Am.  Jour.  Pharra.\ 
D  ecernber,  1899.  J 
Analysis  of  Essential  Oils. 
571 
There  exist,  however,  besides  this,  some  general  points  of  view  after 
which  such  an  investigation  can  be  conducted,  and  which  we  now 
come  to  describe. 
As  the  overwhelming  majority  of  essential  oils  have  at  ordinary 
temperature  the  liquid  form,  the  most  important  process  for  their 
analytical  treatment  is  fractional  distillation.  Useful  and  indispen- 
sable as  the  same  may  be,  it  is  at  the  same  time  a  very  slow  and 
imperfect  process,  and  every  other  known  method  which  leads  to 
the  removal  of  one  or  the  other  constituent  should  be  made  use  of. 
Many  oils  contain  bodies  which  are  solid  at  ordinary  or  lower 
temperatures,  and  will  crystallize  from  the  oil  when  it  is  exposed 
to  cold.  As  such  a  treatment  is  not  liable  to  effect  decomposition, 
this  method,  if  serviceable  at  all,  should  never  be  neglected.  Often 
prolonged  exposure  to  the  cold  of  a  good  freezing  mixture  is  re- 
quired, with  repeated  stirring  by  means  of  a  sharp  glass  rod.  If  the 
odor  indicates  the  presence  of  a  known  crystallizable  body,  the  crystal- 
lization will  be  accelerated  by  the  addition  of  a  minute  fragment  of 
that  body.  In  this  way  crystals  of  borneol,  menthol,  safrol,  anethol, 
apiol,  etc.,  are  obtainable,  which  are  then  separated  from  the  liquid 
by  a  well-cooled  filter  or  centrifugal  machine.  Naturally  such  a 
separation  is  never  complete,  and  we  always  have  to  take  regard  to 
the  dissolved  parts  during  the  further  processes. 
Hereafter,  a  sample  of  the  oil  is  tested  for  the  presence  of  alde- 
hydes and  some,  especially  methylic  ketones,  which  are  combinable 
with  sodium  bisulphite,  and  can  thereby  be  isolated  from  the  oil. 
This  also  should  never  be  neglected,  as  many  of  these  bodies  will 
not  stand  the  subsequent  boiling  with  alkali,  but  are  decomposed 
with  formation  of  acid  and  alcoholic  compounds,  and  would  there- 
fore falsify  the  result  about  the  presence  of  acids  and  esters.  The 
shaking  with  bisulphite  solution  must  be  continued  for  a  consider- 
able length  of  time,  as  some  bisulphite  compounds  are  formed  very 
slowly  (thujone  for  instance  requires  several  days);  addition  of  some 
alcohol  often  accelerates  the  reaction.  Further  attention  is  called 
to  the  property  of  some  unsaturated  aldehydes,  like  citral,  citronellal, 
cinnamic  aldehyde,  to  form  solid  bisulphite  compounds  which,  upon 
standing,  will  combine  with  a  second  molecule  of  bisulphite,  under 
formation  of  liquid  compounds  which  sometimes  cannot  again  be 
decomposed  into  their  components ;  such  bodies  are  therefore 
treated  according  to  the  rules  given  in  the  corresponding  literature. 
