34 THE CHEMISTRY OF CATTLE FEEDING 



They give no coloration with iodine and do not reduce 

 Fehling's solution. They occur in cereal grains, and straws, 

 and in other plants, are digestible by animals, and probably 

 have the same nutritive value as dextrine and other carbo- 

 hydrates. Several varieties of gum are known. The two 

 described below may be taken as typical of the group. 



Gum arabic is completely soluble in water. It forms a 

 viscous solution from which it is reprecipitated by alcohol. 

 It usually contains about 1 2 per cent, of water which is driven 

 off at 100 C. On incineration it yields about 3 per cent, of 

 ash, consisting chiefly of the carbonates of potassium and 

 calcium. The basic elements can be eliminated by the action 

 of hydrochloric acid on aqueous solutions ; and on adding 

 alcohol the arabic acid, or arabin, is precipitated. When 

 purified by reprecipitation, the elementary composition of 

 this substance corresponds to the formula, C^I^Ou or 

 2CeH 10 O5 -f- H 2 O. It is, however, a mixture of at least two 

 compounds, one of which is Isevo-rotatory and on hydrolysis 

 with acids yields a crystallisable pentose, C 5 H 10 O 5 , called 

 arabinose ; the other, which is dextro-rotatory, is converted 

 into an uncrystallisable sugar. Arabinose is an aldose, 

 CH 2 (OH).(CH.OH) 3 .CHO, and can be oxidised to 

 arabonic acid. 



When arabic acid (arabin) is heated, or treated with con- 

 centrated sulphuric acid, it is changed into an insoluble form 

 called meta-arabic acid. By the action of alkalis this 

 substance is reconverted into a soluble salt very similar to 

 the original gum arabic. 



Gum tragacanth, when treated with warm water, undergoes 

 only a partial solution. The soluble portion resembles gum 

 arabic, but is not identical with it. The insoluble portion, 

 which is known as bassorin, absorbs water and swells up, 

 forming a jelly very similar to meta-arabic acid. 



Mucilage. The mucilages have not hitherto been closely 

 investigated and form a very ill-defined group. By some 

 authors they are regarded merely as varieties of gum. They are 

 practically insoluble in water, but swell up, forming jellies 

 like meta-arabic acid. Several of these jellies give a blue 



