GUMMI ACACIA. 2^d 



in water, but not dissolvinj^ even at a boiliao^ heat. If an alkali is 



added, it forms a solution like ordinary gum. Neubaaer who observed 

 these facts (1854^-57) showed that gum arable is essentially an acid 

 calcium salt of ai^abic acid. 



Arabic Acid dried at 100" C. has the composition C^-H^^O^^ and 

 gives up ffO 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 (Q''E''0'J Ca + 3 (C'T'O'' + 5 OH^} 

 would af?3rd by incineration 495 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 27 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 arable 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 arable. There is also a marked 

 difference between gum arable 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 arable 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 

 nave 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 



:ypt. 



Use 



as 



medicine 



gums 



possessing any remedial powers of its own. 



Substitutes— A fvreat number of trees are capable of affording 



arabic. There is to be mentioned for 



p - uiuic ur less similar to gum aiauic. xncic ^^ v^ ^^ "-• ^^ - 



instance Prosojns glandulosa Torrey, a tree growing from 30 to 40 

 leet in height, occurring very abundantly in Texas, and extending as 

 Jar west as the Colorado and the gulf of California. It is universally 

 known by its Mexican name Mesquite. It belongs to the same 

 ' 'er of the Mimosse like the Acacise tribe of the Adenantherete. 



Mesa I 



gum 



since 1854, 



sorts of gum arabic, and is sometimes used in America, 

 ^n the manufacture of confectionery and the arts. 



J^evonia Gum or Wood Apple Gum. This is the produce of i^eroiii« 



^ ^^^ Proceedings of Am. Pharm. Asxor. 1875. 647; Am. Joum. of Pharw. 1S7S. 480. 



