﻿Posnjak 
  and 
  Merwin 
  — 
  Hydrated 
  Ferric 
  Oxides. 
  327 
  

  

  hydrated 
  ferric 
  oxides 
  (except 
  turgite). 
  Crystalline 
  goe- 
  

   thite 
  (No. 
  5) 
  shows 
  the 
  highest 
  decomposition 
  tempera- 
  

   ture, 
  while 
  the 
  "amorphous" 
  hydrated 
  ferric 
  oxide 
  (No. 
  

   22) 
  decomposes 
  at 
  the 
  lowest 
  temperature. 
  No 
  definite 
  

   identification 
  of 
  hydrated 
  ferric 
  oxides 
  is 
  therefore 
  pos- 
  

   sible 
  on 
  the 
  basis 
  of 
  these 
  curves. 
  

  

  As 
  has 
  been 
  stated, 
  the 
  curves 
  for 
  turgite 
  differed 
  

   greatly 
  from 
  those 
  of 
  the 
  other 
  substances. 
  A 
  slight 
  heat 
  

   effect 
  is 
  shown 
  at 
  about 
  110 
  °, 
  which, 
  however, 
  is 
  not 
  sharp 
  

   and 
  the 
  curve 
  falls 
  off 
  gradually 
  ; 
  no 
  other 
  heat 
  effect 
  was 
  

   observed 
  between 
  this 
  temperature 
  and 
  600°. 
  The 
  shape 
  

   of 
  the 
  curve 
  plainly 
  signifies 
  that 
  the 
  water 
  in 
  turgite 
  is 
  

   held 
  in 
  a 
  different 
  way 
  from 
  that 
  in 
  the 
  other 
  hydrated 
  

   ferric 
  oxides. 
  

  

  It 
  has 
  been 
  previously 
  mentioned 
  that 
  Fischer 
  36 
  

   attempted 
  the 
  identification 
  of 
  "limonites" 
  by 
  determin- 
  

   ing 
  their 
  decomposition 
  temperature 
  by 
  slow 
  dehydration 
  

   at 
  fixed 
  temperatures. 
  The 
  choosing 
  of 
  the 
  decomposi- 
  

   tion 
  temperature 
  for 
  purposes 
  of 
  identification 
  is 
  rather 
  

   unfortunate 
  as 
  this 
  temperature 
  may 
  be 
  influenced 
  by 
  the 
  

   physical 
  character 
  of 
  the 
  substance. 
  The 
  rate 
  of 
  reac- 
  

   tion 
  also 
  has 
  to 
  be 
  considered. 
  The 
  longer 
  the 
  substance 
  

   is 
  kept 
  at 
  each 
  temperature, 
  the 
  more, 
  in 
  all 
  probability, 
  

   would 
  the 
  influence 
  of 
  the 
  rate 
  of 
  reaction 
  be 
  reduced. 
  

   However, 
  the 
  irreversibility 
  of 
  the 
  reaction 
  leaves 
  no 
  defi- 
  

   nite 
  criterion 
  and 
  the 
  exact 
  decomposition 
  temperature 
  

   must 
  remain 
  uncertain. 
  

  

  Fischer 
  found 
  that 
  only 
  above 
  about 
  160° 
  did 
  "limon- 
  

   ite" 
  decompose 
  (the 
  decomposition 
  curve 
  however 
  was 
  

   not 
  completed), 
  although 
  an 
  appreciable 
  amount 
  of 
  water 
  

   was 
  lost 
  at 
  lower 
  temperatures. 
  Just 
  before 
  decomposi- 
  

   tion 
  his 
  samples 
  contained 
  between 
  8-5 
  and 
  10-8% 
  water 
  

   which 
  is 
  much 
  less 
  than 
  required 
  by 
  the 
  formula 
  usually 
  

   assigned 
  to 
  "limonite." 
  However, 
  no 
  definite 
  conclu- 
  

   sion 
  could 
  be 
  drawn 
  from 
  these 
  data 
  as 
  the 
  substance 
  was 
  

   not 
  sufficiently 
  defined 
  (only 
  the 
  amount 
  of 
  water 
  deter- 
  

   mined 
  by 
  ignition 
  loss 
  is 
  given). 
  

  

  In 
  view 
  of 
  the 
  sharpness 
  of 
  the 
  curves 
  obtained 
  with 
  

   Le 
  Chatelier 
  's 
  method 
  it 
  was 
  thought 
  that 
  slow 
  dehydra- 
  

   tion 
  at 
  fixed 
  temperatures 
  might 
  be 
  made 
  to 
  yield 
  valu- 
  

   able 
  results. 
  In 
  principle 
  both 
  are 
  identical 
  ; 
  however, 
  as 
  

   the 
  amount 
  of 
  water 
  lost 
  by 
  the 
  substance 
  at 
  each 
  succes- 
  

   sive 
  step 
  is 
  determined 
  by 
  the 
  latter 
  method, 
  the 
  resulting 
  

  

  3G 
  Loc. 
  cit. 
  

  

  Am. 
  Jour. 
  Sci.— 
  Fourth 
  Series, 
  Vol. 
  XLVIT, 
  No. 
  281.— 
  May, 
  1910. 
  

   23 
  

  

  