422    METHODS  FOR  DISCRIMINATING  CINCHONA  ALKALOIDS. 
that  of  Brandes's  chlorine  and  ammonia  reaction,  as  a  means  of 
establishing  the  purity  of  cinchonidin  (miscalled  by  him  quini- 
din, in  common  with  all  German  chemists). 
Be  Vry  has  advised  the  employment  of  hydriodic  acid  or 
iodide  of  potassium,  in  order  to  discover  the  quinidin  of  Pasteur. 
Van  Heijningen  depends  on  oxalate  of  ammonia  to  discrimi- 
nate quinine  from  quinidin. 
All  these  different  tests  the  author  has  examined  most  criti- 
cally, and,  as  far  as  it  is  possible  to  do  so,  determined  the  abso- 
lute numerical  value  of  each  method  experimentally  with  the 
following  results : — 
He  first  explains  MM.  Bouchardat  and  Pasteur's  researches 
on  these  remarkable  alkaloids,  from  which  it  appeared  that  qui- 
nine and  cinchonidin  are  powerfully  laevogyrate,  quinidin  and 
cinchonine  pre-eminently  dextrogyrate,  and  that  quinicine  and 
cinchonicine  are  only  slightly  dextrogyrate  upon  plane-polarized 
light.  These  eminent  experimenters  determined  also  with  accu- 
racy the  amount  of  these  molecular  rotations  for  each  alkaloid. 
Yet  the  expensive  nature  of  the  apparatus,  the  complex  formula 
requisite  to  reduce  the  observed  amount  of  angular  rotation  to 
the  normal  molecular  standard,  and  the  many  interfering  actions 
necessary  to  be  guarded  against,  effectually  prevented  this  from 
ever  becoming  a  process  for  general  adoption,  either  among 
chemists  or  manufacturers. 
Another  method  of  recognizing  the  presence  of  quinine  is 
founded  on  the  optical  phenomena  of  fluorescence,  which  have 
been  investigated  by  Professor  Stokes.  Whilst  endeavoring  to 
turn  this  process  to  account  in  the  quantitative  estimation  of 
quinine  by  means  of  excessive  dilution,  and  marking  the  points 
at  which  the  various  phenomena  of  "  epipolism,"  "fluorescence," 
and  "internal  dispersion"  vanish,  the  author  arrived  at  the  fol- 
lowing extraordinary  results  ;  premising  that  he  employs  the 
term  "  internal  dispersion  "  to  mean  the  positive,  "  fluorescence" 
the  comparative,  and  "epipolism  "  the  superlative  degrees  of  the 
same  optical  power  : — 
I.  Solutions  containing  1  grain  in  35,000  of  either  quinine  or 
quinidin  of  Pasteur,  exhibit  epipolism  and  fluorescence  ;  solutions 
with  1  grain  in  somewhat  less  than  140,000  grains  of  water  are 
still  fluorescent,  with  slight  internal  dispersion. 
