Botanical Origin—The drug under notice is produced by trees belonging to the genus Copaifera, natives of the warmer countries of South America. Some are found in moist forests, others exclusively in dry and elevated situations. They vary in height and size, some being umbrageous forest trees, while others have only the dimension of shrubs; it is from the former alone that the oleo-resin is obtained.

The following are reputed to furnish the drug, but to what extent each contributes is not fully known.

1. Copaifera officinalis L. (C. Jacquini Desf.), a large tree of the hot coast region of New Granada as far north as Panama, of Venezuela and the island of Trinidad.

2. C. guianensis Desf., a tree of 30 to 40 feet high, very closely related to the preceding, native of Surinam, Cayenne, also of the Rio Negro between Manaos and Barcellos (Spruce). According to Bentham it seems to be the same species as the C. bijuga of Hayne.[878]

3. C. coriacea Mart. (C. cordifolia Hayne), a large tree found in the caatingas or dry woods of the Brazilian provinces of Bahia and Piauhy.

4. C. Langsdorffii Desf.[879] (C. nitida Hayne, C. Sellowii Hayne,? C. Jussieui Hayne), a polymorphous species, varying in the form and size of leaflets, and also in dimensions, being either a shrub, a small bushy tree, or a large tree of 60 feet high. Bentham admits, besides the type, three varieties:—β. glabra (C. glabra Vogel), γ. grandifolia, δ. laxa (C. laxa Hayne). The tree grows on dry campos, caatingas and other places in the provinces of S. Paulo, Minas Geraes, Goyaz, Mato Grosso, Bahia and Ceará; it is therefore distributed over a vast area. According to Gardner,[880] the Brazilian traveller, it yields an abundance of balsam.

In addition to these species, must be mentioned a tree described by Hayne and commonly cited under the name of Copaifera multijuga, as a special source of the drug shipped from Pará.[881] As its name implies, it is remarkable for the number of leaflets (6 to 10 pairs) on each leaf. But it is only known from some leaves in the herbarium of Martius which Bentham, who has examined them, informs us are unlike those of any Copaifera known to him, though certainly the leaflets are dotted with oil-vessels as in some species. In the absence of flowers and fruits, there is no sufficient evidence to prove that it belongs even to the genus Copaifera. It is not mentioned by Martius in his Systema Materiæ Medicæ Brasiliensis (1843) as a source of the drug.

History—Among the early notices of Brazil is a treatise by a Portuguese friar who had resided in that country from 1570 to 1600. The manuscript found its way to England, was translated, and was published by Purchas[882] in 1625. Its author notices many of the natural productions of the country, and among others Cupayba which he describes as a large tree from whose trunk, when wounded by a deep incision, there flows in abundance a clear oil much esteemed as a medicine.

Balsam. Copæ. yvæ is already enumerated in the 6th edition of the Pharmacopœa of Amsterdam, a.d. 1636.[883]

Father Cristoval d’Acuña,[884] who ascended the Amazon from Pará, arriving at Quito in 1638, mentions that the country affords very large Cassia fistula, excellent sarsaparilla, and the oils of Andirova (Carapa guianensis Aublet, Meliaceæ), and Copaiba, as good as balsam for curing wounds.

Piso and Marcgraf,[885] who in 1636 accompanied the Count of Nassau to the Dutch establishments in Brazil, each give an account of the Copaiba and the method of obtaining its oleo-resin. The former states that the tree grows in Pernambuco and the island of Maranhon, whence the balsam is conveyed in abundance to Europe.

The drug was formerly brought into European commerce by the Portuguese, and used to be packed in earthen pots pointed at the lower end; it often arrived in a very impure condition.[886] In the London Pharmacopœia of 1677, it was called Balsamum Capivi, which is still its most popular name.

Secretion—Karsten states that he observed resiniferous ducts, frequently more than an inch in diameter, running through the whole stem. He is of the opinion that the cell-walls of the neighbouring parenchyme are liquefied and transformed into the oleo-resin.[887] We are not able to offer any argument in favour of this opinion.

