QUALITIES. _Form:_ white, semi-transparent masses, of a striated or crystalline aspect, which, on exposure to air effloresce; _Odour_, pungent and peculiar; _Taste_, acrid but cooling. CHEMICAL COMPOSITION. It will be found to vary materially in its composition according to the temperature employed for its preparation; the quantity of alkali varying from 20 to 50 per cent. Mr. Phillips considers the _Sub_-carbonate of ammonia to be a _Sesque_-carbonate, composed of 3 atoms of carbonic acid, 2 atoms of ammonia, and 2 of water; or that it is a definite compound of _Carbonate_ and _Bi-carbonate_, one atom of each, with two atoms of water; one hundred parts, by experiment, consist, of 54·2 _carb: acid_, 29·3 _Ammonia_, and 16·5 _water_; if we consider it as a _Sesque-carbonate_, its constitution, according to Dr. Wollaston’s scale, will be 55·72 _Carbonic Acid_, 29 _Ammonia_, 15·28 _water_. SOLUBILITY. According to Duncan it is soluble in twice its weight of cold water; Mr. Phillips states four times; the mean of these will be found nearly correct. Its solubility however is increased by increase of temperature, but when dissolved in boiling water it effervesces, and undergoes a partial decomposition; it is quite insoluble in alcohol, and hence on the addition of spirit to a strong solution, a dense coagulum is produced. INCOMPATIBLE SUBSTANCES. It is decomposed by _acids_, _fixed alkalies_, and their _sub-carbonates_, _lime_, _solution of muriate of lime_, _magnesia_, _alum_, _super-tartrate of potass_, and all the _acidulous salts_, _sulphate of magnesia_, _acetate_, _sub-muriate_, and _oxy-muriate of mercury_, _acetate_, and _sub-acetate of lead_, and the _sulphates of iron and zinc_. If it be added to decoctions and infusions they must be previously cooled. FORMS OF EXHIBITION. Since by exposure to air its virtues are impaired, it ought not to be kept in powdered mixtures; in the form of pill it is preserved much longer, especially if it be combined with some vegetable extract. USES. It is stimulant, antispasmodic, diaphoretic, powerfully antacid, exceeding in this respect the fixed alkalies, and in large doses it is emetic. It is highly useful as a stimulant in those gastric affections which supervene habits of irregularity and debauchery; combined with opium it affords a powerful resource in protracted diarrhœa attended with debility of the alimentary canal: and in cases of muscular atony so frequently witnessed, as the _sequela_ of chronic rheumatism, ammonia, in large doses, offers the best remedy; I have moreover witnessed the beneficial effects of this remedy in hoarseness depending upon relaxed states of the throat. In typhus fever it has been particularly recommended by Huxham, Pringle, and many other physicians, and some have considered it superior to any other stimulant upon such occasions. It is also useful in syncope and hysteria, in the form of smelling salts;[366]