* 
262  ON  CHEMISTRY  APPLIED  TO  THE  ARTS. 
densers,  the  ends  of  which  are  brought  under  the  retorts  to  as- 
sist by  their  combustion  in  the  distillation  of  the  animal  matter. 
By  this  arrangement  not  only  is  char  obtained,  but  oily  matters 
which  are  used  by  curriers,  and  also  ammoniacal  salts  employed 
in  agriculture  and  manufactures.  The  extraordinary  decolorat" 
ing  action  of  animal  blacks  may  be  considered  as  partly  chemical 
and  partly  mechanical — mechanical  because  it  is  proved,  by  some 
interesting  researches  of  Dr.  Stenhouse,  to  which  I  shall  refer  fur- 
ther on,  that  the  action  is  due  to  the  minute  division  of  the  carbon 
and  the  immense  surface  offered  by  its  particles  to  the  coloring 
matter,  char  being  composed  of  90  parts  of  mineral  salts  to  10  per 
cent,  of  carbon.  On  the  other  hand,  the  action  is  proved  also  to  be 
chemical,  by  the  fact  that  water  will  not  remove  the  coloring 
matter,  whilst  a  weak  solution  of  alkali  will  dissolve  it.  Dr. 
Stenhouse's  valuable  researches  not  only  illustrate  fully  this 
fact,  but  also  prove  the  possibility  of  producing  artificially  sub- 
stitutes for  bone-black.  In  1857  he  published  a  paper  describ- 
ing the  production  of  an  artificial  black,  called  by  him  aluminised 
charcoal.  This  he  obtained  by  mixing  intimately  and  heating 
finely  pulverised  charcoal  and  sulphate  of  alumina,  when  he 
obtained  a  powerful  decolorating  agent,  containing  seven  per 
cent,  of  alumina,  and  well  adapted  for  decolorating  acid 
solutions,  such  as  those  of  tartaric  and  citric  acids,  in  chemi- 
cal works.  He  also  prepared  what  he  called  coal-tar  char- 
coal, by  melting  one  pound  of  pitch  in  a  cast-iron  pot,  adding 
to  it  two  pounds  of  coal-tar,  and  mixing  intimately  into  it 
seven  pounds  of  hydrate  of  lime,  then  carrying  the  whole 
to  a  high  temperature,  allowing  it  to  cool,  and  removing 
the  lime  by  washing  the  mass  with  hydrochloric  acid  and 
then  with  water,  when  carbon  in  a  high  state  of  division  was  ob- 
tained, possessing  powerful  decolorating  properties.  The  follow- 
ing series  of  experiments  by  Dr.  Stenhouse  perfectly  illustrate 
the  chemico-physical  action  of  animal  black  as  a  decolorating 
agent.  He  boiled  a  certain  amount  of  char  and  his  two  char- 
coals with  a  solution  of  logwood,  then  treated  each  black  sepa- 
rately with  ammonia,  when  the  following  results  were  obtained  : 
— Aluminised  charcoal  yielded  no  color  ;  bone  black  but  a  slight 
amount;  coal-tar  charcoal  large  quantities.    But  it  would  be 
