﻿of 
  a 
  Heterogeneous 
  Reaction. 
  451 
  

  

  value 
  of 
  the 
  velocity 
  constant 
  K. 
  The 
  method 
  of 
  determining 
  

   and 
  controlling 
  the 
  rate 
  of 
  stirring 
  was 
  as 
  follows 
  : 
  To 
  the 
  axle 
  

   of 
  the 
  stirrer 
  was 
  attached 
  a 
  mechanism 
  which 
  rang 
  a 
  bell 
  at 
  

   every 
  100th 
  revolution. 
  A 
  stop-watch 
  was 
  started 
  at 
  the 
  first 
  

   ring 
  of 
  the 
  bell 
  after 
  the 
  beginning 
  of 
  the 
  reaction 
  period 
  and 
  

   was 
  stopped 
  at 
  the 
  nearest 
  ring 
  to 
  the 
  end. 
  The 
  elapsed 
  time 
  

   so 
  recorded 
  was 
  that 
  required 
  for 
  an 
  even 
  multiple 
  of 
  100 
  

   revolutions, 
  from 
  which 
  the 
  average 
  rate 
  for 
  the 
  period 
  and 
  

   the 
  necessary 
  correction 
  to 
  K 
  were 
  readily 
  calculable. 
  Fur- 
  

   thermore, 
  if 
  the 
  rate 
  were 
  exactly 
  200 
  revolutions 
  per 
  minnte 
  

   the 
  rings 
  would 
  evidently 
  coincide 
  with 
  the 
  passage 
  of 
  the 
  

   second 
  hand 
  of 
  the 
  stop-watch 
  over 
  the 
  even 
  minute 
  and 
  half 
  

   minute 
  on 
  the 
  dial.* 
  Any 
  variation 
  in 
  the 
  speed 
  was 
  imme- 
  

   diately 
  shown 
  by 
  the 
  failure 
  of 
  this 
  coincidence, 
  and 
  was 
  

   promptly 
  corrected 
  by 
  adjusting 
  a 
  rheostat 
  in 
  series 
  with 
  the 
  

   stirring 
  motor. 
  In 
  this 
  way 
  the 
  average 
  rate 
  was 
  kept, 
  in 
  the 
  

   great 
  majority 
  of 
  cases, 
  within 
  0*2 
  per 
  cent 
  of 
  the 
  correct 
  value, 
  

   and 
  only 
  in 
  rare 
  cases 
  did 
  the 
  correction 
  to 
  be 
  applied 
  reach 
  

   0*5 
  per 
  cent. 
  In 
  most 
  of 
  the 
  experiments, 
  therefore, 
  the 
  effect 
  

   of 
  these 
  corrections, 
  sometimes 
  positive 
  and 
  sometimes 
  negative, 
  

   upon 
  the 
  final 
  result 
  (i. 
  e. 
  the 
  average 
  value 
  of 
  K 
  for 
  the 
  

   whole 
  experiment), 
  was 
  nearly 
  or 
  wholly 
  negligible. 
  

  

  Corrections 
  for 
  Evaporation. 
  — 
  Except 
  at 
  the 
  lower 
  tempera- 
  

   tures 
  the 
  effects 
  of 
  evaporation 
  had 
  to 
  be 
  taken 
  into 
  account, 
  

   since 
  the 
  liquid 
  in 
  each 
  experiment 
  was 
  stirred 
  in 
  an 
  open 
  

   vessel 
  for 
  a 
  period 
  varying 
  from 
  40 
  to 
  90 
  minutes. 
  Such 
  

   evaporation 
  may 
  affect 
  the 
  apparent 
  reaction 
  velocity, 
  as 
  cal- 
  

   culated 
  from 
  equation 
  (I), 
  in 
  two 
  ways 
  : 
  first 
  by 
  decreasing 
  the 
  

   volume 
  v, 
  and 
  second 
  by 
  altering 
  the 
  values 
  of 
  the 
  concentra- 
  

   sions 
  of 
  iodine. 
  

  

  The 
  first 
  error 
  drops 
  out 
  if 
  we 
  use 
  the 
  actual 
  value 
  of 
  v 
  in 
  

   calculating 
  K, 
  which 
  has 
  been 
  done 
  in 
  the 
  present 
  work. 
  Since 
  

   the 
  rate 
  of 
  evaporation 
  is 
  proportional 
  to 
  the 
  area 
  of 
  the 
  free 
  

   surface 
  of 
  the 
  liquid, 
  which 
  remains 
  nearly 
  constant,f 
  the 
  

   decrease 
  in 
  volume 
  per 
  minute 
  may 
  be 
  regarded 
  as 
  constant. 
  

   In 
  the 
  experiments 
  of 
  the 
  writer 
  this 
  rate 
  of 
  evaporation 
  was 
  

   determined 
  either 
  by 
  special 
  blank 
  experiments 
  made 
  under 
  

   like 
  conditions, 
  or 
  by 
  determining 
  the 
  actual 
  loss 
  during 
  the 
  

   experiment 
  itself, 
  by 
  carefully 
  measuring 
  the 
  volume 
  remaining 
  

   at 
  the 
  end. 
  Since 
  the 
  rate 
  of 
  evaporation 
  depends 
  somewhat 
  

   on 
  the 
  external 
  conditions 
  the 
  latter 
  procedure 
  is 
  prefer- 
  

   able, 
  and 
  was 
  employed 
  in 
  most 
  cases, 
  especially 
  when 
  the 
  rate 
  

   of 
  evaporation 
  was 
  large. 
  The 
  total 
  change 
  in 
  volume 
  up 
  to 
  

   the 
  middle 
  of 
  each 
  reaction 
  period 
  was 
  then 
  calculated, 
  and 
  the 
  

  

  * 
  Another 
  watch 
  was 
  of 
  course 
  needed 
  to 
  fix 
  the 
  beginning 
  and 
  end 
  of 
  the 
  

   reaction 
  period. 
  

  

  f 
  In 
  reality 
  the 
  concavity 
  of 
  the 
  surface 
  produced 
  by 
  the 
  stirring 
  increases 
  

   slightly 
  as 
  the 
  volume 
  of 
  liquid 
  diminishes. 
  

  

  