236 
PROCESS  FOR  DETERMINING  THE  VALUE  OF  SOAP. 
trate  of  potash,  until  the  fatty  acids  begin  to  separate.  The  larger 
the  quantity  of  bitartrate  requisite,  the  larger  the  quantity  of  free 
alkali.  This  is  certainly  a  mere  comparative  test,  applicable  to 
the  examination  of  a  number  of  different  specimens- of  soap.  It  is 
not  improbable  that  unsaponified  fat  is  sometimes  present  in  soap. 
Dumas  determines  it  by  separating  all  the  fat  by  means  of  hydro- 
chloric acid,  resaponifying  with  baryta-water,  and  extracting  the 
baryta-soap  with  alcohol,  which  dissolves  only  the  unsaponified 
fat. 
The  process  now  to  be  described  does  not  take  into  consideration 
the  quantity  of  free  and  combined  fat  or  alkali,  but  includes  the 
estimation  of  all  the  four  above-named  conditions  which  determine 
the  value  of  a  soap,  and  is  at  the  same  time  serviceable  and  easily 
carried  out. 
A  gramme  of  the  soap  is  weighed  ;  hard  soap  in  shavings ;  soft 
soap  is  weighed  to  near  a  gramme,  because  the  addition  and  ab- 
straction of  small  quantities  is  more  inconvenient  than  a  reduction 
in  the  calculation.  The  soap  is  introduced  into  a  beaker-glass 
holding  about  an  ounce,  treated  with  a  small  quantity  of  ether,  in 
which  it  does  not  dissolve,  and  then  with  a  rather  smaller  volume 
of  pure  acetic  acid;  two  layers  are  thus  formed;  the  soap  is  rapidly 
dissolved  ;  the  upper  layer  containing  the  ether  and  fat  or  resin, 
with  a  little  acetic  acid  ;  the  lower  layer,  water,  alkali  combined 
with  acetic  acid,  free  acetic  acid,  the  salts  usually  formed  in  the 
manufacture  of  soap,  chloride  of  sodium,  alkaline  sulphate,  and 
finally  the  foreign  admixtures,  whether  soluble  in  water  or  not.  If 
sand,  powdered  pumice-stone,  clay,  steatite,  heavy  spar,  &c,  are 
present,  they  remain  at  the  bottom  of  the  glass.  Other  substances 
of  organic  origin,  such  as  starch,  &c,  are  suspended  in  the  layer 
of  liquid  beneath  the  ether.  The  entire  mass  is  then  poured  off 
from  the  undissolved  substances  in  the  beaker  into  a  large  pipette, 
widened  in  the  middle  and  bent  upwards  at  the  lower  extremity 
so  as  to  form  a  kind  of  separator.  The  liquids  are  allowed  to  re- 
main in  the  wide  part  of  the  pipette  until  they  have  perfectly 
separated  into  two  layers,  and  it  may  then  be  so  managed  that 
none  of  the  ethereal  liquid  enters  its  lower  extremity,  which  is 
turned  upwards.  The  beaker-glass,  together  with  the  residue,  if 
any,  is  then  washed  with  ether  and  water,  which  are  poured  into 
the  pipette ;  and  by  inclining  it,  or  blowing  gently  into  the  upper 
