54 
ON  THE  RANCIDITY  OF  FATS. 
is  added  the  coloring  matter  of  the  galls,  and  sometimes  odorous 
matter  derived  from  the  ether ;  and  that,  owing  to  the  insolu- 
bility of  tannin  in  benzine,  its  odor  and  some  of  its  color  may  be 
removed  by  digestion  with  that  liquid. — Proc.  Amer.  Pharm. 
Assoc.,  1864. 
ON  THE  RANCIDITY  OF  FATS. 
By  Thomas  B.  Groves,  F.C.S. 
[Read  at  the  Bath  Meeting  of  the  British  Pharmaceutical  Conference,  Sept.  1864.) 
Some  experiments  relative  to  the  action  of  certain  odorous 
principles  in  preserving  the  neutrality  of  fats,  commenced  in 
December,  1861,  and  not  yet  reported  on,  would,  I  thought, 
form  the  basis  of  a  paper  on  the  above  interesting  subject. 
To  the  invalid  who  suffers  from  applying  to  an  inflamed  sur- 
face an  irritant  instead  of  emollient  substance,  as  well  as  to  the 
pharmaceutist  who  sustains  pecuniary  loss  from  the  spoiling  of 
his  ointments  by  rancification,  the  question  is  doubtless  import- 
ant. * 
Rancidity  may  be  defined  as  the  changed  condition,  assumed 
sooner  or  later  by  .all  natural  fats  exposed  to  air  and  moisture, 
such  change  being  characterized  by  loss  of  blandness  and  neu- 
trality and  development  of  pungency  and  acidity. 
The  cause  of  change  being  hidden,  or  at  best  imperfectly 
understood,  the  change  has  been  termed  spontaneous,  or  classed 
among  those  determined  by  catalytic  influence.  Chemically 
speaking,  it  consists  in  the  separation  of  the  neutral  fats  into 
its  components,  fatty  acids  and  glycerin, — the  latter  substance 
almost  invariably  accompanied  by  obscure  products  of  decom- 
position of  an  offensive  and  noxious  character. 
Palm  oil  is,  I  believe,  the  only  fat  that  has  been  observed  to 
separate  distinctly  into  acid  and  glycerin. 
It  will  throw  much  light  on  the  nature  of  the  change  if  we 
carefully  watch  its  progress  from  the  first  appearance  until  the 
full  development  of  rancidity. 
That  is  done  with  most  ease  in  ointments  containing  coloring 
matter  of  a  vegetable  origin,  such  as  savine,  or  mineral,  as  ox- 
ide of  mercury  ;  but  in  pure  lard  it  is  not  difficult. 
The  best  chemical  test  of  rancidity  I  find  to  be  iodide  of  po- 
