428  Bitter  Substance  of  Hops.  { Am^u8rt;  J^™- 
petroleum  will'  dissolve  has  probably  been  extracted.  The  solutions 
are  put  together  and  the  petroleum  ether  distilled  off  in  vacuo  at  a  low 
temperature,  until  there  remains  in  the  flask  a  dark  brown  syrup, 
which  on  cooling  solidifies  into  a  crystalline  mass.  This  is  pulverized 
and  turned  on  to  a  filter  composed  of  a  large  funnel,  in  which  a  smaller 
funnel  covered  with  muslin  is  inserted.  With  the  aid  of  a  suction 
pump,  the  greater  portion  of  the  thick,  crude  solution  can  be  filtered 
through.  There  remains  on  the  filter  a  highly  colored  crystalline 
"  cake,"  which  should  be  pulverized  with  a  small  quantity  of  petro- 
leum ether  and  again  filtered.  After  this  operation  has  been  repeated 
three  or  four  times,  we  obtain  an  almost  colorless  mass,  consisting  of 
hop-bitter  acid,  contaminated  by  small  quantities  of  a  fatty  substance 
and  a  substance  which  I  could  not  isolate,  and  which  I  had  at  first 
great  trouble  in  separating  from  the  hop-bitter  acid. 
If  we  do  not  wish  to  utilize  this  crude  substance  at  once  it  will  be 
necessary  to  melt  it  in  the  water-bath  and  pour  it  into  a  bottle  under 
close  seal,  where  it  will  at  once  crystallize  and  solidify.  If  it  remains 
exposed  to  the  atmosphere  it  will  soon  become  sticky  and,  turn  partly 
into  resin.  Six  kilos  of  lupulin,  which  included  a  large  proportion 
of  sand,  furnished  400  grams  of  crude  hop-bitter  acid.  The  first 
experiments  in  crystallization  with  petroleum  ether  gave  poor  results ; 
it  is  difficult  to  produce  the  acid  pure  in  large  quantities  by  this  pro- 
cess, as  a  small  quantity  of  the  above  substance  obstinately  clings  to 
it,  and  it  readily  assumes  a  non-crystallizable  form.  Our  object  is 
more  readily  attained  if  we  crystallize  it  once  from  alcohol,  for  which 
purpose  we  dissolve  it  in  a  little  lukewarm  alcohol,  then  quickly  cool 
the  solution ;  flakes  of  a  fatty  substance  will  be  separated,  which  are 
removed  by  filtration  with  the  aid  of  a  suction  pump.  Then  we  throw 
a  few  small  crystals  of  the  acid  into  the  solution,  and  after  a  short 
time  crystallization  commences.  As  soon  as  it  appears  to  be  ended, 
the  mother  solution  is  removed  with  the  aid  of  a  platinum  cone,  and 
the  crystals  washed  with  a  little  cold  alcohol.  The  alcoholic  mother 
solution,  which  still  contains  the  chief  part  of  the  bitter  acid,  must  be 
quickly  evaporated,  and  the  residue  consigned  to  a  flask.  The  acid 
crystallized  from  the  alcohol  is  then  recrystallized  several  times  from 
petroleum  ether.  In  order  to  quickly  dissolve  the  bitter  substance,  it 
should  be  carefully  melted  in  a  flask  and  double  its  volume  of  ether 
gradually  added ;  on  its  cooling,  we  obtain  beautiful  prismatic  crys- 
tals, which  attain  a  length  of  1  cm.,  and  become  perfectly  pure  after 
