tungstates, by the absorption of varying quantities of the trioxide into the molecule of the normal salt (see Chromates, page 663). And in the case of tungsten, the compound metatungstic acid, HW4O13,7HO, is known.

Uranium dioxide and trioxide are both basic oxides, the former yielding the unstable uranous salts, such as uranous sulphate, U(SO4); and the latter producing the uranyl salts, of which the sulphate, (UO)SO4, and nitrate, (UO2)(NO3)2, are well known.

Uranium peroxide, UO4, is an acid oxide which yields per-uranates.

Both molybdic and tungstic acids form complex compounds with phosphoric acid, known as phospho-molybdic and phospho-tungstic acids: thus, when a nitric acid solution of ammonium molybdate (NH),MoO4, is added in excess to a solution of orthophosphoric acid or an orthophosphate, a canary yellow crystalline precipitate of ammonium phospho-molybdate, 2(NH3)3PO4, 22M0O3,12H2O, is obtained (see page 477). It is soluble in alkalies and in excess of phosphoric acid, but insoluble in dilute mineral acids. When this compound is dissolved in aqua-regia the solution deposits yellow crystals of phospho-molybdic acid, 2H,PO,,22M0O.

Compounds with chlorine having the following composition are known

CHAPTER XII

GROUP VII. (FAMILY A.)

Occurrence.

state.

MANGANESE.

Symbol, Mn. Atomic weight=55.0.

This element is never found in nature in the free

It is widely distributed in combination with oxygen, as pyrolusite, MnO2; braunite, MnO3; and hausmannite, Mn3O4. Also as a hydrated oxide in manganite, MnO3, H2O. It is met with also as carbonate in manganese spar, MnCO; and as sulphide in manganese blende, MnS.

Modes of Formation.-Manganese may be obtained by the reduction of the oxide by means of carbon at a very high temperature, as obtained in the electric furnace. The product, however, contains carbon. In a purer state it may be prepared by the reduction of fused anhydrous manganous chloride by means of metallic magnesium, or by reducing the oxide with aluminium at a high temperature.

Properties.-Manganese is a hard, steel-grey, brittle metal. It rapidly oxidises on exposure to moist air, and is readily dissolved by dilute sulphuric or hydrochloric acid, with evolution of hydrogen.

Oxides of Manganese.-The four most important of these

The monoxide and sesquioxide are basic, giving rise to manganous and manganic salts respectively. The oxide, Mn3O4, is also basic, but yields with acids both manganous and manganic

salts. Manganese dioxide or peroxide, MnO2, gives manganous salts with elimination of available oxygen. It also combines with certain more basic oxides, forming unstable compounds known as manganites.

Manganese trioxide, MnO3, and hept-oxide, Mn2O7, have also been obtained. They are both acid oxides, giving rise respectively to the manganates and permanganates.

Manganous Oxide, MnO, is obtained by heating any of the higher oxides in a stream of hydrogen, or by igniting a mixture of manganous chloride, sodium carbonate, and ammonium chloride. It is a light green powder, which, if prepared at a low temperature, oxidises in the air. When perfectly air-free solutions of potassium hydroxide and a manganous salt are mixed, with exclusion of air, hydrated manganous oxide, or manganous hydroxide, Mn(HO), is obtained as a white precipitate, which rapidly oxidises on exposure to air.

Red Manganese Oxide (mangano-manganic oxide), MnO4, is the most stable of the oxides of manganese, being formed when both the higher or lower oxides are strongly heated. Thus, in the preparation of oxygen by heating the dioxide, this compound remains (page 184). With cold sulphuric acid it yields a mixture of manganous and manganic sulphates, but when heated with dilute acid, manganous sulphate and dioxide are formed

Mn3O4+2H2SO,=2MnSO,+MnO2+2H2O.

Manganic Oxide (manganese sesquioxide), MnO3, occurs native as braunite, and in the hydrated condition as manganite, MnO3, H2O. The hydrated oxide is formed by the spontaneous oxidation of manganous hydroxide, and when gently heated it yields the oxide. Both the oxide and the hydrate, on treatment with warm nitric acid, yield manganous nitrate and manganese dioxide.

