TESTING  FOR  QUININE  AND  QUINIDIN. 
27 
British  Alkaloid  Company  has  been  employed — these  severally 
contain  a  very  large  percentage  of  quinidine  ;  the  former  at  least 
fifty,  the  latter  about  twenty ;  but  Howard's  disulphate  of 
quinine  scarcely  contains  five  per  cent,  of  quinidine,  according 
to  my  experiments.  These  substances  are  easily  separated  in 
consequence  of  the  much  greater  solubility  of  the  disulphate  of 
quinidine  in  cold  water. 
One  part  of  disulphate  of  quinine  requires  740  at  60Q ; 
One  part  of  disulphate  of  quinidine  requires... .340  at  55°  ; 
so  that  the  latter  is  more  than  twice  as  soluble. 
If  we  employ  the  selenite  stage  in  the  examination  of  this  ob- 
ject, we  obtain  one  of  the  most  gorgeous  appearances  in  the 
whole  domain  of  the  polarizing  miscroscope;  the  black  cross  at  once 
disappears,  and  is  replaced  by  one  which  consists  of  two  colors, 
being  divided  into  a  cross  having  a  red  and  green  fringe,  whilst 
the  four  intermediate  sectors  are  of  a  gorgeous  orange  yellow. 
These  appearances  alter  upon  the  revolution  of  the  analyzing 
plate  of  tourmaline  :  when  the  blue  stage  is  employed  the  cross 
will  assume  a  blue  or  yellow  hue  according  to  the  position  of  the 
analyzing  plate. 
These  phenomena  are  analogous  to  those  exhibited  by  certain 
circular  crystals  of  boracic  acid,  and  to  those  circular  discs  of 
salicine  (prepared  by  fusion*),  the  difference  being  that  the  salts 
of  quinidine  have  more  intense  depolarizing  powers  than  either 
of  the  other  substances,  besides  which  the  mode  of  preparation 
effectually  excludes  these  from  consideration.  Quinine  pre- 
pared in  the  same  manner  as  the  quinidine  has  a  very  differ- 
ent mode  of  crystallization ;  but  it  occasionally  presents  circular 
corneous  plates,  also  exhibiting  the  black  cross  and  white  sectors, 
but  not  with  one  tenth  part  the  brilliancy,  which  of  course  ena- 
bles us  readily  to  discriminate  the  two. 
Having  shown  in  my  previous  papers  that  none  of  the  vegeta- 
ble alkaloids,  when  treated  with  sulphuric  acid  and  iodine,  pos- 
sess the  power  of  forming  crystalline  compounds  of  similar  pro- 
perties, and  these  artificial  quinine  tourmalines  being  pre-eminent 
in  their  action  on  light,  it  follows  that  the  existence  of  these  crys- 
tals is  a  positive  demonstration  of  the  presence  of  quinine.  It  has 
also  been  proved  by  me  that  quinidine(/3  quinine)  cannot  produce 
*  I  am  indebted  to  my  friend  Mr.  John  Thwaites  for  this  fact. 
