140 THE CARBOHYDRATES 



the character of acids, and it would appear that the molecule 

 of a gum is composed of a number of sugar residues grouped 

 around a nucleus acid in such a way as to leave the acid group 

 exposed. 



The gums are translucent amorphous substances, some of 

 which dissolve in water completely, giving a sticky solution, 

 while others merely swell up in water and form a sort of 

 jelly ; they are all . insoluble in alcohol. 



The natural gums must be distinguished from starch gum 

 or dextrin, which is an artificial product obtained from starch, 

 by the following characteristics : 



1. Solutions of natural gums are laevo-rotatory, whereas 

 those of dextrin are dextro-rotatory. 



2. Basic lead acetate precipitates natural gums from solu- 

 tion, but "has no action on dextrin. 



3. Natural gums on hydrolysis yield chiefly galactose and 

 pentoses such as arabinose or xylose, whereas dextrin yields 

 glucose only. 



The hydrolysis of gums takes a long time to complete 

 from eighteen to twenty-four hours whereas dextrin is easily 

 hydrolysed. 



4. On oxidation with nitric acid, natural gums yield chiefly 

 mucic acid (C 6 H 10 O 8 ) together with some saccharic (C 6 H 10 O 8 ) 

 and oxalic (C 2 H 2 O 4 ) acids, whereas dextrin yields chiefly oxalic 

 acid together with a small quantity of saccharic and tartaric 

 (C 4 H 6 6 ) acids. 



As they occur in nature, the true gums are mostly com- 

 bined with potassium, calcium, or magnesium in the form of 

 salts, from which the true carbohydrate can be isolated by the 

 action of a stronger acid. 



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. 



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

 bassorin. 



(c) Mucilages and pectic bodies which merely swell up with 

 water to form a jelly. 



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

 natural gums being composed of mixtures of several kinds of 

 gums. 



