Hydride of fluorine, chlorine, bromine, and iodine, when dissolved in water, are termed "acids." As already mentioned, this name was originally applied to compounds which possess a sharp taste and change the colour of certain vegetable colouring matters. The word was later extended to apply to compounds similar in function, although not acid to taste, which attack the carbonates, causing them to effervesce, and which yield salts with the oxides of metals. All acids contain hydrogen, and it is now possible to define them in a very simple manner. An acid, in fact, is a compound which yields hydrogen ions when dissolved in water, or in some other solvent capable of causing ionisation. This definition applies to the hydrides of fluorine, chlorine, bromine, and iodine; and also to those of sulphur, selenium, and tellurium; for on solution they ionise thus: HI-F.Aq ; H|-CI. Aq; H -Br.Aq; H|-I.Aq ; H|-SH.Aq ; H|-SeH.Aq ; H-TeH.Aq. But it is not confined to them, for the hydrogen may be united, not with a simple element, but with a complex group of elements, as in H₂|=SO₄.Aq or H|-NO₃.Aq. Now, in dilute solution, a solution of sulphuric acid is less ionised than one of hydrochloric acid, in about the proportion of 1:2, and it is therefore a weaker acid ; so that if a hydroxide, such as sodium hydroxide, be presented to a mixture of equal numbers of these molecules, in quantity requisite for only one*of them, chloride of sodium will be formed in greater quantity than sodium sulphate ; yet, on heating a halide with sulphuric acid, because hydrogen chloride is a volatile compound, it removes itself from the sphere of action in a non-ionised state while the sodium remains as sulphate. Hence these hydrides may be thus prepared. Hydrogen fluoride, H₂F₂, is generally prepared by distilling calcium fluoride, a compound naturally occurring as " fluor-spar," with sulphuric acid in vessels of lead or platinum: CaF₂ + H₂SO₄ = BaSO₄ + H₂F₂. The use of lead or platinum is obligatory on account of the action of hydrogen fluoride on glass or porcelain, the materials of which flasks and retorts are usually made; for hydrogen fluoride attacks the silica which they contain, forming with it silicon fluoride: SiO₂ +2H₂F₂ = SiF₄ + 2H₂O. Gold is almost the only other metal which resists the action of hydrogen fluoride. There is no such difficulty with the other halides. Hydrogen chloride, HC1, is prepared by distilling from a glass retort a mixture of common salt and oil of vitriol: NaCl + H₂SO₄ = HNaSO₄ + HC1. On a large scale this preparation is carried out in rotating circular furnaces, the mixture of salt and vitriol being delivered in through a hopper above, and at the high temperature the action goes further, and di-sodium sulphate is produced: 2NaCl + H₂SO₄ = Na₂SO₄ + 2HCI. The gas is passed up towers filled with coke, and exposed to a descending stream of water, in which it dissolves, forming a saturated solution of hydrochloric acid, or, as it used to be called, "muriatic acid" (from "muria," brine).

Hydrogen bromide, HBr, and iodide, HI, may similarly be produced by distilling together bromide or iodide of sodium or potassium with exactly the right weight of sulphuric acid for the equation 2KBr (or 2KI) + H₂SO₄.Aq= K₂SO₄.Aq + 2HBr (or2HI). But in these cases, the hydrogen bromide or iodide is very apt to exert a reducing action on the sulphuric acid, depriving it of an atom of oxygen, thus: - H₂SO₄ 4- 2HI = H₂SO₃ + H₂O +1₂. Hence it is advisable to use phosphoric acid, H₃PO₄, a compound not thus reduced :-H₃PO₄ + 2KI = HK₂PO₄ + 2HI.

All these halides come over as gases, and may either be collected over mercury or by "downward displacement," i.e. by delivering them to the bottom of a jar containing air, which owing to its less density is forced upwards, and escapes at the mouth of the jar. They cannot be collected over water, for they are readily soluble in it.

The compound HN₃, termed hydrazoic acid (from the French term for nitrogen, "azote"), is also liberated in the gaseous form by warming its sodium salt with sulphuric acid. It, too, is readily soluble in water.

(3) Certain hydrides are set free by the action of an alkali, i.e. the hydroxide of one of the metals of the sodium or the calcium group. It is true that the change may be produced by other hydroxides, but they are not so efficient, and not so generally employed. Among these are ammonia, NH₃, and hydrazine, N₂H₄. These bodies unite with acids; for example, ammonia and hydrogen chloride form ammonium chloride, NH₄C1, when mixed:-NH₃ + HC1 = NH₄C1. This compound is produced by a change in valency of the nitrogen atom; in ammonia it is a triad, N'", but on union with hydrogen chloride the valency of the nitrogen becomes five, N^v; that is, in N-, three electrons are active, and two latent; in N^v, all five electrons are active. On distillation of a mixture of ammonium chloride with caustic soda or with slaked lime, either in presence or absence of water, the following change occurs:- NH₄C1 + NaOH = NaCl + NH₃ + H₂O ; ₂NH₄C1 + Ca(OH)₂ = CaCl₂ + 2NH₃ + 2H₂O. The initial change is the formation of ammonium hydroxide, NH₄OH ; this substance, being unstable when heated, decomposes into ammonia and water. Hydrazine, a compound of the formula N₂H₄, is similarly liberated from its chloride.

The usual source of commercial ammonia is coal-gas. On distillation of coal, all varieties of which contain nitrogen, it may be imagined that when methane, the principal constituent of coal-gas, is strongly heated it splits into carbon and hydrogen. This hydrogen, at the moment of its formation, is in the nascent state, and it unites with the nitrogen, which is also in the nascent condition. As ammonia is very easily soluble in water, while the other constituents of coal-gas are sparingly soluble, the gas is deprived of ammonia by passing it through " scrubbers," pipes containing broken bricks kept moist with water. The ammonia dissolves, while the coal-gas passes on. The solution is next mixed with hydrochloric acid and evaporated to dryness. The residue of ammonium chloride is then distilled with lime, as previously described. The ammonia is received in water, and brought into the market in the form of a concentrated solution, to which the name " liquor ammonias" is given.

(b) Certain double hydrides are formed by the addition of one hydride to another. Ammonia and hydrazine unite with hydrides of the halogens to form salts, such as ammonium chloride, NH₄C1; but as these bodies show analogy with salts of the metals, they will be reserved until the latter are considered.