162  The  Polar  iscope  as  a  Revealer  of  Adulterations.  { Am  April,'  m?™ ' 
question  originated  in  the  city  of  Detroit,  and  had  been  scattered 
throughout  the  country  to  a  vast  extent,  being  transferred  secretly  to 
agents  who  represented  themselves  as  the  bona  fide  producers,  and  the 
quality  as  genuine. 
As  Dr.  Dufneld  kindly  extended  not  only  the  hospitality  of  his 
home,  but  the  use  of  his  laboratory,  for  the  purpose  of  arriving  at 
more  exact  conclusions,  I  took  with  me  a  number  of  samples  of  my 
own  distillation,  besides  others  with  the  quality  of  which  I  was  fully 
acquainted.  The  results  were  that  though  different  pure  samples 
varied  in  the  angle  of  polarization,  there  were  found  to  be  limits  be- 
yond which  pure  samples  did  not  go.  In  the  case  of  oil  of  pepper- 
mint adulterated  with  oil  of  camphor  (the  latter  is  dextrogyre,  while 
the  former,  as  is  well  known,  is  laavogyre),  a  most  valuable  test  was 
found  by  polarization,  especially  valuable  from  the  fact  that  oil  of 
camphor,  being  so  soluble,  limpid  and  volatile,  and  yielding  no  dis- 
tinct chemical  reactions  had  recently  become  the  most  dangerous 
adulterant  known,  being  used  to  an  enormous  extent. 
Regarding  the  instrument  in  question,  a  description  of  its  construc- 
tion may  be  found  in  Tucker  s  work  on  sugar  analysis.  I  will  briefly 
say  for  those  who  cannot  conveniently  gain  access  to  that  work,  that 
its  practical  results  are  that  "  an  angle  of  polarization  "  may  be  found 
by  carefully  revolving  an  index  round  a  graduated  circle,  divided  in 
360  degrees.  A  "  perception  tube"  (which  in  the  case  of  this  instru- 
ment was  200  millimeters  in  length)  is  placed  between  two  refracting 
prisms.  The  graduated  circle  is  marked  off  in  degrees  commencing 
at  a  point  perpendicular  to  the  centre  of  the  circle.  From  this  point 
the  degrees  number  both  to  right  and  left,  meeting  at  180  on  the 
opposite  or  lower  side  of  the  scale.  The  "  polarizing  angle  "  is  found 
by  filling  the  perception  tube  with  the  oil  and  turning  the  index  until 
a  point  is  found  to  the  right  or  left  where  the  violet  and  red  rays  of 
the  spectrum  unite  before  the  90th  degree  is  reached.  For  instance  if 
the  red  and  violet  colors  of  the  spectrum  unite  at  the  50th  degree  to 
the  left,  the  angle  of  polarization  is  said  to  be  — 50°,  and  the  oil  lsevo- 
gyre,  or  "polarizing  50°  to  the  left."  ]t  should  be  observed  that 
this  same  phenomenon  will  be  also  visible  to  the  right  on  the  point 
exactly  opposite,  which,  however,  in  the  case  referred  to,  would  be  be- 
yond the  90th  degree. 
Now,  if  it  is  found  that  a  certain  essential  oil  polarizes  within  cer- 
tain limits,  if  such  oil  is  mixed  with  another  polarizing  with  entirely 
