﻿3M 
  Posnjah 
  and 
  Merwin 
  — 
  Hydrated 
  Ferric 
  Oxides. 
  

  

  into 
  martite. 
  29 
  Properties 
  thus 
  far 
  observed 
  indicate 
  

   that 
  pseudomorphic 
  turgite 
  is 
  intermediate 
  between 
  

   hematite 
  and 
  primary 
  turgite. 
  The 
  character 
  of 
  the 
  fill- 
  

   ing 
  of 
  the 
  space 
  from 
  which 
  water 
  has 
  escaped 
  affects 
  the 
  

   properties. 
  30 
  

  

  These 
  observations 
  together 
  with 
  studies 
  of 
  density 
  

   and 
  thermal 
  properties 
  (p. 
  337) 
  indicate 
  that 
  turgite 
  is 
  

   probably 
  a 
  solid 
  solution 
  of 
  goethite 
  and 
  hematite, 
  often 
  

   containing 
  considerable 
  but 
  minor 
  amounts 
  of 
  capillary 
  

   and 
  adsorbed 
  water. 
  

  

  Thermal 
  Study. 
  

  

  The 
  identification 
  of 
  hydrates 
  is 
  usually 
  accomplished 
  

   by 
  the 
  study 
  of 
  the 
  relation 
  between 
  their 
  vapor 
  pressure 
  

   and 
  composition 
  under 
  definite 
  conditions. 
  The 
  vapor 
  

   pressure 
  can 
  either 
  be 
  measured 
  directly 
  with 
  a 
  gauge 
  or 
  

   according 
  to 
  a 
  method 
  suggested 
  by 
  van 
  Bemmelen 
  by 
  

   placing 
  the 
  substance 
  in 
  a 
  series 
  of 
  known 
  water 
  vapor 
  

   pressures. 
  These 
  methods 
  require 
  prompt 
  reversibility, 
  

   so 
  that 
  equilibrium 
  can 
  be 
  reached 
  from 
  both 
  sides. 
  

   However, 
  as 
  in 
  many 
  other 
  cases, 
  31 
  this 
  is 
  impossible 
  to 
  

   do 
  with 
  the 
  hydrated 
  ferric 
  oxides, 
  because 
  ferric 
  oxide 
  

   under 
  ordinary 
  conditions 
  cannot 
  be 
  hydrated. 
  The 
  ques- 
  

   tion 
  whether 
  such 
  irreversibility 
  is 
  simply 
  due 
  to 
  the 
  

   extreme 
  slowness 
  of 
  the 
  rate 
  of 
  reaction, 
  or 
  whether 
  

   dehydration 
  causes 
  a 
  more 
  stable 
  molecular 
  rearrange- 
  

   ment, 
  is 
  still 
  an 
  open 
  one. 
  32 
  

  

  For 
  irreversibly 
  hydrated 
  substances 
  Le 
  Chatelier 
  33 
  

   suggested 
  a 
  method 
  which 
  is 
  based 
  on 
  the 
  fact 
  that 
  in 
  gen- 
  

   eral 
  the 
  velocity 
  of 
  such 
  reactions 
  increases 
  according 
  to 
  

   an 
  exponential 
  function 
  of 
  the 
  temperature. 
  It 
  consists 
  

   in 
  measuring 
  the 
  temperature 
  at 
  definite 
  intervals 
  while 
  

   the 
  substance 
  in 
  question 
  is 
  heated 
  at 
  a 
  fairly 
  uniform 
  

   rate. 
  In 
  this 
  way 
  temperature-time 
  curves 
  are 
  obtained 
  

   which 
  show 
  a 
  heat 
  effect 
  similar 
  to 
  those 
  of 
  melting" 
  or 
  

   inversion 
  temperature-time 
  curves. 
  

  

  29 
  Some 
  martites 
  studied 
  contained 
  0-6 
  to 
  0-9% 
  of 
  water, 
  and 
  had 
  decidedly 
  

   lower 
  indices 
  of 
  refraction 
  than 
  hematite. 
  See 
  Trans. 
  Am. 
  Inst. 
  Mining 
  

   Engineers, 
  58, 
  431, 
  1917. 
  

  

  80 
  See 
  p. 
  347. 
  

  

  81 
  Johnston, 
  Zs. 
  phys. 
  Chem. 
  62, 
  330, 
  1908. 
  

  

  82 
  The 
  difference 
  in 
  the 
  behavior 
  of 
  irreversibly 
  and 
  reversibly 
  hydrated 
  

   substances 
  is 
  often 
  explained 
  by 
  assuming 
  that 
  the 
  former 
  contain 
  constitu- 
  

   tional 
  water 
  while 
  the 
  latter 
  contain 
  water 
  of 
  crystallization. 
  This, 
  however, 
  

   does 
  not 
  explain 
  anything, 
  as 
  the 
  two 
  words 
  have 
  no 
  exact 
  physical 
  meaning. 
  

  

  88 
  Zs. 
  phys. 
  Chem. 
  1, 
  396, 
  1887. 
  

  

  