52 
Polarimeter  and  its  Use, 
Am.  Jour.  Pharro. 
Jan.,  1880. 
as  to  exclude  air  bubbles,  and  screwed  firmly  down.  It  is  then  placed  in  position 
and  the  instrument  brought  opposite  to  a  sodium  ftame  5  the  operation  must  be  con- 
ducted in  a  dark  room,  or  a  black  covering  cloth  be  used.  The  analyser  is  then  set 
so  that  the  arrowhead  on  the  vernier  points  to  o  on  the  scale  when  the  whole  of  the 
disc  is  at  a  maximum  of  obscurity,  i.e ,  both  halves  equally  obscure ;  it  is  necessary 
to  take  several  readings  of  this  and  note  down  the  results,  taking  the  mean  of  the 
observations,  and  if,  as  sometimes  happens,  there  is  any  difficulty  in  getting  an  exact 
zero  it  is  convenient  to  make  a  note  of  the  error  and  add  or  substract  this  from  the 
subsequent  readings. 
If  now  the  tube  be  replaced  by  one  containing  an  optically  active  liquid,  it  will 
be  found  that  the  field  is  entirely  illuminated,  or  that  one-half  is  so  whilst  the  other 
is  obscure.  The  analyzer  is  then  rotated  until  equal  obscurity  is  regained  and  the 
number  of  degrees,  minutes  or  decimal  parts  of  a  degree,  through  which  it  has  been 
moved,  as  well  as  the  direction,  is  noted.  For  each  of  the  following  results  ten 
readings  were  taken  ;  two  of  these  (the  highest  and  the  lowest)  were  struck  out,  and 
the  sum  of  the  others  divided  by  8  gave  the  mean  reading,  or  where  the  200  mm. 
tube  was  used,  division  by  16  gave  at  once  the  correct  mean  for  100  mm.  It  is,  of 
course,  necessary  from  time  to  time  to  check  the  accuracy  of  the  zero,  just  as  a  care- 
ful dispenser  does  the  correctness  of  his  scales. 
With  .solid  substances,  a  saturated  solution  being  made  in  water  or  other  suitable 
inactive  liquid,  the  specific  rotatory  power  \a\  is  found  by  dividing  the  amount  of 
observed  rotation  by  the  length  of  the  column  in  decimeters  /,  by  the  weight  of 
the  active  body  in  each  unit  of  liquid  w,  and  by  the  density  of  the  solution  thus 
M  = 
For  the  loan  of  authentic    specimens  of  some  of  the  following  oils  I  have  to 
thank  Mr.  E.  M.  Holmes,  Curator  of  the  Pharmaceutical  Society's  Museum. 
Specific  Gra'vity  and  Rotatory  Poiver  oj  Essential  Oils 
[a]  =  ioo  mm.  15  -sfPC 
.  Pimpinella  Anisum 
Illicium  Anisatum 
.  Ptychotis  Ajowan  . 
Archangelica  officinalis 
.  Amygdalus  communis  . 
Do. 
.  Mirbane 
Succinum  .... 
.  Laurus  Nobilis 
Citrus  Limetta  . 
.  Betula  alba  . 
Abies  Balsamea 
.  Dicypellium  Caryophyllatum 
Elettaria  Cardamomum 
.  Citrus  medica 
(DiL  OF 
Anise 
Do. 
Ajowan 
Angelica 
Almond,  Eng. 
Do.      Foreign  . 
Do.  Artificial 
Amber 
Bay 
Bergamotte  . 
Birch 
Canada  Balsam 
Clove  Bark 
Cardamoms  . 
Cedrat 
Cedar,  Commercial 
Do.  Red 
Caraway 
Cassia,  Pure 
Do.,  Commercial 
Rot.  p. 
i°-oo 
o°-82 
o 
i°-78 
o 
o 
o 
o°-85 
:8°-88 
Juniperus  Virginiana 
Carum  Carui 
Cinnamomum  aromaticum 
Sp.  gr. 
0936  + 
©•980 — 
0-919 
0-  897  + 
1-  049 
I  -063 
1-152 
0-859  + 
0-904 — 
o-872+3i°'25 
0872+  2°-i8 
0-  914 — 3o°-07 
1-  052 —  2°-25 
o-976+i4°-59 
0-969 —  3°-oo 
0-968  —  i6°-oo 
0-960 — 28  -75 
0-  940 — 20  -68 
1-  053 —  i°-oo 
1-021+  2°-02 
