108 VARIETIES OF GUM. 



those of the beech and the pine.* Hence the readiness with which a 

 branch of the willow takes root and spiouts, and hence also the occa- 

 sional use of the inner bark of trees for food, especially in northern coun- 

 tries, and in times of scarcity. In some roots which abound in sugar, 

 as in those of the beet, the turnip, and the carrot, only 2 or 3 per cent, 

 of starch can be detected. 



§3. Gum — its constitution and properties. 



The variety of gum with which we are most familiar is gum arabicj 

 or Senegal, the produce of various species of acacia, which grow in the 

 warmer regions of Asia, Africa, and America. It exudes from the 

 twigs, and stems of these trees, and collects in rounded more or less 

 transparent drops or tears. It is also produced in smaller quantities in 

 many of our fruit trees, as the apple, the plum, and the cherry ; it is 

 present in some herbaceous plants, as in the althaea and malva officinalis 

 (common and marsh mallow] ; and it exists in lint, rape, and many 

 other seeds. When treated with boiling water these plants and seeds 

 give mucilaginous solutions. 



Many varieties of gum occur in nature, but they are all characterised 

 by being insoluble in alcohol, by dissolving or becoming gelatinous in 

 hot or cold water, and by giving mucilaginous — viscid and glutinous — 

 solutions, which may be employed as a paste. 



Three distinct species of gum have been recognised by chemists : 



1°. Arahin—o^ which gum arable and gum Senegal almost entirely 

 consisL It is readily soluble in cold ivater, giving a viscid solution, usu- 

 ally known by the name of the mucilage of gum arable. 



2°. Cerasin — which exists in the gum of the cherry-tree. It is inso 

 luble in cold water, but dissolves readily in boiling water. When thus 

 dissolved it may be dried without losing its solubility, and is therefore by 

 boiling supposed to be changed into arabin. 



3°. Bassorin — existing in what is called bassora gum — and forming 

 a large portion of gum tragacanth.f It swells and becomes gelatinous in 

 cold water, but does not dissolve in water either cold or hot. 



By these characters, the three kinds of gum are not only readily dis- 

 tinguished, but may be easily separated from each other. Thus if a 

 native gum or an artificial mixture contain all the three, simple steeping 

 in and subsequent washing with mid water, will separate the arabin — 

 boiling water will then take up the cerasin, and the bassorin will remain 

 behind. 



These different kinds of gum all possess the same chemical constitu- 

 tion. According to the analyses of Mulder, they consist of 

 Carbon . . . 45*10 per cent., or 12 atoms. 

 Hydrogen . . 6-10 " or 10 ♦' 



Oxygen . . . 48-80t " or 10 " 



100 



* Its presence is readily detected in such wood by a drop of the solution of iodine — which 

 ^ives a permanent blue to starch, but tc the woody fibre only a brownish stain. 



 This gum exists along with starch in the roots of the various species of orchia, especially 

 of those which are used for making scdep (Meyen). 



Berzelius Arsberdttclse, 1839, p. 443. 



