Am Aug^!<?2arm  }    Molybdic  Acid  as  a  Color  Reagent.  439 
estimation  or  homogentisinic  acid  in  the  urine,  which  depends  on 
the  reducing  action  of  gallic  acid  on  ammoniated  silver  solution. 
He  finds  that  the  proportion  of  gallic  acid  excreted  in  the  urine 
largely  depends  on  the  amount  given  in  one  dose.  If  a  drachm  to 
a  drachm  and  a  half  be  taken  30  per  cent,  passes  in  the  urine ;  if  30 
grains  only  21  per  cent.  After  22. grains  only  5  per  cent,  could  be 
detected;  after  7—  1 5  grains  only  2  per  cent.,  "whilst  a  dose  of  3 
grains  was  not  followed  by  the  appearance  of  any  gallic  acid  in  the 
urine.  As  he  examined  the  faeces  in  vain  for  any  trace  of  gallic  acid 
it  follows  that  a  certain  portion  is  burnt  up  in  the  system  in  its  pas- 
sage through  the  body.  He  finds  that  after  tannic  acid  has  been 
taken  only  very  little  gallic  acid  passes  into  the  urine.  The  admin- 
istration of  30  grains  to  60  grains  of  tannic  acid  is  indeed  followed 
by  the  appearance  of  a  trace  of  gallic  acid  in  the  urine,  but  so  small 
is  the  quantity  that  he  could  not  estimate  it.  Only  when  two 
drachms  of  tannic  acid  had  been  administered  was  the  quantity 
of  gallic  acid  sufficient  to  allow  of  its  quantitative  estimation,  but 
only  one  per  cent,  of  tannic  acid  taken  was  found  in  the  urine  as 
gallic  acid ;  since  tannic  acid  could  not  be  found  in  the  faeces  he 
concludes  that  the  greater  part  of  the  tannic  acid  which  is  absorbed 
is  burnt  up  in  the  body. 
In  explanation  of  the  very  small  quantity  of  tannic  acid  which 
appears  in  the  urine,  changed  into  gallic  acid,  he  suggests  that  the 
former  forms  with  albumen,  combinations  difficult  of  solution  ;  these 
pass  into  the  intestine,  and  there  slowly  decompose.  The  tannic 
acid  is,  indeed,  converted  into  gallic  acid,  but  only  gradually,  and 
the  small  quantities  of  the  gallic  acid  thus  produced  are  burnt  up 
and  therefore,  never  appear  in  the  urine.  On  the  other  hand,  when 
gallic  acid  is  taken,  owing  to  its  great  solubility  it  is  absorbed,  and 
entering  the  blood  at  once,  a  small  quantity  only  is  burnt  up  and 
the  larger  quantity  excreted. 
MOLYBDIC  ACID  ASA  COLOR  REAGENT  FOR  CER- 
TAIN AROMATIC  OXY-COMPOUNDS. 
i  By  J.  Stahl. 
Hager  some  time  ago  indicated  a  reaction  for  the  tannic  acid  of 
galls  and  other  tannic  acids,  according  to  which  these  substances 
give  fine  reddish-yellow  colors  with  ammonium  molybdate.  I  have 
found   that   the   same    reactions   occur   for  certain  compounds 
