﻿Rhythmic 
  Precipitation. 
  23 
  

  

  the 
  lead 
  nitrate 
  solution 
  in 
  gelatine 
  may 
  be 
  less 
  than 
  that 
  of 
  

   the 
  potassium 
  chromate 
  solution 
  led 
  to 
  a 
  measurement 
  of 
  the 
  

   rates 
  of 
  diffusion 
  of 
  several 
  solutions, 
  in 
  pure 
  gelatine. 
  Well- 
  

   washed 
  gelatine 
  was 
  placed 
  on 
  glass 
  plates, 
  allowed 
  to 
  set, 
  and 
  

   drops 
  of 
  different 
  reagents 
  of 
  different 
  strengths 
  were 
  placed 
  

   upon 
  them. 
  The 
  reagents 
  diffused 
  outwards. 
  In 
  some 
  cases, 
  e. 
  g. 
  

   silver 
  nitrate, 
  lead 
  nitrate, 
  ferric 
  chloride, 
  and 
  sodium 
  hydroxide, 
  

   the 
  diffusing 
  solution 
  had 
  a 
  clearly 
  visible 
  outline 
  in 
  the 
  gela- 
  

   tine, 
  which 
  is 
  susceptible 
  of 
  measurement. 
  The 
  measurements 
  

   were 
  made 
  as 
  before, 
  by 
  means 
  of 
  a 
  millimeter 
  scale, 
  estimat- 
  

   ing 
  to 
  tenths 
  of 
  a 
  millimeter, 
  taking 
  care 
  to 
  eliminate 
  parallax. 
  

   The 
  diameter 
  of 
  the 
  drop 
  was 
  taken 
  and 
  the 
  diameter 
  of 
  the 
  

   circle 
  of 
  diffusion 
  (in 
  the 
  case 
  of 
  the 
  precipitates 
  above, 
  to 
  the 
  

   outer 
  limits 
  of 
  the 
  precipitates). 
  Subtracting 
  the 
  diameter 
  

   of 
  the 
  drop 
  from 
  the 
  total 
  diameters 
  of 
  diffusion, 
  and 
  dividing 
  

   by 
  two 
  the 
  radius 
  of 
  diffusion 
  is 
  obtained. 
  It 
  is 
  the 
  rate 
  of 
  

   elongation 
  of 
  this 
  radius 
  which 
  has 
  been 
  measured 
  and 
  plotted, 
  

   and 
  which 
  is 
  called 
  the 
  radius 
  of 
  diffusion 
  here. 
  The 
  chromate 
  

   solutions 
  did 
  not 
  give 
  a 
  diffusing 
  front 
  which 
  was 
  clearly 
  visi- 
  

   ble 
  and 
  susceptible 
  of 
  easy 
  measurement. 
  The 
  yellow 
  color 
  

   due 
  to 
  the 
  chromat-ions 
  became 
  gradually 
  fainter 
  away 
  from 
  

   the 
  drop, 
  coming 
  to 
  an 
  indistinct 
  and 
  diffuse 
  margin. 
  There- 
  

   fore, 
  in 
  the 
  curves 
  given 
  in 
  fig. 
  9 
  those 
  for 
  the 
  chromate 
  solu- 
  

   tions 
  are 
  dotted, 
  since 
  they 
  are 
  not 
  as 
  reliable 
  as 
  the 
  other 
  

   curves. 
  

  

  The 
  curves 
  (see 
  figs. 
  9, 
  10, 
  11) 
  show 
  that 
  a 
  strong 
  solution 
  

   of 
  any 
  reagent 
  diffuses 
  at 
  a 
  greater 
  rate 
  than 
  a 
  weaker 
  solution 
  

   of 
  the 
  same. 
  They 
  show 
  that 
  silver 
  nitrate 
  diffuses 
  more 
  

   rapidly 
  than 
  potassium 
  chromate 
  solutions 
  of 
  the 
  same 
  molec- 
  

   ular 
  concentrations. 
  (The 
  crossing 
  of 
  the 
  silver 
  and 
  chromate 
  

   curves 
  cannot 
  be 
  regarded 
  as 
  established, 
  for 
  the 
  reason 
  stated 
  

   above. 
  Also 
  the 
  apparent 
  straight 
  lines 
  of 
  the 
  chromate 
  curves 
  

   cannot 
  be 
  considered 
  final 
  determinations. 
  The 
  sudden 
  stop- 
  

   page 
  appears 
  to 
  be 
  correct, 
  though 
  this 
  may 
  be 
  due 
  to 
  invisi- 
  

   bility 
  of 
  the 
  chromat-ion 
  below 
  a 
  certain 
  dilution.) 
  It 
  is 
  

   certain 
  that 
  the 
  lead 
  nitrate 
  solutions 
  diffuse 
  more 
  slowly 
  than 
  

   the 
  potassium 
  chromate 
  solutions 
  of 
  equal 
  molecular 
  strengths. 
  

  

  It 
  was 
  considered 
  that 
  this 
  pair 
  of 
  reagents, 
  the 
  lead 
  in 
  out- 
  

   ward, 
  the 
  chromate 
  in 
  inward 
  diffusion, 
  offered 
  a 
  good 
  chance 
  

   of 
  testing 
  the 
  theory 
  that 
  with 
  such 
  a 
  couple 
  bands 
  of 
  precipi- 
  

   tate 
  might 
  be 
  formed 
  which 
  would 
  become 
  closer 
  together 
  out- 
  

   wards, 
  instead 
  of 
  farther 
  apart. 
  Several 
  different 
  strengths 
  of 
  

   these 
  reagents 
  were 
  tried 
  against 
  each 
  other, 
  the 
  precipitates 
  

   formed 
  being 
  apparently 
  continuous 
  to 
  the 
  naked 
  eye. 
  In 
  the 
  

   case 
  of 
  normal 
  lead 
  nitrate 
  against 
  N/200 
  potassium 
  chromate 
  an 
  

   apparently 
  continuous 
  precipitate 
  was 
  formed. 
  The 
  hand 
  lens 
  

   showed 
  no 
  banding, 
  but 
  under 
  the 
  microscope 
  the 
  desired 
  

  

  