In the vessels already alluded to, the balsam sometimes collects in so large a quantity, that the trunk is unable to sustain the inward pressure, and bursts. This curious phenomenon is thus referred to in a letter addressed to one of us by Mr. Spruce:—“I have three or four times heard what the Indians assured me was the bursting of an old capivi-tree, distended with oil. It is one of the strange sounds that sometimes disturb the vast solitudes of a South American forest. It resembles the boom of a distant cannon, and is quite distinct from the crash of an old tree falling from decay which one hears not unfrequently.”

A similar phenomenon is known in Borneo. The trunks of aged trees of Dryobalanops aromatica contain large quantities of oleo-resin or Camphor Oil,[888] which appears to be sometimes secreted under such pressure that the vast trunk gives way. “There is another sound,” says Spenser St. John,[889] “only heard in the oldest forests, and that is as if a mighty tree were rent in twain. I often asked the cause, and was assured it was the camphor tree splitting asunder on account of the accumulation of camphor in some particular portion.”

Extraction—Balsam Capivi is collected by the Indians on the banks of the Orinoco and its upper affluents, and carried to Ciudad Bolivar (Angostura); some of this balsam reaches Europe by way of Trinidad. But it is obtained much more largely on the tributaries of the Caisquiari and Rio Negro (the Siapa, Içanna, Uaupés, etc.) and is sent down to Pará. Most of the northern tributaries of the Amazon, as the Trombetas and Nhamundá, likewise furnish a supply. According to Spruce, in the Amazon valley it is the tall virgin forest, Caaguaçú of the Brazilians, Monte Alto of the Venezuelans, that yields most of the oils and gum-resins, and not the low, dry caatingas, or the riparial forests. The same observant traveller tells us that in Southern Venezuela, capivi is known only as el Aceite de palo (wood-oil), the name Balsamo being that of the so-called Sassafras Oil, obtained from a species of Nectandra.

Balsam Copaiba is also largely exported from Maracaibo where, according to Engel,[890] it is produced by C. officinalis, the Canime of the natives.

The finest sort, called by the collectors white copaiba, is met with in the province of Pará, where Cross[891] saw a tree of a circumference of more than 7 feet at 3 feet from the ground. Its trunk was clear of branches to a height of at least 90 feet. The collector commenced the work by hewing out with his axe a hole or chamber in the trunk about a foot square, at a height of two feet from the ground. The base or floor of the chamber should be carefully and neatly cut with a gentle upward slope, and it should also decline to one side, so that the balsam on issuing may run in a body until it reaches the outer edge. Below the chamber a pointed piece of bark is cut and raised, which, enveloped with a leaf, serves as a spout for conveying the balsam from the tree to the tin.[892] The balsam, continues Cross, came flowing in a moderate sized cool current, full of air bubbles. At times the flow stopped for several minutes, when a singular gurgling noise was heard, after which followed a rush of balsam. When coming most abundantly a pint jug would have been filled in the space of one minute. The whole of the wood cut through by the axeman was bedewed with drops of balsam; the bark is apparently devoid of it. Trees of the largest size in good condition will sometimes yield four “potos,” equal to 84 English imperial pints.

Description—Copaiba is more or less viscid fluid, varying in tint from a pale yellow to a light golden brown, of a peculiar aromatic, not unpleasant odour, and a persistent, acrid, bitterish taste. Pará copaiba newly imported is sometimes nearly colourless and almost as fluid as water.[893] The balsam is usually quite transparent, but there are varieties which remain always opalescent. Its sp. gr. varies from 0·940 to 0·993, according as the drug contains a greater or less proportion of volatile oil. Copaiba becomes more fluid by heat; if heated in a test tube to 200° C. for some time, it does not lose its fluidity on cooling. It is sometimes slightly fluorescent, it dissolves in several times its weight of alcohol 0·830 sp. gr., and generally in all proportions in absolute alcohol,[894] acetone, or bisulphide of carbon, and is perfectly soluble in an equal volume of benzol. Glacial acetic acid readily dissolves the resin but not the essential oil.

