48 
Polarimeter  and  its  Use. 
(  Am.  Jour.  Phjirm, 
\       Jan.,  1880. 
quite  as  sensitive  as  the  transition  tint,  and  where  observations  are  continued  for  any 
length  of  time  it  is  far  less  fatiguing  to  the  eye  of  the  observer. 
Certain  natural  crystals  possess  high  rotatory  power.  Thus  a  plate  of  quartz  375 
mm.  in  thickness  gives  a  rotation  of  90°,  whilst  a  column  of  English,  oil  of  turpen- 
tine, 100  mm.  in  length,  gives  only  i4°'3o.  Some  few  salts,  such  as  bromate  and 
chlorate  of  sodium,  acetourale  of  sodium,  and  hyposulphite  of  lead  possess  double 
rotatory  power  5  but  most  inorganic  salts,  and  some  liquids,  such  as  water,  alcohol, 
ether  and  chloroform,  are  inactive.  The  activity  in  crystals  and  liquids  depends  on 
different  causes,  the  former  belongs  to  the  domain  of  physics,  the  latter  to  that  of 
chemistry,  and  it  is  this,  viz. :  the  molecular  rotatory  power,  which  we  are  more 
especially  considering.  The  rotation  produced  by  any  given  liquid  (all  else  being 
equal)  depends  on  the  length  of  the  column  j  it  will  be  evident  therefore  that  to 
have  uniformly  correct  results  the  greatest  accuracy  must  be  observed  in  this  respect,, 
and  that  either  the  same  length  of  tube  must  always  be  used  or  the  readings  must 
be  brought  to  the  same  standard  by  calculation.  The  usual  working  length  is  aoo 
mm.,  but  most  operators  supply  themselves  with  tubes  of  100,  50  and  even  25  mm., 
as  some  of  the  fluids  to  be  operated  on  possess  so  much  color  that  light  will  not  pass 
through  a  larger  column  satisfactorily.  It  is  desirable  to  use  the  larger  tube  when- 
ever available,  inasmuch  as  the  error  will  be  thereby  diminished  5  but  whatever  be 
the  dimensions  of  the  tube  used  the  results  should  be  stated  in  terms  corresponding 
to  a  column  of  fluid  100  mm.  in  length,  this  now  being  generally  accepted,  and  \_a\ 
is  used  to  indicate  the  molecularly  rotatory  power  of  such  a  column.  Hesse,  how- 
ever (*' Chem,  Centr.,"  1875,  3^9  >  "Jour.  Chem.  Soc,  1876,  667),  in  referring  to 
the  results  obtained  by  De  Montgolfier,  Weiss  and  Biot,  points  out  the  diff'erence 
obtained  by  the  lay  D,  the  red  and  transition  tints,  and  concludes  that  this  symbol 
is  equivocal,  and  suggests  that  it  is  better  to  use  ao  for  the  rotatory  power  obtained 
by  the  yellow  ray,  as  has  indeed  been  the  practice  for  some  time  in  Germany. 
When  the  transition  tint  was  almost  exclusively  adopted  the  sign  [a]  was  used  to 
indicate  the  rotatory  power  read  by  it,  otherwise  it  would  have  been  more  simple  to 
have  adopted  this  sign  where  the  sodium  ray  was  used,  and  to  have  used  the  qualify- 
ing letter  only  when  other  rays  were  employed,  which  is  now  rarely  done. 
Temperature  influences  the  rotatory  power  to  some  extent,  is'S^C  (6o"F.)  being 
that  at  which  readings  are  usually  taken,  and  it  has  been  found  that  the  rotation 
decreases  as  the  temperature  increases,  and  'vice  'versa ;  but  Landolt  has  shown  that 
the  diminution  is  not  always  uniform  at  all  temperatures  for  the  same  body,  or  equal 
for  all  bodies.    He  gives  as  examples: 
Oil  of  Turpentine  [a]o  36°-6i  :  diminished  rotatory  power  for  an  increase  of 
i°C.=  004437. 
Oil  of  Orange  ii5''-3i:  diminished  rotatory  power  for  an  increase  of  i°C.= 
•12371. 
This  dimunition  being  represented  graphically,  not  by  a  straight,  but  by  a  slightly 
curved  line.  This,  I  think,  would  depend  entirely  on  the  expansion  and  rate  of 
expansion  of  the  liquid,  inasmuch  as  an  increase  of  temperature  would  necessarily 
increase  the  volume  and  reduce  the  number  of  molecules  in  a  column  of  a  given 
length.    The  slight  expansion  of  the  tube  would  tend  in  some  degree  to  compensate 
