238  PROCESS  FOR  DETERMINING  THE  VALUE  OF  SOAP. 
weighed,  and  ought  not  to  suffer  any  further  diminution  of  weight 
when  again  heated  in  the  bath.  The  presence  of  gelatine  may  be 
detected  during  evaporation  by  the  appearance,  starch  by  means 
of  iodine  solution,  cheese-curd  by  the  peculiar  empyreumatic  odor 
evolved  on  the  application  of  a  sufficient  heat;  other  substances 
may  likewise  be  readily  detected.  Their  total  quantity  is  estimated 
by  the  loss  of  weight  on  ignition.  If  the  perfect  clearness  of  the 
aqueous  liquid,  the  small  residue  left  on  evaporation,  and  its 
radiated  crystalline  appearance,  indicate  that  saline  substances  only 
are  present,  the  residue  may  be  ignited  at  once  and  a  previous 
weighing  dispensed  with.  In  this  case  the  ash  should  contain  very 
little  carbon,  and  after  this  has  been  perfectly  burnt  off,  it  may  be 
examined  to  determine  the  quantity  and  kind  of  mineral  substances 
it  contains.  Silica,  if  it  has  been  added  in  the  gelatinous  form, 
will  have  become  insoluble,  and  may  be  separated  by  filtration  and 
weighed.  The  filtrate  must  be  examined  for  sulphate  of  potash 
and  chloride  of  sodium,  and  their  quantity  determined  when  there 
is  reason  to  suppose  that  they  are  greater  than  could  be  accounted 
for  merely  by  the  impurity  of  the  alkali  used  in  the  manufacture  of 
the  soap,  or  the  introduction  of  chloride  of  sodium  during  the  pro- 
cess. In  case  it  is  unnecessary  to  determine  the  admixtures  named, 
the  total  per-centage  of  alkali  may  be  ascertained  by  treating  the 
ignited  residue  with  hydrochloric  acid,  evaporating  to  dryness, 
weighing,  and  calculating  from  the  chloride  of  potassium  or  sodium 
the  quantity  of  soda  or  potash.  "When  it  is  necessary  to  ascertain 
whether  besides  soda  there  is  potash  present,  or  the  reverse,  this 
must  be  done  according  to  the  general  rules  of  analysis,  which  it 
is  not  necessary  to  particularize  here.  The  same  remark  applies 
to  several  other  points.  My  object  in  the  present  instance  is  to 
furnish  the  chemist  with  a  short  method  of  ascertaining  the  value 
of  soap,  leaving  the  detailed  execution  of  it  to  his  own  manage- 
ment. 
For  the  usual  purpose,  then,  this  method  enables  us  to  determine 
the  per-centage  of  fat*  and  its  melting-point,  the  insoluble  admix- 
tures,— sand,  heavy  spar,  pumice-stone,  &c.    The  loss  on  ignition 
*  The  per-centage  of  fat  comes  out  somewhat  higher  than  it  really  is;  since 
the  fatty  acids  are  separated  in  the  hydrated  state,  while  in  combination  with 
potash  and  soda  they  are  anhydrous.  This  circumstance  is  generally  over- 
looked in  the  examination  of  soap;  because  the  high  equivalents  of  the  fatty 
acids  render  its  influence  slight. 
