186 THE CARBOHYDRATES 



GUMS. 



The natural gums were formerly thought to be carbo- 

 hydrates of the general formula (C6Hio05)n ; the researches of 

 O'Sullivan, however, have shown that they are not simple 

 carbohydrates, since on hydrolysis they give rise to sugars 

 mixed with complex acids of high molecular weight. The gums 

 themselves retain 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 arc Igevo-rotatory, whereas 

 those of dextrin are dextro-rotatory. 



2. Basic lead acetate precipitates natural gums from solu- 

 tion, but has no action on dextrin in weak solutions. 



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 (CgHioOg) together with some saccharic (CeHioOg) 

 and oxalic (C2H2O4) acids, whereas dextrin yields chiefly oxalic 

 acid together with a small quantity of saccharic and tartaric 

 (C4H6O6) 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 free acid can be isolated by the action of 

 a stronger acid. 



