The hydrate, Co,O,.HO, is obtained by passing a current of chlorine through water in which hydrated protoxide or carbonate of cobalt is suspended (see Sesquioxide of Nickel).

Sesquioxide of cobalt, whether anhydrous or hydrated, is black; when heated, it is converted into the proto-sesquioxide of cobalt, Co,O,. Sesquioxide of cobalt possesses feeble basic properties; it dissolves in dilute acids, yielding brown liquids which evolve oxygen when heated, leaving salts of the (prot-) oxide of cobalt. When this oxide is heated with hydrochloric acid, chlorine is disengaged.' Sesquioxide of cobalt is capable of combining with the (prot-) oxide, and of forming certain proto-sesquioxides.

It has already been noticed that a black oxide of the formula Co,O,(=CoO.Co2O) is obtained when the sesquioxide is decomposed by heat.

When the (prot-) oxide of cobalt, or its carbonate is heated in air, it is converted into Co,O,, or Co,O,(=4CoO.Co,O,) according to the temperature.

§ 250. This salt may be prepared by dissolving the oxides of cobalt, or the carbonate, in hydrochloric acid; the pink solution yields, on evaporation, rosecolored crystals of the hydrated chloride. If the pink solution be mixed with an excess of acid, it becomes of a blue color, or green if iron or nickel be present. The solution resumes its original red color when largely diluted with water.

When the crystals of hydrated chloride are heated, they lose their water, and evolve hydrochloric acid, oxide of cobalt being left, while a portion of anhydrous chloride sublimes. If only moderately heated, the crystals merely lose their water, becoming blue; this property renders it useful as a sympathetic ink, for letters written with the pink solution are invisible until they are held before the fire, when the chloride loses its water and becomes blue, but resumes its pink color when exposed to air.

Anhydrous chloride of cobalt combines with ammonia, forming the compound CoCl. 2NH,."

The sesquichloride of cobalt formed when the sesquioxide is dissolved in bydrochloric acid, is exceedingly unstable.

1 Some remarkable conjugate compounds of sesquioxide of cobalt have been examined by Genth. By mixing chloride or sulphate of cobalt with a large quantity of chloride of ammonium, adding ammonia in excess, and acidifying the mixture with hydrochloric acid, after four or five weeks' exposure to air, a solution is obtained which, when boiled, deposits a carmine salt of the formula Co2O,.3NH Cl, which its discoverer regards as the chloride of a new radical, Co20.3NH, other compounds of which have been obtained by double decomposition.

2 By the action of ammonia upon a solution of chloride of cobalt, mixed with chloride of ammonium, Claudet has obtained a red crystalline compound, composed of 2CoC1, NH C1,4NH,.

At a later period, in investigating the same subject, Rogojski obtained a chloride of the formula CO,C,,3NH, which is possessed of basic properties.

Fremy has examined the action of hydrochloric acid upon the ammoniacal salts of cobalt; he has obtained a new series of salts, in which part of the oxygen is replaced by chlorine; they have a fine violet color, and are nearly insoluble in water; the chlorine in these compounds cannot be precipitated by nitrate of silver until the solution is boiled, when the chlorine is replaced by oxygen, the original ammonio-cobaltic salts being reproduced.

More recently, the same chemist has published a full investigation of this subject, in which he describes a numerous and interesting series of ammonio-cobaltic salts. (Ann. de Chim. et de Phys. 3d ser. vol. xxxv. p. 257.)

SULPHIDE OF COBALT, COS.

This compound may be obtained by the direct combination of its elements at a high temperature, or by heating one of the oxides of cobalt with excess of sulphur. It is a gray crystalline substance, possessing a metallic lustre.

Hydrated sulphide of cobalt is thrown down as a black precipitate when an alkaline sulphide is added to a solution of cobalt-salt. The hydrate is not dissolved to any great extent by dilute hydrochloric acid, but is easily soluble in nitric acid.

When sulphate of cobalt is reduced by hydrogen, an oxysulphide, CoS, COO, is formed, which is decomposed into its proximate constituents by treatment with acids.

The sesquisulphide, Co,S,, is met with in nature in gray octohedra (cobaltpyrites); it is obtained by passing sulphuretted hydrogen over sesquioxide of cobalt heated to about 500° F. (260° C.)