Copaiba that is rich in resin of an acid character, unites with the oxides of baryum, calcium, or magnesium, to form a gradually hardening mass, provided a small proportion of water is present. Thus 8 to 16 parts of balsam will combine as a stiff compound when gently warmed with 1 part of moistened magnesia; and still more easily with lime or baryta.

Buignet has first shown (1861) that copaiba varies in its optical power. A sample from Trinidad examined by one of us was strongly dextrogyre, and also several samples imported in 1877 from Maturin (near Aragua, Venezuela), and Maracaibo into Hamburg, whereas we found Pará balsam to be levogyre.[895]

The Pará and Maranham balsams are regarded in wholesale trade as distinct sorts, and experienced druggists are able to distinguish them apart by odour and appearance, and especially by the greater consistence of the Maranham drug. Maracaibo balsam is reckoned as another variety, but is now rarely seen in the English market. West Indian copaiba is usually said to be of inferior quality, but except that it is generally opalescent, we know not on what precise grounds.

Chemical Composition—The balsam is a solution of resin in volatile oil; the latter constitutes about 40 to 60 per cent. of the balsam,[896] according to the age of the latter and its botanical origin. The oil has the composition C₁₅H₂₄; its boiling point is 245° C. or even higher. It smells and tastes like the balsam, and dissolves in from 8 to 30 parts of alcohol 0·830 sp. gr. The oil exhibits several modifications differing in optical as well as in other physical properties, but numerous samples of the drug, either dextrogyre or levogyre, invariably afforded us essential oils deviating to the left; their sp. gr. varies from about 0·88 to 0·91.

After the oil of copaiba has been removed by distillation, there remains a brittle amorphous resin of an acid character soluble both in benzol and amylic alcohol, and yielding only amorphous salts. Sometimes copaiba contains a small amount of crystallizable resin-acid, as first pointed out in 1829 by Schweitzer. By exposing a mixture of 9 parts of copaiba and two parts of aqueous ammonia (sp. gr. 0·95) to a temperature of-10° C., Schweitzer obtained crystals of the acid resin termed Copaivic Acid. They were analysed in 1834 by H. Rose, and exactly measured and figured by G. Rose. Hess (1839) showed that Rose’s and his own analyses assign to copaivic acid the formula C₂₀H₃₂O₂. It agrees with Maly’s abietic acid from colophony in composition, but not in any other way. Copaivic acid is readily soluble in alcohol, and especially in warmed copaiba itself; much less in ether. We have before us crystals, no doubt of copaivic acid, which have been spontaneously deposited in an authentic specimen of the oleo-resin of Copaifera officinalis from Trinidad, which we have kept for many years. The crystals may be easily dissolved by warming the balsam; on cooling the liquid, they again make their appearance after the lapse of some weeks. After recrystallization from alcohol they fuse at 116-117 C°., forming an amorphous transparent mass which quickly crystallizes if touched with alcohol.

An analogous substance, Oxycopaivic Acid, C₂₀H₂₈O₃, was examined in 1841 by H. von Fehling, who met with it as a deposit in Pará Copaiba. And lastly Strauss (1865) extracted Metacopaivic Acid, C₂₂H₃₄O₄, from the balsam imported from Maracaibo. He boiled the latter with soda-lye, which separated the oil; the heavier adjacent liquid was then mixed with chloride of ammonium, which threw down the salts of the amorphous resin-acid, leaving in solution those of the metacopaivic acid. The latter acid was separated by hydrochloric acid and recrystallization from alcohol. We succeeded in obtaining metacopaivic acid by washing the balsam with a dilute solution of carbonate of ammonium, and precipitating by hydrochloric acid. The precipitate dissolved in dilute alcohol yields the acid in small crystals, but to the amount of only about one per cent.

These resin-acids have a bitterish taste and an acid reaction; their salts of lead and silver are crystalline but insoluble; metacopaivate of sodium may be crystallized from its watery solution.

