^'^'jiT.'iSjg!'^'}  Polarimeter  and  its  Use,  49 
for  this,  and  in  most  fluids  the  difference  for  two  or  three  degrees  of  temperature  is 
so  slight  that  it  might  be  disregarded  as  being  less  than  the  probable  error  of  obser- 
vation. 
Magnetism,  also,  influences  rotation  ;  indeed,  some  bodies  which  are  void  of  this 
property  under  ordinary  circumstances,  will  under  its  influence  exercise  it  in  a  marked 
degree.  The  discovery  of  this  phenomenon  we  owe  to  Faraday  ("Phil.  Trans.," 
1846,  p.  i),  and  it  has  been  further  investigated  by  De  La  Rive  ("Archives  des 
Sciences,"  etc.,  vol.  xxxii,  p.  193;  "Annales  de  Chimie,"  4th  series,  vol.  xv,  p.  575 
"  Phil.  Mag.,"  4th  series,  vol.  xl,  p  393).  This  is,  however,  a  study  in  itself,  and 
those  who  wish  to  prosecute  it  will  find  abundant  matter  of  interest  in  the  papers 
quoted  j  suffice  it  to  say,  that  under  the  influence  of  magnetism  the  same  law  holds 
good  as  regards  decreased  rotation  for  increased  temperature. 
The  advantage  of  having  certain  commonly  occurring  liquids,  such  as  those  men- 
tioned, void  of  activity  is  obvious,  as  it  enables  us  to  make  concentrated  solutions  of 
most  solid  substances,  such  as  sugar,  camphor,  the  alkaloids,  etc.,  and  to  select  a 
menstruum  in  which  the  body  is  most  soluble,  since  concentrated  solutions  are  most 
desirable,  inasmuch  as  the  calculation  is  made  for  the  solid  substances,  and  any  error 
in  observation  will  be  increased  in  proportion  to  the  dilution.  Not  only  so,  but  it 
has  been  shown  by  Landolt  ("Deut.  Chem.  Ges.  Ber.,"  [9],  901-904)  that  to  obtain 
accurate  results  saturated  solutions  are  absolutely  necessary,  for  as  in  the  case  of 
temperature  so  in  dilution,  the  effect  cannot  be  represented  graphically  by  a  straight 
line.  In  concentrated  solutions  the  divergence  is  only  a  few  tenths  of  a  degree,  and 
the  rotatory  power  of  the  body  remains  the  same  whatever  be  the  (inactive)  solvent 
employed ;  but  observations  taken  with  dilute  solutions  are  utterly  worthless.  He 
further  finds  (Liebig's  "Annalen,"  clxxxix,  241-337)  that  some  substances  have  an 
increased  proportional  rotation  by  dilution,  whilst  others  are  diminished  j  turpentine 
and  ethyl  tartrate  always  show  increase,  nicotin  and  camphor  both  show  diminution, 
and  these  results  are  constant  with  all  solvents. 
Organic  liquids  and  solutions  are  sometimes  so  much  colored  that  light  will  not 
pass  through  even  25  mm.  sufliiciently  for  our  purpose.  In  such  cases  filtration 
through  charcoal  is  usually  resorted  to ;  this,  under  ordinary  circumstances,  removes 
enough  of  the  color  to  admit  of  the  observation  being  made,  or  indeed  sometimes 
entirely  decolorizes.  But  this  procedure  introduces  a  possible  source  of  error,  inas- 
much as  it  has  been  shown  by  Dr.  Stammer  ("American  Chemist,"  from  the  Sugar 
Cane,  "  Pharm.  Journ."  [3],  vol.  i,  p.  926)  that  in  the  case  of  saccharine  solutions  the 
char  absorbs  sugar  from  the  first  portion  of  the  liquid,  which  passes  through  and  so 
reduces  the  strength  and  rotatory  power.  This  would  doubtless  occur  equally  with 
solutions  containing  alkaloidal  bodies,  and  possibly  some  others,-  but  as  the  char 
becomes  saturated  before  it  loses  its  decolorizing  property,  if  a  sufficient  quantity  be 
passed  through,  and  the  latter  portion  be  taken  for  examination,  the  chance  of  error 
on  this  point  is  obviated. 
The  great  commercial  industry  in  which  the  polarimeter  has  been  most  useful  is 
the  sugar  trade,  and  as  the  expenditure  of  large  sums  of  money  is  not  unfrequently 
dependent  on  the  results  so  obtained,  it  is  not  surprising  that  the  greatest  perfection 
4 
