152 
ON  DECOLORIZING  CHARCOALS. 
in  this  way  is  even  more  efficacious  than  that  produced  from 
bone-black.  Such,  then,  was  the  state  of  our  knowledge  in 
regard  to  decolorizing  charcoals  till  about  eighteen  months  ago, 
when  I  first  directed  my  attention  to  the  matter. 
As  the  cost  of  purified  animal  charcoal — about  two  shillings 
per  pound— is  very  considerable,  I  was  induced  to  procure  an 
economical  substitute  for  it,  which  could  also  be  employed  in 
acid  solutions.  This  I  effected  by  combining  alumina  with  or- 
dinary vegetable  charcoal.  The  following  is  the  process  employed. 
Fifty-four  parts  of  the  sulphate  of  alumina  of  commerce,  which 
is  made  by  digesting  the  purest  kinds  of  pipe-clay  in  pretty  con- 
centrated sulphuric  acid,  and  which  usually  contains  about  14 
per  cent,  of  alumina,  were  dissolved  in  water,  and  digested  with 
92 1  parts  of  finely-powdered  ordinary  wood  charcoal.  When 
the  charcoal  had  been  thoroughly  saturated  with  the  solution  of 
sulphate  of  alumina,  the  mass  was  evaporated  to  dryness.  It 
was  then  introduced  into  covered  Hessian  crucibles,  or  large 
muffles,  and  was  heated  to  redness  till  all  the  water  and  acid  had 
been  dissipated.  In  this  way  a  decolorizing  charcoal  was  ob- 
tained, which,  though  it  still  appeared  perfectly  black,  was  tho- 
roughly impregnated  with  anhydrous  alumina.  When  sulphate 
of  alumina  and  charcoal  were  employed  in  these  proportions,  a 
decolorizing  charcoal  was  obtained,  containing  almost  exactly 
seven  and  a  half  per  cent,  of  alumina,  which  is  the  quantity  of 
alumina  which  I  have  found,  after  various  trials,  to  be  the  most 
effective,  as  it  appears  to  be  exactly  the  amount  requisite  tho- 
roughly to  coat  all  the  cells  of  the  charcoal ;  for,  on  increasing 
the  quantity  of  alumina  above  seven  and  a  half  per  cent.,  the 
charcoal  did  not  gain  any  additional  decolorizing  power  ;  and 
on  diminishing  the  amount  of  alumina  below  seven  and  a  half 
per  cent.,  the  decolorizing  power  of  the  charcoal  was  also  di- 
minished. After  calcination  in  the  way  described,  on  being 
again  carefully  pounded,  the  aluminized  charcoal  was  ready  for 
use.  A  more  economical  method  of  preparing  aluminized  char- 
coal is,  instead  of  employing  dried  sulphate  of  alumina  to  prepare 
a  solution  of  sulphate  of  alumina  of  known  strength,  by  simply 
digesting  calcined  pipe-clay  in  pretty  concentrated  sulphuric 
acid.  Such  an  amount  of  the  solution  is  to  be  employed  as  shall 
impregnate  the  charcoal  with  seven  and  a  half  per  cent,  of 
