Fluorine (Fl; 19 or 19.1). Chlorine (Cl; 35.5 or 35.4). Bromine (Br; 80 or 79.75). Iodine (I; 127 or 126.53). These substances form a series in which the atomic weights are nearly in the relation of 1, 2, 4, and 7 (vide also p. 16). They are distinguished by the activity of their chemical affinities and the number of compounds they form.

General Source. - The name halogen (from as, the sea) has been given to the group, because its most important members, chlorine, bromine, and iodine, are derived from the sea; chlorine being obtained from sea-salt, bromine from sea-water, iodine from sea-weed.

General Characters. - They are all very volatile. At ordinary temperatures, chlorine is a gas, bromine a liquid, and iodine a solid, but both bromine and iodine give off vapour freely. On account of their active chemical affinities they unite directly with metals, as is seen in the officinal processes for the preparation of iodide of iron and green iodide of mercury. They have all a great affinity for hydrogen, and are therefore powerful decomposers of organic matter, destroying organic colours and disagreeable emanations of organic origin, as well as decomposing sulphuretted hydrogen (H2S + C12 = 2HC1 + S2) and ammonia which occur amongst the products of decomposition of organic matter. They are therefore used as deodorisers and disinfectants. Chlorine is used for bleaching, but bromine and iodine form coloured compounds with many organic substances, and so are not used for this purpose.

Probably the bleaching power of chlorine is not due to its decomposing organic colours by removing hydrogen from them, but rather to its decomposing water by removing the hydrogen from it, and thus setting free nascent oxygen, which is the direct destroyer of organic matters. The reason for this supposition is that chlorine does not act upon colouring matters when they are dry, but only when moist.

Mode of Peeparation. - Chlorine, bromine, and iodine are all prepared by expelling them from their compounds with the alkaline metals by means of sulphuric acid and manganese dioxide.

Chlorine is prepared by putting sodium chloride, sulphuric acid and manganese dioxide into a retort, applying heat and collecting the chlorine gas in a receiver, by displacement or over warm water, or passing it into cold water which dissolves it freely, forming liquor chlori (B.P.) or aqua chlori (U.S.P.).

Bromine is prepared in a similar manner from the bromides of sodium and magnesium contained in the bittern or mother-liquor left after the salt has crystallised out of sea-water or out of the brine obtained in salt mines. In order to obtain the bromine pure, the bittern is often not treated directly with sulphuric acid and manganese dioxide. Instead of this the bromine is first separated by passing chlorine through the liquid, which is then shaken up with ether. The chlorine decomposes the magnesium bromide and the ether dissolves the bromine thus set free. The bromine is then converted again by potash into bromide, from which bromine is obtained by means of manganese dioxide and acid.

Iodine is prepared in a similar manner to chlorine from the iodides of sodium and magnesium contained in sea-weed. The iodides are obtained from the weed by calcining it in a retort, or by burning it, when the ashes in which they are contained form a hard mass called kelp. This is treated with successive portions of water until the soluble salts are all dissolved out (lixiviation). The solution is filtered, and evaporated to a small bulk, when the less soluble salts, as the sulphates, etc, crystallise out. The mother-liquor containing the iodides of sodium and magnesium is then treated with manganese dioxide and sulphuric acid, and the iodine distils over.

The reactions which occur in the preparations just described are -

Chlorine

2NaCl + 2H.2SO4 + MnO2

= Cl2 + Na2SO4 + MnSO4 + 2H2O.

Bromine

2NaBr + 2H2SO4 + MnO2

= Br2 + Na2SO4 + MnSO4 + 2H2O.

MgBr2 + 2H2SO4 + MnO2

= Br2 + MgSO4 + MnSO4 + 2H2O.

Iodine

2NaI + 2H2SO4 + MnO2

= I2 + Na2SO4 + MnSO4 + 2H2O.

Mgl2 + 2H2SO4 + MnO2

= I2 + MgSO4 + MnSO4 + 2H2O.

General Action. - As chlorine, bromine, and iodine decompose organic compounds having a disagreeable odour, they have been supposed to have a similar action upon the germs of infectious diseases. Chlorine, and sometimes iodine, are therefore used as deodorisers and disinfectants in sick rooms. Bromine cannot well be used on account of its abominable smell.

The objections to chlorine or the vapour of iodine as disinfectants are that we do not at all know that they have any disinfecting power in the dilute state, in which only they can be used in a sick room. When applied to the skin or mucous membranes they cause a greater or less amount of irritation or inflammation, according to the length of time during which they act, and the greater or less degree of concentration in which they are applied. They probably do not enter the blood in the free state, but combine with bases or with albuminous substances at the place of application, and are absorbed as chlorides, bromides, or iodides, or else as albuminous compounds. According to Binz, free chlorine, bromine, and iodine, and all their readily decomposable compounds, have a narcotic action, and paralyse nervous centres in the brain by a direct action on the nervous structures themselves. He considers that they cause death by paralysis of the respiratory centre, and not by paralysis of the heart.