VOLUMETRIC  ESTIMATION  OF  TANNIC  AND  GALLIC  ACIDS,  &C.  315 
a  half,  communicates  with  the  atmosphere  by 
the  bent  tubes  (b)  and  (C),  the  latter  being 
drawn  out  at  the  end  (d)  to  the  diameter  of 
about  one  or  one  and  a-half  millimetres.  The 
two  glass  tubes  are  united  by  means  of  a  mod- 
erately long  piece  of  India  rubber  tubing  (E), 
provided  with  a  pinchcock  (f)  to  close  it  ;  and 
the  lower  glass  tube  is  fixed  in  the  neck  of  the 
bottle  by  a  bored  cork,  or,  better,  a  caoutchouc 
stopper  (g). 
In  executing  the  analysis  it  is  absolutely  ne- 
cessary that  the  air  in  the  bottle  should  be  perfectly  renewed, 
and  the  temperature  of  all  reaching  the  fluid  be  the  same  as 
that  of  the  laboratory.  As  soon  as  the  absorbing  lye  (which 
should  amount  to  150  or  250  c.c.)  is  prepared,  the  bottle  should 
be  perfectly  closed,  and  then  the  pinchcock  opened  just  for  a 
moment,  so  that  the  pressure  of  the  internal  and  external  air 
may  be  equalised.  The  absorption  of  the  oxygen  is  then  has- 
tened by  strongly  shaking  the  bottle,  which  must  be  wrapped 
in  a  cloth  to  avoid  raising  the  temperature  by  the  warmth  of 
the  hand.  After  each  shaking,  water  must  be  allowed  to  flow 
into  the  bottle  (a)  from  a  weighed  quantity  in  a  beaker  (B.  Fig. 
2),  so  that  the  fluid  in  the  two  vessels  may  attain  the  same  level, 
as  shown  in  the  drawing.     The  experiment  is  ended  when,  after 
repeated  shakings,  no  more  water  runs  from  B  to  A,  and  the  dif- 
ference in  the  weight  of  the  water  in  the  beaker  in  grammes 
gives  the  amount  of  oxygen  absorbed  in  cubic  centimetres, 
which  can  be  corrected  for  the  standard  temperature  and 
pressure. 
1.  lor  Gallic  and  Tannic  Acid. — Place  in  the  bottle  about 
200  c.c  of  a  3  per  cent,  potash  or  soda  solution,  then  drop  in 
one  gramme  of  tannic  or  gallic  acid  loosely  wrapped  in  paper 
i 
