Am.  Jour.  Pharm.  1 
December,  1899.  J 
Analysis  of  Essential  Oils. 
575 
of  special  classes  of  compounds,  is  practically  exhausted,  and  for 
further  separation  we  have  to  resort  to  fractional  distillation.  As 
nearly  all  crude  oils,  however,  contain  some  resinous  matter,  part  of 
which  may  also  have  been  formed  by  the  foregoing  treatment,  it  is 
better,  before  further  fractionation,  to  rectify  the  remaining  oil  with 
steam.  By  this  process  we  obtain  a  much  more  complete  separa- 
tion of  the  resin  than  by  direct  distillation,  and  we  also  avoid  un- 
necessary overheating  of  the  oil  and  formation  of  decomposition 
products ;  moreover,  a  crude  fractionation  of  the  oil  is  effected  by 
collecting  the  subsequent  distillates  separately.  The. oil,  which  re- 
mains dissolved  in  the  water,  is  recovered  by  cohobation.  A  careful 
examination  of  the  residue  from  the  steam  distillation  should  not 
be  neglected,  as,  besides  undefinable  resins,  it  may  contain  the 
alkali  salts  of  higher  fatty  acids,  which  were  not  extracted  by  water 
at  the  removal  of  acids  and  phenols ;  also  some  other  bodies  of 
high  boiling  point,  like  paraffines,  which  have  been  blown  over  by 
the  powerful  steam  currents  of  the  chemical  industry,  but  which 
resist  the  volatilizing  power  of  the  steam  in  a  laboratory  flask. 
Now  we  proceed  to  fractional  distillation.  Invaluable  as  this 
method  may  be  to  the  chemist,  it  is  nearly  always  a  slow  and  tire- 
some process,  the  efficiency  of  which  is  often  overrated.  Any  one 
who  stands  for  the  first  time  before  the  task  to  separate,  somewhat 
quantitatively,  even  only  two  bodies  with  a  difference  of,  let  us  say, 
150  in  their  boiling  points,  will  probably  be  badly  disappointed  to 
notice  how  many  times  the  fractionation  has  to  be  repeated,  until 
the  desired  result  is  approximately  obtained  ;  and  if  he  has  once 
completed  the  task  of  thoroughly  separating  the  constituents  of 
such  complex  mixtures  as  essential  oils  usually  are,  and  seen  how 
many  delusions  he  undergoes  from  fractionation  to  fractionation  re- 
garding the  range  of  temperature  in  which  the  chemical  individuals 
which  are  present  are  likely  to  boil,  how  often  the  main  fractions 
are  first  collected  at  temperatures  in  the  neighborhood  of  which, 
finally,  nothing  is  present  at  all — any  such  one  will  only  have  an 
amused  smile  for  the  harmlessness  with  which  sometimes  even  in 
chemical  journals  a  judgment  is  passed  upon  the  composition  of  an 
essential  oil,  merely  from  the  result  of  one  or  two  fractionations. 
If  the  facilities  of  a  laboratory  render  it  possible,  the  fractiona- 
tion of  an  essential  oil  should  be  performed  as  much  as  possible 
under  reduced  pressure.    A  vacuum  of  about  15  millimetres,  which  is 
