ON  DECOLORIZING  CHARCOALS. 
151 
chiefly  that  the  decolorizing  power  of  the  charcoal,  though 
inherent  in  the  charcoal  itself,  is  entirely  dependent  on  its 
physical  state,  especially  on  its  extreme  porosity  and  minute 
state  of  division.  Thus,  charcoal  which  has  been  strongly  heated 
so  as  to  become  hard  and  lustrous  by  undergoing  a  species  of 
fusion,  scarcely  exhibits  any  decolorizing  power.  On  the 
other  hand,  charcoal,  such  as  bone-black,  which  contains  much 
saline  and  earthy  matter,  such  as  phosphate  of  lime,  &c,  as  its 
particles  are  prevented  from  agglutinating,  yields  a  dull  non- 
lustrous  material,  possessing  very  considerable  decolorizing 
power.  Bussy  found  also  that  the  distinction  which  had  pre- 
viously been  drawn  between  vegetable  and  animal  charcoals  as 
decolorizers,  was  erroneous,  and  that  charcoals,  whether  of 
animal  or  vegetable  origin,  should  be  divided  into  two  classes, 
compact  and  lustrous,  and  porous  and  non-lustrous. 
Till  of  late  only  two  species  of  decolorizing  charcoal  have 
been  employed  in  the  arts.    First.  Bone  or  ivory  black,  made 
by  calcining  bones  in  close  cylinders  till  the  organic  matter  they 
contain  is  entirely  carbonized.    This  is  by  far  the  most  common 
of  all  the  decolorizing  charcoals,  and  is  the  only  one  employed 
in  sugar-refining,  and  for  decolorizing  similar  neutral  solutions. 
It  usually  contains  little  more  than  10  per  cent,  of  carbon,  and 
about  90  per  cent,  of  phosphate  and  carbonate  of  lime.  The 
second  species  of  decolorizing  charcoal  is  the  so-called  purified 
animal  charcoal,  which  is  made  by  digesting  bone-black  in  hy- 
drochloric acid,  and  edulcoration  with  water  till  all  the  lime-salts 
are  removed.  Purified  animal  charcoal,  when  carefully  prepared, 
may  be  regarded  as  nearly  pure  charcoal,  which  has  a  dull 
lustrous  appearance,  and  is  extremely  porous,  provided  that, 
after  purification,  it  has  been  dried  at  a  temperature  little  ex- 
ceeding 212°.     It  decolorizes  neutral  and  acid  solutions  ex- 
ceedingly well ;  but,  after  being  heated  to  redness,  its  texture 
becomes  compact,  and  its  decolorizing  power  is  almost  entirely 
destroyed.    A  second  process  for  purified  animal  charcoal  con- 
sists in  intimately  mixing  blood  or  the  fleshy  parts  of  animals 
with  pearlashes,  and  then  calcining  them  in  close  vessels.  The 
alkaline  salt  is  first  washed  out  with  pure  water,  and  then  the 
last  traces  of  the  potash  and  lime-salts  are  removed  by  digestion 
with  hydrochloric  acid.    The  purified  animal  charcoal  obtained 
