The other methods by means of which this acid-radical is identified are the following:

a. Bromine is readily distinguished by the properties which it possesses: it is eliminated from the bromides just as chlorine from the chlorides, by the joint action of sulphuric acid and binoxide of manganese. A remarkable peculiarity, however, belongs to the combinations of this substance with salt-radicals when they are treated with sulphuric acid, which, whether dilute or concentrated, when acting upon chlorides, liberates hydrochloric acid only; but with the bromides the reaction varies with the strength of the acid employed: if the acid be weak, hydrobromic acid is set free, just as in the case of the chlorides—

2MBr+H, SO=2HBr+M, SO1;

but if the acid be concentrated, the hydrobromic acid, which we may suppose to be first produced, is decomposed by a portion of the sulphuric acid, with formation of sulphurous acid and separation of bromine, thus

2MBr+2H, 80, 2Br+H,O+H, SO,+M, SO,.

This is by no means the only method of liberating bromine; for this element is set free equally well when chlorine gas is passed (not in excess) into the solution of a bromide, or when to the latter a few drops of concentrated nitric acid are added: in the first case the bromine is set free by its place being occupied by the chlorine; in the second process by a reaction similar to that exercised by sulphuric acid, given above in the form of an equation. The tests for the detection of bromine have been already given (p. 21), but may be here advantageously recapitulated.

Besides its peculiar odour (which is very suffocating and offensive) and its bleaching properties, the colour of liquid and of gaseous bromine (a colour which it imparts to solvents) may be applied for its recognition, in the following manner :—

1. Some chlorine water is added to the solution of the bromide; the bromine is liberated, and dissolves in the water present, imparting to it a yellow colour. To the coloured solution enough ether is now added to form a distinct stratum upon the

surface of the liquid, and the whole shaken in a narrow vessel, and then allowed to rest in order to admit of the ether rising to the surface and forming a layer there. This will contain all the free bromine of the solution, dissolved in it in virtue of the greater solubility of bromine in ether than in water; and its colour will vary from a pale yellow to a deep orange, according to the proportion of bromine present. The chlorine water in this experiment must be added cautiously; but if, after one treatment as above described, and separation of the etherial layer, the solution should still give a yellow colour with chlorine, the shaking with ether, &c. may be again repeated if it is desired to effect a perfect separation of the bromine. The etherial solution is shaken with a few drops of hydrate of potassium solution; evaporated to dryness; and then the ignition of the residue will give rise to the formation of bromide of potassium. Previously to the ignition, bromate of potassium would be present in addition:

6Br+6KHO=5KBr+KBr0,+3H,0.

bromide bromate of potas. of potas.

2. The starch-test is a good method of distinguishing bromine, as well as iodine, from chlorine. When bromine-vapour is allowed to come into contact with starch paste (as by introducing a glass rod smeared with this substance into a test-tube in which bromine vapour is being disengaged), a yellow colour is produced if that acid-radical be present in minute quantity, or, if in larger proportion, a rich orange tint. Or the test may be applied by mixing the bromide with some starch paste in a porcelain dish, warming the mixture over the lamp, and dropping into its centre one drop of concentrated nitric acid: the liberated bromine immediately colours the starch.

B. When a bromide is distilled with chromate or bichromate of potassium and sulphuric acid, a brown distillate passes over (p. 255), which dissolves in hydrate of ammonium, forming a colourless solution, which is precipitated white by nitrate of silver: this distillate is only bromine set free by the action of the

sulphuric acid on the bromide; for there is no bromochromic acid known.

SALTS OF IODINE, OR IODIDES.

The iodides present the closest analogy with the bromides and chlorides, with which they are, for the most part, isomorphous. A large number of them are soluble in water; a few, chiefly those of Subdivisions I. and II., somewhat soluble in alcohol; but nearly all are insoluble in ether.

Heated before the blowpipe on charcoal, the iodides behave in a similar manner to the corresponding bromides. They colour the flame of a fine green colour when fused with a bead of microcosmic salt saturated with cupric oxide.

THE HYDROGEN SALT (HI), or hydriodic acid, is a transparent colourless gas at ordinary temperatures, which, however, liquefies with cold, and solidifies at about -51° C. It is very soluble in water, the solution possessing the property of dissolving some quantity of iodine, forming a brown solution.

The chief iodides possessing remarkable features are the cuprous, argentic, plumbic, mercurous, mercuric, bismuthic, and palladious salts. The platinous iodide is also insoluble.

