444 
ON  SOME  CONSTITUENTS  OF  RHUBARB. 
solution,  on  slightly  cooling,  deposits  a  still  further  quantity  of 
it  in  the  state  of  a  light  flocculent  precipitate.  The  chrysophane 
is  obtained  by  crystallization,  after  filtering  off  the  flocculent 
matter,  and  concentration  of  the  solution.  In  order  to  obtain 
the  chrysophane  quite  pure,  the  operation  must  be  repeated  sev- 
eral times.  The  flocculent  precipitate  proved  to  be  a  new  body, 
which  appears  to  have  escaped  the  notice  of  previous  investigators 
of  rhubarb. 
In  the  final  purification  of  chrysophane,  both  amyl-alcohol 
and  glacial  acetic  acid  offer  advantages  over  benzol ;  for  -although 
it  is  possible  to  obtain  that  substance  in  beautiful  crystals  from 
a  benzolic  solution,  yet  the  difficulties  are  less  when  either  amyl- 
alcohol  or  acetic  acid  is  employed,  in  consequence  of  temperature 
affecting  the  solvent  powers  of  these  latter  agents  to  a  greater 
extent  than  those  of  benzol.  Although  alcohol  is  not  adapted 
for  the  extraction  of  chrysophane,  it  may  be  nevertheless  used 
in  the  final  crystallization  with  advantage. 
Pure  chrysophane,  when  crystallized  from  benzol  under  favor- 
able circumstances,  forms  six-sided  plates  (monoclinic  prisms,) 
which  bear  great  resemblance  to  iodide  of  lead  ;  the  color  varies 
from  a  pale  yellow  to  a  deep  orange,  according  to  the  size  of  the 
crystals.  From  alcohol,  amyl-alcohol,  and  glacial  acetic  acid,  it 
usually  crystallizes  in  moss-like  aggregations  of  foliated  crystals. 
It  dissolves  in  224  parts  of  boiling  alcohol  of  86  per  cent.,  and 
in  1125  parts  of  alcohol  at  30°  0.  (86°  F.)  When  pure,  it 
melts,  without  decomposition,  at  162°  C.  (323°.6  F.,)  and  solidi- 
fies, on  cooling,  to  a  crystalline  mass. 
Although  chrysophane  dissolves  in  alkalies,  forming  definite 
compounds,  its  acid  properties  are  very  feeble  ;  for  it  not  only 
does  not  drive  off  carbonic  acid  from  its  combinations,  but  even 
parts  with  ammonia,  when  the  ammonia  compound  is  evaporated. 
It  is  well  known,  that  when  chrysophane  is  dissolved  in  caustic 
potash  of  moderate  strength,  a  beautiful  purple  compound  is 
formed,  the  greater  part  of  which  remains  in  solution,  but  a  por- 
tion of  it  gradually  deposits  as  a  flocculent  precipitate.  We 
have  observed,  that  if  grape-sugar  be  added  to  the  compound 
containing  an  excess  of  alkali,  and  the  mixture  kept  for  some 
hours  in  a  well-stoppered  bottle,  the  purple  color  gradually  dis- 
appears, and  the  solution  becomes  of  a  brownish-yellow  tint,  this 
