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heating in air, and UO, by the action of air at ordinary temperatures. These oxides MO, are slightly soluble in acids"; VO, gives salts, e.g. U(SO),, but no definite salts have yet been certainly obtained corresponding to any of the other oxides MO,, although such salts seem to exist.

The trioxides MO, are anhydrides. Cro, is prepared by adding a sufficient quantity of concentrated sulphuric acid to a solution of potassium dichromate, Mo0, and WOg are obtained by oxidising the lower oxides, or better from ammonium molybdate and tungstate, respectively, by heating with nitric acid and then washing out the ammonium nitrate formed. UO, is obtained by heating uranyl nitrate

(UO)(NO3), Cro, is very soluble in water forming a markedly acid liquid; under special conditions the hydrate Crog.H,0–i.e. H. Cró, chromic acid-can be obtained from this liquid.

Cro, interacts with acids to form oxygen and salts corresponding with the oxide Cr,0,; thus

2Cr0,3 + 3H SO,Aq=Cr,(SO2),Aq + 3H,0 + 30. This oxide readily parts with part of its oxygen and therefore acts as an oxidiser, e.g. when it is heated, Cr, og are produced.

M00, is much less soluble in water than Croz; W0, is only very slightly soluble in water; and UO, is insoluble. Hydrates of these oxides exist and exhibit acidic properties (s. Acids, par. 416), but none of them is obtained by the direct addition of water to the oxide. The oxides Mob, and WO; form various complex compounds with several anhydrides such as SO,, P,0,, B,O, &c. The oxide UO, interacts with a few

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acids to form salts (s. Salts, par. 417).

The oxides MO and MO, are on the whole basic; the oxides MO, are acidic, but their acidic character is less marked as the atomic weight of M increases. The change from MO, to Mo, is effected the more easily and directly the greater the atomic weight of M. Of the oxides MO, UO, shews the most clearly marked basic character. Of the oxides MO, UO, is the most stable towards heat and reducing agents, and

Cro is the least stable. 414 The most important sulphides are Cr, Sz; MoS,, Mos.,

MOS: WS, WS,; and Us, Chromic sulphide, Cr, s, is prepared by passing sulphuretted hydrogen over hot chromic oxide (Cr,0); it is not obtainable by reactions between compounds

and oxygen

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in solution. This sulphide is feebly acidic; it combines with certain more basic sulphides, e.g. with ZnS, CaS, &c.

When excess of sulphuretted hydrogen is passed into the solution of an alkaline molybdate, e.g. K M00,, and an acid is then added, molybdenum trisulphide Mos, is precipitated. This sulphide is distinctly acidic; it interacts with K,S, &c. to form thio- (or sulpho-) salts, e.g. K,MOS,

When potassium thiomolybdate, K, MOS, is heated with MoS,, a salt having the composition K MOS, is formed ; this salt interacts with acids to produce molybdenum tetrasulphide Mosc Molybdenum disulphide, Mos,, is obtained by heating together M00, and sulphur.

Tungsten trisulphide WS, is obtained similarly to Mosz. The disulphide ws, is formed by heating together tungsten and sulphur. WS, is distinctly acidic, forming thiotungstates, e.g. K, WS, BaWS, c.

Uranium disulphide, US,, is formed similarly to WS,; it exhibits no acidic functions.

As Crs, has not not yet been prepared it is difficult to compare corresponding sulphides of the four elements; but on the whole it appears that the sulphides become more acidic as the atomic weight of the metals increases.

The haloid compounds of the four elements we considering are important. Their compositions are shewn by the following formulae; CrCl, Crci,, CrBr,, CrBr, Cri, Cre CrF,; Moci,, Moci,, MOCI," MOCI, MoBr,, MoBr,, MoBr"; wci, wci," WCI,, wc, WBr,, W Bry, WI,; UČI, UCI, UCI,, UBr., UF, The following have been gasified ; Moci, wci, wči, uc, UBr,; these formulae are therefore molecular. The formulae of the other compounds are the simplest that can be given, but they are not necessarily molecular.

Chromic chloride, CrCl, is obtained by heating an intimate mixture of chromic oxide and carbon in chlorine; chromous chloride, Crci, is formed by heating CrCl, in hydrogen. The higher chloride is stable in the air, but when strongly heated it gives Cr. Og solutions of this chloride when heated give precipitates of various oxychlorides Cr,O,Cl,; the most important oxychloride of chromium is Croch Chromic chlo

, ride forms either violet crystals by subliming in chlorine or hydrochloric acid gas, or a greenish solid by dissolving chromic hydrate in hydrochloric acid, evaporating nearly to dryness, and heating in chlorine. The violet form is almost insoluble

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in water; the green form readily dissolves in water. Chromous chloride, CrCl,, is very unstable; it removes chlorine readily from various chlorides, e.g. HgCl,, and absorbs oxygen rapidly from the air.

When molybdenum is strongly heated in chlorine the pentachloride, MoCl,, is formed. By heating this chloride in carbon dioxide the tetra- and di-chlorides, MoCl, and MoCl,, are obtained : the trichloride is also got from the pentachloride, by heating in hydrogen. Various oxychlorides of molybdenum are known; the more important are Moo,ci, and MoOci

Tungsten hexachloride, wcie, is produced by strongly heating tungsten in a stream of dry chlorine ; hot water decomposes it to Wo, and hydrochloric acid ; heated in air it yields WOCI, The penta- and di-chlorides, WCl, and WC1,, are obtained from Wci, by heating in hydrogen; the tetrachloride WCl, is produced by heating a mixture of WCl, and WC1, in hydrogen or carbon dioxide. The oxychlorides WO CI, and woCl, are known.

