428     ON  THE  AMOUNT  OF  TANNIN  IN  TANNING  MATERIALS. 
der  examination,  but  this  is  continued  uninterruptedly  until  on 
the  falling  of  a  drop  upon  the  surface,  the  characteristic  ring  of 
tannate  of  gelatin  is  no  more  to  be  detected.  When  this  mo- 
ment occurs,  it  is  necessary  that  the  precipitation  should  be  in- 
terrupted for  a  time,  and  that  the  fluid,  which  appears  limpid 
after  a  lapse  of  a  few  minutes,  should  be  tested  for  tannin. 
The  little  trouble  attending  this  operation  should  not  prevent  its 
being  done  frequently,  in  order  that  no  more  of  the  aluminous 
solution  of  gelatin  may  be  employed  than  is  just  necessary  for 
the  precipitation  of  the  tannin,  as  the  amount  of  the  latter  is 
finally  calculated  from  that  of  the  gelatin. 
In  order  to  ascertain  how  much  aluminous  solution  of  gelatin 
represents  a  given  quantity  of  pure  tannin,  5  grs.  of  the  latter 
were  dissolved  in  half  an  ounce  of  distilled  water,  and  precipi- 
tated with  the  greatest  care.  After  repeating  the  experiment 
five  times,  the  author  found  that  155  grs.  of  the  solution  of  ge- 
latin were  required  for  this  purpose.  Consequently  a  decoction 
of  J  ounce  of  oak-bark,  if  1  ounce  (=480  grs.)  of  aluminous  so- 
lution of  gelatin  were  employed  in  its  precipitation,  would  have 
contained  15  Jj-  grs.  of  tannin,  in  accordance  with  the  formula 
155:5:  :  480  ;  152* 
'  31 
The  quantity  of  solution  of  gelatin  employed  for  the  deter- 
mination of  tannin  is  ascertained  by  accurately  weighing  a  3  or 
4  ounce  glass  filled  with  it,  and  ascertaining  the  difference  of 
weight  after  the  precipitation  ;  this  difference  of  course  indicates 
the  quantity  of  solution  of  gelatin  employed. 
For  the  preparation  of  the  aluminous  solution  of  gelatin,  4 
ounces  of  distilled  water  are  weighed  in  a  phial ;  into  this  1 
drachm  of  ordinary  clean  joiner's  glue,  crushed  a  little,  is  put, 
when  the  glass,  surrounded  with  paper,  is  placed  in  a  vessel  of 
water,  and  set  upon  the  fire.  When  the  glue  is  dissolved,  which 
takes  place  without  any  residue  when  the  so-called  Russian  glue 
is  made  use  of,  15  grs.  of  pure  powdered  alum  are  added  to  the 
hot  fluid,  which  is  then  shaken  round  several  times,  and  may  be 
kept,  well-stoppered,  in  a  dark  place. 
The  substances  (about  50 — 100  grs.),  the  amount  of  tannin 
in  which  is  to  be  ascertained,  are  to  be  extracted  for  a  few  min- 
utes with  as  much  distilled  water  as  will  cover  them  ;  and  this 
