Small, air-dried lumps of gum lose by desiccation over concentrated sulphuric acid (or by heating them in the water-bath) 12 to 16 per cent. of water. If gum independently of its amount of lime, be presented by the formula C₁₂H₂₂O₁₁ + 3H₂O, the loss of 3 molecules of water will correspond to a decrease in weight of 13·6 per cent.; in carefully selected colourless pieces, we have found it to amount to 13·14 per cent. At a temperature of about 150° C., gum parts with another molecule of water, and partly loses its solubility and assumes a brownish hue and empyreumatic taste. Gum already by keeping it for a week at a temperature not exceeding 95° C. gradually acquires a decidedly empyreumatic taste. We have also observed, on the other hand, a fine white gum affording an imperfect solution which was glairy, like the mucilage of marshmallow, but in no other respect could we find that it differed from ordinary gum. On exposing it for some days to a temperature of 95° C., it afforded a solution of the usual character.
When gum arabic is dissolved in cold water and the solution is slightly acidulated with hydrochloric acid, alcohol produces it in a precipitate of Arabin or Arabic Acid. It may be also prepared by placing a solution of gum (1 gum + 5 water), acidulated with hydrochloric acid, on a dialyser, when the calcium salt will diffuse out, leaving behind a solution of arabin.
Solution of arabin differs from one of gum in not being precipitated by alcohol. Having been dried, it loses its solubility, merely swelling in water, but not dissolving even at a boiling heat. If an alkali is added, it forms a solution like ordinary gum. Neubauer who observed these facts (1854-57) showed that gum arabic is essentially an acid calcium salt of arabic acid.
Arabic Acid dried at 100° C. has the composition C₁₂H₂₂O₁₁, and gives up H₂O when it unites with bases. It has however a great tendency to form salts containing a large excess of acid. An acid calcium arabate of the composition (C₁₂H₂₁O₁₁)₂Ca + 3 (C₁₂H₂₂O₁₁ + 5 OH) would afford by incineration 4·95 per cent. of calcium carbonate. Nearly this amount of ash is in fact sometimes yielded by gum. The most carefully selected colourless pieces of it yield from 2·7 to 4 per cent. of ash, consisting mainly of calcium carbonate, but containing also carbonates of potassium and magnesium. Phosphoric acid appears never to occur in gums.
Natural gum may therefore be regarded as a salt of arabic acid having a large excess of acid, or perhaps as a mixture of such salts of calcium, potassium and magnesium. It is to the presence of these bases, which are doubtless derived from the cell-wall from which the gum exuded, that gum owes its solubility.
It still remains unexplained why certain gums, not unprovided with mineral constituents, merely swell up in water without dissolving, thus materially differing from gum arabic. There is also a marked difference between gum arabic and many other varieties of gum or mucilage, which immediately form a plumbic compound if treated with neutral acetate of lead. The type of the swelling, but not really soluble gums, is Tragacanth, but there are a great many other substances of the same class, some of them perfectly resembling gum arabic in external appearance. The name of Bassora gum has also been applied to the latter kinds.
Commerce—The imports of Gum Arabic into the United Kingdom have been as follows:—
| 1871 | 1872 |
| 76,136 cwt. | 42,837 cwt. |
| value £250,088. | value £123,080. |
The country whence by far the largest supplies are shipped, is Egypt.
Uses—Gum is employed in medicine rather as an adjuvant than as possessing any remedial powers of its own.