VARIOUS GUMS 141 



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 ; Dragendorff 

 found that much more arabic acid could be extracted after 

 the material had been treated with an alcoholic solution of 

 tartaric acid 



Micro chemical Reactions. 



Microchemically, gum and mucilage may be recognize^ 

 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 

 cuprammonia. Many of these substances stain well with coral- 

 lin soda, and they also, especially the mucilages, show a great 

 avidity for stains such as aniline blue and aniline violet. 



GUM ARABIC. 



This substance is a mixture of calcium, magnesium, and 

 potassium salts of a weak acid of unknown constitution, to 

 which earlier writers gave the name of arabic acid or arabin. 

 O'Sullivan, however, applied the term arabic acid to a sub- 

 stance of the formula C 23 H 38 O 22 , 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 arabic acid ; the natural gum 

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

 arabic acid of the formula 2C 10 H 10 O 8 , 4C 12 H 20 O n , C 23 H 30 O 18 , 

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

 sium. The arabic acid of the earlier 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 