Bisulphide of cobalt, CoS,, is formed when a mixture of carbonate of cobalt and sulphur is heated to a certain point; it is a black, amorphous powder, which is decomposed by heat into sulphur and sulphide of cobalt. The bisulphide is not readily attacked by acids or alkalies.

$251. Technical History of Cobalt.-The chief minerals containing cobalt are the following:

White cobalt ore, which is the most common, and contains cobalt associated with arsenic, iron, and sulphur.

Gray cobalt ore, containing arsenic, iron, cobalt, and silica.

Glance cobalt, or Tunaberg cobalt, which, when pure, has the formula CoAs,, CoS. This is the richest of the cobalt ores. It can be dissolved only by nitric acid. These ores generally contain more or less nickel.

Since cobalt is never used in the metallic state, the reduction of the ores of this metal has for its object the production of a pretty pure oxide.

It is generally thought sufficient to roast the ore in order to expel the greater part of its arsenic and sulphur, and thus to convert it into an impure oxide, which is sent into commerce under the name of zaffre. Cobalt is also extracted by a process similar to that described for nickel.

Cobalt is extensively employed in the preparation of colors; two of the most important of these are smalt and Thenard's blue.

Smalt is a kind of glass colored with oxide of cobalt, and reduced to a fine powder.

In order to prepare this pigment, a quantity of zaffre is fused with sand and carbonate of potassa in a large earthen crucible; the silicic acid (sand) combines with the potassa, forming a vitreous silicate, which dissolves the oxide of cobalt, while the arsenic, iron, and nickel contained in the ore, are deposited as a metallic-looking mass of speiss, at the bottom of the crucible. The fused mass is reduced to a fine powder, and subsequently levigated.

Thenard's blue consists of phosphate of cobalt and phosphate of alumina, and is prepared by calcining an intimate mixture of the precipitates obtained by adding phosphate of soda to solutions of the bases in question.

VANADIUM.

Sym. V. Eq. 68.6.

§ 252. This rare metal is found in certain Swedish iron-ores, remarkable for their malleability; it is also met with in the form of vanadiate of lead.

It may be obtained by heating vanadic acid with potassium in a platinum crucible, and extracting the potassa with water.

Vanadium is a white metal which dissolves in nitric acid, yielding a blue solution. It is not readily attacked by sulphuric or hydrochloric acid.

Three independent oxides of vanadium are known.

The (prot-) oxide, VO, is formed when vanadic acid is reduced by carbon or hydrogen at a red heat. It is an indifferent oxide.

Binoxide of Vanadium, VO,, may be precipitated as a hydrate by adding potassa to a solution of the bichloride, prepared by heating vanadic acid with hydrochloric acid. It is white when freshly precipitated, and becomes brown upon drying. When binoxide of vanadium is exposed to air, it absorbs oxygen, and assumes a greenish color, due to the formation of an intermediate oxide; it sometimes plays the part of an acid, but generally that of a base, for it dissolves in acids, forming crystallizable salts, which have a blue color.

VANADIC ACID, VO,.

To prepare this acid, the natural vanadiate of lead is heated with nitric acid, evaporated, and the residue extracted with water, which leaves vanadic acid undissolved; this latter is dissolved in ammonia, when crystals of the ammonia-salt are obtained on evaporation; this salt, ignited in air, leaves the vanadic acid.

This acid has a yellow color; it fuses at a red heat, and when cooled, solidifies again, with evolution of light; it is not decomposed at a high temperature. It is very slightly soluble in water, giving a yellow solution, which reddens litmuspaper. Organic matters, and reducing agents in general, convert vanadic acid into the oxide.

When vanadic acid is dissolved in hydrochloric acid, and the solution heated, chlorine is disengaged, and bichloride of vanadium formed :

VO,+3HCl=3HO+VCI,+Cl.

Vanadic acid combines with bases, forming crystallizable vanadiates. The vanadiates of the alkalies are soluble in water.

Vanadic acid also behaves like a basic teroxide, in combining with acids to form definite salts, which may be crystallized; thus, the compound with sulphuric acid has the composition, VÕ,.3SO,. Similar compounds have been obtained with nitric, arsenic, and phosphoric acids. They have a yellow or red color, and are soluble; their solutions lose their color when heated, and are rendered blue by sulphuretted hydrogen and organic matters, in consequence of the reduction of the acid to the state of binoxide of vanadium.

