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_. ACETICA. L.E.D. _Preparations of Vinegar._ These preparations consist of vegetable principles dissolved in vinegar. OFFICINAL PREPARATIONS. _Acetum Aromaticum._ E. _Acidum Acetosum camphoratum._ E. Medicated vinegars were formerly much extolled; the first London Dispensatory contained no fewer than ten, at present the number is reduced to two, viz. _Acetum Colchici._ L. _Acetum Scillæ._ L.E.D. _which see_. ACETIS HYDRARGYRI. E. Acetas Hydrargyri. D. _Acetate of Mercury._ QUALITIES. _Form_, small flaky crystals; _Colour_, silvery white; _Taste_, acrid. CHEMICAL COMPOSITION. Acetic Acid, and Oxyd of Mercury. SOLUBILITY. It is soluble in hot, but very sparingly in cold water, and quite insoluble in Alcohol. FORMS OF EXHIBITION. It should be always given in pills,[323] it is however seldom used. DOSE, gr. j. As an external application, a solution of it, in the proportion of grs. j. to f℥i of rose water, has been commended as a cosmetic.