ACACIA 449 



part of the tears of small fissures. In consequence of these they 

 easily break up into a number of small, transparent, angular frag- 

 ments with glistening, vitreous surfaces. The drug is practically 

 inodorous, and has a bland, mucilaginous taste. Whilst the finest 

 qualities are white, or have at most only a yellowish tinge, inferior 

 grades have a decided yellow or reddish or brownish red colour and 

 then contain traces of tannin. 



Acacia gum is insoluble in alcohol, but dissolves freely in water, 

 forming a translucent, viscid, but not glairy or ropy liquid, that 

 feebly reddens litmus paper. A 10 per cent, aqueous solution of 

 good qualities is slightly laevorotatory, and when boiled with an 

 equal volume of Fehling's solution throws down a slight but dis- 

 tinct deposit of cuprous oxide. Solution of lead acetate produces 

 no precipitate, but subacetate produces a copious white one ; whilst 

 a saturated solution of borax forms with a strong solution of gum a 

 clear, translucent jelly. Inferior (brown) gum usually contains tannin 

 which may be detected by solution of ferric chloride. 



Constituents. Acacia gum consists almost entirely of a gluco- 

 sidal acid of high molecular weight, which has been termed arabic 

 acid, combined with potassium, magnesium, and calcium ; by hy- 

 drolysis each molecule yields two molecules of the sugar arabinose 

 and four of galactose together with an organic acid to which the name 

 of arabic acid has also been given, but which is better termed iso- 

 geddic acid, as it is isomeric with the corresponding acid, geddic acid, 

 obtained from geddah gum. The glucosidal acid of acacia, is there- 

 fore, a diarabinan-tetragalactan-isogeddic acid, the termination ' an ' 

 indicating the anhydride of the corresponding sugar. This acid can 

 be obtained from the gum by acidifying an aqueous solution with a 

 mineral acid, dialysing it until the mineral constituents are removed, 

 and fractionally precipitating with alcohol. Whilst moist it dissolves 

 in water, but the dried acid only swells in water, dissolving on the 

 addition of an alkali. 



Gum acacia also contains an oxydase enzyme, and hence readily 

 turns powdered guaiacum resin, or the tincture diluted with water, 

 blue. It loses about 14 per cent, of moisture when dried at 100 

 and yields from 2*7 to 4'0 per cent, of ash. It contains further a 

 small percentage of nitrogen, but this does not enter into the com- 

 position of the gum itself (distinction from gelatin, &c.) ; it is probably 

 due to the enzyme, from which the gum cannot be entirely freed. 



Uses. Acacia gum is used medicinally as a demulcent and as a 

 means of suspending oils, resin, &c., in aqueous fluids. 



Varieties, Impurities, &C. Gum-yielding acacias are widely dis- 

 tributed over tropical and subtropical countries, and furnish large 

 quantities of gums which, though in most cases unsuitable for 

 medicinal use, are extensively employed for certain technical purposes. 



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