446  ON  SOME  CONSTITUENTS  OF  RHUBARB. 
hoeinic  aek1  prepared  from  a  Mexican  root  called  raiz  del  pipit- 
zahuac,  and  described  by  Weld  ;  but  the  analysis  differs  too 
much  to  admit  of  the  two  bodies  being  considered  identical. 
Weld  obtained  the  following  numbers  : — 
i.  *  ii. 
Carbon,  .  .  .  72.58  73,24 
Hydrogen,   .       .       .         8.06  8.27 
From  the  foregoing  numbers,  Weld  derives  the  formula  C30 
06.    Rochleder  and  Heldt's  formula  for  chrysophane  is  C20 
H8  06,  making  a  difference  in  the  composition  of  the'  bodies  of 
We  cannot  refrain  from  pointing  out  the  possibility  of  these 
two  bodies  being  homologues,  and  differing  by  5  (C2H2) ;  this 
would  make  the  formula  of  pipitzahoeinic  acid  to  be  C30  H18  06, 
which  requires  73.17  carbon  and  7.31  hydrogen. 
III. — Emodin. 
In  the  description  of  the  preparation  of  chrysophane,  it  was 
mentioned  that  on  treating  the  crude  chrysophane  with  benzol, 
there  is  left  undissolved  a  yellowish-red  residue,  and  that  a 
further  quantity  taken  up  by  the  hot  benzol  is  deposited  as  the 
benzolic  solution  of  the  chrysophane  becomes  somewhat  cooled. 
Although  the  insolubility  of  this  body  in  benzol  led  to  its  detec- 
tion, the  complete  separation  of  it  from  chrysophane  is  attended 
with  many  difficulties,  on  account  of  the  latter  substance  modi- 
fying to  a  considerable  extent  the  properties  of  the  new  body. 
The  best  mode  of  separating  the  new  body  from  chrysophane 
is  to  dissolve  the  precipitate  entirely  in  hot  benzol,  and  to  allow 
the  benzolic  solution  to  cool  slowly ;  the  deposit  which  forms  is 
then  separated  by  filtration,  and  dissolved  by  heat  in  glacial 
acetic  acid,  from  which  it  is  obtained,  in  a  state  of  considerable 
purity,  and  in  beautiful  crystals,  as  the  solution  cools.  This 
treatment  with  acetic  acid  appears  to  be  essential,  on  account  of 
its  removing  some  foreign  matter,  the  presence  of  which  inter- 
feres with  the  crystallization. 
After  purification  in  the  foregoing  manner,  the  new  body  may 
be  dissolved  in  boiling  alcohol,  from  which  it  separates  on  cooling 
in  splendid  prismatic  crystals,  sometimes  two  inches  long,  asso- 
ciated in  concentric  aggregations. 
