﻿364 
  ME. 
  A. 
  JOHNSTONE 
  ON 
  THE 
  ACTION 
  OF 
  WATER 
  ON 
  

  

  Division 
  II., 
  or 
  Hydromicas. 
  

  

  The 
  micas 
  assigned 
  to 
  this 
  section 
  are, 
  in 
  general 
  appearance, 
  

   quite 
  similar 
  to 
  those 
  of 
  the 
  first 
  division, 
  except 
  that 
  they 
  show 
  a 
  

   less 
  splendent 
  degree, 
  and 
  a 
  more 
  pearly 
  kind, 
  of 
  lustre, 
  and 
  are 
  

   wanting 
  in 
  elasticity 
  or 
  at 
  least 
  have 
  that 
  property 
  poorly 
  deve- 
  

   loped. 
  The 
  fine 
  laminae 
  are 
  in 
  many 
  specimens 
  somewhat 
  talc-like 
  

   in 
  being 
  tough 
  and 
  flexible, 
  but 
  in 
  other 
  cases 
  are 
  distinctly 
  brittle, 
  

   as 
  can 
  be 
  proved 
  by 
  their 
  quickly 
  breaking 
  to 
  pieces 
  when 
  pressed 
  

   between 
  the 
  finger 
  and 
  thumb. 
  The 
  " 
  feel 
  " 
  is 
  usually 
  more 
  or 
  less 
  

   greasy. 
  The 
  hardness 
  and 
  density 
  are 
  generally 
  slightly 
  less 
  in 
  

   this 
  section 
  than 
  in 
  Division 
  I. 
  

  

  Margarodite, 
  gilbertite, 
  damourite, 
  and 
  sericite 
  are 
  mineralogists' 
  

   names 
  for 
  varieties 
  possessing 
  the 
  same 
  composition 
  as 
  muscovite, 
  

   differing 
  from 
  the 
  latter 
  mineral 
  merely 
  in 
  containing 
  at 
  least 
  about 
  

   5 
  per 
  cent, 
  of 
  water. 
  All 
  of 
  these 
  varieties, 
  it 
  is 
  evident 
  enough, 
  

   ought 
  to 
  be 
  known 
  under 
  one 
  term. 
  The 
  common 
  name 
  proposed 
  

   for 
  them 
  by 
  the 
  Author 
  is 
  hydromuscovite. 
  

  

  Paragonite 
  is 
  a 
  hydromuscovite 
  with 
  soda 
  replacing 
  part 
  or 
  all 
  of 
  

   the 
  potash. 
  

  

  Voigite 
  is 
  the 
  usual 
  mineralogical 
  name 
  for 
  a 
  hydrous 
  mica 
  

   having 
  the 
  same 
  composition 
  as 
  biotite, 
  and 
  which 
  therefore 
  the 
  

   Author 
  prefers 
  to 
  call 
  hydrobiotite. 
  

  

  The 
  minerals 
  usually 
  termed 
  vermiculite 
  and 
  jeflreysite 
  are 
  other 
  

   examples 
  of 
  hydrobiotites. 
  

  

  Eahlunite 
  and 
  the 
  micaceous 
  chlorites 
  are 
  related 
  to 
  the 
  hydro- 
  

   biotites. 
  

  

  In 
  the 
  Table 
  on 
  p. 
  365 
  the 
  Author 
  has 
  attempted 
  to 
  give 
  in 
  a 
  

   concise 
  form 
  his 
  classification 
  of 
  micas. 
  

  

  Experimental 
  and 
  Observational. 
  

  

  Two 
  pieces 
  of 
  muscovite, 
  detached 
  from 
  a 
  larger 
  piece 
  *, 
  having, 
  

   as 
  ascertained 
  by 
  analysis, 
  the 
  chemical 
  composition 
  given 
  below, 
  

   were 
  suspended, 
  one 
  in 
  a 
  wide-mouthed 
  vessel 
  containing 
  two 
  litres 
  

   of 
  jjure 
  distilled 
  water, 
  and 
  the 
  other 
  in 
  a 
  like 
  vessel 
  containing 
  

   two 
  litres 
  of 
  distilled 
  water 
  saturated 
  ivitli 
  carbonic 
  acid 
  gas. 
  These 
  

   two 
  muscovite 
  fragments, 
  which 
  were 
  equal 
  in 
  weight, 
  were 
  allowed 
  

   to 
  hang 
  completely 
  immersed 
  for 
  a 
  whole 
  year 
  (from 
  October 
  1 
  , 
  

   1887, 
  to 
  October 
  1, 
  1888) 
  in 
  their 
  respective 
  liquids. 
  The 
  liquids, 
  

   which 
  had 
  an 
  average 
  temperature 
  all 
  the 
  year 
  round 
  of 
  about 
  60° 
  

   Fahr., 
  were 
  gently 
  agitated 
  almost 
  every 
  alternate 
  day 
  during 
  the 
  

   period 
  of 
  immersion 
  for 
  about 
  ten 
  minutes 
  at 
  a 
  time. 
  

  

  When 
  the 
  specimens 
  at 
  the 
  end 
  of 
  the 
  year 
  were 
  removed 
  from 
  

   their 
  baths 
  they 
  were 
  observed 
  to 
  have 
  changed 
  physically 
  in 
  much 
  

   the 
  same 
  manner 
  and 
  to 
  nearly 
  the 
  same 
  extent. 
  They 
  bad 
  super- 
  

   ficially 
  dimmed 
  considerably 
  in 
  lustre, 
  had 
  decreased 
  slightly 
  in 
  

   hardness, 
  and 
  had 
  lost 
  a 
  good 
  deal 
  of 
  their 
  original 
  elasticity 
  and 
  

  

  * 
  Which 
  vas 
  removed 
  from 
  the 
  heart 
  of 
  a 
  large 
  compact 
  mass 
  of 
  granite. 
  

  

  