liquid, and appear there, after standing, as a red or beautiful violet-coloured stratum. The iodine may then be converted into iodide of potassium, and tested with nitrate of silver in the usual

manner.

2. The starch-test is as good a test for iodine as for bromine, and perhaps more delicate. If a glass rod which has been dipped in starch paste be immersed into a tube containing the violet vapour of iodine, a blue colour will be produced, which passes into a black if much iodine be present. The same effect is arrived at by dissolving a small quantity of an iodide in water, adding a little starch paste, then a drop or two of pure sulphuric acid, and lastly a very minute trace of nitrous acid, or of a soluble nitrite (e. g. nitrite of potassium KNO,), or else a very small quantity of chlorine water.

B. When an iodide is distilled with bichromate of potassium and sulphuric acid, no iodochromic acid is obtained, vapours of iodine only being evolved.

SALTS OF FLUORINE, OR FLUORIDES.

This acid-radical never having been isolated, the precise analogies which it may present to the three preceding elements are unknown; its salts have, however, some features in common with the chlorides, bromides, and iodides, although presenting also many points of difference. The salts of fluorine are often isomorphous with these last-named salts, crystallizing generally in the regular system.

Heated upon charcoal, many of the fluorides fuse, but without suffering decomposition. They impart no colour to the blowpipe flame when added to a bead of microcosmic salt saturated with cupric oxide.

The perfectly dry HYDROGEN SALT of fluorine (HF), or hydrofluoric acid, is believed to be a transparent colourless gas under ordinary conditions, and to have no action on glass, &c.; but, as usually obtained in the form of an aqueous solution, hydrofluoric acid is remarkable as one of the most powerful solvents known, dissolving such bodies as the silicic, titanic,

molybdic, and tungstic anhydrides, which are quite unattacked by all other acids.

Fluorides vary considerably as to their solubility in water. The fluorides of the first subdivision are soluble; those of the second subdivision insoluble, thus exhibiting a remarkable contrast with the chlorides, bromides, and iodides of the same group of metals. Most of the other protofluorides (MF) are insoluble, or but sparingly soluble in water, though slightly more soluble in aqueous hydrofluoric acid; they are often resolved by excess of water, or on the application of heat, into oxyfluorides. The sesqui-, bi-, and terfluorides are very soluble the terfluoride of antimony, unlike the corresponding chloride, bromide, and iodide, is not decomposed by water with formation of the oxyfluoride.

Since the fluorides of the second subdivision of the basic radicals are insoluble, fluorine may be recognized by the formation of these salts, as also by that of the cuprous and plumbie fluorides.

The Barium Salt is produced by the action of a soluble barium salt upon hydrofluoric acid or a soluble fluoride: it is a white precipitate.

Its composition is BaF.

It is insoluble in water, but readily dissolves in hydrochlorie, hydrofluoric, or nitric acid.

THE STRONTIUM SALT is produced by the action of a soluble strontium salt upon a solution of a fluoride: it is a white precipitate, insoluble in water and most acids.

The Calcium Salt is produced by adding a soluble calcium salt to hydrofluoric acid or a soluble fluoride: it is generally a mere gelatinous precipitate, hardly perceptible; but on the addition of a little hydrate of ammonium, it becomes more visible. Its composition is CaF.

This precipitate is somewhat soluble in solutions of ammonium salts; it is almost insoluble in water or in hydrofluoric acid, sparingly soluble in cold hydrochloric or nitric acid, but more soluble in these acids when hot.

THE MAGNESIUM SALT is produced by the action of a soluble magnesium salt on solutions of fluorides: it is a white precipitate, nearly insoluble in water and acids.

THE CUPROUS SALT appears to be insoluble both in water and in hydrofluoric acid, but soluble in strong hydrochloric acid. THE SILVER SALT is soluble.

THE LEAD SALT is produced by the action of soluble lead salts upon hydrofluoric acid or a soluble fluoride: it is a white precipitate, which easily fuses into a thick yellow liquid. Its formula is PbF. It is but slightly soluble in water or hydrofluoric acid, more abundantly in hydrochloric or nitric acid.

THE MERCUROUS SALT is not produced by the action of soluble mercurous salts upon solutions of fluorides, but only by sublimation. THE MERCURIC SALT is soluble.

THE BISMUTH SALT appears to be soluble.

THE PLATINOUS and PALLADIOUS SALTS are not known.

The other insoluble salts are not employed in testing for fluorine.
Many of the remaining fluorine salts are soluble in water.

