a. The negative evidence mentioned above as being employed in the identification of the compounds of oxygen, will not need further description; but the method of recognizing the salts of the radical now under consideration, by its elimination, may be here noticed.

6. The oxides which can be decomposed into metal and oxygen by the mere action of heat are few in number. Of these the oxygen salts of silver, mercury, and gold are the most important. Lead also, under certain circumstances, may be obtained by heating its oxide. If a small quantity of oxide of silver (Ag2O) be heated in a dry test-tube, it will readily split into metallic silver and oxygen gas, the latter being easily recognizable by the usual test, the insertion, into the vessel used for the experiment, of the glowing end of a wooden match which has been previously lighted, allowed to burn for a minute, and then blown out. If there has been only a small quantity of oxygen liberated, the redhot end of the match will only glow more brightly; but if a considerable amount, it will be rekindled.

SALTS OF SULPHUR, OR SULPHIDES.

A great resemblance exists between these salts and the oxides, some of them possessing remarkably brilliant colours, others again being of an intense brown colour, or even black. When fused, or naturally occurring in crystals, many sulphides have a peculiar metallic lustre. To these salts belong the same formulæ as those given under the salts of oxygen.

When heated before the blowpipe, sulphides are generally decomposed, the sulphur they contain being oxidized into the sulphurous or sulphuric radical, which remains combined with the metal; but, on the other hand, when the sulphites and sulphates thus produced are heated on charcoal, they are for the most part reduced to the state of sulphide. Sulphides, when heated alone in the air, often yield the oxide, and sometimes even the metal.

THE HYDROGEN SALT (H, S), hydrosulphuric or sulphydric acid or sulphuretted hydrogen, is a gas at ordinary temperatures and pressures, possessing a peculiarly disagreeable and sickly

smell, and being very poisonous and narcotic. It may be condensed, by combined cold and pressure, into a colourless transparent liquid, more mobile and less adhesive even than ether, while by further cold it may be made to assume the solid form, and then appears as a white crystalline mass. The gas has a density of 1.191. Water dissolves about its own volume of sulphuretted hydrogen at the common temperature, forming a solution which, by exposure to the air, oxidizes, a deposit of sulphur taking place. The gas is combustible, burning with a blue flame, and may be ignited at a comparatively low temperature, as by a red-hot coal; the products of its combustion are sulphurous anhydride (SO2) and water:

H2S+30=H20+SO2.

Most sulphides are insoluble in water, although many are soluble in acids. The sulphides of the first and second subdivisions are, however, soluble in water, though it may be presumed that they really undergo a decomposition during solution, becoming partially converted into sulphydrates, just as certain oxides yield hydrates under similar circumstances:

KKS+HHO=KHS+KHO.

With hydrosulphuric acid solution instead of water only, 2 eqs. of the sulphydrate are formed:

KKS+HHS=KHS+KHS.

The chief insoluble sulphides have been already described under their respective basic radicals; for the sake of comparison, however, with the salts of other acid-radicals, we give a list of the more important, premising that, of the insoluble sulphides given, all but the six first are precipitable from their neutral solutions by sulphuretted hydrogen.

THE POTASSIUM, SODIUM, AMMONIUM, BARIUM, STRONTIUM, CALCIUM, and MAGNESIUM SALTS are soluble.

THE FERROUS (Fe, S) and FERRIC ([Fe2], S2) SALTS are black. THE MANGANOUS SALT (Mn, S) is buff or flesh coloured, and soluble in dilute acids.

THE COBALT (CO, 8) and NICKEL (Ni, S) SALTS are black, and soluble in dilute acids.

THE ZINC SALT (Zn, 8) is white, insoluble in water, but soluble in dilute acids.

THE CADMIUM SALT (Cd, S) is bright yellow.

THE CUPROUS ([Cu2], S), CUPRIC (Cu, S), SILVER (Ag, S), MERCUROUS ([Hg,], S), MERCURIC (Hg, S), LEAD (Pb, S), and PALLADIOUS (Pd, S) SALTS are black or brownish black, and insoluble or nearly so in cold water and in dilute acids. The cupric sulphate is easily oxidized by atmospheric oxygen into the sulphate. THE STANNOUS SALT (Sn, S) is brownish black.

THE STANNIC SALT (Sn, S.)) is white, inclining to yellow.

THE ANTIMONIOUS (Sb, S,), ANTIMONIC (Sb, S,), ARSENIOUS (As, S,), and ARSENIC (AS, S,) SALTS are of a fine orange yellow colour, inclining to red.

The presence of sulphur in its compounds may be recognized also by processes of decomposition:

a. Most sulphides, when warmed with strong hydrochloric or sulphuric acid, decompose with evolution of hydrosulphuric acid gas (the odour of which is so characteristic), and occasionally also with separation of sulphur:

M2S+H, SO1=H2S+M, SO1.

When nitric acid is employed, the sulphide is generally oxidized and converted into the corresponding sulphate or nitrate (see d.). 6. When an insoluble sulphide is fused with carbonate of sodium on charcoal, sulphide of sodium (Na, S) is formed; and if the mass be then scooped out, laid on a silver coin and moistened with water, a black stain (Ag, S) is immediately produced.

