﻿340 
  Posnjak 
  and 
  Merwin 
  — 
  Hydrated 
  Ferric 
  Oxides. 
  

  

  cessful. 
  The 
  conditions 
  of 
  formation 
  of 
  hydrated 
  ferric 
  

   oxides 
  are, 
  therefore, 
  not 
  known 
  and 
  the 
  numerous 
  

   attempts 
  to 
  synthesize 
  them 
  produced 
  apparently 
  only 
  

   substances 
  of 
  an 
  indefinite 
  composition, 
  which 
  gave 
  rise 
  

   to 
  the 
  idea 
  of 
  the 
  existence 
  of 
  a 
  large 
  number 
  of 
  hydrated 
  

   compounds. 
  In 
  regard 
  to 
  these 
  synthetic 
  products 
  one 
  

   fact, 
  however, 
  stands 
  out 
  more 
  definitely, 
  and 
  that 
  is 
  that 
  

   they 
  were 
  either 
  red 
  or 
  yellow. 
  Van 
  Bemmelen 
  50 
  inves- 
  

   tigated 
  the 
  behavior 
  of 
  these 
  hydrated 
  ferric 
  oxides 
  and 
  

   found 
  their 
  composition 
  to 
  vary 
  considerably 
  with 
  the 
  

   surrounding 
  conditions. 
  According 
  to 
  prevalent 
  ideas 
  

   he 
  called 
  the 
  red 
  and 
  the 
  yellow 
  substances 
  colloids 
  and 
  

   believed 
  that 
  as 
  such 
  they 
  were 
  not 
  definite 
  compounds 
  

   and 
  therefore 
  could 
  not 
  essentially 
  differ 
  from 
  one 
  

   another. 
  Our 
  present 
  knowledge, 
  however, 
  shows 
  that 
  

   colloids 
  are 
  not 
  particular 
  chemical 
  substances 
  or 
  a 
  dis- 
  

   tinct 
  form 
  of 
  the 
  substance, 
  as 
  was 
  previously 
  thought, 
  

   but 
  that 
  any 
  substance 
  under 
  certain 
  conditions 
  can 
  be 
  

   made 
  to 
  have 
  properties 
  characteristic 
  of 
  colloids 
  ; 
  the 
  

   term 
  colloid, 
  therefore, 
  does 
  not 
  characterize 
  the 
  sub- 
  

   stance 
  as 
  such, 
  but 
  only 
  indicates 
  properties 
  which 
  are 
  

   due 
  to 
  a 
  certain 
  condition 
  of 
  the 
  substance. 
  It 
  is 
  of 
  inter- 
  

   est 
  now 
  that 
  van 
  Bemmelen 
  noticed 
  a 
  certain 
  difference 
  

   in 
  the 
  behavior 
  of 
  the 
  red 
  and 
  yellow 
  substances. 
  He 
  

   found 
  that 
  between 
  50° 
  and 
  200° 
  the 
  latter 
  held 
  its 
  water 
  

   much 
  more 
  tenaciously 
  than 
  the 
  former 
  and 
  that 
  only 
  

   above 
  about 
  200° 
  the 
  two 
  did 
  become 
  identical 
  in 
  their 
  

   composition 
  and 
  behavior. 
  Colloidal 
  properties 
  probably 
  

   can 
  not 
  be 
  responsible 
  for 
  this 
  difference 
  and 
  in 
  view 
  of 
  

   the 
  marked 
  difference 
  in 
  color 
  it 
  is 
  more 
  likely 
  that 
  the 
  

   chemical 
  nature 
  of 
  these 
  two 
  substances 
  is 
  different, 
  

   although 
  both 
  of 
  them 
  may 
  possess 
  similar 
  colloidal 
  

   properties 
  due 
  to 
  their 
  physical 
  condition. 
  Fischer 
  51 
  

   tried 
  to 
  verify 
  this 
  assumption 
  and 
  found 
  that 
  on 
  dehy- 
  

   dration 
  the 
  yellow 
  synthetic 
  substance 
  behaves 
  similarly 
  

   to 
  the 
  natural 
  "limonites'' 
  and 
  at 
  about 
  200° 
  changes 
  

   its 
  color 
  and 
  becomes 
  red. 
  We 
  were 
  able 
  to 
  confirm 
  the 
  

   observation 
  that 
  the 
  yellow 
  synthetic 
  substance 
  behaves 
  

   like 
  a 
  hydrated 
  compound 
  ; 
  in 
  f^o;. 
  1 
  (No. 
  24) 
  the 
  dehydra- 
  

   tion 
  curve 
  of 
  a 
  yellow 
  hydrated 
  ferric 
  oxide 
  (formed 
  by 
  

   the 
  hydrolysis 
  of 
  a 
  dilute 
  solution 
  of 
  ferric 
  sulphate) 
  is 
  

   given 
  and 
  its 
  similarity 
  with 
  the 
  monohydrates 
  of 
  ferric 
  

   oxide 
  is 
  readily 
  seen. 
  The 
  optical 
  properties 
  of 
  such 
  

  

  r,0 
  Loc. 
  cit. 
  

   Bl 
  Loc. 
  cit. 
  

  

  