142 THE CARBOHYDRATES 



concentrated aqueous solution of gum arable with hydro- 

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

 white amorphous glassy mass which dissolves in water to 

 give a laevo-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; (b] 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 

 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 arabic acid produces a precipitate which is soluble in 

 excess. 



4. A solution of neutral sulphate of aluminium (10 per 

 cent) generally gives a precipitate which is, in many cases, 

 soluble in potash. 



5. Basic lead acetate yields a precipitate which is entirely 

 or partially soluble in excess. 



GUM TRAGACANTH.. 



This gum occurs in species of Astragalus, and consists 

 of about 8-10 per cent of soluble calcium, magnesium, and 

 potassium salts, together with about 60-70 per cent of in- 

 soluble salts, which only swell up in water to a jelly. The 

 water soluble portion is said to contain a substance, poly- 

 arabinon-trigalactan-geddic acid, which on hydrolysis breaks 

 up into arabinose, galactose, and geddic acid, an isomer of 



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

 Book of Pharmacy," 1881, rtji. 



