The liquid known by this name is an aqueous solution of the volatile gas Ammonia. Ammoniacal gas contains one atom of Nitrogen combined with three of Hydrogen: these two elementary bodies exhibit no affinity for each other, but they can be made to unite under certain circumstances, and the result is Ammonia. _Properties of Ammonia._--Ammoniacal gas is soluble in water to a large extent; the solution possessing those properties which are termed alkaline (see page 308). Ammonia however differs from the other alkalies in one important particular--it is volatile: hence the original colour of turmeric-paper affected by Ammonia is restored on the application of heat. Solution of Ammonia absorbs Carbonic Acid rapidly from the air, and is converted into Carbonate of Ammonia; it should therefore be preserved in stoppered bottles. Besides Carbonate, commercial Ammonia often contains Chloride of Ammonium, recognized by the white precipitate given by Nitrate of Silver after acidifying with pure Nitric Acid. The strength of commercial Ammonia varies greatly; that sold for pharmaceutical purposes under the name of Liquor Ammoniæ, contains about 10 per cent, of real Ammonia. The sp. gr. of aqueous Ammonia diminishes with the proportion of Ammonia present, the Liquor Ammoniæ being usually about ·936. Ammonia, although forming a large class of salts, appears at first sight to contrast strongly in composition with the alkalies proper, such as Potash and Soda. Mineral bases generally are _protoxides of metals_, as already shown at page 308, but Ammonia consists simply of Nitrogen and Hydrogen united without Oxygen. The following remarks may perhaps tend somewhat to elucidate the difficulty:-- _Theory of Ammonium._--This theory supposes the existence of a substance possessing the properties of a _metal_, but differing from metallic bodies generally in being _compound_ in structure: the formula assigned to it is NH{4}, one atom of Nitrogen united with four of Hydrogen. This hypothetical metal is termed "Ammonium;" and Ammonia, associated with an atom of water, may be viewed as its _Oxide_, for NH{3} + HO plainly equals NH{4}O. Thus, as Potash is the Oxide of _Potassium_, so Ammonia is the Oxide of _Ammonium_. The composition of the _salts_ of Ammonia is on this view assimilated to those of the alkalies proper. Thus, Sulphate of Ammonia is a Sulphate of the Oxide of Ammonium; Muriate or Hydrochlorate of Ammonia is a Chloride of Ammonium, etc. AMMONIO-NITRATE OF SILVER. _See_ Silver, Ammonio-Nitrate of. AQUA-REGIA. _See_ Nitro-Hydrochloric Acid. BARYTA, NITRATE OF. _See_ Nitrate of Baryta. BICHLORIDE OF MERCURY. _See_ Mercury, Bichloride of. BROMINE. Symbol, Br. Atomic weight, 78. This elementary substance is obtained from the uncrystallizable residuum of sea-water, termed _bittern_. It exists in the water in very minute proportion, combined with Magnesium in the form of a soluble Bromide of Magnesium.