164         The  Polariscope  as  a  Revealer  of  Adulterations.  {A 
6 
+  10.5  (to  right) 
.9385 
202 
7 
+  1  " 
.9444 
207 
8 
—  8.5  (to  left) 
.9480 
211 
9 
—21.5  " 
.9526 
214.5 
10 
None. 
.9734 
When  nine  full  fractions  had  been  distilled,  it  was  found  that  no 
more  could  be  recovered,  which  was  sufficiently  translucent  to  admit 
of  polarization.  The  residuum  in  the  still  being  of  a  tarry  consistency 
and  quite  opaque,  distillation  was  stopped  and  the  residuum  drained 
from  the  retort,  this  representing  the  10th  fraction. 
Another  feature  of  the  fractions  of  the  adulterated  oil  is  that  the 
specific  gravity  rises  continually  with  each  succeeding  fraction,  whereas 
in  the  fractional  distillation  of  pure  oil  of  peppermint,  the  specific 
gravity  rises  with  the  first  few  fractions,  then  falls  until  a  specific 
gravity  is  reached,  about  equivalent  to  the  third  fraction,  and  then 
again  commences  to  rise,  this  time  with  increased  rapidity.  This  phe- 
nomenon I  had  the  pleasure  of  calling  attention  to  in  the  Proceedings  of 
the  American  Pharmaceutical  Association  for  1885,  page  579. 
After  an  examination  of  a  great  number  of  samples  of  pure  oil  of 
peppermint,  collected  specially  from  all  parts  of  the  world,  I  find  but 
one  sample  the  impurity  of  which  is  not  established,  that  polarizes  at  a 
less  angle  than  -38°.  The  highest  polarizing  point  obtained  in  pure 
oil  of  peppermint  was  a  sample  distilled  from  plants  which  I  had 
dried  for  six  months  before  distillation,  which  experiment  is  referred 
to  in  the  September  number  of  The  American  Druggist.  This 
polarized  at  -74°. 
Upon  comparing  the  same  oil  before  dementholization  and  after,  I 
found  the  angle  of  polarization  lessened  three  degrees  by  dementho- 
lization, the  amount  of  menthol  extracted  being  16  per  cent.  Pure 
menthol  (being  melted,  of  course,  to  form  a  liquid),  I  find  does  not 
polarize  distinctly;  but  when  added  to  natural  oil  increases  the  polar- 
izing angle.  I  farther  found  that  the  effects  of  temperature  on  the 
polarizing  angle  are  very  slight,  and  not  (as  has  been  supposed)  such 
as  to  materially  affect  the  test.  For  determining  this,  oils  both  dex- 
trogyre  and  lsevogyre  were  used,  with  the  result  that  for  a  variation 
of  115°  F.  the  angle  of  polarization  was  changed  but  three  degrees,  or 
but  little  more  than  one-fourth  of  one  degree  for  each  ten  degrees  of 
temperature  ;  so  that  experiments  made  with  the  polariscope  at  ordi- 
nary temperatures,  say  from  55  to  7 5°  F.,  would  vary  but  a  fraction  of  a 
