58o 
Analysis  of  Essential  Oils. 
(  Am.  Jour.  Phar  n. 
I  December,  1899. 
much  regard  to  their  boiling  points.  The  latter  may  be,  for 
instance,  i70°-i85°,  i85°-2io°,  2io°-240°,  240°-28o°.  Now  we 
start  again  with  the  first  traction.  It  will  begin  to  boil  below  1700, 
say  at  1600;  nevertheless  we  take  again  a  fraction  to  1 85 °.  At 
this  temperature  the  flask  will,  at  all  events,  still  contain  quite  some 
liquid;  we  nowadd  fraction  i85°-2io°.  On  starting  again,  we  aresure 
to  find  a  lower  boiling  point  than  1 85  °,  may  be  175  0  ;  that  which  passes 
over  between  I75°and  1 85 0  is  added  to  fraction  i6o°-i85°.  Now  we 
take  fraction  i85°-2io°,  add  to  the  residue  fraction  2io°-240°,  and 
so  on.  In  this  way  we  obtain  again  only  four  fractions,  but  prob- 
ably already  in  proportions  rather  different  from  those  of  the  first 
distillation.  It  is  quite  safe  to  repeat  fractionation  within  the  same 
intervals  for  a  third  time.  If  certain  fractions  become  specially  strong, 
perhaps  that  of  i6o°-i85°,  we  divide  it  into  fractions  i6o°-i72°, 
and  i72°-i85°,  otherwise  the  work  is  continued  as  above,  always 
adding  the  next  fraction  to  the  residue  in  the  flask  when  the  lowest 
boiling  point  of  this  fraction  is  reached.  It  is  recommended  to 
keep  all  fractions  in  bottles  of  equal  size,  so  that  their  relative  pro- 
portion can  be  estimated  at  a  glance. 
Oaly  if,  upon  further  fractionation,  the  relative  quantity  of  the 
fractions  remains  materially  unchanged,  we  separate  the  distillates 
within  smaller  intervals,  say  from  5°-5°.  Gradually  it  then  becomes 
noticeable  about  at  what  temperatures  the  boiling  points  of  the  indi- 
vidual constituent  can  probably  be  assumed,  and  the  fractions  are  then 
taken  in  such  intervals  which  are  symmetrical  to  the  presumable 
boiling  points.  In  this  way  we  proceed  till  all  the  single  fractions 
pass  over  within  intervals  of  2°  or  30,  which  usually  is  the  case 
after  about  a  dozen  fractionations.  A  rather  good  indication  that  a 
fraction  consists  of  a  tairly  uniform  body  is  that  the  distilling 
temperature  becomes  practically  independent  from  the  speed  of  dis- 
tillation. In  mixtures,  a  relatively  greater  part  of  the  higher  boil- 
ing bodies  is  run  back  by  the  dephlegmator  at  a  slow  rate  of  dis- 
tillation rather  than  at  a  high  one,  therefore  in  the  first  case  the  vapors 
which  pass  over  will  have  a  lower  temperature  than  in  the  latter, 
while,  of  course,  with  a  body  of  uniform  composition  the  distilling 
temperature  remains  unchanged  at  different  speeds.  To  push 
fractionation  much  farther  than  to  fractions  within  2°  or  30  is 
generally  unnecessary,  in  the  first  line  because,  as  it  is  evident  for 
theoretical  reasons,  perfectly  pure  compounds  cannot  be  obtained 
