Am'A«g.r;Sarm"}^^;^  and  Chemistry  of  Essential  Oils.  401 
Oil  of  rue  is  a  ketone,  and  consists  of  methyl  nonyl  ketone, 
MeC9H19CO,  but  its  constitution  is  not  fully  known,  as  it  is  possible 
for  several  isomers  of  this  formula  to  exist,  and  the  particular 
formula  for  C9H19  has  not  yet  been  worked  out. 
(/5)  We  pass  on  to  the  terpenes,  with  which  are  ever  associated 
the  name  of  Tilden  and  Wallach. 
These  occur  in  very  many  essential  oils,  particularly  those  of  the 
Coniferce,  which  consist  almost  wholly  of  terpene,  but  many  oils — 
oil  of  lemon,  thyme,  fennel,  etc. — contain  one  or  more  of  these 
terpenes. 
They  may  be  represented  by  the  formula  (C5H8)n,  and  are  closely 
related  to  cymene. 
The  number  of  isomers  is  much  smaller  than  was  at  first  sup- 
posed, and  I  only  propose  to  mention  the  more  important  of  them  : 
Pinene  is  contained  in  German  and  American  oil  of  turpentine, 
oils  of  juniper,  eucalyptus,  sage,  etc.,  and  is  obtained  by  distillation. 
Limonene  is  found  in  oils  of  lemon  and  orange  peel,  oil  of  dill,  oil 
of  caraway,  oil  of  bergamot,  etc. 
Silvestrene  occurs  chiefly  in  Swedish  and  Russian  oil  of  turpentine. 
Phellandrene  occurs  in  eucalyptus  oil,  elemi  and  fennel,  and  is 
distinguished  from  others  by  forming  a  crystalline  compound  with 
HN02.  These  different  terpenes  often  exist  in  dextro-rotatory  and 
laevo-rotatory  forms,  and  often  by  a  mixture  of  tnese  we  may  get 
inactive  bodies.  Wallach  has  separated  them  and  identified  them 
by  formation  of  their  hydrochlorides,  bromides  and  nitrites. 
They  have  not  yet  been  synthesized,  and  their  chemical  constitu- 
tion is  not  quite  settled,  but  they  are  probably  isomerides  of  dihy- 
dromethylisopropyl  benzene,  or  dihydrocymene : 
/CH2-CHX 
CH3C/  >OCH(CH3)2 
Terpene. 
/CH-CH. 
CH3C/  >OCH(CH3)2 
Cymene. 
Though  cymene  has  been  prepared  synthetically  by  Widman, 
from  1-4  bromcumene  and  Mel  (Ber.,  xix),  it  has  not  yet  been 
found  possible  to  hydrogenate  it  and  produce  the  terpenes.1 
1  Since  the  above  was  in  type,  Baeyer  claims  to  have  synthesized  dihydro- 
cymene (Ber.,  26,  232). 
