54  New  Method  for  Preparing  Pepsin.  {AFeb°A%8?2iB^ 
bonate  of  soda  to  neutralize  the  acid,  and  then  obtained  by  alcohol  a 
precipitate  which  I  believed  to  be  pure  pepsin ;  at  that  time  I  had 
not  studied  the  change  which  carbonate  of  soda  produces  in  pepsin. 
When  I  say  above  the  pepsin  is  destroyed  I  mean  its  action  on 
fresh  coagulated  albumen.  A  pepsin  solution,  made  entirely  neutral, 
or  rather  a  little  alkaline  by  addition  of  carbonate  of  soda,  which 
afterwards  is  acidulated  again  with  hydrochloric  acid,  has  lost  its 
power  to  dissolve  fresh  coagulated  albumen. 
The  alkaline  solution  assumes  a  foul  odor  after  a  short  time  ;  it  does 
not  acton  fresh  coagulated  albumen,  except  when  purification  sets  in, 
and  then  the  more  putrid  the  solution  becomes,  the  more  it  seems  to 
act  on  albumen :  at  the  same  time  the  most  natural  odor  of  healthy 
human  faeces  will  show  itself. 
But,  on  the  other  hand,  the  alkaline  solution,  by  itself  as  well  as 
when  acidulated,  dissolves  partly  digested  albumen. 
Coagulated  albumen,  put  into  pepsin  solution  until  half  gone,  then 
taken  out  on  a  cloth  and  washed  and  put  into  an  alkaline  pepsin  so- 
lution, will  dissolve;  it  will  likewise  dissolve  in  an  alkaline  solution 
which  has  been  again  acidulated  by  the  addition  of  hydrochloric  acid. 
But  these  solutions  have  a  different  appearance  from  a  solution  by 
pepsin ;  they  are  not  as  clear  and  thin  a  liquid  as  the  latter. 
An  alkaline  (modified)  pepsin  solution  does  not  get  precipitated  by 
chloride  of  sodium,  but  by  addition  of  hydrochloric  acid  immediately 
a  copious  gelatinous  precipitate  will  be  formed. 
Digestive  power  of  Pepsin. — In  my  former  experiments  the 
strength  of  pepsin  was  ascertained  by  allowing  its  solution  at  a  certain 
temperature  to  act  upon  a  convenient  quantity  of  coagulated  albumen 
for  a  given  time,  and  determining  the  quantity  dissolved  by  weighing 
that  undissolved  ;  the  albumen  by  this  method  was  only  partially  dis- 
solved. In  my  recent  experiments  I  determined  the  strength  by 
ascertaining  the  amount  of  albumen  that  would  be  fully  dissolved  in 
a  certain  time  and  at  a  given  temperature.  I  had  found  that  the 
solvent  power  of  pepsin  is  not  inverse  proportional  to  the  time ;  for 
if  a  pepsin  dissolves  X  albumen  in  S  time,  2a  pepsin  will  not  dissolve 
X  albumen  in  =|  time,  as  might  be  supposed,  but  require  longer  time. 
The  last  portion  of  coagulated  albumen  to  be  dissolved  in  an  experi- 
ment requires  much  longer  time  in  proportion,  even  when  pepsin  is 
in  excess. 
Having  used  heretofore,  in  my  experiments  with  pepsin,  10  drops 
