ON  SOME  CONSTITUENTS  OF  RHUBARB. 
447 
This  substance,  for  which  for  the  present  we  propose  the  name 
of  Emodin,  is  of  a  bright  deep  orange  color ;  in  thick  crystals 
it  assumes  almost  a  red  tint.  The  crystals  are  monoclinic  prisms, 
extremely  brittle,  and  resembling  isatin  in  appearance. 
Emodin  does  not  melt  at  a  temperature  below  250°  C.  (428°  F.), 
while  chrysophane  melts  at  162°  C.  (323.6  F.) ;  during  the  fusion, 
a  small  portion  volatilizes  undecomposed,  giving  a  yellow  vapor, 
which  first  condenses  to  a  yellow  liquid,  and  subsequently  solidi- 
fies to  a  crystalline  mass. 
In  its  chemical  properties,  emodin  closely  resembles  chryso- 
phane, the  different  behaviour  to  some  solvents  being  the  princi- 
pal distinction  between  them.  Emodin  is  much  more  soluble 
than  chrysophane  in  alcohol,  glacial  acetic  acid,  and  amylic 
alcohol.  In  benzol,  however,  emodin  is  much  less  soluble  than 
chrysophane  ;  hence  the  value  of  the  solvent  in  the  separation 
of  the  two  bodies. 
Caustic  alkalies  dissolve  emodin  with  the  same  phenomena  as 
occur  in  the  solution  of  chrysophane,  and  ammonia  behaves  to  it 
as  it  does  to  the  latter  substance,  giving  a  purple  liquid,  and 
flying  off  completely  on  evaporation  of  the  solution. 
On  combustion,  emodin  gave  the  following  numbers  : — 
I.  0.165   gramme  substance  gave  : — 
0.4035      «      Carbonic  acid  =  66.69  Carbon. 
0.0605      «      Water  =  4.07  Hydrogen. 
II.  0.1688  gramme  substance  gave : — 
0.4120      «      Carbonic  acid  =  66.57  Carbon. 
0.0603      «      Water  =  4.13  Hydrogen. 
The  purely  empirical  formula  C40  H15  013  agrees  well  with  the 
foregoing  analysis,  as  will  be  seen  from  the  following  table  : — 
Theory.  Experiment.  Mean. 
C^--  240  —  66.85  66!'69  66.5;  66.63 
H15  —  15  —  4.18  4.07  4.13  4.10 
Q13  —  104  —  28.97         ...  ...  29.27 
359     100.00  100.00 
Up  to  the  present  time  we  have  not  been  able  to  determine 
any  rational  formula  for  emodin. 