Commerce—The balsam is imported in barrels direct from Pará and Maranham, sometimes from Rio de Janeiro, and less often from Demerara, Angostura, Trinidad, Maracaibo, Savanilla, and Cartagena. It often reaches England by way of Havre and New York. In 1875 there were exported 10,150 kilogrammes from Savanilla, 99,800 lb. from Ciudad Bolivar (Angostura), and 65,243 kilos. from Pará.

Uses—Copaiba is employed in medicine on account of its stimulant action on the mucous membranes, more especially those of the urino-genital organs.

Adulteration—Copaiba is not unfrequently fraudulently tampered with before it reaches the pharmaceutist; and owing to its naturally variable composition, arising in part from its diverse botanical origin, its purity is not always easily ascertained.

The oleo-resin usually dissolves in a small proportion of absolute alcohol: should it refuse to do so, the presence of some fatty oil other than castor oil may be surmised. To detect an admixture of this latter, one part of the balsam should be heated with four of spirit of wine (sp. gr. 0·838). On cooling, the mixture separates into two portions, the upper of which will contain any castor oil present, dissolved in alcohol and the essential oil. On evaporation of this upper layer, castor oil may be recognized by its odour; but still more positively by heating it with caustic soda and lime, when œnanthol will be formed, the presence of which may be ascertained by its peculiar smell. By the latter test an admixture of even one per cent. of castor oil can be proved.

The presence of fatty oil in any considerable quantity is likewise made evident by the greasiness of the residue, when the balsam is deprived of its essential oil by prolonged boiling with water.

The admixture of some volatile oil with copaiba can mostly be detected by the odour, especially when the balsam is dropped on a piece of warmed metal. Spirit of wine may also be advantageously tried for the same purpose. It dissolves but very sparingly the volatile oil of copaiba: the resins of the latter are also not abundantly soluble in it. Hence, if shaken with the balsam, it would remove at once the larger portion of any essential oil that might have been added. For the recognition of Wood Oil if mixed with copaiba, see page 233, note 1.

Substitutes—Under this head two drugs deserve mention, namely Gurjun Balsam or Wood Oil, described at p. 88, and Oleo-resin of Hardwickia pinnata Roxb.—The tree, which is of a large size, belongs to the order Leguminosæ and is nearly related to Copaifera. According to Beddome,[897] it is very common in the dense moist forests of the South Travancore Ghats, and has also been found in South Canara. The natives extract the oleo-resin in exactly the same method as that followed by the aborigines of Brazil in the case of copaiba,—that is to say, they make a deep notch reaching to the heart of the trunk, from which after a time it flows out.

This oleo-resin, which has the smell and taste of copaiba, but a much darker colour, was first examined by one of us in 1865, having been sent from the India Museum as a sample of Wood Oil; it was subsequently forwarded to us in more ample quantity by Dr. Bidie of Madras. It is a thick, viscid fluid, which, owing to its intense tint, looks black when seen in bulk by reflected light; yet it is perfectly transparent. Viewed in a thin layer by transmitted light, it is light yellowish-green, in a thick layer vinous-red,—hence is dichromic. It is not fluorescent, nor is it gelatinized or rendered turbid by being heated to 130° C., thus differing from Wood Oil.[898] Broughton[899] obtained by prolonged distillation with water an essential oil to the extent of 25 per cent. from an old specimen, and of more than 40 per cent. from one recently collected. The oil was found to have the same composition as that of copaiba, to boil at 225° C., and to rotate the plane of polarization to the left. The resin[900] is probably of two kinds, of which one at least possesses acid properties. Broughton made many attempts, but without success, to obtain from the resin crystals of copaivic acid.

The balsam of Hardwickia has been used in India for gonorrhœa, and with as much success as copaiba.

GUMMI ACACIÆ.