Although the isolation of fluorine cannot be accomplished, we are at no loss for a method of recognizing this elementary saltradical, on account of the very remarkable affinity which it possesses for silicon; this fact is taken advantage of in two ways:

1. In the etching test. A soluble or insoluble fluoride is placed in the dry state in a platinum capsule. A watch-glass is then taken to serve as a cover; a little wax is melted in it, and allowed to run over every part, and through this coating a device is scratched with a wooden point, so as to expose the glass beneath. Several drops of strong sulphuric acid are then poured into the capsule, which is closed immediately afterwards with the coated glass, and then gently warmed, taking care not to melt the wax. The design traced on the cover will be found distinctly etched after the removal of the wax from the glass by means of turpentine or other solvent. The change described may be thus expressed, in the form of two equations :

2MF+H, SO=2HF+M, SO1;

Si, 0,+6HF=2SiF,+3H,0.

N

Thus the hydrofluoric acid first formed acts upon the silicon contained in the glass, yielding water and the gaseous fluoride of silicon, the formation of which is itself an excellent test for fluorine.

2. The terfluoride of silicon test. A fluoride mixed with some quantity of sand (Si,O,) and of sulphuric acid, when heated, evolves, as has been already shown, gaseous terfluoride of silicon. The above mixture may be made in a test-tube (perfectly dry), fitted with a cork and bent tube dipping under water in another glass vessel. When heat is applied, the terfluoride is immediately evolved, and, passing through the water, suffers the following decomposition:

3SiF,+2H20=HSiO2+H, Si, F..
gelat. ppt.

So rapid is the decomposition when the gas meets the water, that the aperture of the delivery tube soon becomes blocked with the silica deposited; to avoid this, the point of the delivery-tube dips beneath a layer of mercury (fig. 7, p. 65). The formation of hydrofluosilicic acid (H, Si, F,), in the experiment just described, furnishes another proof of the presence of fluorine in the substance examined; this acid may be recognized by adding to the filtrate from the gelatinous precipitate of silica, a few drops of chloride of barium solution, when, after agitation, we obtain a gelatinous precipitate of silicofluoride of barium (Ba Si̟ F ̧), a salt with which the student has already become familiar.

2

SECTION II.-The hypochlorites, chlorites, chlorates, perchlorates, hypobromites, bromates, iodites, iodates, and periodates.

SALTS OF THE COMPOUND ACID-RADICALS WHICH CONTAIN CHLORINE, BROMINE, AND IODINE COMBINED WITH OXYGEN.

Of the acid-radicals containing chlorine combined with oxygen, those existing in the acids or hydrogen salts, termed hypochlorous, chloric, and perchloric, are the most important in an analytical point of view.

SALTS OF THE HYPOCHLOROUS RADICAL, OR HYPOCHLORITES.

A few only of these salts are known; they are extremely unstable. The mode in which they are commonly produced is by passing chlorine either into a solution of hydrate of potassium or over solid hydrate of calcium; the temperature of the materials must not be allowed to be more than lukewarm.

2CaHO+2C1=CaClO+CaCl+H20.

The hypochlorites are decomposed on a slight increase of temperature, and give no characteristic blowpipe reactions.

The anhydrous hypochlorous acid (ClO) is a yellow gas, very soluble in water, and possessing a peculiar odour distinct from, and yet recalling, that of chlorine. Water dissolves more than 100 times its volume of the gas, and yields the hydrogen salt, or hypochlorous acid (HClO). The gas (Cl,O) is a powerful bleaching agent, yielding up both oxygen and chlorine, and indeed, from its great instability, decomposing readily, with explosion, when gently heated.

No insoluble hypochlorites are known; the aqueous solutions of the hypochlorites acquire a most sickly and disagreeable odour by contact with organic matter; and in sunlight, or when heated, decompose into chloride and chlorate, with evolution of oxygen and chlorine.

a. Hypochlorous acid, or a hypochlorite, may be immediately detected by warming the solution of the former, or by adding an acid to that of the latter, when, in either instance, their own peculiar odour is converted into the characteristic odour of chlorine. The following equations represent the metamorphoses of a hypochlorite :

MCIO+HCl=HC10+ MC1;

2HC10=H20+201+0.

B. If a solution of a hypochlorite be mixed with a solution of a manganese salt, a brownish black precipitate of the bihydrate (MnH,O,) falls, thus

CaC10+2MnCl+3H,O=CaCl+2MnH,O,+2HC1.

brown-black ppt.