910.  When  milk  is  saturated  with  common  salt  and  filtered,
a  clear  liquid  is  obtained,  which  holds  in  solution  the  casein,
sugar,  and  salts;  while  the  butter  rests  upon  the  filter  in  the
form  of  globules,  which  are  enclosed  in  an  albuminous  mem-
brane, and  are  insoluble  in  ether.  If,  however,  the  milk  is
first  boiled,  the  albuminous  coating  appears  to  be  dissolved,
and  the  whole  of  the  butter  is  dissolved  by  agitation  with
ether,  leaving  the  milk  transparent.  After  a  few  hours'  repose
the  greater  part  of  the  globules  rise  to  the  surface  in  the  form
of  cream.     In  describing  casein,  (875,)  the  efi'ect  of  acids  in


628  ORGANIC   CHEMISTRY.
producing  the  coagulation  of  milk  has  been  already  noticed :
^he  whey  contains  all  the  sugar,  and  the  soluble  salts.  The
spontaneous  coagulation  of  milk  depends  upon  the  forma-
tion of  a  little  lactic  acid  from  the  sugar  :  in  the  prepara-
tion of  cheese,  an  infusion  of  the  lining  membrane  of
a  calf's  stomach,  called  rennet,  is  added,  which  causes  an
almost  immediate  separation  of  the  casein  in  an  insoluble
form ;  this  reaction  does  not  depend  upon  the  formation  of
lactic  acid,  but  may  take  place  in  the  presence  of  an  excess
of  alkali  ]  milk  in  its  recent  state  has  an  alkaline  reaction.
In  cheese  which  has  been  long  kept,  there  are  found  seve-
ral products  of  the  decomposition  of  casein,  among  which
are  salts  of  butyric  and  valeric  acids,  and  probably  leucine ;
besides  butter  from  the  milk,  cheese  often  contains  a
portion  of  fat,  from  the  transformation  of  the  casein  already
described.
