PROCESS  FOR  DETERMINING  THE  VALUE  OF  SOAP.  237 
end,  the  liquids  may  be  partially  separated.  The  addition  of  suc- 
cessive quantities  of  distilled  water  will  then  wash  the  ethereal 
liquid  perfectly  free  from  saline  matters.  This  being  effected,  the 
ethereal  solution  of  fat,  together  with  the  small  quantity  of  water 
beneath  it,  are  poured  back  into  the  empty  beaker-glass,  and  the 
pipette  is  washed  out  with  a  mixture  of  strong  alcohol  and  ether.  A 
great  advantage  is  thus  gained,  inasmuch  as  the  small  layer  of  water 
beneath  the  ether  is  rendered  miscible  with  it  by  means  of  the 
alcohol ;  and  if  this  is  not  the  case  at  first,  a  few  drops  of  alcohol 
must  be  added.  It  is  for  this  reason  advisable  to  avoid  introducing 
too  much  water  into  the  mixture  when  washing  out  the  contents 
of  the  pipette  into  the  beaker.  It  is  also  convenient  to  have  the 
weight  of  the  beaker  marked  upon  it  with  a  diamond.  The  ethereal 
liquid  is  then  placed  upon  a  water-bath,  and  left  until  nothing  re- 
means  but  the  fat  or  resin,  wThich,  without  altering  the  general 
principle  of  the  process,  may  readily  be  recognized.  When  a  trace 
of  aqueous  liquid  remains  beneath  the  fat,  it  is  very  difficult  to 
remove  it  by  evaporation,  and  the  addition  of  alcohol  to  the  ether 
is  a  very  appropriate  means  of  obviating  this  difficulty.  When 
the  smell  of  ether,  alcohol,  and  acetic  acid  has  become  very  feeble, 
the  residue  is  weighed,  and  the  weighing  repeated  after  a  longer- 
continued  application  of  heat;  it  rarely  happens  that  any  decrease 
of  weight  is  perceived  on  the  second  weighing,  when  the  evapo- 
ration is  carried  far  enough  in  the  first  instance.  When  several 
experiments  are  made  successively  with  the  same  soap,  the  per- 
centage of  fat  comes  out  in  the  several  determinations  agreeing  in 
the  second  decimal  place. 
The  fatty  acid  from  1  grm.  of  soap  forms  a  layer  of  such  thick- 
ness, that  by  slightly  inclining  the  beaker-glass,  the  bulb  of  a 
small  thermometer  can  be  introduced,  so  as  to  determine  the  melt- 
ing-point, by  which  means  some  idea  may  be  formed  of  the  kind 
of  fat.  If  any  insoluble  residue  remains  in  the  beaker  after  the 
first  treatment  of  the  soap,  it  is  dried  and  weighed,  and  its  nature 
determined.  The  aqueous  liquid  which  is  separated  from  the 
etherial  solution  of  fat  is  introduced  into  a  small  capsule*,  and 
carefully  evaporated  to  dryness  in  a  wTater-bath.    The  residue  is 
*  A  platinum  or  silver  capsule,  two  inches  in  diameter,  with  a  cover  so 
that  it  can  be  used  as  an  evaporating  dish,  or  crucible,  is  the  most  con- 
venient. 
