AmjJu°nei;'Sarm"}   Iodine  Absorption  of  Essential  Oils.  303 
the  amount  of  hypo  solution  required  for  the  blank  experiment  (when 
no  oil  is  present)  and  that  required  for  the  liquids  containing  oil,  but 
otherwise  exactly  similar  to  the  blank  experiment,  the  amount  of  io- 
dine that  has  gone  from  the  free  to  the  combined  condition  through 
the  agency  of  the  oil  can  easily  be  calculated. 
This  amount  is  usually  expressed  as  proportional  to  100  parts  by 
weight  of  oil,  and  constitutes  the  "  iodine  absorption  equivalent "  of 
the  oil.  It  is  not  to  be  assumed  that  the  number  represents  the 
amount  of  iodine  actually  absorbed  by  the  oil,  since,  without  doubt, 
the  process  is  usually,  if  not  invariably,  a  substitution  process,  in 
which,  for  each  iodine  atom  entering  into  the  composition  of  the  oil  a 
second  combines  with  the  hydrogen  thus  displaced,  and  this  ultimately 
forms  the  familiar  mercuric  iodide,  which  can  be  obtained  in  crystals 
upon  evaporation  of  the  aqueous  liquid. 
At  the  time  I  commenced  this  work,  and  indeed,  for  some  time 
later,  I  was  not  aware  that  the  application  of  this  process  to  essential 
oils  had  been  attempted,  but  Mr.  A.  H.  Allen,  F.  I.  C,  whom  I  con- 
sulted on  this  point,  eventually  forwarded  a  reference  which,  when 
followed  up,  showed  that  in  1886  Mr.  C.  Barenthin,  a  pharmacist  of 
Berlin,  had  applied  the  process  to  some  eighteen  essential  oils,  and 
had  published  his  results  in  the  Archiv  der  Pharmacie  for  October  of 
that  year. 
Some  preliminary  experiments  showed  at  once  that  there  was  a 
very  great  difference  in  the  amount  of  absorption  of  iodine  by  differ- 
ent essential  oils. 
In  order  to  get  some  idea  as  to  whether  much  or  little  iodine  would 
be  taken  up,  2  drops  of  essential  oil  were  dissolved  in  about  5  cc. 
chloroform  in  a  small  flask,  and  the  "  Hubl  reagent "  added,  noting 
the  volume  required  before  the  decolorization  began  to  become  slow. 
This  preliminary  examination  enabled  me  to  divide  the  oils  roughly 
into  four  classes,  according  to  their  reaction  with  the  Hubl  reagent. 
In  the  first  class  the  absorption  was  very  little  or  none,  under  1  cc. 
of  iodine  solution  being  required.  This  comprised  essential  oil  of 
almonds,  natural  and  artificial,  oils  of  cassia,  chamomile,  cinnamon, 
cummin,  cloves,  pimento,  mustard,  American  peppermint,  Japanese 
mint,  and  gaultheria. 
In  the  second  class,  comprising  English  dill  and  caraway  oils,  the 
reaction  was  slow  in  starting,  but  afterwards  considerable. 
In  the  third  class  the  absorption  was  to  a  moderate  extent  only ; 