Botanical Origin—Among the plants abounding in mucilage, numerous Acaciæ of various countries are in the first line. The species particularly known for affording the largest quantities of the finest gum arabic is Acacia Senegal[901] Willdenow (syn. Mimosa Senegal L., A. Verek Guillemin et Perrottet), a small tree not higher than 20 feet, growing abundantly on sandy soils in Western Africa, chiefly north of the river Senegal, where it constitutes extensive forests. It is called by the negroes Verek. The same tree is likewise found in Southern Nubia, Kordofan, and in the region of the Atbara in Eastern Africa, where it is known as Hashab. It has a greyish bark, the inner layers of which are strongly fibrous, small yellowish flowers densely arranged in spikes 2 to 3 inches long, and exceeding the bipinnate leaves, and a broad legume 3 to 4 inches in length containing 5 to 6 seeds.

According to Schweinfurth,[902] it is this tree exclusively that yields the fine white gum of the countries bordering the Upper Nile, and especially of Kordofan. He states that only brownish or reddish sorts of gum are produced by the Talch, Talha or Kakul, Acacia stenocarpa Hochstetter, by the Ssoffar, A. fistula Schweinf. (A. Seyal Delile, var. Fistula), as well as by the Ssant or Sont, A. nilotica Desfont (A. arabica Willd.). These trees grow in north-eastern Africa; the last named is, moreover, widely distributed all over tropical Africa as far as Senegambia,[903] Mozambique and Natal, and also extends to Sindh, Gujarat[904] and Central India. We find even the first sort, “Karami,” of gum exported from the Somali coast,[905] to be inferior to good common Arabic gum. Hildebrandt (1875) mentions that gum is there largely collected from Acacia abyssinica Hochst. and A. glaucophylla Steudel.

History—The history of this drug carries us back to a remote antiquity. The Egyptian fleets brought gum from the gulf of Aden as early as the 17th century b.c. Thus in the treasury of king Rhampsinit (Ramses III.) at Medinet Abu, there are representations of gum-trees, together with heaps of gum. The symbol used to signify gum, is read Kami-en-punt. i.e. gum from the country of Punt. This, in all probability, includes both the Somali coast as well as that of the opposite parts of Arabia (see article Olibanum, p. 136). Thus, gum is of frequent occurrence in Egyptian inscriptions; sometimes mention is made of gum from Canaan. The word kami is the original of the Greek κόμμι, whence through the Latin our own word gum.[906]

The Egyptians used gum largely in painting; an inscription exists which states that in one particular instance a solution of Kami (gum) was used to render adherent the mineral pigment called chesteb,[907] the name applied to lapis lazuli or to a glass coloured blue by cobalt.

Turning to the Greeks, we find that Theophrastus in the 3rd and 4th century b.c. mentioned Κόμμι as a product of the Egyptian Ἂκανθα, of which tree there was a forest in the Thebaïs of Upper Egypt. Strabo also, in describing the district of Arsinöe, the modern Fayûm, says that gum is got from the forest of the Thebaïc Akanthe.

Celsus in the 1st century mentions Gummi acanthinum; Dioscorides and Pliny also describe Egyptian gum, which the latter values at 3 denarii [2s.] per lb.

In those times gum no doubt used to be shipped from north-eastern Africa to Arabia; there is no evidence showing that Arabia itself had ever furnished the chief bulk of the drug. The designation gum arabic occurs in Diodorus Siculus (2, 49) in the first century of our era, also in the list of goods of Alexandria mentioned in our article on Galbanum.

Gum was employed by the Arabian physicians and by those of the school of Salerno, yet its utility in medicine and the arts was but little appreciated in Europe until a much later period. For the latter purpose at least the gummy exudations of indigenous trees were occasionally resorted to, as distinctly pointed out about the beginning of the 12th century, by Theophilus or Rogker:[908] “gummi quot exit de arbore ceraso vel pruno.“

During the middle ages, the small supplies that reached Europe were procured through the Italian traders from Egypt and Turkey. Thus Pegolotti,[909] who wrote a work on commerce about a.d. 1340, speaks of gum arabic as one of the drugs sold at Constantinople by the pound not by the quintal. Again, in a list of drugs liable to duty at Pisa in 1305,[910] and in a similar list relating to Paris in 1349,[911] we find mention of gum arabic. It is likewise named by Pasi,[912] in 1521, as an export from Venice to London.

