448  Gum  Arabic  and  Gum  Senegal.  {AmS;Srm' 
brownish,  and  strongly-refractive  little  lumps ;  whilst  gum  Senegal 
is  usually  in  long,  straight  or  curled  cylindrical  pieces,  but  occasion- 
ally in  mulberry- shaped  nodules,  and  is  either  colorless  or  faintly- 
yellow  or  white,  like  etched  glass,  superficially,  and  lustrous  and 
transparent  internally.  The  two  gums  are  therefore  readily  detected 
in  the  uncrushed  condition,  but  under  other  circumstances  they 
require  further  investigation  for  their  identification. 
Water  dissolves  both  gums,  leaving  a  residue  of  wood  fibres,  these 
being  usually  red  if  from  gum  arabic  and  black  from  gum  Senegal. 
Potassium  hydroxide  and  copper  sulphate  produce  a  blue  precipi- 
tate in  both  solutions  ;  the  gum  arabic  precipitate  is  more  consider- 
able than  the  Senegal  precipitate.  Moreover,  the  former  is  coherent, 
and  rises  to  the  surface,  whereas  the  latter  is  more  flocculent,  and 
remains  disseminated  in  the  liquid.  The  precipitates  are  only  very 
slightly  soluble  on  warming,  and  are  not  reduced  even  on  boiling. 
Under  similar  treatment,  dextrin  also  gives  a  bluish  precipitate 
insoluble  in  the  cold,  but  soluble  to  a  clear,  dark-blue  solution  on 
warming,  which  solution  is  completely  reduced  by  prolonged  boil- 
ing. By  heating  with  dilute  potassium  hydroxide  for  some  time, 
solutions  of  gum  arabic  or  dextrin  become  amber-yellow;  solutions 
of  gum  Senegal,  on  the  other  hand,  scarcely  alter,  or  are  but  very 
faintly  yellow. 
Mixtures  of  the  gums  arabic  and  Senegal  behave,  with  potassium 
hydroxide  alone,  like  gum  arabic ;  with  potassium  hydroxide  and 
copper  sulphate  like  gum  Senegal.  The  blue  precipitates  from  mix- 
tures of  dextrin  with  gum  arabic  or  gum  Senegal  are  reduced  on 
boiling,  provided  the  quantity  of  dextrin  is  not  too  small;  but  when 
the  latter  is  the  case,  after  thorough  warming,  the  precipitate  must 
first  be  filtered  off,  then,  on  boiling  the  filtrate,  reduction  takes  place 
if  dextrin  is  present.  When  both  gums  as  well  as  dextrin  are  pres- 
ent, the  precipitate  is  washed,  dissolved  in  dilute  hydrochloric  acid, 
and  the  gums  precipitated  by  means  of  a  large  excess  of  alcohol ; 
when  settled,  they  are  washed  and  examined  by  the  above  methods. 
The  examination  of  a  sample  of  gum  arabic  may  be  conducted 
in  the  following  manner  :  Dissolve  the  powdered  substance  in  luke- 
warm water,  examine  residue — any  gelatinous  matter  indicating 
foreign  gums  ;  treat  the  solution  with  excess  of  potassium  hydroxide 
and  copper  sulphate,  warm,  filter,  and  examine  for  dextrin  and  Sene- 
gal as  described  above. 
