622 
The  Chemistry  of  Rhubarb. 
(  Am.  Jour.  Pnarm.. 
I   December,  1895. 
than  the  mono-acetyl  derivative,  especially  on  heating,  and  was  to 
some  extent  soluble  in  dilute  acetic  acid.  It  was  readily  soluble, 
also,  in  potassium  carbonate  solution,  caustic  potash,  or  ammonia, 
and  with  purple  color.  On  heating  in  potash  solution,  rhei'n  and 
acetic  acid  are  reproduced  according  to  the  equation — 
C15H8(GH30)206>-  <CI5H10Oe 
2H,0        )  ~ 
'6 
\ 
Diacetyl-rhein  melts  at  2360  C.  Analysis  gave  the  following 
results  : 
Calculated  for 
CjsHsCCoHaO^Og.  Found. 
Carbon   61*62  6175 
Hydrogen   378  3*60 
It  may  be  assumed  that  by  longer  continued  heating,  or  by  some 
other  modification  of  the  treatment,  rhei'n  would  take  up  two  more 
acetyl  groups  with  formation  of  tetracetyl  rhein.u  Taking  that  as- 
sumption as  well  founded  it  would  appear  that  rhubarb  contains  a 
series  of  substances  the  relations  of  which  to  each  other  may  be 
represented  by  the  following  formulae: 
Chrysophanic  acid  Cj 5H802.(OH)5. 
Emodin  C15H702.(OH)3. 
Rhein  C15H602.(OH)2. 
It  may  probably  also  be  inferred  that  these  substances  originate 
from  a  still  unknown  constituent  of  rhubarb  by  oxidation,  and,  in 
fact,  Proctor  considers  there  is  reason  for  believing  that  after  ex- 
tracting rhubarb  by  means  of  benzene,  chrysophanic  acid  is  again 
formed  on  exposing  the  exhausted  residue  to  the  action  of  the  air.. 
The  entire  process  of  change  would  then  be 
X  O,  un^nown  chrysophanic  acid,^^  emodin, >SV  rhei'n. 
constituent,/  /  / 
It  must  be  noted,  however,  that  Proctor  does  not  say  that  he  has 
actually  obtained  chrysophanic  acid  in  the  way  suggested.  As  for 
Kubly's  opinion  in  respect  to  the  origin  of  chrysophanic  acid,  I  con- 
sider that  it  is  open  to  considerable  doubt,  and  that  his  chrysophan 
was  probably  nothing  else  than  impure  rhei'n. 
It  is  well  known  that  rhubarb  frequently  presents  a  streaked  ap- 
pearance, and  it  is  stated  that  at  one  time  red  rhubarb  was  brought 
14  Owing  to  want  of  material,  I  have  been  unable  to  try  this  experiment. 
