42      PUEITY  OF  SULPHATE  OF  QUININE  OF  COMMERCE. 
the  latter  (quinidine)  is  of  no  consequence ;  this,  however,  is 
not  the  question  at  issue,  and  the  points  of  this  report  are 
strictly  confined  to  the  commercial  purity  of  quinine  and  its 
freedom  from  the  cheaper  salts. 
In  carrying  out  this  object  the  desired  results  are  three- 
fold :— 
First,  Qualitative,  or  to  find  an  easy  and  reliable  test  for  the 
presence  of  the  three  most  common  cinchona  alkaloids,  quinine, 
quinidine,  and  cinchonine. 
Secondly,  Quantitative,  to  find  the  most  practical  and  reli- 
able mode  of  separating  and  estimating  these  alkaloids ;  and 
Thirdly,  an  application  of  the  above  to  the  examination  of 
the  sulphate  of  quinine  as  made  by  the  principal  manufactures. 
Although  the  cinchona  barks  contain  many  alkaloids,  only 
four  occur  in  sufficient  frequency  and  quantity  to  merit  notice 
in  a  commercial  investigation — quinine,  quinidine,  cinchonine, 
and  cinchonidine.  In  actual  practice  the  two  last  may  be  esti- 
mated together. 
The  most  prominent  impurity  in  quinine  is  quinidine ;  in 
none  of  the  after-mentioned  samples  was  cinchonine  discovered 
in  any  quantity  except  one.  The  slight  solubility  of  the  cincho- 
nine salts  as  compared  with  those  of  quinine  and  quinidine,  and 
the  boldness  of  its  crystallization  would,  to  the  practiced  eye, 
soon  lead  to  its  detection ;  experiments  will  easily  show  the 
truth  of  this,  and  that  quinidine  and  not  cinchonine  must  be 
generally  sought  as  the  chief  impurity  in  commercial  sulphate 
of  quinine. 
Notwithstanding  tests  for  the  purity  of  quinine  are  so  nu- 
merous and  in  some  instances  so  trustworthy,  still  few  apply  to 
the  separation  of  quinidine  from  quinine  ;  nearly  all  are  pro- 
posed for  the  indication  of  quinine  only,  or  its  detection  when 
mixed  with  other  substances. 
The  polariscope  tests  of  Bouchardat  and  Pasteur,  and  the 
fluorescent  test  of  Professor  Stokes,  require  too  much  study  and 
practice  to  come  into  general  use  for  qualitative  analysis  among 
manufacturers  and  retailers. 
The  same  remarks  apply  to  the  exquisitely  beautiful  experiments 
of  Dr.  W.  B.  Herapath.  It  is  quite  true  that  to  the  expert 
the  iodide  test  will  detect  the  presence  of  a  very  minute  portion 