Gum also reached Europe from Western Africa, with which region the Portuguese had a direct trade as early as 1449.

Production—Respecting the origin of gum in the tribe Acaciæ, no observations have been made similar to those of H. von Mohl on tragacanth.[913]

It appears that gum generally exudes from the trees spontaneously, in sufficient abundance to render wounding the bark superfluous. The Somali tribes of East Africa, however, are in the habit of promoting the outflow by making long incisions in the stem and branches of the tree.[914] In Kordofan the lumps of gum are broken off with an axe, and collected in baskets.

The most valued product, called Hashabi gum, from the province of Dejara in Kordofan, is sent northward from Bara and El Obeid to Dabbeh on the Nile, and thence down the river to Egypt; or it reaches the White Nile at Mandjara.

A less valuable gum, known as Hashabi el Jesire, comes from Sennaar on the Blue Nile; and a still worse from the barren table-land of Takka, lying between the eastern tributaries of the Blue Nile and the Atbara and Mareb; and from the highlands of the Bisharrin Arabs between Khartum and the Red Sea. This gum is transported by way of Khartum or El Mekheir (Berber), or by Suakin on the Red Sea. Hence, the worst kind of gum is known in Egypt as Samagh Savakumi (Suakin Gum).

According to Munzinger,[915] a better sort of gum is produced along the Samhara coast towards Berbera, and is shipped at Massowa. Some of it reaches Egypt by way of Jidda, which town being in the district of Arabia called the Hejaz, the gum thence brought receives the name of Samagh Hejazi; it is also called Jiddah or Gedda Gum. The gums of Zeila, Berbera and the Somali country about Gardafui, are shipped to Aden, or direct to Bombay. A little gum is collected in Southern Arabia, but the quantity is said to be insignificant.[916]

In the French colony of Senegal, gum, which is one of its principal productions, is collected chiefly in the country lying north of the river, by the Moors who exchange it for European commodities. The gathering commences after the rainy season in November when the wind begins to set from the desert, and continues till the month of July. The gum is shipped for the most part to Bordeaux. The quantity annually imported into France since 1828 from Senegal is varying from between 1½ to 5 millions of kilogrammes.

Description—Gum arabic does not exhibit any very characteristic forms like those observable in gum tragacanth. The finest white gum of Kordofan, which is that most suitable for medicinal use, occurs in lumps of various sizes from that of a walnut downwards. They are mostly of ovoid or spherical form, rarely vermicular, with the surface in the unbroken masses, rounded,—in the fragments, angular. They are traversed by numerous fissures, and break easily and with a vitreous fracture. The interior is often less fissured than the outer portion. At 100° C. the cracks increase, and the gum becomes extremely friable. In moist air, it slowly absorbs about 6 per cent. of water.

The finest gum arabic is perfectly clear and colourless; inferior kinds have a brownish, reddish or yellowish tint of greater or less intensity, and are more or less contaminated with accidental impurities such as bark. The finest white gum turns black and assumes an empyreumatic taste, when it is kept for months at a temperature of about 98° C., either in an open vessel, or enclosed in a glass tube, after having been previously dried over sulphuric acid or not.

An aqueous solution of gum deviates the plane of polarization 5° to the left in a column 50 mm. long; but after being long kept, it becomes strongly acid, the gum having been partly converted into sugar, and its optical properties are altered. An alkaline solution of cupric tartrate is not reduced by solution of gum even at a boiling heat, unless it contains a somewhat considerable proportion of sugar, extractable by alcohol, or a fraudulent admixture of dextrin.

We found the sp. gr. of the purest pieces of colourless gum dried in the air at 15° C., to be 1·487; but it increases to 1·525, if the gum is dried at 100°.

