QUALITIES. It is dry, semi-transparent, brittle and insipid; by exposure
to the air it undergoes no other change than loss of colour. _Specific
Gravity_, 1·515. SOLUBILITY. It is soluble in water in every proportion,
forming a viscid solution, (_mucilage_). One part dissolved in water
affords a fluid of the consistence of syrup; and in two parts, a medium
well calculated for the union of dry powders. Gum is also soluble in
pure alkalies and lime water, as well as in vegetable acids, especially
vinegar, with which it forms a mucilage that may be used as a cement,
like the watery solution, and with the additional advantage of not being
susceptible of mouldiness.[321] It is insoluble in alcohol, as well as
in æther and oils. By strong sulphuric acid the gum is decomposed, and a
considerable proportion of carbon deposited.[322] For a farther history
of its habitudes see _Mucilago Acaciæ_. MEDICINAL USES. It is demulcent
and nutritious; although it appears in certain states of the body to
pass through the bowels without change. When triturated with gum-resins
it assists their mechanical division, as in _Form: 30_. OFFICINAL
PREPARATIONS. _Mucilago Acaciæ._ L. E. D. _Emulsio Mimosæ Niloticæ._ E.
_Emulsio Arabica._ D. _Mist. Corn. ust._ L. D. (=O=) _Mist. Cretæ._ L.
D. (=O=) _Mist. Moschi._ L. (=O=) _Confect. Amygdal._ L. (=O=) _Pulv.
Cret. co._ L. (=O=) _Pulv. Tragacanth co._ L. (=B=) Trochisci _Carbonat.
Calcis_ E. (=O=) _Troch._ _Glycyrrh. Glab._ E. (=O=) _Troch. Glycyrrh.
cum Opio_ E. (=O=) _Troch. Gummos._ E. (=O=). ADULTERATIONS. Gum Senegal
is not unfrequently substituted for it, but this may be distinguished by
its clammy and tenacious nature; whereas genuine _gum arabic_ is dry and
brittle; the fraud is of no consequence in a medical point of view. It
is also occasionally mixed with the gum of plum and cherry trees; this
fraud however, is to be easily detected, for such gum has peculiar
properties by which it may be chemically distinguished; see _Mucilago
Tragacanth_.