METHOD  OF  OBTAINING  GLUE  FROM  LEATHER.  358 
of  lime,  and  a  considerable  quantity  of  water,  the  leather  is 
almost  entirely  decomposed  ;  its  tannic  acid  combining  with  the 
lime  forms  an  insoluble  precipitate,  and  the  gelatine  dissolving 
in  the  water  forms  a  pretty  concentrated  solution,  which  on  eva- 
poration yields  an  excellent  glue. 
The  glue  obtained  by  this  process  amounts  to  about  25  per 
cent.,  but  varying  considerably ;  sometimes  as  much  as  36  per 
cent,  has  been  obtained,  at  other  times  as  little  as  15  per  cent., 
operating,  of  course,  on  different  parcels  of  leather  ;  the  reason 
of  these  variations  will  speedily  be  apparent. 
It  was  naturally  to  have  been  expected  that  the  thicker  kinds 
of  leather,  such  as  those  employed  for  the  soles  of  shoes  and 
similar  purposes,  would,  when  digested  with  chalk  under  pressure, 
have  undergone  a  similar  decomposition  ;  this,  however,  was  found 
not  to  be  the  case,  as  this  species  of  leather,  however  minutely 
divided,  yielded  little  more  than  traces  of  glue,  and  this  was  the 
case  even  when  caustic  baryta  was  employed.  It  is  evident, 
therefore,  that  the  thicker  kinds  of  leather  have  a  very  different 
constitution  from  that  of  the  lighter  varieties,  and  are  in  fact 
different  substances.  This  diversity  in  constitution  probably 
arises  from  changes'  which  the  thick  leather  undergoes  in  the 
tan-pit,  where  it  is  usually  kept  from  six  to  eighteen  months, 
while  the  tanning  of  the  thinner  kinds  of  leather  is  usually  com- 
pleted in  a  few  weeks.  This  singular  metamorphosis,  which  the 
thick  leather  undergoes  during  the  long-continued  maceration 
with  tannin  to  which  it  is  subjected,  explains  the  reason  why  the 
numerous  attempts  which  have  been  made  to  shorten  the  length 
of  the  tanning  process  for  thick  leather  have  all  been  unsuccess- 
ful, the  quality  of  the  leather,  as  is  well  known,  being  deteriorated 
exactly  in  proportion  as  the  tanning  process  was  shortened ;  the 
leather  produced  by  these  quick  processes  being  of  a  uniformly 
soft  and  spongy  character,  long-continued  exposure  in  the  tan- 
pit  being  essential  to  the  formation  of  the  altered,  dense  variety 
of  leather.  Even  the  lighter  varieties  of  leather,  which  have 
been  quickly  tanned,  when  long  kept,  say  for  ten  or  twelve  years, 
appear  to  undergo  a  similar  modification,  so  that  when  digested 
under  pressure  with  lime,  they  yield  but  very  small  quantities  of 
glue.  This  alteration  appears  to  take  place  more  rapidly  when 
the  leather  is  exposed  to  air  and  moisture ;  the  leather,  under 
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