GUMS 187 



The classification of gums is, for want of more accurate 

 knowledge, based chiefly on their solubility in water : — 



(a) Gums, such as arabin, which are completely soluble. 



(b) Gums which are partially soluble, such as cerasin and 

 bassorin. 



{c) Mucilages and pectic bodies which swell up with water 

 and dissolve, and in concentrated solution form a jelly. 



The classification, however, is by no means rigid, many 

 natural gums being composed of mixtures of several kinds of 

 gums. 



In the separation of gums from the tissues of the plant 

 advantage is taken of their solubility in water ; it is found in 

 practice, however, that in many cases mere maceration in 

 water does not remove all the gum present. 



Microchemical Reactions. 



Microchemically, gum and mucilage may be recognized 

 by their solubility and swelling respectively in water. Both 

 are insoluble in alcohol and ether. With other reagents the 

 results differ in different examples. Thus with iodine either 

 a blue or a yellow colour may result, while in other cases the 

 blue coloration is only obtained after treatment with chlor- 

 zinc iodide or sulphuric acid and iodine, indicating a close 

 association with cellulose ; this type of mixed gum, e.g. gum 

 tragacanth, is not stained by such dyes as ruthenium red (an 

 ammoniacal solution of ruthenium sesquichloride), whereas 

 true gums, such as those of apricot, cherry, peach, etc., are 

 stained red. They show different degrees of solubility in 

 cuprammonia. Many of these substances stain well with 

 corallin soda, and they also, especially the mucilages, show a 

 great avidity for stains such as aniline blue and aniline violet. 



GUM-ARABIC. 



This substance is a mixture of calcium, magnesium, and 

 potassium salts of a weak acid of unknown constitution, to 

 which earlier writers gave the name of arable acid or arabin. 

 O'SuUivan,* however, applied the term arable acid to a 



*0'Sullivan: "J. Chem. Soc," 1884, 45, 41 ; 1891, 59, 1029. 



