﻿Posnjak 
  and 
  Merwin 
  — 
  Hydrated 
  Ferric 
  Oxides. 
  321 
  

  

  If 
  instead 
  of 
  the 
  random 
  orientation 
  there 
  is 
  a 
  tendency 
  

   toward 
  parallelism, 
  then 
  there 
  will 
  be, 
  for 
  any 
  bundle 
  of 
  

   fibers 
  considered 
  as 
  a 
  unit, 
  three 
  principal 
  refractive 
  

   indices 
  : 
  y 
  = 
  240, 
  p 
  variable 
  between 
  240 
  and 
  2-33, 
  and 
  

   a 
  variable 
  between 
  2-26 
  and 
  2-33. 
  For 
  example 
  y 
  = 
  240, 
  

   p 
  = 
  2-35, 
  a 
  = 
  2-31 
  ; 
  or 
  y 
  = 
  240, 
  p 
  = 
  2-34, 
  a 
  = 
  2-32. 
  

  

  The 
  presence 
  of 
  variable 
  amounts 
  of 
  impurities 
  in, 
  and 
  

   the 
  physical 
  behavior 
  of, 
  all 
  our 
  analyzed 
  samples 
  of 
  

   bladed 
  or 
  fibrous 
  goethite 
  indicate 
  that 
  the 
  fibers 
  are 
  not 
  

   so 
  tightly 
  packed 
  as 
  just 
  supposed 
  but 
  that 
  the 
  impurities 
  

   (excess 
  of 
  water, 
  silica, 
  etc.) 
  are 
  in 
  thin 
  films 
  among 
  the 
  

   fibers. 
  By 
  calculating 
  as 
  nearly 
  as 
  possible 
  the 
  volumes 
  

   of 
  the 
  various 
  impurities 
  and 
  assuming 
  the 
  additive 
  rela- 
  

   tion 
  for 
  refractive 
  index, 
  the 
  calculated 
  mean 
  refractive 
  

   indices 
  for 
  several 
  analyzed 
  specimens 
  have 
  been 
  found. 
  

   Comparison 
  of 
  them 
  with 
  the 
  observed 
  values 
  is 
  found 
  in 
  

   Table 
  IV. 
  Most 
  of 
  these 
  specimens 
  were 
  called 
  limonite 
  

   by 
  the 
  collectors. 
  

  

  Table 
  IV. 
  

  

  

  Mean 
  refractive 
  

  

  i 
  index 
  

  

  Mean 
  refractive 
  

  

  No. 
  

  

  calculated 
  from 
  

  

  analyses 
  

  

  index 
  measured 
  

  

  9 
  

  

  2-28„ 
  

  

  

  2-30 
  

  

  10 
  

  

  2-27 
  2 
  

  

  

  2-28 
  

  

  13 
  

  

  2-22 
  5 
  

  

  

  2-21 
  

  

  14 
  

  

  2-22 
  2 
  

  

  

  2-19 
  

  

  15 
  

  

  2-20 
  2 
  

  

  

  2-19 
  

  

  16 
  

  

  217 
  8 
  

  

  

  217 
  

  

  17 
  

  

  216 
  8 
  

  

  

  217 
  

  

  18 
  

  

  2-11, 
  

  

  

  212 
  

  

  Limonite: 
  — 
  When 
  Fe 
  2 
  3 
  -H 
  2 
  is 
  precipitated 
  and 
  hard- 
  

   ened 
  under 
  natural 
  conditions 
  which 
  do 
  not 
  permit 
  defi- 
  

   nite 
  crystallization, 
  a 
  porous 
  mass 
  is 
  formed 
  containing 
  

   considerable 
  excess 
  of 
  water, 
  besides 
  other 
  impurities. 
  

   So 
  indefinite 
  is 
  the 
  material 
  that 
  it 
  cannot 
  be 
  character- 
  

   ized 
  very 
  satisfactorily. 
  Its 
  color 
  in 
  fine 
  powder 
  is 
  

   scarcely 
  distinguishable 
  from 
  that 
  of 
  fibrous 
  goethite. 
  

   Although 
  often 
  occurring 
  in 
  compact 
  layered 
  or 
  botry- 
  

   oidal 
  forms 
  it 
  does 
  not 
  possess 
  decided 
  fibrous 
  fracture. 
  

   The 
  earthy 
  varieties 
  consist 
  of 
  minute 
  loosely 
  agglom- 
  

   erated 
  spheroidal 
  grains. 
  Air-dried 
  material 
  possesses 
  

   some 
  pores 
  of 
  such 
  size 
  that, 
  although 
  microscopically 
  

   invisible, 
  they 
  readily 
  fill 
  with 
  such 
  liquids 
  as 
  are 
  used 
  to 
  

   immerse 
  the 
  grains 
  for 
  microscopic 
  study. 
  Thus, 
  refrac- 
  

   tive-index 
  determinations 
  cannot 
  be 
  definitely 
  inter- 
  

   preted; 
  air-dried 
  material 
  does 
  have 
  observed 
  apparent 
  

  

  