376 
Polygonum  Hydropiper. 
Am.  Jour.  Pharm. 
Aug.,  1886 
3d. — Tannic  acid  does. 
4th. — Tannic  acid  does. 
3d. — Polygonic  acid  does  not  pre- 
cipitate a  neutral  solution  of  a  ferric 
salt. 
4th. — Polygonic  acid  does  not  pre- 
cipitate a  solution  of  tartar  emetic. 
With  the  above  reactions,  I  hope  the  gentlemen  will  be  able  to 
differentiate  between  tannic  acid  and  polygonic  acid. 
1st. — Gallic  acid  produces  a  deep 
bluish-black  color,  with  solutions  of 
the  salts  of  sesquioxide  of  iron. 
2d. — When  a  solution  of  gallic  acid 
is  added  to  a  solution  of  proto-sul- 
phate  of  iron,  and  to  this  solution  a 
little  acetate  of  sodium  is  added,  a 
deep  violet  color  will  appear. 
3d. — If  a  dilute  solution  of  gallic 
acid  is  added  to  a  dilute  solution  of 
caustic  potash,  an  intense  green  color 
is  produced. 
1st. — Polygonic  acid  does  not. 
2d. — When  polygonic  acid  is  treated 
the  same  way,  the  violet  color  does 
not  appear. 
3d. — If  a  dilute  solution  of  caustic 
potash  is  treated  with  a  dilute  solu- 
tion of  polygonic  acid,  no  green  color 
is  produced,  but,  to  the  contrary,  an 
intense  yellow  color  is  produced. 
Besides  the  above  reactions,  the  difference  in  crystalline  appearance 
will  be  noticed  (for  gallic  acid,  see  figure  below;  for  polygonic  acid, 
see  page  280  of  June  number). 
GALLIC  ACID  x700. 
