The

the metallic chlorides formed dissolved out with acidulated water. residual tungstic acid (which also contains a little silicic and niobic acids) is washed with alcohol, and treated with ammonia, which dissolves the tungstic acid, leaving the silicic and niobic acids undissolved. These are filtered off, the filtrate evaporated and ignited, when pure tungstic anhydride is obtained. Another method is to fuse three parts of the finely powdered mineral with two parts of carbonate, and half a part of nitrate of potassium in a platinum crucible, by which process the tungstic acid is entirely converted into tungstate of potassium, which is dissolved out by water, and separated by filtration from the insoluble oxides. The filtrate is nearly neutralised with nitric acid, and precipitated by subnitrate of mercury, a few drops of ammonia being added to neutralise the free nitric acid. The precipitate is thoroughly washed, (first with water, and lastly with a very dilute solution of subnitrate of mercury), and ignited, when tungstic anhydride is obtained. Insoluble tungstates may also be decomposed by heating with concentrated sulphuric acid, or by fusion with acid sulphate of potassium.

Wolfram is always estimated as tungstic anhydride, in which form it is usually separated.

8. Molybdenum. Mo. Atomic weight, 48.

Occurs in nature principally as sulphide of molybdenum and as molyb date of lead. It is a silver-white metal, very difficultly fusible, and slightly malleable. It is not affected by exposure to the air at the ordinary temperature: but when heated in contact with air it becomes first brown, then blue, and finally white, passing through various stages of oxidation till it is converted into molybdic anhydride, Mo2O3. It is insoluble in hydrochloric acid: nitric acid or aqua-regia convert it into molybdic acid if sufficient nitric acid be present; if not, nitrate of molybdenum is formed.

OXIDES OF MOLYBDENUM. Protoxide, Mo2O. Binoxide, Mo2O2. Molybdic Anhydride, Mo2O3.-The oxides of molybdenum correspond with those of wolfram. There is also a blue oxide of molybdenum, Mo105, intermediate between the binoxide and the anhydride.

a. Protoxide of Molybdenum. Molybdous Oxide. Mo2O.-A black powder, obtained by digesting molybdic anhydride with a quantity of hydrochloric acid not sufficient to dissolve it, and with metallic zinc. It is insoluble in acids. The hydrate, MoHO, is formed when an alkaline molybdate is dissolved in hydrochloric acid and reduced by zinc. It is a black powder, soluble in acids, forming molybdous salts. It absorbs oxygen readily from the air: when heated in contact with air it takes fire, and is converted into molybdic anhydride. When heated out of contact with the air it is converted into molybdous oxide, insoluble in acids.

b. Binoxide of Molybdenum. Molybdic Oxide. Brown Oxide. Mo2O2. -A brown powder, obtained by fusing an alkaline molybdate with chloride of ammonium, and treating the fused mass with water: or by reducing molybdic anhydride by hydrogen, at a moderate temperature. It is insoluble in acids. The hydrate is precipitated by ammonia from

an aqueous solution of bichloride of molybdenum. It is also brown, soluble in acids, forming molybdic salts: slightly soluble in water. By ignition out of contact with the air, it is converted into molybdic oxide.

Molybdous and molybdic salts resemble each other very closely in their reactions. Both form dark-brown solutions, which become blue on exposure to the air, from which sulphide of molybdenum is slowly precipitated by hydrosulphuric acid. The best reagent for distinguishing them is carbonate of potassium, which precipitates the hydrate both from molybdous and molybdic salts; but in the former case the precipitate is difficultly soluble in excess of the reagent, while in the latter it is readily soluble. Both the lower oxides of molybdenum, as well as the sulphide, are converted into molybdic anhydride by ignition in the air.

c. Molybdic Anhydride, Mo2O3.-A white crystalline powder, slightly soluble in water, soluble in acids and in alkalis. When ignited in a covered vessel, it fuses to a yellow liquid, which on cooling forms a light yellow crystalline mass, much less soluble in acids than before ignition. When heated in the air it sublimes at no very high temperature into crystalline needles. It combines with water, forming a white compound, Mo2O+H2, which has acid properties. This is molybdic acid: it is bibasic, and combines with metals, forming a class of salts called molybdates. The alkaline molybdates are soluble in water: most others are insoluble. Nitric or hydrochloric acid precipitates molybdic acid from a solution of a molybdate; the precipitate is soluble in excess of acid. Metallic zinc immersed in the hydrochloric acid solution, produces a blue colour, which gradually becomes green, and finally dark-brown, owing to the reduction of the molybdic acid to molybdous hydrate, which remains dissolved in the hydrochloric acid. Hydrosulphuric acid gradually precipitates sulphide of molybdenum from an acid solution of molybdic acid: the supernatant liquid has a blue or green colour. The same precipitate is produced when an aqueous solution of an alkaline molybdate is saturated with sulphide of ammonium or hydrosulphuric acid, and hydrochloric acid added to it. Chloride of tin gives with an alkaline molybdate a greenish-blue precipitate, which hydrochloric acid dissolves to a green solution: if very little chloride of tin be used, the solution is blue. Subnitrate of mercury gives a yellowish precipitate, soluble in nitric acid, blackened by ammonia. (c) When molybdate of ammonium is mixed with a small quantity of a phosphate or arsenate, excess of nitric acid added, and the whole heated, a yellow solution and a yellow precipitate are formed (p. 60).

