814  VOLUMETRIC  ESTIMATION  OF  TANNIC  AND  GALLIC  ACIDS,  AC. 
anic  acid  obtained  is  more  nearly  the  theoretical  quantity  than 
is  the  process  of  M.  Gay  Lussac. 
2.  The  apparatus  of  Gea  Pessina  can  be  modified  very  easily 
to  furnish  the  anhydrous  acid. 
8.  Medicinal  prussic  acid,  obtained  with  this  anhydrous  acid, 
is  more  stable  than  that  which  is  obtained  by  the  acid  of  Gay 
Lussac. 
4.  The  stability  of  the  acid  of  Gea  Pessina  has  always  a  rela- 
tive stability,  the  two  acids  being  subject  to  the  same  phe- 
nomena of  decomposition,  and  differ  only  in  the  time  necessary 
to  manifest  these  phenomena. 
5.  The  acid  of  Gea  Pessina  does  not  owe  its  relative  stability 
to  the  influence  of  the  water,  which  is  uuited  with  it,  in  the 
process  of  preparing  it. 
6.  When  the  decomposition  of  hydrocyanic  acid  has  commenced, 
under  the  influence  of  the  light,  it  continues  itself  rapidly  in 
the  dark. 
7.  Hydrocyanic  acid  can  be  exposed  to  the  light  during  a  cer- 
tain time  without  its  appearance  being  modified,  but  in  this  case 
it  has  submitted  to  an  influence  which  predisposes  it  to  alteration 
and  renders  it  more  easily  decomposable  in  the  dark. 
8.  In  measure  as  medicinal  prussic  acid  is  clouded  and  col- 
ored, its  proportion  of  hydrocyanic  acid  is  lessened  gradually, 
and  after  a  time,  not  very  long,  none  of  the  acid,  free  or  com- 
bined, can  be  found  in  it. — Jour,  de  Chim.  Med.,  Mars,  1864. 
ON  THE  VOLUMETRIC  ESTIMATION  OF  TANNIC  AND 
GALLIC  ACIDS,  IRON,  MANGANESE,  &c. 
By  Moritz  Mittenzwey. 
The  known  disposition  of  tannic  acid  in  alkaline  solution  to 
absorb  oxygen  from  the  air  affords  an  excellent  means  of  esti- 
mating this  body  for  technical  purposes.  The  estimation  can 
be  conveniently  made  in  the  simple  apparatus  here  figured  and 
described. 
The  air  in  a  bottle  (a),  capable  of  holding  about  a  litre  and 
