52       ON  THE  CONSTITUTION  OF  TANNIN  AND  GALLIC  ACID. 
means  of  fermentation,  this  substance  is  found  in  the  liquors,  but 
if  the  conversion  is  effected  by  means  of  diluted  sulphuric  acid,  as 
substance  similar  to  humic  acid  is  produced  at  the  same  time  as 
the  gallic  acid,  and  we  are  naturally  led  to  think  that  the  sulphuric 
acid  has  resolved  the  tannin  into  gallic  acid  and  a  gummy  sub- 
stance which  is  afterwards  carbonized.  It  may  be  added  that  by 
no  process,  whatever  care  may  be  taken,  is  it  possible  to  produce 
a  quantity  of  gallic  acid  that  shall  be  more  than  half  the  weight 
of  the  tannin  employed*  This  last  fact,  the  uniform  result  of  ex- 
periments, is  much  relied  upon  by  those  Chemists  who  believe  in 
the  pre-existence  of  gallic  acid  in  tannin,  in  the  same  way  as 
benzoic  acid  is  supposed  to  exist  in  hippuric  acid.  The  formula 
for  tannin  assimilates  to  that  for  gallic  acid  dried  at  212°  Fahr., 
consequently,  if  the  latter  is  but  an  isomeric  transformation  of  the 
former,  the  product  obtained  in  gallic  acid  should  at  least  weigh 
as  much  as  the  tannin  used.  I  say  at  least  as  much,  because,  as 
the  transformation  takes  place  in  water,  the  gallic  acid  obtained 
is  more  highly  hydrated  than  that  dried  at  212°. 
But  on  examining  the  facts  more  closely  it  is  found,  that  in  con- 
verting tannin  into  gallic  acid  it  is  impossible  to  avoid  the  decom- 
position of  a  large  portion  of  the  former,  and  it  is  evident  that  the 
tannin  is  changed  as  a  whole,  and  is  not  split  in  two.  When 
once  it  is  thus  modified  the  tannin  is  no  longer  susceptible  of  con- 
version into  gallic  acid,  but  it  is  transformed  into  an  amorphous 
mucilaginous  substance.  It  is  on  this  account  that  the  gallic 
acid  made  by  Scheele's  process  is  not  even  equal  to  half  the 
weight  of  the  tannin  contained  in  the  galls  used.  For  the  same 
reason,  also,  the  tannin,  already  in  an  altered  state,  contained  in 
oak  bark,  yields  no  gallic  acid. 
The  following  experiments  appear  to  me  to  confirm  these 
views  :— 
An  aqueous  solution  of  pure  tannin  was  submitted,  in  a  close 
vessel,  to  the  action  of  pectase,  at  a  temperature  ranging  from  77° 
to  86°  Fahr.,  for  eight  days.  The  small  quantity  of  tannin  not 
yet  attacked  at  the  end  of  this  time  was  precipitated  with  gelatine, 
and  the  gallic  acid  with  neutral  acetate  of  lead.  After  standing 
for  twenty-four  hours,  the  complex  precipitate  was  completely  se- 
parated from  the  supernant  liquor,  which  was  clear  and  colorless. 
