
  of 
  one 
  

   ion 
  a 
  certain 
  definite 
  concentration 
  of 
  the 
  other 
  causes 
  pre- 
  

   cipitation. 
  Thev 
  calculated 
  that 
  when 
  precipitation 
  takes 
  

  

  N 
  

   place 
  the 
  concentrations 
  of 
  the 
  solutions 
  are 
  — 
  for 
  the 
  chrom- 
  

  

  ST 
  

   ate 
  and 
  — 
  for 
  the 
  silver. 
  Both 
  solutions 
  must 
  be 
  fully 
  disso- 
  

  

  200 
  J 
  

  

  ciated 
  at 
  these 
  dilutions. 
  They 
  calculated, 
  further, 
  that 
  H 
  is 
  

   a 
  definite 
  super-saturation 
  limit, 
  which 
  is 
  1*4 
  X 
  10~ 
  6 
  gram 
  

   molecules 
  per 
  liter, 
  at 
  16° 
  C. 
  They 
  found 
  that 
  the 
  diffusion 
  

   is 
  quicker 
  in 
  gelatine 
  than 
  in 
  water, 
  the 
  diffusion 
  constant 
  cal- 
  

   culated 
  for 
  silver 
  nitrate 
  being 
  1*24 
  cms. 
  per 
  day, 
  that 
  observed 
  

   being 
  1*54 
  (They 
  also 
  suggested 
  that 
  the 
  precipitate 
  is 
  col- 
  

   loidal, 
  but 
  this 
  appears 
  not 
  to 
  be 
  the 
  case.) 
  

  

  Small 
  finer 
  white 
  lines 
  are 
  mentioned 
  as 
  forming 
  between 
  

   the 
  main 
  precipitates, 
  which 
  become 
  red 
  and 
  then 
  are 
  oblit- 
  

   erated 
  when 
  the 
  main 
  precipitate 
  forms 
  over 
  them. 
  These 
  are 
  

   said 
  to 
  be 
  due 
  to 
  impurities, 
  and 
  are 
  not 
  formed 
  in 
  pure 
  gela- 
  

   tine. 
  They 
  found 
  that 
  gelatine 
  is 
  not 
  essential 
  to 
  the 
  formation 
  

   of 
  the 
  banded 
  precipitates, 
  but 
  that 
  capillary 
  tubes 
  containing 
  

   aqueous 
  solutions 
  (jive 
  similar 
  results, 
  and 
  that 
  the 
  precipitates 
  

  

  