ON  FERMENTATION. 
329 
different  view.  In  the  first  place,  he  endeavors  to  prove,  tha 
in  the  same  way  that  there  is  a  special  alcoholic  ferment — beer- 
yeast — which  is  met  with  wherever  sugar  is  converted  into  alco- 
hol and  carbonic  acid  ;  so  in  like  manner  there  is  a  lactic  fer- 
ment, which  presents  itself  wherever  sugar  is  converted  into 
lactic  acid,  and  if  any  nitrogenous  substance  is  capable  of  inducing 
this  latter  change,  it  is  by  furnishing  the  material  requisite  for 
the  development  of  the  appropriate  ferment. 
During  the  progress  of  the  ordinary  lactic  acid  fermentation, 
it  is  sometimes  possible  to  recognize,  above  the  sediment  of  chalk 
and  nitrogenous  substance,  particles  of  a  grey  substance,  form- 
ing a  zone  upon  the  surface  of  the  sediment.  When  these  par- 
ticles are  examined  by  the  aid  of  the  microscope,  it  is  scarcely 
possible  to  detect  any  disintegrated  casein  or  gluten,  so  that  there 
is  nothing  to  indicate  that  this  is  a  peculiar  substance  that  has 
originated  during  the  progress  of  the  fermentation.  Neverthe- 
less, M.  Pasteur  regards  this  substance  as  exercising  a  principal 
and  essential  influence.  The  following  is  the  plan  he  adopted 
for  obtaining  it  apart,  and  in  a  state  of  purity. 
Beer-yeast  was  deprived  of  its  soluble  portion  by  digesting  it 
for  some  time  with  fifteen  or  twenty  times  its  weight  of  boiling 
water.  In  the  filtered  liquid,  sugar  was  dissolved,  in  the  pro- 
portion of  fifty  grammes  to  the  litre.  Some  chalk  was  then 
added,  and  a  very  small  portion  of  the  grey  substance  spoken  of 
above  taken  from  a  liquid  in  an  active  state  of  lactic  fermenta- 
tion. By  the  next  day  active  and  regular  fermentation  had  set 
in.  The  liquid,  which  at  first  was  perfectly  clear,  had  become 
turbid,  the  chalk  gradually  disappeared,  and  at  the  same  time  a 
sediment  was  deposited,  increasing  in  proportion  to  the  solution 
of  the  chalk.  In  addition,  all  the  well  known  characteristic  fea- 
tures of  lactic  fermentation  were  presented  by  the  liquid.  With 
the  same  result,  the  decoction  of  beer-yeast  might  be  replaced 
by  a  decoction  of  any  other  nitrogenous  substance,  either  in  a 
fresh  or  stale  condition. 
The  characters  of  this  substance,  whose  production  is  correla- 
tive with  the  phenomena  included  by  the  term  lactic  fermenta- 
tion, are  thus  described  by  M.  Pasteur.  In  appearance  it 
resembles  beer-yeast  in  a  strained  or  pressed  state.  Examined 
by  the  aid  of  the  microscope,  it  is  seen  to  consist  of  little 
