(a.)  They  are  soluble  in  water;  not  coagulated  by  heat;  and
are  precipitated  by  saturation  with  neutral  ammonium  sulphate.
The  precipitate  with  (NH^)^S04  partly  disappears  on  heating,  and
reappears  on  cooUng.  They  are  precipitated  but  not  coagulated
by  alcohol.
(b.)  Add  nitric  acid  =  a  white  precipitate  which  dissolves  witli
heat  (yellow  fluid)  and  reappears  on  coohng.  Run  tap  water  on
the  tube,  the  precipitate  reappears.  This  is  a  characteristic  re-
action, and  occurs  best  in  the  presence  of  NaCl.
(c.)  It,  like  peptone,  gives  a  rosy -pink  with  Piotrowski's  test.
{d.)  It  is  precipitated  by  acetic  acid  and  ferrocyanide  of  potas-
sium, but  the  precipitate  disappears  on  heating,  and  reappears  on
coohng.
(e.)  It  is  precipitated  by  acetic  acid  and  saturation  with
NaCl.  The  precipitate  disappears  on  heating,  and  reappears  on
cooling.
10.  VI.  Peptones  are  hydrated  proteids,  and  are  usually  produced
by  the  action  of  proteolytic  ferments  on  proteids.  They  are  exceed-
ingly soluble  in  water.  Their  solutions  are  not  precipitated  by
sodic  chloride,  acids,  or  alkalies,  nor  are  they  coagulated  by  heat.
They  are  precipitated  by  tannic  acid,  and  with  difficulty  by  a  large
excess  of  absolute  alcohol.     !N"ot  precipitated  by  (NH^)^SO^.
Preparation  (see  "  Digestion  "). — For  applying  the  tests  dissolve
a  small  quantity  of  Darby's  fluid  meat  or  commercial  peptone  in
warm  water.  Commercial  peptone  contains  only  a  small  amount
of  peptone,  and  much  albumose.
{a.)  Boil  a  portion  ;  it  is  not  coagrdated.
{h.)  Xanthoproteic  Reaction. — Add  nitric  acid,  and  boil  =  a
faint  yellowish  colour,  and  rarely  any  previous  precipitate ;  cool,
and  add  ammonia  =  orange  colour.
(c.)  Acidify  strongly  with  acetic  acid,  and  add  ferrocyanide  of
potassium  =  no  precipitate.