The foregoing remarks apply chiefly to the fine white gum of Kordofan, the Picked Turkey Gum or White Sennaar Gum of druggists. The other sorts which are met with in the London market are the following:—

1. Senegal Gum—As stated above, this gum is an important item of the French trade with Africa, but is not much used in England. Its colour is usually yellowish or somewhat reddish, and the lumps, which are of large size, are often elongated or vermicular. Moreover Senegal gum never exhibits the numerous fissures seen in Kordofan gum, so that the masses are much firmer and less easily broken. In every other respect, whether chemical or optical, we find[917] Senegal gum and Kordofan gum to be identical; and the two, notwithstanding their different appearance, are produced by one and the same species of Acacia, namely Acacia Senegal.

2. Suakin Gum, Talca or Talha Gum, yielded by Acacia stenocarpa, and by A. Seyal var. Fistula, is remarkable for its brittleness, which occasions much of it to arrive in the market in a semi-pulverulent state. It is a mixture of nearly colourless and of brownish gum, with here and there pieces of a deep reddish-brown. Large tears have a dull opaque look, by reason of the innumerable minute fissures which penetrate the rather bubbly mass. It is imported from Alexandria.

3. Morocco, Mogador or Brown Barbary Gum—consists of tears of moderate size, often vermiform, and of a rather uniform, light, dusky brown tint. The tears which are internally glassy become cracked on the surface and brittle if kept in a warm room; they are perfectly soluble in water. The above mentioned Acacia nilotica is supposed to be the source of the gum exported from Morocco, and also from Fezzan.

Gums of various kinds, including the resin Sandrac, were exported from Morocco in the year 1872 to the extent of 5110 cwt., a quantity much below the average.[918]

4. Cape Gum—This gum, which is uniformly of an amber brown, is produced in plenty in the Cape Colony, as a spontaneous exudation of Acacia horrida Willd. (A. Karroo Hayne, A. capensis Burch.), a large tree, the Doornboom, Wittedoorn or Karródoorn of the Cape colonists, the commonest tree of the lonely deserts of South Africa. The Blue Book of the Cape Colony, published in 1873, states the export of gum in 1872 as 101,241 lb.

5. East India Gum—The best qualities consist of tears of various sizes, sometimes as large as an egg, internally transparent and vitreous, of a pale amber or pinkish hue, completely soluble in water. This gum is largely shipped from Bombay, but is almost wholly the produce of Africa; the imports into Bombay from the Red Sea ports, Aden and the African Coast in the year 1872-73, were 14,352 cwt. During the same year the shipments from Bombay to the United Kingdom amounted to 4,561 cwt.[919]

6. Australian Gum, Wattle Gum—This occurs in large hard globular tears and lumps, occasionally of a pale yellow, yet more often of an amber or of a reddish-brown hue. It is transparent and entirely soluble in water; the mucilage is strongly adhesive, and said to be less liable to crack when dry than that of some other gums. The solution, especially that of the darker and inferior kinds, contains a little tannin, evidently derived from the very astringent bark which is often attached to the gum.

A. pycnantha Benth.; A. decurrens Willd. (A. mollissima Willd., A. dealbata Link), Black or Green Wattle-tree of the colonists, and A. homalophylla A. Cunn., are the trees which furnish the gum arabic of Australia.[920]

Chemical Characters and Composition—At ordinary temperatures gum dissolves very slowly and without affecting the thermometer in an equal weight of water, forming a thick, glutinous, slightly opalescent liquid, having a mawkish taste and decidedly acid reaction. At higher temperatures the dissolution of gum is but slightly accelerated, and water does not take up a much larger quantity even at 100°. The finest gum dried at 100° C. forms with two parts of water a mucilage of sp. gr. 1·149 at 15° C.

This solution mixes with glycerine, and the mixture may be evaporated to the consistence of a jelly without any separation taking place. Solid gum in lumps, on the contrary, is but little affected by concentrated glycerine. In other liquids, gum is insoluble or only slightly soluble, unless there is a considerable quantity of water present. Thus 100 parts of spirit of wine containing 22 volumes per cent. of alcohol, dissolve 57 parts of gum; spirit containing 40 per cent. of alcohol takes up 10 parts, and spirit of 50 per cent. only 4 parts. Aqueous alcohol of 60 per cent. no longer dissolves gum, but extracts from it a small quantity (⅓ to ½ per cent. according to the variety) of resin colouring matter, glucose, calcium chloride, and other salts.

