﻿24 
  J. 
  Stansfield 
  — 
  Retarded 
  Diffusion 
  and 
  

  

  result 
  was 
  observed. 
  Toward 
  the 
  outer 
  part 
  of 
  the 
  precipitate 
  

   a 
  very 
  fine 
  banding 
  was 
  seen 
  and 
  the 
  bands 
  were 
  closer 
  together 
  

   outwards. 
  An 
  attempt 
  to 
  reproduce 
  this 
  was 
  not 
  successful, 
  

   owing 
  to 
  different 
  hygroscopic 
  character 
  of 
  the 
  atmosphere, 
  

   but 
  a 
  recent 
  paper 
  by 
  Bradford 
  (10) 
  describes 
  an 
  experiment 
  in 
  

   which 
  the 
  diffusion 
  of 
  N/5 
  lead 
  nitrate 
  against 
  N/10 
  potassium 
  

   chromate, 
  in 
  a 
  agar 
  gel. 
  gave 
  rise 
  to 
  bands 
  of 
  precipitate 
  which 
  

   became 
  more 
  closely 
  spaced 
  in 
  the 
  later 
  deposited 
  layers. 
  The 
  

   first 
  eight 
  bands 
  were 
  l'15 
  mm 
  apart, 
  the 
  ninth 
  one 
  being 
  '9 
  mm 
  

   from 
  the 
  eighth. 
  No 
  chromate 
  was 
  left 
  between 
  the 
  bands. 
  

  

  The 
  same 
  strengths 
  of 
  lead 
  nitrate 
  and 
  potassium 
  chromate 
  

   used 
  by 
  Bradford 
  were 
  tried 
  in 
  gelatine. 
  Three 
  gelatine 
  layers 
  

   were 
  placed 
  in 
  a 
  test-tube, 
  the 
  lower 
  one 
  being 
  a 
  N/10 
  solution 
  

   of 
  potassium 
  chromate, 
  the 
  middle 
  layer 
  being 
  clear 
  gelatine, 
  

   and 
  the 
  upper 
  one 
  a 
  N/5 
  solution 
  of 
  lead 
  nitrate. 
  Another 
  

   test-tube 
  was 
  prepared 
  with 
  the 
  same 
  solutions 
  reversed, 
  the 
  

   lower 
  one 
  being 
  lead 
  nitrate 
  and 
  the 
  upper 
  one 
  potassium 
  

   chromate. 
  In 
  both 
  cases 
  the 
  chromate 
  solution 
  diffused 
  the 
  

   more 
  rapidly 
  so 
  that 
  the 
  first 
  formed 
  precipitate 
  was 
  nearer 
  to 
  

   the 
  starting 
  point 
  of 
  the 
  lead 
  solution 
  than 
  to 
  that 
  of 
  the 
  chrom- 
  

   ate 
  solution, 
  while 
  in 
  both 
  cases 
  the 
  later 
  formed 
  precipitate 
  

   was 
  on 
  the 
  side 
  remote 
  from 
  the 
  lead 
  solution. 
  With 
  the 
  

   chromate 
  solution 
  above 
  the 
  precipitate 
  was 
  a 
  continuous 
  mass, 
  

   but 
  when 
  the 
  lead 
  solution 
  was 
  above 
  the 
  precipitate 
  was 
  

   formed 
  in 
  bands, 
  which 
  became 
  closer 
  and 
  closer 
  together, 
  pass- 
  

   ing 
  into 
  a 
  continuous 
  mass. 
  These 
  experiments 
  were 
  repeated 
  

   with 
  the 
  same 
  results. 
  This 
  illustrates 
  a 
  controlling 
  effect 
  of 
  

   gravity, 
  which 
  does 
  not 
  come 
  into 
  play 
  when 
  the 
  diffusion 
  

   takes 
  place 
  horizontally, 
  as 
  in 
  the 
  glass 
  plate 
  experiments. 
  

  

  Miscellaneous 
  Diffusions. 
  

  

  In 
  comparing 
  the 
  rates 
  of 
  diffusion 
  of 
  ferric 
  chloride 
  and 
  

   sodium 
  hydroxide 
  the 
  observation 
  was 
  made 
  that 
  ferric 
  chloride 
  

   possesses 
  in 
  a 
  marked 
  degree 
  the 
  property 
  of 
  inducing 
  coagula- 
  

   tion 
  in 
  gelatine, 
  a 
  property 
  possessed 
  by 
  silver 
  and 
  lead 
  nitrate 
  

   solutions 
  in 
  a 
  much 
  smaller 
  degree. 
  This 
  coagulant 
  property 
  

   of 
  ferric 
  chloride 
  is 
  well 
  known. 
  In 
  comparing 
  the 
  rates 
  of 
  

   diffusion 
  of 
  the 
  two 
  reagents 
  it 
  was 
  found, 
  contrarv 
  to 
  expecta- 
  

   tion, 
  that 
  the 
  sodium 
  hydroxide 
  diffuses 
  much 
  more 
  slowly 
  

   than 
  ferric 
  chloride. 
  (See 
  fig. 
  10.) 
  But 
  the 
  diffusion 
  of 
  the 
  

   sodium 
  hydroxide 
  is 
  not 
  a 
  simple 
  case 
  of 
  diffusion. 
  A 
  reaction 
  

   takes 
  place 
  with 
  the 
  gelatine, 
  and 
  this 
  is 
  at 
  once 
  apparent 
  in 
  

   the 
  unexpected 
  nature 
  of 
  the 
  curve. 
  

  

  Diffusion 
  of 
  normal 
  ferric 
  chloride 
  against 
  N/100 
  and 
  N/200 
  

   sodium 
  hydroxide 
  — 
  (stronger 
  solutions 
  prevent 
  the 
  setting 
  of 
  

   the 
  gelatine) 
  — 
  gave 
  a 
  colloidal 
  precipitate 
  of 
  ferric 
  hydroxide. 
  

  

  