Blowpipe reactions.- Molybdic anhydride, when heated on charcoal in the outer flame, is volatilised, and forms a white crystalline sublimate on the charcoal; in the inner flame, it may be reduced (even without carbonate of sodium) to metallic molybdenum, which is separated as a grey powder on levigating the charcoal. With borax all oxides of molybdenum give, in the outer flame, a bead which is yellow when hot, and colourless on cooling: in the inner flame, a dark-brown bead, which is opaque if excess of molybdenum be present. By long continued heat the molybdic oxide may be separated as dark-brown flakes, swimming in a clear yellow glass. With microcosmic salt, in the outer flame, all oxides

of molybdenum give a bead which is greenish when hot, and colourless on cooling in the inner flame, a clear green bead, from which molybdic oxide cannot be separated by continued heat.

Separation and estimation of Molybdenum.-For the extraction of molybdic acid from minerals, various processes are employed. Sulphide of molybdenum (molybdenite) is converted into molybdic anhydride by ignition in an open crucible, placed in a slanting position over the lamp, the anhydride is then dissolved out with ammonia. If the sulphide in lumps is heated in an open glass tube, through which a stream of air is drawn by an aspirator, molybdic anhydride sublimes in crystals in a state of perfect purity. (Wöhler. Ann. Ch. Pharm., c.) Molybdate of lead is finely powdered, and freed from carbonates of zinc, iron, and calcium by digestion with diiute hydrochloric acid, and thoroughly washed by decantation. It is then heated with 14 part concentrated sulphuric acid, with constant stirring, till it is perfectly white, and the sulphuric acid begins to evaporate. It is then allowed to cool, and a considerable quantity of water added, and the sulphate of lead which remains insoluble filtered off. The filtrate is mixed with nitric acid, and evaporated with constant stirring in a porcelain basin, till the sulphuric acid begins to evaporate, when the molybdic acid separates as a white powder: it is freed from phosphoric acid by washing with water, to which at the last a few drops of nitric acid have been added. Or the mineral, after treatment with dilute hydrochloric acid, may be evaporated to dryness with strong hydrochloric acid: the dry residue (containing basic chloride of lead and molybdic acid) is exhausted with ammonia, and the ammoniacal solution crystallised, or evaporated as before with nitric acid, and treated with water. Or the mineral is fused with its own weight of calcined tartrate of potassium, the fused mass exhausted with water, and the aqueous solution evaporated to dryness after saturation with nitric acid. The nitrate of potassium is dissolved out of the residue with water, and molybdic acid remains behind. For the estimation of molybdenum, the molybdic acid is dissolved in dilute ammonia, and the solution precipitated by subnitrate of mercury: the precipitate is thoroughly washed with a dilute solution of the subnitrate, dried, and ignited at a moderate heat in a stream of hydrogen, when it is reduced to brown binoxide of molybdenum, Mo2O2, and weighed in that form.

There is so close a resemblance between the reactions of molybdenum and wolfram compounds, that their distinction is attended with some difficulty. Molybdic acid is distinguished from tungstic acid by its fusibility and volatility, and by its solubility in excess of acid when precipitated by nitric or hydrochloric acid from an alkaline solution. For the separation of the two acids Rose gives the following method. Tartaric acid is added to the alkaline solution of the two acids, and the whole saturated with hydrochloric acid. The molybdenum is then precipitated as sulphide by repeated saturation with hydrosulphuric acid (a long and difficult process), the sulphide filtered off, and gently ignited in a stream of hydrogen till it no longer loses weight. It is then in the form of bisulphide, MoS2, and is weighed as such. The filtrate is evaporated to dryness and the residue ignited in the air: if the residue be at all black (owing

to the carbon of the tartaric acid not being entirely burnt) it must be fused with a little nitrate, and excess of carbonate, of potassium. The fused mass is dissolved in water, the tungstic acid precipitated by subnitrate of mercury, ignited, and weighed. The same process which effects the separation of antimony from arsenic and tin, serves also for its separation from wolfram and molybdenum, the alkaline tungstates and molybdates being soluble in water. From arsenic they are separated by converting the arsenic into arsenic acid, and precipitating it by a magnesiumsalt.

9. Vanadium. V. Atomic weight, 68.5.

Occurs as vanadate of lead in the brown lead-ore from Zimapan: in small quantities in many iron- and copper-ores, and in the slags from blastfurnaces. It is a white hard metal, that may be reduced to an iron-grey powder. It is not oxidised by exposure to moist air at the ordinary temperature, but acquires a reddish tint. Heated in the air it burns vividly, and is converted into black oxide, V2O2. It is not attacked by sulphuric, hydrochloric, or hydrofluoric acid: nitric acid and aqua-regia dissolve it readily, forming a light-blue solution.

