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  Posnjak 
  and 
  Merwin 
  — 
  Hydrated 
  Ferric 
  Oxides. 
  

  

  ognized. 
  The 
  compositions 
  of 
  these 
  products, 
  however, 
  

   agreed 
  remarkably 
  well 
  with 
  the 
  formulas 
  given 
  for 
  

   hydrates 
  of 
  ferric 
  oxide, 
  and 
  the 
  density 
  determined 
  upon 
  

   this 
  material 
  was 
  almost 
  exactly 
  as 
  given 
  in 
  text-books 
  

   for 
  the 
  corresponding 
  minerals. 
  The 
  yellow 
  colloid 
  did 
  

   not 
  change 
  under 
  the 
  pressure 
  applied 
  by 
  Buff 
  in 
  the 
  

   temperature 
  interval 
  of 
  40-70° 
  and 
  he 
  therefore 
  assumes 
  

   that 
  it 
  is 
  "not 
  a 
  colloid 
  but 
  a 
  true 
  hydrate' 
  ' 
  which 
  he 
  

   suspects 
  is 
  xanthosiderite. 
  But 
  specimens 
  of 
  the 
  two 
  

   hydrated 
  mineral 
  ferric 
  oxides, 
  investigated 
  under 
  similar 
  

   conditions, 
  — 
  "limonite'' 
  at 
  a 
  temperature 
  of 
  70-75° 
  and 
  

   turgite 
  at 
  40-50°, 
  — 
  did 
  not 
  show 
  any 
  change 
  whatsoever 
  

   (turgite 
  and 
  ''limonite," 
  respectively, 
  should 
  be 
  formed 
  if 
  

   the 
  two 
  foregoing 
  stability 
  relations 
  were 
  true). 
  This, 
  

   however, 
  did 
  not 
  deter 
  Ruff 
  from 
  assuming 
  that 
  the 
  prob- 
  

   lems 
  of 
  composition 
  and 
  the 
  stability 
  relations 
  of 
  the 
  

   hydrated 
  ferric 
  oxides 
  were 
  thus 
  satisfactorily 
  solved, 
  

   and 
  he 
  discusses 
  on 
  that 
  basis 
  the 
  formation 
  of 
  natural 
  

   deposits. 
  8 
  

  

  Another 
  investigation 
  was 
  undertaken 
  by 
  H. 
  W. 
  

   Fischer. 
  9 
  By 
  hydrolyzing 
  ferric 
  chloride 
  solutions 
  he 
  

   came 
  to 
  the 
  conclusion 
  that 
  at 
  least 
  three 
  entirely 
  differ- 
  

   ent 
  substances 
  exist 
  among 
  the 
  colloidal 
  hydrated 
  ferric 
  

   oxides: 
  the 
  yellow, 
  reddish-black 
  and 
  vanBemmelen's 
  

   reddish-brown 
  colloids. 
  10 
  Now 
  Fischer 
  found 
  that 
  limon- 
  

   ite 
  has 
  a 
  characteristic 
  dehydration 
  curve 
  ; 
  water 
  is 
  lost 
  

   rapidly 
  below 
  100°, 
  then 
  sparingly, 
  although 
  continu- 
  

   ously, 
  up 
  to 
  about 
  165° 
  ; 
  above 
  that 
  temperature 
  the 
  loss 
  

   is 
  very 
  large 
  and 
  is 
  accompanied 
  by 
  a 
  change 
  in 
  color 
  

   from 
  yellow 
  to 
  red. 
  Just 
  before 
  the 
  substance 
  is 
  heated 
  

   above 
  165° 
  it 
  has 
  approximately 
  1 
  mol. 
  of 
  water. 
  This 
  

   behavior 
  of 
  limonite 
  corresponds, 
  according 
  to 
  Fischer, 
  

   to 
  that 
  of 
  a 
  colloidal 
  hydrate, 
  11 
  and 
  as 
  the 
  synthetic 
  yel- 
  

   low 
  colloid 
  shows 
  a 
  similar 
  dehydration 
  curve 
  he 
  con- 
  

   cludes 
  that 
  they 
  are 
  identical. 
  The 
  reddish-black 
  colloid 
  

   he 
  thinks 
  is 
  identical 
  with 
  hematite 
  and 
  the 
  reddish-brown 
  

   colloid 
  he 
  believes 
  (no 
  experiments 
  made) 
  is 
  identical 
  

  

  8 
  As 
  a 
  result 
  of 
  the 
  present 
  investigation 
  the 
  ' 
  l 
  data 
  ' 
  ' 
  presented 
  by 
  Ruff 
  

   concerning 
  the 
  compositions 
  and 
  densities 
  of 
  the 
  hydrated 
  ferric 
  oxides 
  can 
  

   only 
  be 
  considered 
  accidental. 
  

  

  9 
  Zs. 
  anorg. 
  Chem., 
  66, 
  37, 
  1910. 
  

  

  10 
  Fischer 
  defines 
  colloids 
  as 
  "chemical 
  individuals 
  whose 
  total 
  energy 
  

   depends 
  besides 
  the 
  variables 
  of 
  state 
  (Zustandsvariablen) 
  on 
  an 
  additive 
  

   quantity 
  K, 
  which 
  may 
  be 
  interpreted 
  as 
  a 
  function 
  of 
  time. 
  ,, 
  

  

  "Hintze, 
  in 
  his 
  "Handbuch 
  der 
  Mineralogie, 
  ' 
  ' 
  vol.l 
  (2), 
  p. 
  2008, 
  draws 
  

   the 
  conclusion 
  that 
  limonite 
  is 
  the 
  colloidal 
  form 
  of 
  goethite. 
  

  

  