CHAPTER XXVII.

GUMS.

ARABIC-SENEGAL-TRAGACANTH-KUTEERA-CAROB

STARCH.

It is

GUM is a thick transparent tasteless fluid, which gradually hardens without losing its transparency. It exudes from certain species of trees, and from its adhesive quality is extremely useful in the arts. moderately hard and somewhat brittle, so that when cold it may readily be reduced to a fine powder. This substance is extremely soluble in water, but insoluble in alcohol, being exactly opposed in this respect to the resins. On the application of heat it swells and softens: it is infusible.

Gums are largely used in topical or calico printing to give a proper consistency to the cloth previously to the application of the mordants, by which means they can be evenly laid on the surface and are prevented from running and mixing with each other, and thus rendering the pattern indistinct and imperfect.

Gum Arabic is most extensively employed for this purpose. The common appearance of this gum is too well known to need a particular description; when of a pale yellow colour it is most esteemed.

It is obtained from the Acacia nilotica, or Egyptian acacia. This tree is not of very large growth, rising only to somewhat beyond twelve feet in height. The bark of the trunk is smooth and of a grey colour, that of the branches has a slight purple tinge. The leaves are double-winged, and placed alternately on the branches. The single leaves grow in pairs on stalks, which are likewise placed in pairs opposite to each other on the thicker stalk that grows out from the branches. Long white spines proceed from each side of the base of the leaves. The flowers are

of a globular form, and stand four or five together upon slender stalks, which arise from the insertion of the leaves. It blooms in July.

The fruit is a long pod resembling that of the lupin and contains many flattish brown seeds.

This tree is indigenous to Arabia, and found abundantly spread over the vast extent of Africa, but the gum requires for its production the intense heat of the torrid zone. It is said that in Lower Egypt the solar rays are never sufficiently powerful to produce this effect.

The gum exudes, in a liquid state, from the bark of the trunk and the branches of the tree, and concretes

by exposure to the air in a similar manner to the gum which is often produced from the cherry and some other trees growing in this country. When the tree first opens its flowers the gum begins to flow, and continues to exude during the rainy season, until the month of December. It is then collected for the first time. In the month of March incisions are made in the bark, an operation rendered necessary, it is said, by the extreme dryness of the weather, and the gum issuing from these wounds is soon after collected.

Gum arabic is imported into this country from the East Indies, Turkey, and Africa. The EastIndian is the worst. It is distinguished in commerce by its colour, the lightest being most esteemed; its price is from £2. 2s. to £12. 12s. according to its quality. The quantity retained for home consumption in 1830 was 12,193 cwt. It is admitted under a duty of 6s. per cwt. from British possessions, the duty. being doubled on that coming from foreign countries.

Gum Senegal is extremely similar to gum arabic, though of rather an inferior quality. It is the product of another species of acacia. The Senegal mimosa is a native of Guinea. Its flowers are yellow, globular, and fragrant; the pods are brown, rounder, and smaller than those of the nilotica. On incisions being made in the bark of the tree, the gum exudes but less plentifully than the gum arabic.

It is likewise not so transparent and is of a darker colour. This gum is much used for all those purposes to which gum arabic is applied. It is subject to the same rate of duty as gum arabic; its price is from £3. 16s. to £4. 4s. per cwt. The quantity reserved for home consumption for 1830 was 3,493 cwt. It is imported from Senegal and Barbary.

The Astragalus tragacantha, a small prickly shrub not exceeding three feet in height, growing in many

parts of the Levant, affords a gum in some respects superior to either of the above. Though the native of a warmer climate, this plant can endure with impunity the colder temperature of England, refusing, however, to yield any of that juice which here it produces only sufficiently for its own nourishment. This gum is called tragacanth or adragant, and sometimes it is vulgarly termed gum-dragon. Tournefort tells

us, that the naked hillocks of Mount Ida in Candia produce this plant abundantly. The gum exudes spontaneously towards the end of June and in the following months, during which period the nutritious juice of the plant, thickened by the summer heat, bursts most of the vessels in which it is confined. This juice coagulates in threads, which make their way into the pores of the bark, through which being pushed forward by fresh juice they issue forth, and are at length hardened in the air, either in irregular lumps, or in long vermicular pieces bent into a variety of shapes *. The best sort is white and semitransparent, dry, but somewhat soft to the touch. It is extremely different in many of its properties to gum arabic; one part of this diffused in one hundred parts of water affords a fluid of the same consistency as one part of gum arabic dissolved in ten parts of water. Water is, however, but an imperfect solvent to it, not forming the same intimate union with it as with other gums. When tragacanth is put into water it slowly imbibes a great quantity, swells into a large volume, and forms a soft, but not fluid mucilage. On the addition of more water, and if the mixture be agitated, the gum will be more generally diffused throughout the liquor, which will appear turbid. If left at rest the mucilage will again separate and subside; the supernatant water appearing limpid, and holding only a very small portion of the gum.

This is more costly than gum arabic or senegal, but its employment is highly beneficial in topical dyeing, when the mordant is prepared with nitrous acid; since other gummy solutions are coagulated by the application of this active alterative.

In 1830 the quantity of tragacanth retained for home consumption was 29,725 lbs. It is admitted on a duty of 1s. per lb.; its price being from £16 to £18 per cwt.

* Voyage Du Levant, tom. i. p. 64.