620  The  Chemistry  of  Rhubarb.  {ADeclmbefhia^' 
ing  laminae,  which  melt  between  2300  and  2400  C,  and  become 
dark-colored. 
Calculated  for 
C15H1204.  Found. 
Carbon    70*36  69*59 
Hydrogen   4*68  4*94 
The  formation  of  this  product  from  emodin  is  represented  by  the 
equation : 
ClsH10O5 1  s  ciA  A 
4hi  )   -    £h2o  +  4i 
3.  RHElN. 
In  treating  the  mixture  of  emodin  and  rhein  with  hot  toluene, 
the  rhein  remains  undissolved,  and  may  then  be  purified  by  recrys- 
tallization  from  hot  glacial  acetic  acid.  It  forms  microscopic  yel- 
lowish-brown scales,  is  only  sparingly  soluble  in  hot  or  cold  alcohol, 
almost  insoluble  in  toluene,  benzene,  ether  or  cold  glacial  acetic 
acid,  and  is  but  slightly  soluble  in  the  boiling  acid.  When  heated 
to  2800  C.  it  does  not  undergo  any  alteration ;  considerably  above 
that  temperature  it  melts  and  is  decomposed,  forming  a  black 
liquid.  Rhein  is  quite  insoluble  in  water ;  its  alcoholic  solution  is 
distinctly  acid.  At  the  normal  temperature  it  dissolves  readily  in 
caustic  potash  or  soda,  also  in  solution  of  potassium  or  sodium  car- 
bonate, with  a  deep  purple-red  color.  When  a  solution  of  rhein  in 
potassium  carbonate  is  mixed  with  solid  potassium  carbonate  a 
compound  of  rhein  with  potassium  is  separated  as  an  amorphous 
purple  mass.  Rhein  also  dissolves  in  a  solution  of  potassium  or 
sodium  bicarbonate,  but  not  so  rapidly  as  in  a  solution  of  neutral 
carbonate.  It  dissolves  also  with  purple  color  in  ammonia,  and  the 
solution  forms  flocculent  amorphous  precipitates  of  a  purple  color 
on  addition  of  lead  acetate,  agentic  nitrate,  or  barium  chloride. 
When  a  solution  of  rhein  in  caustic  alkali  or  in  other  basic  sol- 
vents is  mixed  with  hydrochloric  acid  the  rhein  is  separated  in  a 
colloid  condition,  and  from  very  concentrated  solution  it  presents  the 
appearance  of  a  translucent,  slimy  mass.  When  these  solutions  are 
boiled  with  excess  of  hydrochloric  acid  the  rhein  soon  assumes  the 
sparingly  soluble  crystalline  condition  above  described.  In  the 
colloid  condition  rhein  is  much  more  soluble  in  ether  than  the  crys- 
talline substance.    It  has  a  sweetish  acid  taste,  and  does  not  appear 
