20 
ON  IODO-SULPHATE  OF  QUININE . 
this  succession  of  change  is  known  by  philosophers  to  depend 
upon  the  thickness  of  the  plate  of  crystal,  and  the  changes  pass 
through  a  certain  order  of  colors ;  namely,  those  commonly  known 
as  Newton's  rings,  so  that  a  color  of  a  certain  order  will  always 
indicate  a  certain  thickness  of  selenite  plate." 
A  pupil  of  Dr.  Herapath,  (Mr.  W.  H.  Phelps,)  accidentally 
noticed  a  peculiar  crystalline  formation  of  a  green  color  in  a 
bottle  of  solution  of  sulphate  of  quinia,  which  on  investigation 
proved  to  be  the  iodide  of  the  disulphate  of  quinia. 
It  is  obtained  readily  by  dissolving  five  grains  of  ordinary  di- 
sulphate of  quinia  in  a  fluid  drachm  of  acetic  acid,  1.041,  (the  com- 
mercial acid,)  adding  a  few  drops  of  tincture  of  iodine  to  the  so- 
lution and  heating  to  ebullition.  As  the  liquid  cools  the  iodo-suL 
phate  of  quinia  crystallizes  out  in  extremely  thin  transparent 
plates  which  reflect  the  rich,  iridescent  green  color  noticed  in 
the  elytra  of  the  Spanish  fly.  By  transmitted  light  they  have 
only  a  faint  olive  color.  By  collecting  these  crystals  on  a  filter, 
and  redissolving  them  in  alcohol,  and  evaporating,  the  salt  may 
be  obtained  in  a  purer  state.  The  form  of  the  crystals  vary  ex- 
ceedingly; sometimes  they  are  injlong  flattened  rectangular  figures, 
at  others  in  square  tables  ;  again  the  plates  are  rhomboidal,  and 
very  frequently  hexagonal ;  occasionally  stellae,  the  radii  of  which 
are  rhombic  prisms,  are  formed.  When  the  materials  for  a  larger 
quantity  are  suffered  to  cool  gradually,  large,  very  thin  tabular 
crystals,  composed  of  parallel  rectangular  figures,  arrayed  edge 
to  edge,  are  developed. 
Optical  properties.— These  crystals  when  examined  by  trans- 
mitted light  possess  scarcely  any  color,  . 
having  a  slight  olive  tinge,  but  if  two  of 
the  crystals  cross  each  other  at  right 
angles  the  spot  where  they  cross  appears 
perfectly  black,  as  at  fig.  1,  although  the 
crystals  may  not  exceed  500th  of  an  inch 
in  thickness.  If  the  light  used  be  but 
slightly  polarized,  as  by  reflection  from  a 
glass  mirror  at  the  angle  of  56°  45',  these 
crystals  assume  complimentary  colors, 
one  appears  green,  and  the  other  pink,  and  at  the  point  at  which 
