428     THE  ANIMAL  CHARCOAL  USED  IN  SUGAR  REFINING. 
water.  These  facts  prove  that  the  pores  become  either  smaller 
or  less  numerous  as  the  charcoal  is  used,  and  point  to  the  con- 
clusion that,  to  keep  animal  charcoal  in  the  most  efficient  state, 
it  should  be  re-burned  in  such  a  way  as  to  lessen  the  porosity  as 
little  as  possible. 
Dr.  Wallace  finds  that,  although  the  action  of  heat  is  the 
main  cause,  it  is  not  the  only  one  concerned  in  producing  an 
increase  in  the  apparent  gravity  of  the  charcoal.  The  propor- 
tion of  carbon  may  increase  during  use  from  8  or  9  per  cent.,  to 
14,  15,  or  even  19  per  cent., — this  increase  being  derived  from 
the  organic  impurities  in  the  sugar.  The  carbon  obtained  in  the 
carbonization  of  these  impurities  is  deposited  partly  on  the  sur- 
face, but  largely  also  in  the  pores.  This  is  found  to  be  a  great 
evil,  and  a  great  point  is  gained  if  it  can  be  prevented.  That 
it  can  be  prevented  is  proved  by  the  fact  that  in  some  refineries 
the  amount  of  carbon  does  not  increase,  while  in  others  it  even 
decreases,  but  this  is  owing  to  mismanagement  in  the  re-heating. 
When  the  retorts  are  quite  tight,  and  the  heat  not  excessive,  the 
carbon  necessarily  increases  if  the  precaution  be  not  taken  to 
wash  well  before  re-burning,  so  as  to  remove  all  the  organic  matter 
absorbed  from  the  sugar  liquor.  To  do  this,  boiling  water  is 
requisite,  and  one  of  the  most  advanced  of  the  Clyde  sugar-re- 
finers insists  that  the  charcoal  should  even  be  boiled  with  the  water. 
The  author's  experience  accords  with  that  of  the  sugar-refiner 
referred  to. 
In  ordinary  cane  sugars  there  is  from  half  to  one  per  cent,  of 
soluble  mineral  matter,  consisting  of  salts  of  potash,  soda,  lime 
and  magnesia  ;  and  in  beet  sugars  there  is  much  more — from  1J 
to  3  per  cent.,  and  sometimes  as  much  as  6  or  7  per  cent.  The 
highly  soluble  salts,  such  as  the  salts  of  potash,  do  not  affect 
the  charcoal,  and  only  annoy  the  refiner  by  accumulating  in  the 
syrups  ;  but  the  sulphate  of  lime  is  detrimental,  owing  to  its 
comparative  insolubility,  and  to  the  fact  that  it  is  readily 
absorbed  from  the  sugar  liquor  by  the  charcoal.  It  may  be  re- 
moved, however,  by  copious  washing  and  boiling,  and  it  is  even 
removed  in  solution  by  washing  with  weak  sugar  liquors.  Such 
charcoal  is  sure  to  have  its  sulphate  of  lime  increased  if  it  be 
washed  with  water  naturally  containing  that  substance  in  large 
