312  The  Rotatory  Power  of  Volatile  Oils.  {AmjJ^-If7h7arm- 
the  magnitude  of  rotating  power  of  such  mixed  bodies  as  ethereal  oils 
is  the  product  of  different  co-operating  factors.  If  the  constituents  of 
the  oils  were  always  mixed  in  the  same  proportions,  the  rotating  power 
might  also  remain  the  same.  The  author  discusses  also  the  effect  of 
solvents  upon  the  rotating  power,  the  variation  in  this  respect  of  un- 
doubtedly genuine  volatile  oils,  the  similarity  in  the  behavior  of  very 
different  oils,  etc.,  and  concerning  the  value  of  the  rotating  power  as 
a  test  for  volatile  oils,  arrives  at  the  following  conclusions  : 
I.  Among  the  constituents  of  volatile  oils  are  both  rotating  and 
non-rotating. 
II.  The  rotating  power  of  an  oil  is  the  product  of  the  rotatory 
power  of  its  single  constituents. 
III.  These  constituents,  being  present  in  changeable  proportions,  it 
is  on  this  account  that  one  and  the  same  oil  does  not  always  exhibit  the 
same  rotating  power. 
IV.  A  second  reason  is  to  be  looked  for  in  the  fact  that  a  body  of 
a  definite  chemical  composition,  for  instance  the  molecule  C10H16,  on 
keeping,  may  undergo  chemical  changes  (by  taking  up  O  or  OH2)  which 
would  also  affect  the  optical  qualities  of  the  oil. 
V.  The  rotation  is  further  influenced  by  the  quality  and  quantity  of 
substances,  which  themselves  have  no  effect  on  the  plane  of  polariza- 
tion. 
VI.  The  same  influence  is  to  be  expected  in  mixtures,  in  which 
several  optically  active  substances  are  present.  How  complicated 
these  qualities  may,  and  in  ethereal  oils  certainly  must  be,  will  be 
illustrated  by  the  following  considerations  :  A  is  an  optically  indiffer- 
ent stearopten,  dissolved  in  B,  a  left  turning  terebene,  and  accompanied 
by  C — perhaps  produced  by  oxidation  of  the  former,  and  acting  to  the 
right.  The  rotation  of  the  oil  will  in  the  first  line  depend  on  the 
relative  proportions  of  B  and  C  ;  should  C  have  a  much  higher  boiling 
point  than  B,  a  slight  variation  in  the  process  of  distillation  alone  would 
be  sufficient  to  produce  considerable  differences  in  the  volatile  oil  of 
the  same  plant.  Another  question  would  be  whether  the  presence  of 
"  A,"  regardless  of  the  simple  fact  of  dilution,  would  not  affect  the 
optical  qualities  of  "  B  "  and  "  C." 
VII.  Although  the  rotating  power  is  the  result  of  different  co- 
operating powers,  we  must  further  consider  that  even  these  results, 
according  to  "  IV,"  cannot  be  accepted  as  unchangeable. 
