1 88 THE CARBOHYDRATES 



substance of the formula C23H38O22, 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 arable acid ; the natural gum 

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

 arabic acid of the formula 2C10H10O8, 4C12H20O11, C23H30O18, 

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

 sium. The arable acid of the earher 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 arable acid into metarabic acid, which swells up 

 in water, but does not dissolve. 



Reactions. 



Solutions in water (10 per cent) of arable 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 solu- 

 tion of copper acetate ; [h) 10 per cent solution of lead 

 acetate ; [c) solution of ferric chloride (sp. gr. 1-2). 



2. A 5 per cent solution of silicate of potash produces a 

 cloudiness or a precipitate which is partially or wholly soluble 

 on adding an excess. Arabic acid either does not respond to 

 this reagent, or merely gives a slight turbidity, and the same 

 applies to the gums obtained from certain species of Cactus, 

 Albizzia, Acacia catechu, Acacia leucophlcea, and other plants. 



3. Stannate of potash gives similar reactions, and in the 

 case of arable acid produces a precipitate which is soluble in 

 excess. 



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

 " Year Book of Pharmacy," 1881, 191, 



