ON  DECOLORIZING  CHARCOALS. 
155 
digested  with  dilute  hydrochloric  acid,  thrown  upon  a  filter,  and 
finally  washed  with  distilled  water  till  everything  soluble  is  re- 
moved. In  this  way  an  extremely  porous,  very  light,  nearly  pure 
charcoal  is  obtained,  which  for  some  purposes,  such,  for  instance, 
as  decolorizing  logwood  and  similar  solutions,  is  four  times  as  effect- 
ive as  the  most  carefully  prepared  purified  animal  charcoal.  It  like- 
wise decolorizes  impure  gallic  acid  solutions  admirably.  This 
charcoal  from  coal-tar  may  likewise  be  prepared  with  much 
weaker  hydrochloric  acid  than  is  required  to  remove  the  phos- 
phate of  lime  from  bone-black.  Quick-lime  in  fine  powder  may 
likewise  be  employed  instead  of  hydrate  of  lime,  and  also  calcined 
magnesia  and  the  light  sub-carbonate  of  magnesia  of  the  shops, 
but  chalk,  even  when  finely  powdered,  answers  very  badly  indeed. 
Instead  of  coal-tar  and  coal-tar  pitch,  some  other  vegetable 
substances,  such  as  maize,  wheat,  and  the  various  kinds  of  flour, 
common  resin  or  colophonium,  pitch,  wood-tar,  or  bitumen,  may 
be  employed.  When  flour,  resin,  &c,  are  intimately  mixed  with 
carbonate  of  potash  and  calcined  in  close  vessels,  very  good 
decolorizing  charcoals  are  obtained,  but  I  was  surprised  to  find 
that  carbonate  of  soda,  either  hydrated  or  dried,  did  not  produce 
a  similar  result. 
As  was  first  observed  by  Messrs.  Bussy  and  Payen,  I  find  that 
every  species  of  decolorizing  charcoal  acts  with  varying  effi- 
ciency on  each  colored  solution  to  which  it  is  applied.  Thus,  for 
instance,  one  charcoal  decolorizes  a  solution  of  indigo  better 
than  it  does  one  of  logwood,  syrup,  or  tartaric  acid,  while 
another  charcoal  will  decolorize  a  solution  of  logwood  more 
readily  than  one  of  molasses  ;  and  so  of  others. 
As  the  result  of  this  investigation,  I  think  that  decolorizing 
charcoals  may  be  very  properly  divided  into  three  classes  :  First, 
into  those  which,  like  the  purified  and  the  coal-tar  charcoals, 
above  described,  may  be  regarded  as  merely  pure  charcoals  in  an 
extremely  minute  state  of  division,  and  which  decolorize  by 
their  porosity  alone.  Second,  into  those  mordaunted  charcoals 
which,  like  aluminized  charcoal,  and  artificial  phosphate  of  lime 
charcoal,  decolorizes  solely  by  the  mordaunts  or  bases  which 
they  contain.  Third,  into  bone-black,  which  is  a  composite  char- 
coal, and  decolorizes,  partly,  by  the  large  quantity  of  phosphate 
of  lime,  and  partly  by  the  ten  per  cent,  of  minutely  divided 
charcoal  which  it  contains. 
