f2 REPORTS ON THE STATE OF SCIENCE.—-1917. 
Gum Arabie. 
The commercial gum arabic is often a mixture of gums from several 
species of trees, and is therefore a variable product. The best gum arabic 
is that obtained from Acacia Senegal, known as Hashab gum, which comes 
from Kordofan and the Blue Nile District. Gums are also obtained from 
several other species of acacia, and, although these resemble gum arabic 
in appearance, they differ from it more or less in constitution and also 
notably in yielding solutions which are less viscous and much less adhesive. 
They therefore rarely appear in commerce except in admixture as above 
stated. 
Gum arabic contains normally about 15 per cent. of water and 33 per 
cent. of mineral matter, which consists largely of the carbonates of potash 
and lime. It has long been regarded as an acid—arabic acid—in com- 
bination with bases. The same acid is supposed to be present in beet- 
root juice. More recently the researches of O'Sullivan and others have, 
according to H. H. Robinson (‘ Report on the Chemistry of Gums,’ British 
Association, 1906), appeared to prove that the gums are composed of 
complex acids built up of a nucleus acid associated with the sugars 
galactose, arabinose, and xylose. Arabic acid is the nucleus acid in gum 
arabic and geddic acid in gedda-gum. Without subscribing to this 
decision, it may be stated that gum arabic functions as an acid, and 
either combines with or adsorbs metallic bases; it is a complex of the 
two carbohydrates galactan and araban. 
Gum arabic is employed more for its adhesiveness than for any other 
property, but closely allied to adhesiveness is viscosity, the gums which 
yield the most viscous solutions being the most valuable. It is employed in 
painting (Church, ‘ Chemistry of Paints and Painting ’) asa glaze or varnish, 
in the manufacture of sweets, and for emulsifying oils, for which purposes 
it is eminently fitted. 
Gum arabic is usually entirely soluble in water. The gum from the 
earliest exudation is, however, not entirely soluble, yielding a glairy mucus- 
like fluid, from which a perfect solution separates after a time. After 
storage for two to three months a change has taken place in the product, 
probably due to enzymes, so that it dissolves entirely (I. Reinitzer, 
‘ Zeitschr. f. Physiolog. Chem.’ 1909, 352-392, also ‘ 3rd Report of the 
Wellcome Research Laboratories, Khartoum *). Dr. Beam (‘ 2nd Report 
of the Wellcome Research Laboratories ’) notes the fact, which, however, 
was previously well known, that the viscosity of a solution prepared in the 
cold is greater than when heat is employed. 1t may be mentioned that 
gum arabic contains at least three enzymes, an oxidase, a peroxidase, and a 
diastase (I". Reinitzer, ‘ J. Physiol. Chem.’ 1909, 61, 352-392), the functions 
of which have not yet been determined, but these render the gum incom- 
patible for use with several medicinal preparations; incidentally they also 
serve for detecting gum arabic in admixture with other gums, e.g., traga- 
canth, which are free trom these enzymes. 
Hashab gum was found to be composed of two gums, a hard and a 
soft one. On exposure to the sun the hard gum remains glassy, but the 
soft kind becomes white and opaque, owing to the development of 
innumerable tiny fissures. By this treatment it appears to be bleached, 
although this is really not the case. There is a marked difference in the 
a 
