THE  ANIMAL  CHARCOAL  USED  IN  SUGAR  REFINING.  427 
pound  which  greatly  damages  sugar  run  through  it.  Such  char 
should  be  well  washed  and  reburned  before  being  used.  The 
washing  removes  common  salt,  ammonia,  and  the  sulphide  of 
calcium  which  is  likely  to  be  present  if  the  char  has  been  over- 
burnt.  The  last-mentioned  substance  is  due  to  the  decomposi- 
tion of  the  sulphate  of  lime,  and  acts  injuriously  on  the  sugar. 
Sulphuretted  hydrogen  is  also  given  off  from  overburnt  charcoal 
when  treated  with  water  or  acid.  A  sample  of  new  char  gave 
•08  per  cent,  of  sulphuretted  hydrogen  on  treating  it  with  hydro- 
chloric acid.  Combustible  gases  are  frequently  given  off  by 
both  old  and  new  char,  and  sometimes  they  form  explosive  mix- 
tures with  the  air  of  the  cisterns. 
Speaking  of  the  mechanical  properties  of  animal  charcoal, 
Dr.  Wallace  said  that  he  had  long  regarded  the  bulk  occupied  by 
the  char  as  compared  with  its  weight  as  a  property  of  great  im- 
portance. He  stated  that  a  ton  of  new  and  dry  char  fills  a  space 
of  about  48  or  50  cubic  feet,  while  a  ton  of  old  char  may  fill 
no  more  space  than  40,  35,  30,  or  even  28  cubic  feet,  the  appar- 
ent density  of  dry  charcoal  thus  becoming  sometimes  nearly 
double  what  it  was,  But  the  absolute  specific  gravity  of  old 
and  new  charcoal  varies  but  very  slightly.  That  the  charcoal 
rapidly  diminishes  in  bulk  while  the  real  gravity  remains  prac- 
tically unaltered  is  a  point  of  great  importance,  and  one  to 
which  the  author  thinks  he  was  the  first  to  direct  the  attention 
of  sugar  refiners.  The  inference  is  that,  by  frequent  re-heating, 
the  particles  of  charcoal  become  smaller,  owing  to  the  "diminution 
of  the  pores  ;  hence  the  apparent  gravity  of  char  gives  a  ready 
and  certain  indication  of  its  value  in  su°;ar  refining.  From  ex- 
periments  made  by  Dr.  Wallace,  a  specimen  of  a  new  char  lost 
as  much  of  its  porosity  by  burning  in  a  covered  crucible  during 
eleven  hours  as  it  would  have  lost  by  re-burning  about  one  hun- 
dred times  in  a  sugar-house.  He  is  of  opinion  that  the  porosity 
of  the  charcoal  is  diminished  by  a  sort  of  agglutination  of  the 
particles  of  phosphate  of  lime  during  the  re-heating.  New 
charcoal  will  hold  from  80  to  100  per  cent,  of  water,  and  is 
made  preceptibly  wet  with  20  per  cent,  of  water,  while  old 
charcoal  (two  or  three  years  in  use)  will  only  hold  from  30  to  45 
per  cent.,  and  is  made  preceptibly  wet  with  only  5  per  cent,  of 
