CHEMICAL  EXAMINATION  OF   THE  RUBIACE^E. 
245 
the  Stellatce,  for  the  same  reason  that  the  rubichloric  acid  is  a  cha- 
racteristic constituent  of  this  group.    If  the  hydrate  of  citric  acid 
be  written=C12HG+012,  it  represents  a  member  of  the  formula 
ci2hg  ? 
C2H25 
Just  as  the  Stellatce,  together  with  the  characteristic  tannic  acids 
of  the  formulario  TTo  {  On,  contain  the  rubichloric  and  citric  acids, 
analogous  in  their  composition,  but  differing  in  their  properties,  so 
in  the  Cinchonacece  the  tannic  acids  are  accompanied  bykinic  and 
kinovic  acids.  The  kinic  acid,  C14H808,  like  the  rubichloric  acid, 
is  nearly  allied  in  its  composition  to  the  tannic  acid  of  all  Rubiacece, 
of  the  formula  CI4H8On,  but  like  it  also  differs  from  them  in  all  its 
reactions.  The  place  of  the  citric  acid  of  the  Stellatce  is  occupied 
in  this  group  by  an  acid,  which,  like  citric  acid,  contains  12 
atoms  of  carbon ;  its  formula  is  C12H903. 
In  the  true  Coffeacece,  Cephaelis  ipecacuanha,  Coffea  arabica 
and  Chiococca  racemosa,  no  common  characteristic  substance  ac- 
companies the  tannic  acids.  In  the  seeds  of  Coffea  arabica,  the 
citric  acid  of  the  Stellatce  is  present,  although  in  inconsiderable 
quantity ;  the  kinovic  acid  of  the  Cinchonacece,  combined  with 
hydrate  of  carbon,  is  contained  in  the  root  of  Chiococca  racemosa 
in  the  form  of  caincic  acid.  In  the  root  of  Cephaelis  ipecacuanha 
both  acids  are  wanting.  The  large  quantity  of  starch  and  gum, 
which  partake  of  the  formula  C12H10O10,  contained  in  this  plant, 
is  perhaps  to  be  regarded  as  the  representative  of  the  citric  acid. 
Citric  acid  (anhydrous)  with  2  additional  equivs.  of  water  =C12 
H'O13,  is  starch  or  gum  in  which  3  equivs.  of  hydrogen  are  re- 
placed by  3  equivs.  of  oxygen. 
If  the  plants  of  these  three  groups  be  brought  together  with 
their  characteristic  constituents,  the  following  tabular  view  is  ob- 
tained : — 
RlJBIACEiE. 
I.  Stellatce. 
I.  Characteristic  II.  Characteristic  III.  Characteristic 
constituent.  constituent.  constituent. 
Tannic  acid  of  the  for-  Rubichloric 
mula —  acid —         Citric  acid — 
C12H6  }  ^  „_0  ft  „  ^  C12H6  ?  ^  C12HW2. 
C2  R2  ^  On.n=8,  9  or  10.  q2 
H25°' 
