GUMMI ACACLE. 239 



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 C^-W^O^^, and 

 gives up H-0 when it unites with btises. It has however a great 

 tendency to form salts containing a large excess of acid. An acid 

 calcium arabate of the composition (e'H-'0")2 Ca + 3 (CH-Q^^ + 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., value £250,088. 42,837 cwt., 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 Mimosae like the Acacise tribe of the Adenanthereae. 

 Mesquite gum agrees not with the fine description, but with the inferior 

 sorts of gum arabic, and is sometimes used in America,^ since 1854, 

 in the manufacture of confectionery and the arts. 



Feronia Ghim or Wood Apple Gutti. This is the produce of Feronia 



^ See Proceedings of Am. Pliarm. >4.«or. 1875. 647; Am. Joum. of Pharm. 1878. 480. 



