16 
GUMS. 
shapes made by pressing these together. The natural form 
is very often stalactitic, as may be seen in satinwood gum, 
owing to the running down of the exuding gum ; sometimes 
it is almost leaf-like, as in gum tragacanth, owing to its 
exudation at a narrow crack ; at other times it takes the 
form of small tears. The surface is sometimes quite smooth. 
e.g.y in the gums of Moringa and Anacardium, sometimes 
striated, as in gum tragacanth, or marked by rents or grooves ; 
these markings are generally due to the shrinkage in 
drying. 
The cleavage of a gum is generally conchoidal ; the colour 
usually pale yellow, to brownish red ; the streak usually 
white. 
Gums are generally transparent or translucent ; the surface 
has commonly a vitreous lustre, but several are dull. Many 
are doubly refractive or rotate the plane of polarization. The 
specific gravity of samples of the same gum varies owing tq 
the different amounts of air enclosed in them. When dry 
they are mostly easily powdered. 
Nearly all gums are scentless; they have a characteristic 
slimy taste, often with a peculiar after taste. 
All gums swell with water, but only some actually dissolve. 
The viscosity, a most important quality, is tested by making 
a solution of a given strength, say 10 per cent., and deter¬ 
mining the time a given quantity takes to flow through the 
stopcock of a burette, as compared with that taken by the 
same quantity of a similar solution of gum arabic. The 
longer the time, the greater the viscosity. 
As regards chemical constitution, gums may be classified 
according to the proportions of arabin, cerasin, and bassorin 
they contain. Arabin, really a mixture of glucosides, is fully 
soluble in water, and gums containing large quantities of it 
are the best for adhesive purposes. The best is gum arabic 
(Acacia), and other good gums of this class are those of 
Feronia and Anacardium. Cerasin is insoluble in water, 
merely swelling up. It is characteristic of the gums of the 
cherry (Cerasus), almond, peach, and allied plants. Bassorin 
