124 THE CARBOHYDRATES 



from eighteen to twenty-four hours — whereas dextrin is easily 

 hydrolysed. 



4. On oxidation with nitric acid, natural gums yield chiefly 

 mucic acid (C^jHi^Og) together with some saccharic (CgHj^^Og) 

 and oxalic (C2H2O4) acids, whereas dextrin yields chiefly oxalic 

 acid together with a small quantity of saccharic and tartaric 

 (C4H6O6) acids. 



As they occur in nature, the true gums are mostly com- 

 bined with potassium, calcium, or magnesium in the form of 

 salts, from which the true carbohydrate can be isolated by the 

 action of a stronger acid. 



The classification of gums is, for want of more accurate 

 knowledge, based chiefly on their solubility in water : 



{a) Gums, such as arabin, which are completely soluble. 



{b) Gums which are partially soluble, such as cerasin and 

 bassorin. 



{c) Mucilages and pectic bodies which merely swell up with 

 water to form a jelly. 



The classification, however, is by no means rigid, many 

 natural gums being composed of mixtures of several kinds of 

 gums. 



In the separation of gums from the tissues of the plant, 

 advantage is taken of their solubility in water ; it is found in 

 practice, however, that in many cases mere maceration in 

 water does not remove all the gum present ; Dragendorfif 

 found that much more arable acid could be extracted after 

 the material had been treated with an alcoholic solution of 

 tartaric acid. 



Microche^nical Reactions. 



Microchemically, gum and mucilage may be recognized 

 by their solubility and swelling respectively in water. Both 

 are insoluble in alcohol and ether. With other reagents the 

 results differ in different examples. Thus with iodine either 

 a blue or a yellow colour may result, while in other cases the 

 blue coloration is only obtained after treatment with chlor- 

 zinc iodide or sulphuric acid and iodine. Then again different 

 degrees of solubility are found to obtain on treatment with 