Binoxide of vanadium also combines with vanadic acid, forming compounds which dissolve in water with a fine green color; these compounds contain, respectively, VO,.2VO, and VO,.4VO,, and are obtained either by heating vanadic acid with the binoxide, or by adding a solution of an alkaline vanadiate to a salt of binoxide of vanadium.

Terchloride of Vanadium, VCI,, is a volatile, yellow, fuming liquid, obtained by passing chlorine over a mixture of vanadic acid and charcoal at a red heat. Bisulphide of Vanadium, VS,, constitutes the black precipitate which is produced by an alkaline sulphide in a salt of the binoxide; it dissolves in an excess of the precipitant, forming a purple solution.

REACTIONS OF VANADIUM.-Potassa, soda, and their carbonates; a grayish precipitate of hydrate, soluble in excess, yielding a blue or brown solution. Ammonia; a brown precipitate, insoluble in excess.

Sulphide of ammonium; a dark-brown precipitate, soluble in excess, yielding a dark purple solution.

Ferrocyanide of potassium; a yellowish-green precipitate.

Hydrosulphuric acid, in acid solutions of vanadic acid, a blue color, due to

reduction.

With a borax-bead, in the outer flame, a yellow glass, becoming green in the inner flame, and brown while hot, if much vanadium be present. If the bead contain but little vanadium, it may be perfectly decolorized in the outer flame.

METALS OF THE FOURTH GROUP.

CADMIUM.

Sym. Cd. Eq. 56. Sp. Gr. 8.6.

§ 253. THIS metal is associated in nature with the ores of zinc, and does not occur very frequently or in large quantities.

Preparation. In the description already given of the extraction of zinc from its ores, it was mentioned that the first portions of the vapor which passed over when the roasted ore was distilled with charcoal, burnt with a brown flame (brown blaze), due to the presence of cadmium. In order to obtain this metal in a pure state, these vapors are condensed, and the mixture of zinc, cadmium, and a little copper, thus obtained, is dissolved in hydrochloric or sulphuric acid, and the solution treated with sulphuretted hydrogen, which precipitates the sulphides of copper and cadmium; these are washed, redissolved in concentrated hydrochloric acid; after evaporating the solution to expel excess of acid, carbonate of ammonia in excess is added, which precipitates the carbonate of cadmium, and redissolves that of copper; the carbonate is calcined to expel carbonic acid, and distilled with carbon, when metallic cadmium distils over.

Cadmium may also be obtained by collecting the oxides of zinc and cadmium resulting from the brown blaze, and distilling these, at a moderate heat with charcoal, when most of the zinc is left in the residue; by repeating the operation, the cadmium may be still further purified.

Properties.-Cadmium has a grayish-white color, and much resembles tin in its physical properties. It is very soft, malleable, and ductile; when bent, it emits a crackling sound, like tin; its internal structure is crystalline.

This metal fuses very easily, and is converted by a higher temperature into an inodorous vapor; the fused metal, if slowly cooled, crystallizes in octohedra.

Cadmium is not sensibly affected by dry air at the ordinary temperature; when heated in air, it oxidizes more readily than zinc, burning with a luminous flame, and producing a red-brown oxide. It dissolves readily in dilute acids, with disengagement of hydrogen. Hydrated alkalies also dissolve cadmium at a high temperature.

Only one oxide of cadmium, CdO, is known.

OXIDE OF CADMIUM, CdO.

The oxide is formed when cadmium is heated in air; if the metal be heated in a close crucible, the oxide condenses, as a red-brown film, upon the cover. It has a yellow, brown, or black color, according to the temperature to which it has been exposed; it is sometimes crystallized in needles.

This oxide is infusible, and does not volatilize; it combines with acids, forming well-defined salts. It is capable of absorbing carbonic acid from the air. The hydrated oxide is obtained as a gelatinous precipitate, when an alkali is solution of a salt of cadmium; the hydrate (CdO.HO) is white, but becomes brown when heated, from loss of water. It is easily reduced by charcoal at a low red heat.