THE CUPROUS SALT is produced by the action of a soluble cuprous salt on a solution of an iodide. The cuprous salt employed is often prepared extemporaneously, by adding to a solution of the cupric salt sulphurous acid or ferrous sulphate, before mixing it with the iodide: the ferrous sulphate is most frequently employed for this purpose, and acts as follows:

2Cu, SO1+2Fe, SO1=(Cu)1⁄2 SO2+(Fe2)2 (SO4)3⋅ The cuprous iodide is a white precipitate, with a tinge of brown. Its formula is Cu, I. It is slightly soluble in hydrochloric acid; but when heated with nitric or sulphuric acid, it is completely decomposed, with evolution of iodine.

The Argentic or Silver Salt is produced by the action of a soluble silver salt on the solution of hydriodic acid or of a soluble iodide. It is a pale yellow precipitate.

Its formula is AgI.

It is soluble to a considerable extent in concentrated solutions of chloride of potassium or ammonium, also in concentrated solutions of mercuric nitrate. It is scarcely soluble in concentrated hydrate of ammonium solution, 1 part requiring 2510 parts of the strongest ammonia-water to dissolve it. Dilute acids are without action on it; but concentrated nitric and sulphuric acids dissolve it with separation of a small proportion of iodine.

THE MERCUROUS SALT is produced by the action of a soluble mercurous salt on hydriodic acid or a soluble iodide; but the salts should be mixed in equivalent proportions, to obtain a precipitate free from metallic mercury or a mercuric salt. Mercurous iodide is a green precipitate. Its formula is Hg, I. It dissolves partially in concentrated solutions of most soluble iodides, and in hydriodic acid; but it is decomposed into mercuric iodide and mercury by the action of the reagents just mentioned, -by certain chlorides and by hydrochloric acid. It dissolves in about 2375 parts of water, but is insoluble in alcohol.

The Mercuric Salt is produced by the action of a soluble mercuric salt on hydriodic acid or a soluble iodide; the addition of equivalents of the iodide and of the mercuric salt causes a perfect precipitation. It is a magnificent scarlet crystalline precipitate, which, when it begins to form, has a bright yellow tint.

Its formula is HgI.

It is very soluble in the alkaline iodides and chlorides, also in many ammonium salts; in most mercuric salts it is also very soluble. In water and ether it dissolves sparingly, but more abundantly in hot alcohol. It is soluble in hydriodic and hydrochloric acids.

The Lead Salt is obtained by the action of a soluble lead salt upon hydriodic acid or a soluble iodide. It is an orange-yellow crystalline precipitate.

Its formula is PbI.

It dissolves in concentrated solutions of most soluble iodides, also in chloride of ammonium, but not in other ammonium salts. 1 part dissolves in 1235 parts of cold, and in 194 parts of

boiling water, and may thus be purified from the oxyiodide, which is insoluble in water; it is slightly soluble in alcohol. Acids exert a slight decomposing influence upon it.

THE BISMUTH SALT is produced by the action of a soluble salt of bismuth on a solution of a soluble iodide. It is a brown crystalline precipitate. Its formula is BiI..

The Palladious Salt is obtained by the action of palladious chloride on hydriodic acid or a soluble iodide.

Its formula is PdI.

It is insoluble in water, alcohol, ether, and hydriodic acid. The other insoluble iodides are not employed in testing for iodine. Many of the remaining iodides are soluble in water or in hydriodic acid. The recognition of iodides by the separation of their acidradical, is even more easy and certain than the detection of bromides by similar means. Iodine is liberated just as chlorine and bromine by the conjoint action of sulphuric acid and binoxide of manganese; but iodides differ from chlorides, and resemble bromides, in the elimination of their acid constituents by the action of concentrated sulphuric acid employed without the addition of binoxide of manganese.

a. Iodine, by whatever method liberated, may be recognized by the properties which have been already stated as belonging to this acid-radical (p. 22).

1. Not only by its characteristic odour may this element be detected, but, if it be present in some quantity in the substance under examination, it may be recognized by its separation in minute bluish black crystals when strong sulphuric acid is added to the aqueous solution of an iodide. If this solution become very hot on the addition of the acid, or if it be afterwards heated over the lamp, the beautiful rich violet-coloured vapour of iodine will float over the surface of the liquid, and gradually condense upon cooler parts of the test-tube in glistening crystals of a bluish black. If chlorine (not in excess) be passed into the aqueous solution of an iodide, and a little ether (or, better, benzole) be shaken up with the mixture, the ether or benzole will carry up the whole of the free iodine present to the surface of the