370 
7he  Preparation  of  Gelatin. 
J  Am.  Jour.  Pharm. 
\     Aug.  1,  1873. 
to  the  action  of  superheated  steam  under  a  pressure  of  two  atmos- 
pheres in  a  closed  vessel ;  and  after  standing  quietly  half  an  hour,, 
the  liquid  is  drawn  off.  After  skimming  off  the  supernatant  grease, 
the  strong  ammoniacal  glue  solution  is  strained  and  evaporated  on  a. 
steam  bath,  and  then  furnishes  the  before  mentioned  blackish  glue. 
When  perfectly  dry,  it  is  very  brittle  and  easily  rubbed  off  between 
the  fingers.  Attempts  to  bleach  it  have  yielded  unfavorable  results. 
It  shows  that  it  is  already  decomposed  and  is  no  longer  gluten,  or 
contains  only  very  little  of  it.  A  large  quantity  of  sulphurous  acid 
partially  bleaches  it,  but  to  employ  this  on  a  large  scale  would  involve 
many  technical  difficulties.  The  fragile  apparatus  for  making  sul- 
phurous acid  would  soon  be  broken  in  the  hands  of  the  workmen.  Sul- 
phite of  soda  could  be  dissolved  in  a  very  dilute  glue  solution,  and 
muriatic  acid  added  to  decompose  this  salt,  if  the  quantity  of  the  sul- 
phite of  soda  required  were  not  too  large;  but  fifty  kilogrammes  of 
glue  would  require  at  least  2,500  grammes  sulphite  of  soda  and  2,250 
grammes  muriatic  acid.  The  salts  formed,  which  are  sulphate  of  soda 
and  chloride  of  sodium,  as  also  the  free  acid,  would  in  no  case  increase 
the  quality  of  glue,  but  on  the  contrary  would  render  it  utterly  use- 
less for  many  purposes  in  the  arts.  The  process  of  bleaching  with 
mineral  acids  would  also  destroy  the  iron  evaporating  pans,  so  that 
this  method  must  be  given  up  entirely. 
All  attempts  at  giving  to  the  glue,  when  finished,  the  color  desired 
having  failed,  no  other  course  remained  but  to  ascertain  the  cause  of 
its  becoming  so  dark-colored.  The  presence  of  sulphur  and  of  con- 
siderable quantities  of  ammoniacal  salts  in  the  glue  solution  was  too 
striking  to  escape  notice  very  long.  They  could  only  have  been 
caused  by  allowing  the  steam  to  act  too  long  and  too  violently,  where- 
by not  only  were  the  cartilages  and  gristle  converted  into  glue,  but 
the  hair  too  had  been  dissolved,  and  thus  caused  the  dark  color.  In 
order  to  reduce  the  decomposition  of  the  glue  and  formation  of  ammo- 
nia to  a  minimum,  the  process  may  be  varied  in  such  a  manner  that,, 
instead  of  drawing  off  the  contents  of  the  digester  once  at  the  end  of 
three  hours,  they  shall  be  drawn  off  hourly.  On  standing  a  little,  the 
grease  rises  to  the  top  and  can  be  skimmed  off,  and  then  a  quantity  of 
fresh  wood  charcoal  mixed  with  25  per  cent,  bone  black  is  put  into 
the  liquid  and  left  over  night  for  the  purpose  of  absorbing  ammonia 
and  other  impurities.  The  following  morning  it  is  heated  to  a  tem- 
perature at  which  gelatin  melts,  about  70°  to  85°  Fahr.,  strained  and 
