﻿Mack 
  and 
  Hulett 
  — 
  Water 
  Content 
  of 
  Coal. 
  99 
  

  

  suitable 
  density. 
  There 
  was 
  no 
  evidence 
  of 
  a 
  combination 
  

   between 
  the 
  water 
  and 
  gels. 
  The 
  results 
  showed 
  that 
  the 
  

   water 
  was 
  continuously 
  removed 
  as 
  the 
  surrounding 
  water 
  

   vapor 
  pressure 
  was 
  lowered, 
  and 
  in 
  general 
  when 
  the 
  vapor 
  

   pressure 
  was 
  increased 
  the 
  gel 
  would 
  take 
  on 
  water 
  ; 
  but 
  this 
  

   "hydration" 
  curve 
  fell 
  above 
  the 
  dehydration 
  curve, 
  that 
  is, 
  

   the 
  vapor 
  pressure 
  of 
  a 
  gel 
  with 
  a 
  given 
  amount 
  of 
  water 
  had 
  

   different 
  values 
  depending 
  on 
  its 
  previous 
  history 
  and 
  on 
  

   whether 
  it 
  was 
  being 
  dehydrated 
  or 
  hydrated. 
  Porter 
  and 
  

   Ralston* 
  have 
  found 
  this 
  same 
  phenomena 
  with 
  different 
  vari- 
  

   eties 
  of 
  coal, 
  so 
  that 
  in 
  this 
  respect 
  also 
  coal 
  behaves 
  like 
  a 
  col- 
  

   loid. 
  Whether 
  or 
  not 
  there 
  is 
  here 
  a 
  coagulation 
  of 
  colloidal 
  

   particles 
  during 
  dehydration 
  of 
  such 
  a 
  nature 
  that 
  there 
  is 
  less 
  

   surface 
  available 
  when 
  hydrating, 
  the 
  fact 
  is 
  that 
  the 
  process 
  

   is 
  not 
  reversible, 
  and 
  if 
  we 
  allow 
  a 
  coal 
  sample 
  to 
  lose 
  or 
  gain 
  

   water 
  we 
  are 
  not 
  in 
  a 
  position 
  to 
  return 
  it 
  to 
  the 
  original 
  con- 
  

   dition 
  and 
  water 
  content, 
  with 
  a 
  definite 
  vapor 
  pressure. 
  These 
  

   facts 
  and 
  considerations 
  make 
  it 
  necessary 
  to 
  fix 
  some 
  base 
  line 
  

   or 
  starting 
  point 
  in 
  determining 
  the 
  moisture 
  content 
  of 
  a 
  coal. 
  

  

  There 
  seems 
  to 
  be 
  no 
  evidence 
  that 
  extraneous 
  water 
  pene- 
  

   trates 
  the 
  coal 
  beds 
  or 
  coal. 
  It 
  would 
  be 
  only 
  locally 
  that 
  

   spring 
  water, 
  or 
  an 
  unusually 
  warm 
  zone, 
  might 
  alter 
  the 
  

   normal 
  content 
  of 
  a 
  coal 
  bed. 
  In 
  the 
  main 
  the 
  water 
  content 
  

   would 
  depend 
  on 
  the 
  per 
  cent 
  and 
  texture 
  of 
  the 
  humus 
  and 
  

   colloidal 
  matter 
  in 
  the 
  coal, 
  both 
  of 
  which 
  are 
  determined 
  by 
  

   its 
  previous 
  history. 
  It 
  would 
  seem 
  therefore 
  entirely 
  satis- 
  

   factory 
  to 
  define 
  the 
  w 
  T 
  ater 
  content 
  of 
  coal 
  as 
  the 
  amount 
  of 
  

   water 
  present 
  in 
  that 
  coal 
  as 
  it 
  is 
  found 
  in 
  the 
  bed. 
  Care 
  

   must 
  be 
  taken 
  that 
  the 
  sample 
  is 
  from 
  a 
  fresh 
  break, 
  and 
  that 
  

   it 
  does 
  not 
  lose 
  or 
  gain 
  water 
  during 
  the 
  subsequent 
  handling 
  

   and 
  preparation 
  of 
  the 
  analytical 
  sample. 
  

  

  On 
  exposure 
  to 
  air 
  coal 
  begins 
  to 
  lose 
  or 
  gain 
  moisture 
  

   depending 
  on 
  the 
  kind 
  of 
  coal, 
  the 
  temperature, 
  and 
  the 
  mois- 
  

   ture 
  content 
  of 
  the 
  surrounding 
  air. 
  In 
  view 
  of 
  the 
  forego- 
  

   ing 
  considerations 
  we 
  could 
  not 
  expect 
  attainment 
  of 
  a 
  condi- 
  

   tion 
  of 
  equilibrium 
  even 
  with 
  fixed 
  air 
  conditions 
  or 
  even 
  

   attainment 
  .of 
  a 
  "steady" 
  state. 
  If 
  the 
  air 
  is 
  saturated 
  with 
  

   moisture 
  any 
  coal 
  takes 
  on 
  moisture, 
  the 
  amount 
  and 
  rate 
  

   depending 
  on 
  the 
  nature 
  and 
  thickness 
  of 
  the 
  water 
  layer 
  on 
  

   the 
  humus 
  and 
  colloidal 
  particles 
  and 
  on 
  the 
  per 
  cent 
  of 
  humus 
  

   present. 
  If 
  this 
  absorbed 
  layer 
  is 
  very 
  thin 
  it 
  has 
  a 
  very 
  low 
  

   vapor 
  pressure, 
  increasing 
  with 
  the 
  thickness 
  of 
  this 
  layer, 
  and 
  

   approaching 
  but 
  never 
  reaching 
  the 
  vapor 
  pressure 
  of 
  a 
  plane 
  

   water 
  surface. 
  Under 
  ordinary 
  conditions 
  air 
  is 
  generally 
  not 
  

   saturated 
  with 
  water 
  vapor 
  and 
  for 
  the 
  most 
  part 
  is 
  far 
  from 
  

   it, 
  so 
  that 
  generally 
  a 
  coal 
  loses 
  water 
  on 
  exposure 
  to 
  air. 
  As 
  

  

  *Tech. 
  Paper 
  113, 
  U. 
  S. 
  Bureau 
  of 
  Mines, 
  1915. 
  

  

  