ON  THE  RANCIDITY  OF  FATS. 
55 
tassium,  which  when  mixed  with  fat  in  the  least  degree  affected, 
quickly  assumes  an  orange  color,  indicative  of  the  presence  of 
free  iodine,  the  tint  being  directly  proportional  to  the  amount 
of  rancidity.  Thus  I  have  found  that  when  added  to  fat  ex- 
tremely rancid,  the  coloration  is  intense  and  immediate  ;  when 
added  to  mixtures  of  this  fat  and  other  perfectly  neutral,  the 
coloration  is  less  and  less  according  to  its  dilution,  and  is 
plainly  perceptible  when  the  fat  contains  only  one-twentieth, 
but  a  longer  time  is  required  for  its  development. 
Acidity  cannot  be  relied  on  as  the  index,  for  it  will  be  found, 
that  whereas  the  highly  rancid  fat,  when  boiled  with  alcohol, 
yields  a  solution  reddening  decidedly,  though  not  promptly 
litmus-paper,  a  mixed  fat,  such  as  the  above,  gives  a  negative 
result. 
When  rancidity  first  shows  itself  in  a  pot  of  lard  that  has 
been  filled  with  the  melted  fat  and  not  subsequently  disturbed, 
the  following  phenomena  may  be  observed.  Here  and  there 
throughout  its  contents,  but  more  especially  at  the  points  where 
the  lard  touches  the  sides  of  the  stoneware  jar,  small  patches 
of  a  whiter  and  more  crystalline  aspect  than  the  bulk  have 
made  their  appearance.  If  these  patches  are  tried  by  iodide 
of  potassium,  they  will  be  found  to  be  highly  rancid,  whilst  the 
surrounding  parts  retain  unimpaired  their  original  freshness. 
As  the  process  advances,  these  patches  increase  in  size  and 
number,  until  the  whole  mass  has  become  infected.  How  far 
the  action  would  proceed  if  time  were  given  it  is  uncertain,  but 
ordinary  cases  of  rancidity  are  caused  by  the  splitting  up  of  a 
small  portion  only  of  the  fat.  From  the  change  taking  place 
in  situations  to  which  the  air  cannot  be  supposed  to  penetrate, — 
for  instance,  the  interior  of  a  homogeneous  mass  of  fat, — it  may 
be  inferred  that  air  is  not  necessary  to  its  propagation.  Ana- 
logy coupled  with  the  observation  that  rancidity  first  makes  its 
appearance  at  the  sides  of  the  vessel,  where  possibly  from  con- 
traction during  cooling  a  small  "air-space  may  have  been  left, 
point  to  the  necessity  of  air  being  present  at  its  commencement. 
In  fact,  it  is  pretty  sure  that  the  phenomena  are  caused  by  a 
species  of  fermentation  involving  the  joint  action  of  air,  water, 
albuminous  matter,  and  a  moderate  degree  of  heat.  Such  is 
the  opinion  of  M.  Pelouze,  who  has  written  much  on  the  ques- 