7. When hydrosulphuric and sulphurous acids meet in the presence of water, a white or very pale yellow precipitate of sulphur is produced, while the solution will be found to contain an acid known as pentathionic (H, S,0,):

5H, S+5H, SO,=H, S,O,+9H20+5S.

white ppt.

d. Very generally, when sulphides are boiled with nitric acid, a separation of sulphur takes place, and the latter substance often fuses and floats as an oily globule upon the surface of the liquid, solidifying, on cooling, in the form of a little circular cake.

This action of nitric acid also gives rise to the simultaneous formation of sulphuric acid, and to the most highly oxygenated combination which the metal of the sulphide employed is capable of forming thus with ferrous sulphide

2

2Fe, S+10HNO, ̧=2Fe, (NO3),+H, SO2+4H2O+S+2N2O2. ferric nitrate. sulphuric

ferrous sulphide.

acid.

ppt.

excellent test for the presence of sulphur in the €. A very form of a soluble sulphide (and by process ß. all insoluble sulphides may be made to yield the soluble sulphide of sodium) is found in a solution of nitroprusside of sodium. The most minute trace of sulphur is discoverable by the magnificent violet or purple colour which a drop of a dilute solution of this reagent immediately produces.

Hydrosulphuric acid and other soluble salts of sulphur are usually detected by the formation of coloured metallic sulphides, and particularly by their action on lead salts (which, for this purpose, may be applied in the form of aqueous solution to strips of white paper), and also by the tests a, ß, y, e.

SALTS OF SELENIUM, OR SELENIDES.

A most marked resemblance exists between the selenides and sulphides; they do not, however, possess the same variety of brilliant colours, most of them, when freshly precipitated by hydroselenic acid (H, Se) being brownish black or flesh-coloured. Natural selenides often possess a metallic lustre.

When heated on charcoal in the air, the selenium of these salts burns slowly with a reddish blue flame, with evolution of a most powerful odour of horse-radish, due to the formation of a gaseous oxide of selenium, probably SeO. Selenium is more slowly expelled from selenides, by this process of "roasting," than sulphur from sulphides.

THE HYDROGEN SALT (H, Se), hydroselenic acid or seleniuretted hydrogen, is a gas under ordinary circumstances, of specific gravity 2.795. At first, its smell seems to resemble that of the corresponding sulphur acid; but it subsequently painfully affects the mucous membrane of the nose, and destroys the sense of smell. It is intensely poisonous. It burns with a bluish flame. It is more soluble in water than hydrosulphuric acid; the aqueous solution decomposes by exposure to the air, depositing selenium in red flakes.

Most selenides are insoluble in water; but they have not been much studied. THE POTASSIUM and SODIUM SALTS are the best known soluble selenides.

THE BARIUM and STRONTIUM SALTS are soluble, THE CALCIUM and MAGNESIUM SALTS nearly insoluble in water.

THE MANGANESE SALT is pale red, THE ZINC SALT yellow; they are both insoluble in water.

THE CUPRIC, SILVER, MERCURIC, and LEAD SALTS are black, becoming grey on drying; they are insoluble in water or neutral solutions.

The presence of selenium in its compounds may be readily recognized by several processes of decomposition :

:

a. Upon treatment of a selenide with hydrochloric or sulphuric acid, hydroselenic acid is evolved, which may be recognized by its peculiar odour. Nitric acid generally converts selenides into selenites, with occasional separation of selenium.

B. A selenide fused with carbonate of sodium on charcoal, transferred to a silver surface, and moistened with hydrochloric acid, will produce a black stain.

y. Sulphurous acid produces, in an aqueous solution of hydroselenic acid, a bright reddish precipitate of selenium.

d. Selenides heated on charcoal with carbonate of sodium or microcosmic salt, give the characteristic odour of horse-radish.

e. To detect traces of selenium, it is recommended to fuse the substance under examination with hydrate of potassium, to dissolve in water, filter, and expose the solution to the air;' if selenium be present it will slowly appear, causing a reddish turbidity, or it will separate in reddish flakes. Selenium may be conveniently recognized by the tests d. and e.

SALTS OF TELLURIUM, OR TELLURIDES.

These salts greatly resemble the selenides; they, however, partake more of the character of alloys (or combinations of metals with one another) than of that of saline compounds. In tellurides the metallic appearance of the basic radical is scarcely masked by the tellurium; even the potassium salt has a metallic lustre. The tellurides of the heavy metals, obtained by passing telluretted hydrogen through their aqueous solutions, are generally brown or black, and insoluble in water.

When tellurides are heated on charcoal before the blowpipe, the tellurium burns with a blue flame edged with green, and emits a peculiar odour, different from that of selenium.

THE HYDROGEN SALT (H, Te), hydrotelluric acid or telluretted hydrogen, is a gas of specific gravity 4.489, its odour somewhat resembling that of the corresponding sulphur compound. It is poisonous. It is absorbed by water, imparting to that solvent a pale red colour; the aqueous solution, by exposure to the air, becomes brown, from separation of tellurium.

Most tellurides are insoluble in water; but they have not been much studied. THE POTASSIUM and SODIUM SALTS are soluble in water.

THE CUPRIC, SILVER, MERCURIC, and LEAD SALTS are brown or black precipitates, insoluble in water.