I20 THE CARBOHYDRATES 



AMYLOID. 



Amyloid * is the name given to a substance occurring in 

 the seeds of paeonies and certain cresses,"f which yields on 

 hydrolysis with dilute sulphuric acid a mixture of galactose, 

 glucose, and xylose. It is a colourless substance, and is in- 

 soluble in cold water, but swells up into a slimy mass in hot 

 water ; it is soluble in cuprammonia solution. Amyloid does 

 not reduce Fehling's solution, but is oxidized by nitric acid to 

 mucic and trihydroxy-glutaric acids. It gives a blue colour 

 with iodine. 



GUMS. 



The natural gums were formerly thought to be carbo- 

 hydrates of the general formula (C6Hj()05)n ; the researches of 

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

 hydrates, but are rather substances of a glucosidal nature, 

 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 are Isevo-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 — 



* Cf. footnote, p. 132. 



f Winterstein : " Z. physiol. Chem.," 1893, 17, 353. 



