122 THE CARBOHYDRATES 



cuprammonia. Many of these substances stain well with coral- 

 lin 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 arabic acid or arabin. 

 O'Sullivan, however, applied the term arabic acid to a sub- 

 stance of the formula C23H3SO22, which he regarded as the 

 nucleus acid around which a number of sugar residues are 

 grouped ; by hydrolysis under varying conditions, it is possible 

 to split off successive sugar residues with the formation of 

 acids of gradually decreasing molecular weight, until finally 

 the nucleus acid free from all carbohydrate residues remains, 

 and it is this acid that he calls arabic acid ; the natural gum 

 itself would, according to him, be a diarabinan-tetragalactan- 

 arabic acid of the formula 2CiQHiQ0g, 4CJ2H2QO11, CogH^yOig, 

 which is combined with the calcium, magnesium, and potas- 

 sium. The arabic acid of the earlier authors, which is the acid 

 set free from the natural gum by the removal of the calcium, 

 magnesium, and potassium, may be prepared by acidifying a 

 concentrated aqueous solution of gum arabic with hydro- 

 chloric acid, and adding alcohol. The pure substance is a 

 white amorphous glassy mass which dissolves in water to 

 give a Isevo-rotatory solution. Ten per cent sulphuric acid 

 converts this arabic acid into metarabic acid, which swells up 

 in water, but does not dissolve. 



Reactions. 



Solutions in water (10 per cent) of arabic acid and other 

 varieties of gum arabic give, according to Masing,* certain 

 more or less definite reactions. 



1. They are not precipitated by {a) a cold saturated so- 

 lution of copper acetate; (/;) 10 per cent solution of lead 

 acetate ; {c) solution of ferric chloride (sp. gr. i •2). 



2. A five per cent solution of silicate of potash produces a 



* Masing: " Archiv d. Pharm.," 1879, [3], 15, 216; 1880, 17,34, 41; "Year 

 Book of Pharmacy," 1881, igi. 



