156  The  Testing  of  Pepsin.        •  {'"^■^Z'^l'^"'' 
by  drop,  to  a  peptic  solution  of  albumen  no  longer  gives  a  precipitate  it 
may  be  concluded  that  all  the  albumen  is  transformed  into  peptones. 
The  report  upon  pepsin  presented  in  1865  by  M.  Guibourt  to  the 
Societe  de  Pharmacie,  did  not  sufficiently  bring  out  the  importance  of 
this  reaction.  M.  Guibourt  thought  that  the  solution  of  the  fibrin  was 
sufficient,  and  it  appears  (p.  102)  that  i  gram  of  pepsin  prepared  by  the 
Commission,  in  presence  of  8*40  grams  of  lactic  acid  and  20  grams  of 
water,  very  incompletely  modified  12  grams  of  fibrin  after  heating  for 
twelve  hours  at  40  to  45°C.  The  solution  was  in  fact  semigelatinous, 
and  was  strongly  precipitated  by  nitric  acid.  Certainly,  at  the  present 
time,  such  a  pepsin  would  be  considered  to  be  of  mediocre  quality. 
Rapid  solution  indicates  that  the  pepsin  is  of  good  quality,  but 
between  this  phenomenon  and  that  of  transformation  the  difterence  is 
essential. 
Let  us  now  examine  the  various  methods  of  testing  that  have  been 
proposed  for  pepsin,  and  see  which  is  most  suitable  for  adoption. 
1.  Test  by  Coagulation  of  Milk. — This  process  ought  to  be  rejected. 
iVl.  Guibourt  had,  in  1865,  come  to  the  conclusion  from  experiments 
made  on  rennet,  that  the  principle  in  rennet,  which  produces  the  coag- 
ulation of  milk,  is  not  that  which  dissolves  and  transforms  fibrin.  I 
have  observed  also  that  a  pepsin,  twelve  times  more  active  than  another 
prepared  from  calves'  rennet,  was  much  less  active  than  the  latter  in 
respect  to  the  coagulation  of  milk.  It  is  probable  that  this  special 
action  is  due  to  a  particular  ferment. 
2.  Test  by  Coagulated  White  of  Egg. — This  test  is  universally  adopted 
in  England  and  in  Germany.  I  have  made  some  experiments  to  deter- 
mine : 
(a)  The  temperature  most  favorable  to  solution. 
(b)  The  action  of  various  acids. 
{c)  The  acidimetric  strength  which  it  is  advisable  to  give  to  the  liquors. 
With  respect  to  temperature,  experiments  were  made  at  from  30° 
to  8o°C.  Even  at  elevated  temperatures  the  action  of  acidified  pepsin 
is  produced,  but  the  maximum  occurs  at  50°. 
With  liquors  containing  2  to  15  per  mille  of  acetic  or  butyric  acid, 
I  have  been  able  to  convince  myself  that  in  the  presence  of  these 
acids  pepsin  is  without  action  upon  coagulated  white  of  egg.  Tartaric, 
lactic,  and  especially  hydrochloric  acids,  on  the  contrary,  facilitate  the 
action  of  pepsin. 
The  solution  of  tartaric  acid  ought  to  be  of  the  strength  of  about  10 
