426     THE  ANIMAL  CHARCOAL  USED  IN  SUGAR  REFINING. 
ents  of  the  natural  bones.  The  carbon  varies  with  the  amount 
of  grease  and  gelatin  left  in  the  bones  after  boiling,  and  it  also 
varies  in  the  different  parts  of  the  same  bone, — the  hard  exte- 
rior containing  less  than  the  extremities  and  spongy  interior  of 
the  bone.  Charcoal  as  met  with  in  commerce  usually  contains 
about  ten  per  cent  of  water,  as  water  is  used  to  cool  the  char  as 
it  is  drawn  from  the  retorts. 
The  author  then  stated  generally  the  method  of  conducting 
the  analysis.  It  presents  certain  difficulties,  notwithstanding 
the  apparent  simplicity  of  the  composition  of  bone  charcoal.  He 
instanced  carbonic  acid  as  offering  some  difficulty,  and  said  that 
his  own  method  of  estimating  it  was  less  complicated  than  the 
one  given  by  Fresenius,  although  the  latter  is  otherwise  equally 
good. .  The  method  used  by  some  chemists  of  precipitating  the 
phosphate  of  lime,  and  then  throwing  down  the  lime  in  the  fil- 
trate in  the  usual  way,  he  characterized  as  giving  entirely  falla- 
cious results.  Free  lime  has  been  occasionally  found  by  the 
author,  but  only  in  minute  quantity.  Before  estimating  the 
carbonic  acid  the  finely  pulverized  charcoal  should  be  treated 
with  solution  of  carbonate  of  ammonia. 
Chemists  have  long  been  aware  of  the  presence  of  nitrogen 
in  animal  charcoal,  but  the  custom  hitherto  has  been  to  include 
it  in  the  carbon.  Dr.  Wallace  finds  it  to  vary  in  amount,  and 
to  diminish  in  quantity  as  the  char  is  used.  He  found  it  to  be 
1*55  per  cent  in  a  total  of  8*5  of  so-called  carbonaceous  matter 
in  char  made  from  home-collected  bones ;  and  in  another  sample 
made  from  foreign  bones  it  was  1*08  out  of  9  parts  of  carbona- 
ceous matter.  Two  samples  of  moderately  old  charcoal  gave  re- 
spectively '3  and  -55  of  nitrogen,  while  the  carbon  was  reported 
at  15  and  17,  respectively.  The  author  did  not  venture  to  say 
whether  or  not  the  nitrogen  plays*  any  important  part  in  the  de- 
colorising action,  although,  he  said,  no  really  good  decolorising 
a^ent  exists  that  is  not  made  from  an  animal  substance.  It  is 
not  enough  that  the  charcoal  should  be  porous,  for  wood  char- 
coal is  so,  and  yet  it  is  practically  useless  as  a  decoloriser. 
Traces  of  ammonia  always  exist  in  new  char,  but  the  amount 
is  so  minute  that  the  author  has  never  estimated  it.  Frequently, 
however,  it  exists  in  the  form  of  sulphide  of  ammonium,  a  com- 
