270  Methods  of  Studying  Coal.        {  Arful^nT;^arm' 
carbolic  acid  is  then  washed  out  with  repeated  changes  of  warm 
water.  Heat  and  subsequent  treatment  with  water  after  neutraliza- 
tion by  means  of  an  acid  are  likewise  necessary  in  the  case  of  ma- 
terial treated  with  alkaline  alcohol,  as  described  above.  The  ad- 
vantages of  the  use  of  phenol  in  softening  coal  are  that  less  swelling 
and  cracking  results  than  in  the  case  of  alkaline  alcohol,  and  the 
material  is  in  better  condition  for  subsequent  manipulations. 
The  removal  of  mineral  substances  from  the  coal  is  the  next 
stage  and  for  this  purpose  hydrofluoric  acid  is  most  generally  em- 
ployed. The  fragments  of  coal  remain  in  strongest  commercial 
hydrofluoric  acid  for  some  days  or  even  a  week  or  more.  In  the 
case  of  coals  neither  much  carbonized  nor  possessing  a  very  high 
proportion  of  ash,  the  processes  indicated  suffice.  In  most  coals, 
particularly  those  of  the  Paleozoic  period,  after  treatment  with  hydro- 
fluoric acid,  the  combustibles  must  be  washed  for  a  day  or  two  in 
running  water  and  then  returned  to  the  phenol  for  a  renewed  sojourn 
in  the  heat.  This  second  softening  in  many  cases  is  sufficient,  but 
where  a  higher  degree  of  carbonization  is  present,  a  second  treatment 
with  hydrofluoric  acid  is  needed.  In  still  more  resistant  coals  the 
processes  must  be  further  repeated  and  the  acid  is  reinforced  in  its 
action  by  adding  crystals  of  chlorate  of  potash  or  soda,  which  brings 
into  play  the  activity  of  nascent  chlorine.  With  anthracites  and 
other  coals  of  an  extreme  degree  of  carbonization,  nitric  acid  may 
be  added  with  advantage  to  the  hydrofluoric  acid  and  chlorate  of 
potash,  but  in  moderation  so  that  maceration  may  not  result.  The 
treatment  with  hydrofluoric  acid  and  accompanying  reagents,  where 
these  are  necessary  in  the  case  of  more  refractory  coals,  is  carried 
on  in  wax  bottles  or  in  glass  bottles  coated  both  externally  and  inter- 
nally, with  hard  paraffine  or  beeswax.  A  fume-cupboard  with 
heavily  painted  windows  is  safe  and  convenient  for  this  work,  par- 
ticularly if  it  is  built  over  a  soapstone  sink. 
After  the  coal  is  softened  and  bleached  (as  is  the  case  where 
chlorates  and  aqua  regia  are  used),  it  is  carefully  washed  in  running 
water  until  quite  free  from  the  reagents.  In  the  case  of  highly 
bituminous  coals,  particularly  cannels  and  the  like,  the  pieces  may  be 
returned  to  melted  phenol  for* some  days.  With  most  coals,  especially 
those  of  later  geological  ages,  it  is  necessary  to  wrap  the  specimens 
with  bands  of  cotton  fabric,  held  in  place  by  stout  linen  thread.  This 
precaution  prevents  the  coal  from  going  to  pieces  in  the  phenol. 
After  the  last  treatment  with  carbolic  acid,  the  combustible  is  washed 