Manganese Dioxide, MnO2, is the most important of the manganese ores. It may be obtained by the cautious ignition of manganous nitrate

Mn(NO3)2=2NO2+ MnO2.

Manganese dioxide is a hard black solid which conducts electricity and is strongly electro-negative to metals. On this account it is employed in certain forms of voltaic battery. When heated it loses oxygen, and forms first the sesquioxide and finally MnO1

Manganese dioxide dissolves in cold concentrated hydrochloric acid, forming a dark-brown solution which is believed to contain the compound MnCl ̧. On warming it evolves chlorine, and leaves manganous chloride, MnCl2.

Manganites.-Manganese dioxide combines with certain metallic oxides, forming unstable compound oxides. Thus, with lime it forms CaO,MnO2; CaO,2MnO2, and CaO,5MnO. These compounds are produced in the Weldon recovery process (page 359).

MANGANOUS SALTS.

Manganous Chloride, MnCl2, is the only chloride of this metal that has been isolated. It is obtained by dissolving any of the oxides or the carbonate in hydrochloric acid, and on evaporation is deposited as pink crystals of MnCl2,4H,O. The anhydrous salt is prepared by heating the crystals in a stream of hydrochloric acid. Manganese chloride forms double salts with chlorides of the alkalies, the ammonium salt MnCl2,2NH4Cl, H2O being the best known.

Manganous Sulphate, MnSO4, is prepared by strongly heating a pasty mixture of the dioxide and strong sulphuric acid. The iron present is thereby converted into ferric oxide, and on treating the calcined mass with water manganous sulphate dissolves. The solution on evaporation deposits, at ordinary temperatures, large pink crystals of MnSO4,5H,O (isomorphous with copper sulphate). Below 6° rhombic crystals are formed (also pink) of the composition MnSO,,7H,O (isomorphous with ferrous sulphate).

When these salts are heated to 200°, or when their solutions are boiled, the anhydrous salt is formed. With sulphates of the alkalies, manganous sulphate forms double salts, as potassium manganous sulphate, K2SO4, MnSO4,6H,O; and with aluminium sulphate it yields a pseudo-alum (see page 620), MnSOAl(SO4)37 24H2O.

MANGANIC SALTS.

Manganic Chloride is obtained as a dark-brown solution when the dioxide is dissolved in cold hydrochloric acid. It has never been isolated, and is believed to have the composition MnCl

Manganic Sulphate, Mn,(SO4)3, is obtained as a green deliquescent powder by the action of sulphuric acid upon the pre

cipitated peroxide. On exposure to the air the deliquesced mass becomes muddy, by the precipitation of hydrated manganic oxide, thus

Mn(SO4)3+4H,O=3H,SO,+MnO3, H2O.

On the addition of potassium sulphate to a solution of manganic sulphate in dilute sulphuric acid, potassium manganese alum is obtained, K2SO4, Mn(SO4)3,24H,O, which deposits in violet regular octahedra. In the presence of much water the salt is decomposed, and deposits the hydrated manganic oxide.

MANGANATES.

These salts are derived from the hypothetical manganic acid, H2MnO4. The oxide corresponding to this acid is known, viz., MnO2. It is an unstable compound, obtained as a reddish amorphous mass, by adding a solution of potassium permanganate in sulphuric acid to dry sodium carbonate.

The manganates of the alkalies are obtained by fusing manganese dioxide with potassium or sodium hydroxide. If air be excluded the following reaction takes place

3MnO2+2KHO=K2MnO4 + Mn2O3+H2O.

In the presence of air or oxygen, or by the addition of potassium nitrate or chlorate, more of the manganese is converted into manganate. The fused mass has a dark-green colour, and dissolves in a small quantity of cold water to a deep green solution, which is only stable in the presence of free alkali.

When a solution of potassium manganate is largely diluted or gently warmed, it changes from green to pink, owing to the conversion of the manganate into permanganate, thus

3KMnO4+2H,O=2K MnO4 + MnO2+4KHO.

The same change takes place when carbon dioxide is passed through the solution.

PERMANGANATES.

These salts are derived from permanganic acid, HMnO4. When potassium permanganate is cautiously added to cold strong sulphuric acid, green oily drops of the unstable manganese heptoxide