126 THE CARBOHYDRATES 



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 leucophloea 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 

 arabic acid. The part soluble in water, when treated with 

 baryta water, gives two isomeric tragacanthan-xylan-bassoric 

 acids, which on hydrolysis yield a pentose sugar tragacanthose, 

 xylose, and bassoric acid C 14 H 20 O 13 . 



WOOD GUM AND CERASIN OR CHERRY GUM. 



These are other examples of pentosanes, but too little is 

 known of their chemistry to warrant any description. 



WOUND GUM. 



A gum-like substance, termed wound gum, is frequently 

 found in the tracheae of plants, in the immediate neighbour- 

 hood of wounds, and stopping up the lumina ; it is secreted 

 by the surrounding living cells. Wound gum does not swell 

 in water, and is insoluble in sulphuric acid and in caustic soda. 

 On oxidation with nitric acid it yields both mucic and oxalic 

 acids, and it responds to lignin tests ; e.g., on treatment with 



