162      PHYSIOLOGICAL  EXHAUSTION  OF  BEER  YEAST,  ETC. 
eluded  that  100  grammes  of  dry  yeast  contained  the  elements  of 
4-28  gr.  of  anhydrous  phosphoric  acid.  But  this  analysis  does 
not  prove  that  the  acid  was  actually  formed  there. 
The  infusion  of  yeast  is,  no  doubt,  always  acid,  and  this  acidity 
may  be  attributed  to  phosphoric  acid,  if  the  yeast  does  not 
ceaselessly  engender  other  acids.  To  ascertain  that  the  yeast 
actually  contained  pre-formed  phosphoric  acid,  and  phosphates, 
boil  the  well-washed  yeast  in  a  large  quantity  of  distilled  water ; 
this  will  kill  it,  and  cause  it  to  abandon  various  products,  and 
among  them  phosphoric  acid,  which  estimate  by  the  process 
presently  to  be  described,  whence  it  will  be  found  that  100 
grammes  of  dry  yeast  disengages  from  2*8  gr.  to  3*1  gr.  of  phos- 
phoric acid,  of  which  a  portion  is  free. 
But  though  the  yeast  abandons  large  quantities  of  phosphoric 
acid,  at  a  temperature  of  100°,  when  It  is  killed,  this  is  not  the 
case  when  it  is  left  in  water,  either  cold  or  even  heated  to  30  or 
40°  ;  under  these  circumstances  it  abandons  the  phosphoric  acid 
and  other  matters  very  gradually,  not  as  an  inert  precipitate,  but 
as  a  living  creature,  offering  vital  resistance  to  destruction.  By 
renewing  the -water  every  twenty-four  hours,  and  estimating  the 
phosphoric  acid  in  each  lixivium,  it  will  be  found  that  the  quan- 
tity  of  this  acid,  small  at  first,  augments  with  each  successive 
treatment,  and  rapidly  attains  a  maximum,  after  which  it  de* 
creases,  until  it  altogether  disappears.  The  following  is  the 
experiment : 
In  the  first  place  I  determined  the  amount  of  phosphoric  acid 
which  could  be  found  in  matters  adhering  to  the  yeast,  proceed- 
ing from  the  medium  in  which  the  yeast  was  formed,  or  which 
had  been  excreted  by  the  yeast  itself  and  impregnated  it  ex- 
ternally. 500  grammes  of  brewers'  yeast,  new  and  in  a  paste, 
were  soaked  in  cold  water  and  washed  on  a  filter.  The  yeast 
having  been  well  drained,  there  remained  four  litres  of  lixivium. 
In  this  liquid  the  phosphoric  acid  was  estimated  by  the  same 
process  which  was  applied  to  the  following  estimations  ;  and 
there  were  obtained  of  anhydrous  phosphoric  acid,  0*095.  gr. 
This  result  is  invariable;  all  things  being  equal,  500 grammes 
of  yeast  in  a  paste,  representing  about  100  grammes  of  dry 
