3  io  The  Rotatory  Power  of  Volatile  Oils.  {Am)^;^m 
unexceptionally  common  to  all  these  bodies,  we  must  admit  the  cor- 
rectness of  the  negative  affirmation,  that  not  one  of  the  so-called 
ethereal  oils  is  an  unmixed  body — a.  pure,  definite  chemical  compound. 
Nearest  to  it  are  the  volatile  oils  of  mustard  and  bitter  almond.  Even 
the  numerous  terebens  of  tlfe  simple  formula  (C5H8)  seem  to  be  mix- 
tures, probably  of  C10H16,C15H24,C20H32,  and  the  slight  generation  of 
gas,  which  commences  in  the  cold  when  oil  of  turpentine  is  acted  upon 
by  sodium,  indicates  oxygen,  although  oxidized  substances  (water  ?) 
are  only  present  in  a  very  limited  quantity  in  the  hydrocarbons  of  the 
abietineae  and  of  most  aurantiaceae.  We  must  therefore  always  remem- 
ber that  volatile  oils  are  mixtures,  and  naturally  mixtures  in  various 
proportions,  a  fact  which  is  readily  observed  in  the  oils  of  parsley  fruit 
and  of  Mentha  rotundifolia.  The  yield  of  the  former  is  one-half  per 
cent.,  and  if  the  distillate  is  left  undisturbed  at  about  25°C,  it  will 
separate  in  nearly  two  equal  parts,  one  rising  to  the  surface,  the  other 
sinking  to  the  bottom  of  the  water  ;  both  portions  turn  the  plane  of 
polarization  to  the  left,  the  last  one  much  less  than  the  first.  The 
oil  of  wild-grown  Mentha  rotundifolia,  which  was  prepared  in  July, 
1876,  showed  at  a  column  length  of  50  mill,  a  deviation  of  39*2°  to 
the  left,  while  a  sample  prepared  in  September  of  the  same  year  showed 
only  half  as  much.  Daily  experience  teaches  us  further  that  the 
chemical  and  physical  properties  of  ethereal  oils,  and  especially  the 
fluid  ones,  are  more  subject  to  considerable  changes  than  many  other 
organic  substances.  These  observations  bring  the  author  to  the  con- 
clusion that  the  rotatory  power  of  ethereal  oils  cannot  serve  as  a  means- 
of  recognition. 
A  small  minority  of  ethereal  oils  is  incapable  of  turning  the 
plane  of  polarization.  It  is  remarkable  that  among  these  are  the  oils 
of  mustard  and  bitter  almond,  and  several  more,  whose  chemical  consti- 
tution is  also  better  known  than  the  majority  of  the  others.  To  this 
class  belong  the  oils  of  anise,  gaultheria,  cloves,  thyme  and  cinnamon, 
as  far  as  their  well-investigated  principal  constituents  are  concerned,, 
which  are  lacking  the  rotating  power.    This  is  in  fact  wanting  in 
Anethol  C6H4  j  g"™s  Salicylate  of  Methyl  C6H4  j  cooCH3,  E"genol 
(OCH3  rCH3 
C,HJ  OH  Thymol  C,HA  OH     and  also  in  the  Cinnamic 
6    31CH=CH-CH3,       /  6  3lCsH7, 
Aldehyd  C6H5 — CH=CH — COH  and  the  principal  constituent  of 
