402  Botan y  and  Chemistry  o f  Essential  Oils.  {Am'j^;^xm' 
The  terpenes,  then,  are  a  class  of  bodies  of  formula  (C5H8)n,  of 
the  probable  composition  dihydroparacymene,  and  exist  in  a  number 
of  isomeric  forms  which  are  identified  by  the  compounds  they  form 
with  bromine,  hydrochloric  and  nitric  acids. 
I  have  treated  of  the  terpenes  in  a  very  cursory  way,  but  as  my 
time  is  limited  I  must  now  pass  on  to  consider  the  next  group  [y)y 
the  members  of  which  are  generally  the  aromatic  constituents  of 
the  oil.  Of  late,  much  light  has  been  thrown  on  these  constituents, 
but  much  remains  to  be  done  ;  but  here,  as  in  the  terpenes,  it  is 
probable  the  number  of  substances  present  is  actually  less  than 
was  at  first  supposed.  Since  the  camphors  as  a  class  are  very  much 
allied  to  the  terpenes,  I  shall  treat  those  first. 
They  include  camphor,  Borneol  camphor,  menthol  and  thymol. 
Of  these  we  have: 
C10H16O   Camphor,  thymol,  carvacrol. 
<C10H18O,  Borneol  camphor  and  cineol. 
C10H20O,  Menthol. 
The  constitution  of  camphor  is  still  an  open  question  amongst 
chemists,  and  several  formulae  have  been  proposed  for  it.  It  has 
not  yet  been  synthesized  from  cymene,  to  which  it  certainly  bears 
a  very  close  relation.  The  facts  that  camphor  combines  readily 
with  HCN,  phenyl  hydrazine,  and  NaHS03,  and  that  on  oxidation  it 
yields  camphoric  acid,  and  also  that  by  treatment  with  ZnCl2  and 
other  dehydrating  agents  we  get  as  a  chief  product  paracymene, 
tend  to  show  that  it  is  a  ketone  of  which  Borneol  camphor  is  the 
secondary  alcohol,  as  camphor  on  reduction  with  Na  yields  Borneol 
camphor,  which,  on  oxidation,  yields  camphor.  There  are  several 
objections  to  this  view,  but  Cazeneuve,  in  a  masterly  review  of 
the  whole  state  of  our  knowledge  on  this  subject  {Bull.  [3],  ix, 
x,  38),  ascribes  to  it  the  formula  : 
/C0-CH3x 
CHcny 
CHSC(X  )CHCH(CH3)2 
2 
CH3CH/  2^CCH(CH3): 
CH2-CH 
Camphor. 
Thymol — one  of  the  official  stearoptens,  occurs  in  ajowan  and 
other  oils.  It  is  probably  a  tertiary  alcohol,  and  is  metahydroxy- 
cymene  1,  3,  4. 
