Am.  Jour.  Pharm. 
Feb.,  1889. 
Soluble  Gum  of  Tragacanth. 
73 
found  also  its  way  into  the  present  British  Pharmacopoeia,  published 
in  1885,  the  statement  differing  from  the  above  quotation  mainly  in 
describing  the  color  produced  by  iodine  to  be  violet  or  blue. 
It  was  to  be  expected  that  the  statements  of  recent  English  and 
American  writers  on  Materia  Medica  and  on  Chemistry  would  differ 
in  regard  to  the  question  involved,  and  that  some  of  them  would  have 
adopted,  without  reserve,  the  characterization  of  tragacanth  as  given  by 
the  two  pharmacopoeias,  while  others  would  give  it  as  ascertained  by 
experiment  or  from  the  writings  of  others.  Such  has  indeed  been 
found  to  be  the  case  as  will  be  seen  from  the  following  brief  summary. 
Regarding  merely  the  effect  of  water  upon  tracaganth,  the  latter  is 
described  by  Bloxam,  Edes,  Fownes  and  Wood,  as  being  not  soluble, 
but  merely  swelling  in  water.  The  property  of  swelling  with  water 
to  a  gelatinous  mass  is,  as  a  matter  of  course,  mentioned  by  all  writers  ; 
but  in  addition  to  this  tragacanth  is  described  to  be  sparingly  or  very 
sparingly  soluble  in  water  by  Allen,  Brunton,  and  Milne,  (edit,  by 
Craig).  These  writers  do  not  allude  to  the  effect  of  alcohol  upon  the 
soluble  portion. 
In  several  works  the  correct  statement  is  made  that  tragacanth  is 
insoluble  in  alcohol ;  but  this  property  does  not  throw  any  light  upon 
the  behavior  of  the  aqueous  solution,  since  some  of  the  vegetable 
mucilages  are  insoluble  in  alcohol  when  treated  directly  with  this 
liquid,  while  their  aqueous  solutions  are  not  disturbed  on  the  addition 
of  alcohol. 
A  nearer  approach  to  the  behavior  of  the  water-soluble  portion  of  traga- 
canth is  found  in  Attfield,  Biddle,  and  in  Bentley  and  Trimen's  Medicinal 
Plants,  where  that  portion  is  said  to  resemble  arabin,  but  the  points  of 
resemblance  or  difference  are  not  given.  Potter  and  Prescott  (proxi- 
mate analysis)  state  the  soluble  gum  to  be  arabin. 
The  language  of  the  British  Pharmacopoeia  is  quoted  in  the  works  on 
Materia  Medica  by  Bentley,  Bruce,  Gerrard  and  Thorowgood,  the  last 
two  stating  that  the  alcohol  test  (non-precipitation  of  the  aqueous 
solution)  shows  the  absence  of  gum  arabic. 
The  correct  behavior  towards  alcohol  is  given  in  Pharmacographia 
and  in  the  manuals  of  Muter,  Royle  (edited  by  Harley)  and  Wills. 
About  half  a  century  ago  Pereira,  in  his  Materia  Medica,  gave  the 
following  account,  which  was  reproduced  in  the  later  editions : — 
"  The  soluble  gum  of  tragacanth  is  usually  regarded  as  similar  to 
gum  arabic,  and  hence  it  is  called  arabin  ;  but  is  distinguished  by 
silicate  of  potash  and  perchloride  of  iron  producing  no  change  in  it, 