OXIDES OF VANADIUM. Protoxide, V2O. Binoxide, V2O2. Vanadic Anhydride, V2O3. — Like wolfram and molybdenum, vanadium forms one or more oxides intermediate between V2O2 and V2O3, which are soluble in water with a blue, green, or orange-red colour.

a. Protoxide of Vanadium. Vanadous Oxide. V2O.-A black powder, obtained by heating vanadic anhydride in a stream of hydrogen. When heated in the air it is converted into binoxide. It does not combine either with acids or alkalis; but on digestion with nitric acid or potash it absorbs oxygen and is dissolved, in the former case giving a light blue solution, in the latter, a solution of vanadate of potassium.

b. Binoxide of Vanadium. Vanadic Oxide. Vanadous Anhydride. V2O2. — A black earthy powder, obtained by heating vanadium or the protoxide in contact with air. It is slowly but perfectly soluble in acids, forming vanadium-salts; their solutions are blue, and by exposure to the air absorb oxygen and become green. It is also soluble in fixed alkalis, forming vanadites. Vanadium-salts give the following reactions. Hydrosulphuric acid gives no precipitate in a neutral or acid solution.* Sulphide of ammonium gives a black-brown precipitate, soluble in excess to a dark purple-red solution, whence brown sulphide of vanadium is cipitated by a dilute acid. Ferrocyanide of potassium give a yellow precipitate, insoluble in acids, which turns green by exposure to the air. Gallic acid gives a black precipitate, which settles slowly, leaving a bluish solution. The fixed alkalis and their normal carbonates give a greyishwhite precipitate of hydrate, soluble in moderate excess, reprecipitated by a large excess as an alkaline vanadite. Ammonia gives a brown precipitate, somewhat soluble in water, insoluble in water containing ammonia.

pre

*Though vanadium, like wolfram, is not precipitated from an acid solution by hydrosulphuric acid, it is nevertheless included in this group, because its sulphide is soluble in sulphide of ammonium, forming a sulpho salt, and is precipitated from this solution by hydrochloric acid.

Only the alkaline vanadites are soluble in water. Their solution gives with sulphide of ammonium a fine purple-red colour, owing to the formation of a soluble sulpho-salt. By acids they are coloured blue, owing to the formation of a double salt of vanadium and the alkaline metal. Insoluble vanadites, when moistened with water, rapidly become green, being converted into vanadates.

c. Vanadic Anhydride, V2O3. — A red powder, obtained by the ignition of vanadate of ammonium. It is fusible, non-volatile, and does not lose oxygen at a white heat. It is slightly soluble in water: the solution, which contains vanadic acid, reddens litmus strongly. It dissolves both in acids and alkalis, in the former case forming vanadic salts, in the latter, vanadates. Its solution in acids is generally red or yellow it is very easily reduced by hydrosulphuric, sulphurous, or oxalic acid, sugar, alcohol, and many other organic compounds, the solution being coloured blue. Sulphide of ammonium precipitates brown sulphide, soluble in alkaline sulphides, or caustic alkalis, to a purple-red solution, whence light-brown sulphide of vanadium is precipitated by dilute acids. Ferrocyanide of potassium gives a green precipitate, insoluble in acids.

Vanadates are mostly soluble in water, all in nitric acid. Alkaline vanadates are difficultly soluble in water containing free alkali or an alkaline salt: thus vanadate of ammonium is insoluble in a saturated solution of chloride of ammonium. Aqueous solutions of vanadates are coloured red by stronger acids, but the colour frequently disappears after a time. Hydrosulphuric acid in neutral solutions gives a mixed precipitate of sulphur and bihydrate of vanadium: in acid solutions, it separates sulphur and gives a blue solution. Sulphide of ammonium behaves as with vanadic salts. Terchloride of antimony, lead-salts, protosalts of copper and of mercury, give orange-red precipitates. Vanadates of a fixed base are not decomposed by heat.

Blowpipe reactions.- When vanadic anhydride is heated on charcoal in the inner flame, it is reduced to protoxide. With borax and microcosmic-salt, all oxides of vanadium give in the outer flame a clear colourless bead, which is yellowish if a large amount of the vanadium compound be present: in the inner flame a green bead, which, if a large amount of the vanadium compound be present, is brown while hot, and becomes green on cooling. On heating the green bead in the outer flame, it becomes colourless or slightly yellow. A blue bead cannot be obtained before the blowpipe with an oxide of vanadium.

Separation and estimation of Vanadium. To extract vanadium from iron-ore, the powdered mineral is fused for an hour at a red heat with one third its weight of nitrate of potassium, the fused mass when cool boiled with water and filtered. The filtrate, which is yellow, contains vanadate, chromate, phosphate, nitrite, and silicate of potassium and aluminium: it is nearly neutralised with nitric acid, the precipitated silica and alumina filtered off, and the filtrate precipitated with excess of chloride of barium. The precipitate (of vanadate, chromate, and phosphate of barium) is washed, boiled while moist with dilute sulphuric acid, and filtered. The reddish-yellow acid filtrate is neutralised with ammonia, concentrated by evaporation, and a fragment of chloride of ammonium placed in it, when vanadate of ammonium gradually separates as a yellow