_Gummi Arabicum_; _Gum Arabic_; F. _Gomme Arabique_; G. _Arabisches
Gummi_, _Acacien-Gummi_, _Kordofan-Gummi_.
=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.
[898] It may be further distinguished from Wood Oil as well as from
copaiba, if tested in the following simple manner:—Put into a tube
19 drops of bisulphide of carbon and one drop of the oleo-resin, and
shake them together. Then add one drop of a mixture of equal parts of
strong sulphuric and nitric (1·42) acids. After a little agitation the
appearance of the respective mixtures will be as follows:—
_Copaiba_—Colour faint reddish-brown, with deposit of resin on sides of
tube.
_Wood Oil_—Colour intense purplish-red, becoming violet after some
minutes.
_Oleo-resin of Hardwickia_—No perceptible alteration; the mixture pale
greenish yellow.
By this test the presence in copaiba of one-eighth of its volume of
Wood Oil may be easily shown.
[899] Beddome, _op. cit._
[900] See also Hazlett, _Madras Monthly Journ. of Med. Science_, June
1872.
[901] Figures in Guillemin and Perrottet _Floræ Senegamb. tent._ 1830,
p. 246, tab. 56; also Bentley and Trimen, _Med. Plants_, part 17
(1877).
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_ [2_s._] 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.
[902] _Aufzählung und Beschreibung der Acacien-Artendes
Nilgebiets.—Linnæa_, i. (1867) 308-376, with 21 plates. Schweinfurth’s
observations are strongly confirmed by an account of the commerce of
Khartum in the _Zeitschrift für Erdkunde_, ii. (1867, Berlin) 474.
[903] The _A. Adansonii_ Guill. et Perr. is the same tree.
[904] The “_Kikar_” of the Punjaub, or “_Babul_” or “_Babur_” of
Central India.
[905] As presented to me by Capt. Hunter of Aden, July 1877.—F. A. F..
[906] We have to thank Professor Dümichen for most of the information
relating to Egypt, which may be partly found in his own works, and
partly in those of Brugsch, Ebers, and Lepsius.
[907] Lepsius, _Abhandl. der Akademie der Wissensch. zu Berlin_ for
1871, p. 77. 126. Metalle in den Aegyptischen Inschriften.
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]
[908] _Schedula diversarum artium_, Ilg’s edition in Eitelberger’s
_Quellenschriften für Kunstgeschichte_, vii. (1874) 60.
[909] _Della Decima e di varie altre gravezze imposte dal commune di
Firenze_, iii. (1766) 18.
[910] Bonaini, _Statuti inediti della città di Pisa_, Firenze, iii.
(1857) 106. 114.
[911] _Ordonnances des Rois de France_, ii. (1729) 310.
[912] _Tariffa de pesi e misure_, Venet. 1521. 204. First edition, 1503.
[913] See, however, Möller, Academy of Vienna, _Sitzungsberichte_, June
1875.
[914] Vaughan (Drugs of Aden), _Pharm. Journ._ xii. (1853) 226.
[915] Private information to F. A. F..
[916] Vaughan, _l.c._
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]
[917] Flückiger, in the _Jahresbericht_ of Wiggers and Husemann, 1869.
149.
[918] _Consular Reports_, August, 1873. 917.
[919] _Statement of the Trade and Navigation of the Presidency of
Bombay for_ 1872-73, pt. ii. 34. 77.
[920] P. von Müller, _Select Plants for industrial culture in
Victoria_. 1876; 2. 4.
=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:—
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.
=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_.
[921] See _Proceedings of Am. Pharm. Assoc._ 1875. 647; _Am. Journ. of
Pharm._ 1878. 480.
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.