Neutral acetate of lead does not precipitate gum arabic mucilage; but the basic acetate forms, even in a very dilute solution, a precipitate of definite constitution.

Soluble silicates, borates, and ferric salts render gum solution turbid, or thicken it to a jelly. It is not a compound of gum with any of these substances which is formed, but in the cases of the first, basic silicates separate. No alteration is produced by silver salts, mercuric chloride or iodine. Ammonium oxalate throws down the lime contained in a solution of gum. Gum dissolves in an ammoniacal solution of cupric oxide. Acted upon by nitric acid, mucic acid is produced.

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:—

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.

Substitutes—A great number of trees are capable of affording gums more or less similar to gum arabic. There is to be mentioned for instance Prosopis glandulosa Torrey, a tree growing from 30 to 40 feet in height, occurring very abundantly in Texas, and extending as far west as the Colorado and the gulf of California. It is universally known by its Mexican name Mesquite. It belongs to the same suborder of the Mimosæ like the Acaciæ tribe of the Adenanthereæ. Mesquite gum agrees not with the fine description, but with the inferior sorts of gum arabic, and is sometimes used in America,[921] since 1854, in the manufacture of confectionery and the arts.

Feronia Gum, or Wood Apple Gum. This is the produce of Feronia Elephantum Correa, a spiny tree, 50 to 60 feet high, of the order of Aurantiaceæ, common throughout India from the hot valleys of the Himalaya to Ceylon, and also found in Java. There exudes from its bark abundance of gum, which appears not to be collected for exportation per se, but rather to be mixed indiscriminately with other gum, as that of Acacia.

Feronia gum sometimes forms small roundish transparent, almost colourless tears, more frequently stalactitic or knobby masses, of a brownish or reddish colour, more or less deep. In an authentic sample, for which we are indebted to Dr. Thwaites of Ceylon, horn-shaped pieces about ½ an inch thick and two inches long also occur.

Dissolved in two parts of water, it affords an almost tasteless mucilage, of much greater viscosity than that of gum arabic made in the same proportions. The solution reddens litmus, and is precipitated like gum arabic by alcohol, oxalate of ammonium, alkaline silicates, perchloride of iron, but not by borax. Moreover, the solution of Feronia gum is precipitated by neutral acetate of lead or caustic baryta, but not by potash. If the solution is completely precipitated by neutral acetate of lead, the residual liquid will be found to contain a small quantity of a different gum, identical apparently with gum arabic, inasmuch as it is not thrown down by acetate of lead. If the lime is precipitated from the Feronia mucilage by oxalate of potassium, the gum partially loses its solubility and forms a turbid liquid.

From the preceding experiments, it follows that a larger portion of Feronia gum is by no means identical with gum arabic. The former, when examined in a column of 50 mm. length, deviates the rays of polarized light 0°·4 to the right,—not to the left as gum arabic. This was, we believe, the first instance of a dextrogyre gum;[922] Scheibler has afterwards shown (1873) that there are also dextrogyre varieties among the African gum from Sennar. Gum arabic may be combined with oxide of lead; the compound (arabate of lead) contains 30·6 per cent. of oxide of lead, whereas the plumbic compound of Feronia gum, dried at 110° C., yielded us only 14·76 per cent. of PbO. The formula (C₁₂H₂₁O₁₁)₂Pb + 2(C₁₂H₂₂O₁₁) supposes 14·2 per cent. of oxide of lead.

Feronia gum repeatedly treated with fuming nitric acid produces abundant crystals of mucic acid. We found our sample of the gum to yield 17 per cent. of water, when dried at 110° C. It left 3·55 per cent. of ash.

CATECHU.