Am  ApS;  W7?m' }  The  Polarisoope  as  a  Eevealer  of  Adulterations.  165 
degree.  In  the  test  of  specific  gravity  such  variation  in  temperature 
would  make  considerable  d inference. 
The  one  single  sample  of  oil  of  peppermint  regarding  which  I  am 
not  satisfied  on  account  of  its  polarization,  was  grown  in  Kalamazoo 
county,  Michigan.  It  polarized  at  -29°.  Whether  this  is  due  to  pecu- 
liar soil,  or  some  subtle  adulteration,  is  yet  to  be  investigated. 
Among  other  notable  results,  was  the  test  of  an  oil  marked  "  German  " 
polarizing  at  — 14°,  heavily  adulterated  with  oil  of  pennyroyal  and 
u  Japanese  peppermint "  (Mentha  arvensis).  Another  was  the  fraud- 
ulent brand  called  "  Michigan  county  oil  peppermint,"  to  which  I  have 
formerly  referred,  showing  by  fractional  distillation,  50  per  cent,  oil 
of  turpentine,  and  which  polarized  at  — 5°.  Another  sample  repre- 
senting one  thousand  pounds  of  dementholized  oil,  from  Detroit,  polar- 
ized at  —38°. 
While  carrying  on  the  experiments  with  the  polariscope,  Dr. 
Duffield  made  some  most  interesting  experiments  with  an  optical  in- 
strument invented  by  Prof.  Abbe  of  Germany,  named  the  "  Refrac- 
tometer."  This  delicate  and  beautiful  instrument  arrived  at  the 
doctor's  laboratory  during  the  week  of  my  visit ;  he  having  worked 
with  it  recently  with  Dragendorff  at  Dorpat,  Russia,  ordered  one  con- 
structed for  himself  before  returning  home.  The  determinations  of 
this  instrument  are  based  upon  the  refractive  power  of  a  liquid,  which 
is  shown  on  a  finely  graduated  scale,  similar  to  that  of  the  polariscope. 
This  instrument,  like  the  polariscope,  is  quite  expensive  and  rare  in 
this  country.  While  it  requires  more  skill  and  care  in  adjusting  and 
using,  its  utility  consists  in  the  fact  that  but  a  single  drop  of  liquid  is 
required. 
Through  the  courtesy  of  Dr.  Dunield,  I  have  been  furnished  with 
a  table  of  the  "  indices  of  refraction  "  of  many  of  the  samples  already 
mentioned,  and  the  relationship  of  polarization  to  refraction  will  form 
an  interesting  subject  for  future  investigation. 
Beside  the  value  of  the  polariscope  in  determining  adulterations  of 
laevogyre  essential  oils  with  oil  of  camphor  it  is  also  most  valuable  in 
determining  adulterations  of  oil  of  erigeron  with  oil  of  turpentine,  as 
the  former  is  lsevogyre ;  also  in  determining  the  genuineness  of  oil  of 
true  fireweed,  for  which,  heretofore,  erigeron  oil  has  been  mostly  sub- 
stituted, and  for  which  no  chemical  test  had  yet  been  known,  while 
oil  of  erigeron  polarizes  about  50°  to  the  left,  the  oil  of  true  fire  weed 
(Erechthites  hieracifolia)  polarizes  44°  to  the  right ;  consequently  those 
