r 
pi 
164      PHYSIOLOGICAL  EXHAUSTION  OF  BEER  YEAST,  ETC. 
fourths  than  that  produced  by  the  incineration  of  the  yeast,  as  in 
Mitscherlich's  process  ;  and  if  it  is  observed  that  the  yeast  elimi- 
nates at  the  same  time  various  other  products,  proportionately  to 
the  quantity  of  phosphoric  acid,  an  idea  may  be  formed  of  the 
degree  of  exhaustion  undergone  by  each  globule.  The  exhaus- 
tion is  such  that  under  the  microscope  the  yeast  seems  reduced 
to  merely  its  envelope ;  it  is  then  barely  visible,  and  its  color  so 
faint  that  it  looks  like  shrivelled  skins,  with  no  clear  outlines,  and 
with  a  taste  like  raspberries.  From  the  nuclei  or  interior 
granulations  which  remain,  the  form  of  the  envelope  may  be 
divined. 
If  by  the  use  of  the  apparatus,  elsewhere  to  be  described,  the 
air  is  perfectly  excluded,  this  exhaustion  can  be  effected,  without 
what  is  called  the  putrefaction  of  the  yeast,  or,  more  correctly, 
without  the  appearance  of  the  organisms  which  cause  the  putre- 
faction of  the  organic  matters  eliminated.  But  if  air  enters, 
these  products  acquire  a  foetid  odor,  and  there  may  be  observed 
a  disengagement  of  sulphuretted  hydrogen,  and  the  formation  of 
.the  infusoria,  which  are  the  cause  of  a  particular  fermentation  of 
the  nitrogenized  and  sulphuretted  organic  matters  of  yeast. 
However,  if  the  other  products  accompanying  phosphoric  acid 
are  different,  the  quantity  of  the  latter  in  -each  washing  corres- 
ponds to  the  numbers  of  the  above  table. 
The  yeast  exhausted  in  the  experiment  which  furnished  the 
numbers  given  in  the  table  appeared  to  be  dead ;  but  this  was 
not  the  case,  for  it  was  still  capable  of  transforming  cane  sugar 
into  glucose — that  is  to  say,  of  forming  zymase  and  then  pro- 
ducing alcohol  by  the  glucose  engendered.  But  the  products  of 
alcoholic  fermentation  by  exhausted  yeast  are  notably  different 
in  nature  and  in  quantity  from  those  obtained  from  normal  yeast. 
The  same  as  in  alcoholic  fermentation  by  mother  of  vinegar,  a 
crystallizable  compound  is  formed  possessing  the  properties  of 
mannite. 
These  results  prove  contrary  to  Mitscherlich,  (who  supposed 
that  the  globules  of  ferment,  well  washed  in  water  are  entirely 
deprived  of  the  property  of  saccharifying  cane  sugar),  that  yeast 
continues  to  change  cane  sugar  until  it  ceases  to  live,  and  that 
when  it  is  so  much  exhausted  that  it  may  be  said  to  be  reduced 
