368 
COMPOSITION  OP  VEGETABLE  GUMS. 
lime  is  produced.  It  may,  therefore,  be  readily  comprehended 
why  the  body  obtained  by  M.  Ge'lis  is  reconverted  by  the  simple 
action  of  boiling  water,  while  metagummic  acid,  obtained  by  the 
method  described,  is  only  transformed  under  the  influence  of 
bases.  Considering  the  very  feeble  saturating  power  of  the 
class  of  acids  to  which  these  bodies  approximate,  it  is  not 
astonishing  that  the  lime  salt  should  only  contain  three  per  cent, 
of  lime.  M.  Guerin-Vary  has  shown  that  the  gum  of  the  cherry- 
tree  contains  a  soluble  substance,  which  is  identical  with  gum 
arabic,  and  also  that  the  insoluble  gelatinous  substance  or  cera- 
sine  is,  by  prolonged  ebullition,  rendered  soluble,  and  converted 
also  into  gum  arabic.  M.  Fremy  shows  that  the  alkaline  carbo- 
nates react  rapidly  upon  the  cerasine,  giving  rise  to  carbonate 
of  lime,  and  a  gum  entirely  comparable  to  that  obtained  by  the 
direct  action  of  the  base  on  metagummic  acid.  The  dilute  acids 
in  the  cold  decompose  cerasine,  removing  the  lime  and  elimina- 
ting metagummic  acid,  which,  by  boiling  with  lime,  yields  ordi- 
nary gum.  Cerasine,  then,  is  not  an  immediate  neutral  princi- 
ple, but  a  combination  of  lime  with  metagummic  acid.  It  is 
identical  with  the  insoluble  product  obtained  by  M.  Ge'lis  in 
heating  gum  to  150°  C.  The  two  bodies  reproduce  gum  arabic 
under  the  same  circumstances.  This  transformation,  which  is 
effected  by  the  action  of  boiling  water,  may  also  take  place  un- 
der the  influence  of  vegetation  ;  it  is  therefore  natural  to  meet 
in  the  vegetable  organism  with  mixtures  of  gum  arabic  and  cera- 
sine— they  constitute  two  isomeric  states  of  the  same  lime  salt. 
There  remain  other  gums,  such  as  tragacanth,  which  are  ap- 
parently distinct  from  the  preceding  in  their  properties  and  their 
constitution.  It  results  from  M.  Fremy's  experiments  that 
tragacanth  contains  an  acid  gelatinous  substance,  presenting  a 
certain  analogy  to  metagummic  acid,  but  which  is  nevertheless 
distinct.  When  the  insoluble  matter  of  gum  tragacanth  is  sub- 
mitted to  the  action  of  the  alkalies  and  alkaline  earths,  verita- 
ble gummy  substances  are  obtained,  soluble,  insipid,  uncrystal- 
lizable,  insoluble  in  alcohol,  like  gum  arabic,  but  precipitated  by 
neutral  acetate  of  lead,  which,  as  is  known,  has  no  action  on 
ordinary  gum.  It  results,  then,  from  these  latter  observations, 
that  the  gelatinous  bodies  contained  in  the  gums  may  be  con- 
verted into  soluble  compounds  by  the  action  of  boiling  water  or 
