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  Posnjak 
  and 
  Merwin 
  — 
  Hydrated 
  Ferric 
  Oxides. 
  

  

  although 
  Lacroix 
  2 
  makes 
  a 
  definite 
  optical 
  distinction. 
  

   Hydrogoethite 
  has 
  been 
  supposed 
  to 
  contain 
  more 
  water 
  

   than 
  goethite 
  but 
  in 
  its 
  optical 
  properties 
  is 
  said 
  to 
  be 
  

   similar 
  to 
  lepidocrocite. 
  The 
  other 
  names 
  in 
  the 
  list 
  have 
  

   been 
  given 
  because 
  of 
  certain 
  indefinite 
  physical 
  differ- 
  

   ences 
  associated 
  with 
  plausible 
  but 
  not 
  well 
  established 
  

   chemical 
  formulas. 
  

  

  Below 
  we 
  have 
  attempted 
  to 
  bring 
  together 
  the 
  signi- 
  

   ficant 
  results 
  of 
  synthetic 
  chemical 
  work 
  already 
  done 
  on 
  

   the 
  hydrated 
  ferric 
  oxides. 
  

  

  Work 
  previously 
  done. 
  

  

  Early 
  investigations 
  were 
  brought 
  to 
  a 
  conclusion 
  by 
  

   Tomasi 
  3 
  who 
  summarized 
  the 
  synthetic 
  researches 
  of 
  his 
  

   predecessors 
  and 
  his 
  own 
  by 
  classifying 
  the 
  hydrated 
  fer- 
  

   ric 
  oxides 
  in 
  accordance 
  with 
  their 
  color 
  in 
  two 
  series, 
  

   differing 
  in 
  their 
  properties: 
  the 
  red 
  and 
  the 
  yellow 
  

   hydrates. 
  For 
  each 
  series 
  a 
  number 
  of 
  definite 
  hydrates 
  

   and 
  their 
  methods 
  of 
  preparation 
  were 
  described. 
  Gen- 
  

   erally, 
  according 
  to 
  Tomasi, 
  the 
  red 
  hydrates 
  were 
  formed 
  

   by 
  precipitation 
  of 
  ferric 
  salts 
  with 
  alkalies, 
  while 
  the 
  

   yellow 
  hydrates 
  were 
  produced 
  by 
  oxidation 
  of 
  ferrous 
  

   oxide 
  or 
  carbonate. 
  

  

  More 
  recently, 
  vanBemmelen 
  4 
  investigated 
  such 
  syn- 
  

   thetic 
  hydrated 
  ferric 
  oxides 
  in 
  respect 
  to 
  their 
  water 
  

   content. 
  He 
  came 
  to 
  the 
  conclusion 
  that 
  the 
  reddish- 
  

   brown 
  substance 
  is 
  a 
  colloid 
  and 
  not 
  a 
  hydrate 
  (or 
  

   hydrates) 
  as 
  previously 
  described. 
  This 
  substance 
  he 
  

   says 
  "may 
  undergo 
  various 
  changes 
  and 
  therefore 
  has 
  

   no 
  definite 
  composition 
  but 
  always 
  comes 
  to 
  an 
  equili- 
  

   brium 
  depending 
  upon 
  the 
  surrounding 
  vapor 
  tension." 
  

   The 
  yellow 
  substance 
  is 
  also 
  a 
  colloid. 
  However, 
  "it 
  is 
  

   not 
  a 
  variety 
  of 
  the 
  brownish-red 
  substance, 
  since 
  it 
  is 
  

   from 
  the 
  time 
  of 
  its 
  formation 
  in 
  a 
  denser 
  and 
  more 
  

   stable, 
  though 
  variable 
  condition.' 
  ' 
  He 
  found 
  that 
  

   between 
  50° 
  and 
  200° 
  the 
  yellow 
  colloid 
  retains 
  its 
  water 
  

   much 
  more 
  tenaciously 
  than 
  the 
  brownish-red, 
  but 
  above 
  

   200° 
  both 
  colloids 
  attain 
  the 
  same 
  composition 
  and 
  

   become 
  identical. 
  5 
  

  

  2 
  Mineralogie 
  de 
  la 
  France, 
  3, 
  360. 
  

  

  8 
  Bicerche 
  sulle 
  f 
  ormule 
  de 
  constituzione 
  dei 
  compositi 
  f 
  errici, 
  Florence, 
  

   1879; 
  Ber. 
  deutsch. 
  chem. 
  Ges., 
  12, 
  1929, 
  2334, 
  1879. 
  

  

  4 
  Van 
  Bemmelen's 
  collected 
  papers, 
  published 
  under 
  the 
  title, 
  "Die 
  

   Absorption," 
  pp. 
  70-77. 
  

  

  "In 
  a 
  later 
  paper 
  (Zs. 
  anorg. 
  Chem., 
  20, 
  185, 
  1899; 
  Die 
  Absorption, 
  p. 
  

   370), 
  which 
  deals 
  with 
  the 
  hydration 
  and 
  dehydration 
  of 
  the 
  reddish-brown 
  

  

  