iodine with a solution of ammonia. A part or all of the hydrogen is thereby forthwith removed and replaced by iodine. NH, +61= NI, + 3HI, or 3 NH, + 4 = NHI, + 2HI. Two chlorides of iodine are known. If chlorine be passed over iodine, the iodine melts, and is converted into a brown liquid consisting of ICl, which dissolves in water. The same body is formed when dry iodine and chlorate of potassium are heated in a retort. KC10, + 21 = KIO, + ICI. 3 If excess of chlorine be passed over iodine or protochloride of iodine, the latter is converted into the terchloride of iodine, ICl, a bright orange-yellow solid body, soluble in water. Comparison between Chlorine, Bromine, and Iodine. § 214. These three elements form a well-defined group, no less remarkable for the dissimilarity of its members to other elements than for the manner in which they resemble and differ from one another. They all unite directly with metals, and form neutral salts, hence they are sometimes called halogens. They are always found together in nature. They all form more or less insoluble salts with lead, mercury, and silver, and with no other metals. They are similar and peculiar in smell; they are soluble in the same media; they combine with almost all the other elements in the same numerical proportions. In fine, if any compound containing any one of the three is known, we may look with almost perfect assurance for the existence or discovery of similar compounds containing each of the others. They differ from one another chemically more in degree than in kind; and bromine is, so to speak, the chemical mean between chlorine and iodine. Thus, iodine being a solid and chlorine a gas, bromine is a liquid, and the equivalent of bromine is nearly exactly half the sum of the equivalents of chlorine and iodine. Cl= 35.5 2)162.5 81.25 Bromine 80. If the chloride of a metal is more soluble in water or any other medium than the bromide, then the bromide is more soluble than the iodide, and vice versa. Chlorine expels bromine and iodine from combination with metals; bromine expels iodine, &c. The body which most closely resembles the halogens is the compound body cyanogen, CN (see §§ 173 et seq.), and, in some respects, fluorine. CHAPTER XI. FLUORINE AND ITS COMPOUNDS WITH THE PRECEDING ELEMENTS. Symbol, Fl. Equivalent, 19. Density, 19 (compared with air 1.310). § 215. Fluorine is not well known in the separate state, on account of the energy with which it combines with other elements. It is believed to be a permanent gas of yellowishgreen colour. Fluorine occurs in nature chiefly combined with calcium-as fluor-spar or Derbyshire spar, which is fluoride of calcium, CaFl. It is also found in small quantities in some other rocks (such as cryolite), in bones, and especially in the enamel of teeth. Combination of Fluorine with Oxygen. § 216. No oxides of fluorine are known. Combination of Fluorine with Hydrogen. § 217. Fluoride of hydrogen, HFl, or hydrofluoric acid, is obtained from fluoride of calcium on treating it with sulphuric acid. CaFl2 + H2SO1 = CaSO, 2HF1. 4 Owing to the affinity of fluorine for silicon, which exists in all glass, porcelain, &c., vessels of these substances cannot be used for the preparation of hydrofluoric acid. The acid is generally made in vessels of lead, upon which metal hydrofluoric acid has little action. Fig. 64 shows a convenient arrangement. The vessel V is a cylinder of lead closed at the bottom. Its upper edge, T, is double, so as to form a trough running round the vessel. Into this circular trough fits the edge of the leaden cap C, bearing the leaden tube P. In V powdered fluor-spar and strong sulphuric acid are placed and mixed. The cap C is put in its place, and the trough T filled with sulphuric acid. X shows the arrangement of the trough and edge of the cap. On warming V, hydrofluoric acid escapes by the tube P, and may be condensed in the nearly anhydrous state, or absorbed by water held in a platinum, silver, or gutta-percha vessel at the extremity of P. To obtain the acid in the perfectly anhydrous state, it is distilled in a platinum retort from anhydrous phosphoric acid, P2O. § 218. Anhydrous HF is a colourless liquid, having the density of 1.3, and boiling at 30° C. It fumes in the air, and mixes with water in all proportions, combining with that body with great violence. § 219. Hydrofluoric acid and fluorides are detected by the power which the free acid has of attacking glass and other silicates. This is owing to the affinity of the fluorine for the silicon of such bodies. The substance under examination is powdered and warmed in a platinum vessel with strong sulphuric acid. There is held over the mouth of the vessel a plate of glass partly covered with wax. The liberated acid eats away or etches the exposed portion of the glass; so that when the wax is removed, those parts of the glass are found to be dull. The action may be represented thus: The tetrafluoride of silicon being a gas, escapes from the surface of the glass as it is formed thereon (see Silicon, §§ 291-294). § 220. The fluorides of Mg, Ca, Ba, and Pb are insoluble in water. Solutions of these metals accordingly give precipitates with solutions of soluble fluorides, including hydrofluoric acid and fluoride of silver, which is soluble. Compounds of fluorine with other preceding elements need not be described. CHAPTER XII. SULPHUR AND ITS COMPOUNDS WITH THE PRECEDING ELEMENTS. Symbol, S. Equivalent, 32. Density of vapour, 32 (compared with air 2.207). § 221. Sulphur in the free or uncombined state is found in the neighbourhood of volcanoes. It is chiefly obtained from Sicily and South America. In combination with metals it forms sulphides, many of which are found in nature, forming minerals often employed for the extraction of the metals which they contain. metallic sulphides are, Such Zinc blende...... sulphide of zinc, &c. In combination with oxygen, and forming therewith an acid, sulphuric acid, it is found united with bases in rocks, as sulphate of calcium (plaster of Paris, gypsum, or selenite). It also occurs abundantly in the sea, and in many mineral springs, as sulphate of magnesium. In plants it is frequently found combined with carbon &c., especially in the natural order of the Cruciferæ. Sulphur also forms an essential part of animals, being found in albumen, in the horny tissues, and in blood. It does not form an essential part of the air, but is often found in impure air, owing to the putrefaction of animal and vegetable matter containing it, also to the artificial burning of sulphurous coal. § 222. Sulphur is generally prepared from the native sulphur by submitting it to distillation in earthenware retorts, whereby the rocky impurities are left behind; the sulphur melts, boils, and condenses in the receiver. Before being used, it is generally subjected to another distillation from iron retorts. If the vapour of sulphur be allowed to enter a large cool room, the sulphur condenses and solidifies as a coarse semicrystalline powder called "flowers of sulphur." If the vapours are only sufficiently cooled to become liquid, and the liquid is subsequently cast in moulds, it is called stick sulphur or "brimstone." Sulphur is also extensively prepared by heating large heaps of copper pyrites partly exposed to the air; the sulphur collects in the liquid form in cavities made in the heaps, and is thence tapped off. § 223. Sulphur is capable of assuming different modifications or allotropic conditions. (a) Native sulphur, or such as has been crystallized from a solution in sulphide of carbon or sulphide of chlorine, has the density of 2.05; it is nearly transparent, and crystallizes in rhombic octahedra. (3) Common brimstone (that is, sulphur which has solidified from a state of fusion) has a density of 1.98, and crystallizes in prisms. (7) When sulphur is heated it melts to an amber-coloured liquid; if it be further heated, it deepens in colour and thickens. On still applying heat (which becomes latent), it deepens further in colour and thickens until it becomes almost black and solid. At a still higher temperature it becomes again somewhat liquid. If it be now suddenly cooled by pouring it in a thin stream into cold water, it is found to be |