344 
Commercial  Tannin. 
[Am.  Jour.  Pliarm. 
X       July,  1885. 
All  the  samples  were  soluble  iu  rectified  spirit.  No.  2  disappeared 
almost  instantaneously^  1,  5,  and  7  very  rapidly^  and  3,  4,  8,  and  9 
somewhat  slowly.  When  diluted  with  an  equal  bulk  of  water  all  the 
solutions  remained  perfectly  clear. 
The  samples  were  also  soluble  in  glycerin.  Employing  the  pro- 
portions for  glycerinum  acidi  tannici,  at  a  temperature  of  60^  F.,  it  was 
found  that  No.  1  dissolved  in  about  three  hours,  and  No.  5  in  four 
hours.  All  the  others  were  more  or  less  undissolved  at  four  hours,  but 
when  placed  in  a  bath  of  hot  water  they  dissolved  in  a  very  few  minutes. 
Tannic  Acid. — In  coming  to  the  consideration  of  the  composition  of 
the  samples,  I  may  refer  to  the  uncertainty  of  our  chemical  knowledge 
of  the  tannins  in  general,  and  of  the  gallotannic  acids,  as  commonly 
met  with,  in  particular.  It  would  be  out  of  place  for  me  to  attempt 
to  summarize  the  different  opinions  that  prevail  regarding  gall  tannin, 
but  it  does  not  seem  as  yet  to  have  been  determined  whether  it  is  or  is 
not  a  glucoside,  and  if  a  glucoside,  whether  of  gallic  or  digallic  acid. 
The  researches  of  Paul  and  Kingzett^  led  them  to  the  conclusion 
that  the  gallotannic  acid  of  commerce  \vas  not  a  glucoside,  and,  further, 
that  the  supposition  that  tannin,  as  it  exists  in  gall  nuts  is  a  gluco- 
side is  rendered  doubtful,  and  the  sugar  met  witli  in  some  samples'of 
tannic  acid  is  more  probably  referable  to  an  impurity,  as  Rochleder 
and  Kawalier  assumed.  Schiff  leans  to  the  opinion  that  the  tannin  in 
gall  nuts  is  a  glucoside,  but  he  states  that  the  gallotannic  acid  met  with 
in  commerce  is  not  a  glucoside,  but  digallic  acid." 
This  uncertainty,  coupled  witli  the  extreme  difficulty  of  procuring 
pure  gallotannic  acid,  has  always  been  a  barrier  in  the  way  of  analytical, 
and  especially  gravimetric  methods  for  its  estimation.  A  large  num- 
ber of  methods  have  been  proposed,  but  most  of  these  are,  according 
to  competent  authorities,  more  or  less  faulty.  Proctor,  in  the  paper 
already  quoted,  gives  a  decided  preference  to  Lowenthal's  volumetric 
method,  and  DragcndorflP,^  after  summarizing  the  more  important  pro- 
cesses and  indicating  precisely  the  strong  and  weak  points  of  each,  also 
seems  to  prefer  the  permanganate  method.  My  own  experience  of  the 
gravimetric  methods  has  been  that  I  found  a  difficulty  in  obtaining 
concordant  results,  and  these,  moreover,  were  generally  too  high,  which 
is  doubtless  to  be  accounted  for  by  the  varying  composition  of  the  pre- 
cipitates, and  the  liability  of  the  tannin  and  its  salts  to  undergo  decom- 
1  "  Joiirn.  Chem.  Soc,"  vol.  xxxiii,  p.  217. 
2  "  Plant  Analysis,"  pp.  41-46. 
