304  Iodine  Absorption  oj  Essential  Oils.  {Amji°ne;im&Tm' 
not  more  than  2  cc.  was  required,  though  reaction  commenced  at  the 
first  addition.  This  class  comprised  oils  of  calamus,  eucalyptus,  fir 
wool,  pennyroyal,  sassafras,  sanclal  wood,  spearmint. 
In  the  fourth  class  the  decolorization  of  iodine  solution  was  rapid 
and  abundant,  3  to  6  cc.  of  reagent  being  required.  This  class  con- 
tained oils  of  English  aniseed,  star-anise,  bergamot,  copaiba,  cubeb, 
fennel,  juniper,  lavender,  English,  foreign  and  "  spike/'  lemon,  ori- 
ganum, rosemary,  rue,  nutmeg,  turpentine  and  terebene. 
The  estimation  of  the  amount  of  "  absorption  "  was  then  proceeded 
with,  a  weighed  quantity  of  the  oil  being  allowed  to  stand  in  contact 
with  an  excess  of  the  iodine  solution  during  one  night.  It  was  soon 
found  that  the  differences  in  absorption  power  were  very  much 
greater  in  the  case  of  esseutial  than  in  that  of  fixed  oils.  In  the  latter 
the  lowest  recorded  is  about  9  per  cent.,  and  the  highest  about  160 
per  cent. ;  but  amongst  volatile  oils,  whilst  some  do  not  absorb  any 
appreciable  amount  of  iodine,  others  will  remove  from  solution  four 
times  their  weight,  or  400  per  cent. 
On  this  account  the  uniform  quantity  of  oil,  0*4  gram,  always 
taken  when  fixed  oils  are  operated  upon,  is  not  applicable.  A  very 
much  smaller  quantity  must  be  taken  of  certain  oils,  e.  g.}  those  of  the 
fourth  class  above,  or  a  much  larger  quantity  of  the  reagent  than  the 
prescribed  20  cc,  which  is  not  so  convenient. 
The  estimations  that  have  so  far  been  made  are  shown  in  the  table 
on  the  opposite  page. 
In  some  cases  (following  JBarenthin)  I  have  used  a  solution  of  dou- 
ble the  strength  in  iodine  and  mercuric  chloride  given  above ;  it  is 
possible  by  this  means  to  work  with  a  larger  quantity  of  oil ;  the  re- 
sults do  not  materially  differ.  In  this  case  a  correspondingly  greater 
quantity  of  the  potassium  iodide  must  be  employed  in  the  final  opera- 
tion. The  alcohol  I  have  used  was  the  strongest  commercial  spirit  of 
wine,  containing  usually  93  per  cent,  alcohol  by  weight  (i.  e.,  67  O. 
P.),  having  ascertained  experimentally  that  this  strength  answered  as 
well  as  absolute  alcohol  and  gave  similar  results. 
It  is  desirable  to  make  two  blank  experiments  and  take  the  mean, 
rather  than  depend  upon  one,  when  taking  the  absorptions  of  a  ser- 
ies of  oils. 
As  an  example,  the  numbers  obtained  in  working  one  of  these  will 
perhaps  not  be  out  of  place.    Taking  sample  28  : — 
Oil  of  lavender,  French  grown. 
