100 



are indicated by the more or less darkened colour of the gum. 

 They are so obstinately attached to it as to be removable only by 

 bleaching, which would destroy also part of the gum. Fortunately, 

 the quantity of the adhering pigment is always so minute as to be 

 practically of no consequence in the quantitative analysis. 



By the repeated precipitation with alcohol any sugar present in 

 the gum may be expected to be thoroughly removed. Any 

 remaining traces may be discovered as follows: — Dissolve from 

 10 to 20 gi-ains of the dried and pulverised gum in a little water, 

 add one drop of a solution of sulphate of copper (1 part to 9 parts 

 water), and enough of solution of caustic potash or soda to give it 

 a decidedly alkaline reaction. It must yield with pure gum a 

 clear, blue liquid, and must remain so when heated to the boiling 

 point; while in the presence of sugar the colour will be changed, 

 and a red powder (suboxyd of cojiper) subsides. Gum free from 

 sugar has no disoxydising effect upon the salts of oxyd of copper, 

 but effects only a blue solution with excess of potash or soda. 

 The best kinds even of commercial gum arable contain traces of 

 sugar, which may be thoroughly removed from the finely pul- 

 verised gum by treating with alcohol. 



Another and very frequent impurity of gum obtained as above, 

 or exudating spontaneoiisly, consists in mineral substances 

 (alkalies and alkaline earths), which are combined either vdth the 

 gum itself, while this acts as a weak acid, and has, therefore, 

 an acid reaction when dissolved in water; or, less frequently, these 

 alkalies are fixed to stronger acids (phosphoric acid, &c.) forming 

 compounds which dissolve slowly or not at all in alcohol. Their 

 presence is best asceitained by incinerating a portion of the gum 

 at the ail". If a remnant is left, mineral substances were present, 

 and the weight of the ashes thus obtained has, therefore, to be 

 subtracted from that of the gum. To prepare a gum free from 

 mineral substances, its aqueous solution has to be strongly 

 acidified with, hydrochloric acid and precipitated with alcohol; the 

 deposit is washed with alcohol and redissolved in water, again 

 acidified with hydrochloric acid and precipitated with alcohol, and 

 these operations are repeated several times. 



Different gums have different i-eactions with the same reagents. 

 Gum Arabic (from Acacia Ai'abica, A. Seyal and other species) 

 is precipitated from its aqueous solution by subacetate of 

 lead, chloride of iron, and silicate of potash; it becomes thick 

 with borax solution, and is not affected by acetate of lead; 

 whereas many other gums, on the contrary, are precipitated by 

 acetate of lead, yet are not affected by subacetate of lead, chloride 

 of ii'on, silicate of potash and borax. A cold concentrated 

 sulphuric acid colours pulverised gum, only after several houi\s, 

 l)ut renders it black instantly on heating; when boiled with 

 diluted sulphuric acid it is converted into grape sugar. Nitric acid 

 produces oxalic and mucic acids. Iodine shows itself inactive to gum. 