ühen uranium dioxide, UO, is mixed with carbon and heated in chlorine the tetrachloride UCI, is formed; this chloride is decomposed by hot water to UO, and hydrochloric acid; it is an energetic reducing agent, e.g. it reduces ferrie chloride to ferrous chloride. The pentachloride UCI, is formed by the direct addition of chlorine to UCI,; but when UCI, is heated to 230° in hydrogen or carbon dioxide it is again separated into the tetrachloride and chlorine. Only a few oxyhaloid compounds of uranium have been prepared ; the chief are UO,X, where X =Cl, Br, or F.

Acids and salts derived therefrom. Many of the hydroxides of chromium, molybdenum, tungsten, and uranium, are acidic.

The precipitate obtained by adding ammonia to a solution of a chromic salt varies in composition according to the conditions, but it is always a hydrate of the oxide Cr,0,(Cr,0.H,O).

.XH This compound is basic, as it interacts with acids to form sålts. By dissolving Cro, in a little cold water, warming, and again cooling, crystals of the hydrate Crog.H.O(=H.Cro) are said to be formed. This compound is distinctly acidic, from it is derived a well marked series of salts, the chromates, MCro, M = K,, Ba, &c.

Ammonia ppts. UO,«H,0 from solutions of the tetrachloride UCI,; this hydrate dissolves in acids to form salts, e.g. U(SO2),;

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it is therefore basic. The hydrate UO,.H2O( = H, UO) is obtained indirectly from UO,(NO3),; this hydrate interacts with some acids to form salts, e.g. UO,(80) &c. and it also interacts with alkalis to form salts of the form MUO. (M= Na,, Ba, &c.), it is therefore both basic and acidic.

Hydrates of the dioxide and sesquioxide of molybdenum M00,.H.O and MoO,..H,0 are known, but their interactions with acids have been little examined ; they appear however to possess only basic properties. Various hydrates of MoO, are prepared indirectly (that is not by addition of water to the oxide); the chief are M002.H,0 and M00g. 2H,0= H M00, and H M00,; these compounds are acidic; they may however also shew basic functions; their interactions with acids have not been much investigated.

Two hydrates of tungsten trioxide are known, WO .H O and WO,.2H,O=H, WO, and H.WO,; these compounds are acidic, and possibly also basic. Another compound HW.0,3.7H,0 has been obtained; it is acidic.

The chief salts derived from the acidic hydroxides of the elements under consideration, by replacing hydrogen by metals, are the chromates and dichromates, the molybdates, di- tri- tetra&c. molybdates, the di-tri- &c. tungstates, and the uranates.

The chromates MCro, where M = Ba, Ag,, K,, &c. &c. are prepared by double decomposition from potassium chromate which may be obtained by neutralising a solution of chromium trioxide with potash.' The alkaline chromates are also formed by fusing chromic oxide, Cr,0g, or a chromic salt, with potash and a little potassium nitrate or chlorate. When potassium chromate (K,

Cro) is treated with dilute sulphuric acid potassium dichromate, K, Cr,0,, is formed; from this salt a series of dichromates MCr,0, is obtained. Trichromates MCr, io, and tetrachromates MCr,O, are also known ; but the best marked salts are the chromates and dichromates.

The molybdates MM00, are obtained by double decomposition from the alkali salts; most of these salts form non-crystalline masses. Dimolybdates MM0,0,, trimolybdates MMO, O, and tetramolybdates MMO 0,, are obtained by boiling various

MM0,0 metallic carbonates with molybdenum trioxide under various conditions: these salts crystallise well and are more stable and definite than the molybdates.

A few tungstates MWO, are obtained by heating WOg with alkali or alkaline carbonates ; several series of condensed tungstates or polytungstates exist belonging to the

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forms MW,0, M,WO,

M.W.0172

and M.W.O., (M=K, Na,'Ba, Cà, &c.). * Many of these are obtained by boiling tungstates with tungsten trioxide.

A few uranates MUO, are known; they are produced by adding alkalis or alkaline earths to solutions of uranyl salts. The diuranates MU,0, are more definite and marked salts than the uranates. A few polyuranates are also known.

Salts. Chromium and uranium replace the hydrogen of acids forming well marked salts; salts of molybdenum and tungsten almost certainly exist, but they have been very little investigated.

The salts of chromium form two series, the chromous salts CrX, and the chromic salts Cr 3X, where X = SO, 2NO, CO3, 2010.,, PO,, sAso, &c. A few so called basic salts of chromium also exist, e.g. Cr,0,80, Cr,O(SO2),, Cr,O(NO3), &c.

The salts of uranium also form two series, the uranic salts UX,, and the uranyl salts UO,X, where X = SO, 2NO, ŠPO, &c. "The uranyl salts belong to the class of basic salts.

The chromous salts are very easily oxidised to chromic salts; chromous acetate Cr(C,H,O2). H,O is obtained by adding a concentrated solution of sodium acetate to chromous chloride solution produced by reducing chromic chloride by zinc and hydrochloric acid. Chromous sulphate Crso,.7H,O, chromous

CrSO0 oxalate Crce, and a few other salts may be obtained from the acetate.

The chroinic salts are well marked and stable compounds; among the more important are the sulphate Cr (SO2),.18H,0, and the phosphates Cr,(PO),.12H0,, Cr,(P0,378,0,"Cr,(POS) These salts are generally prepared by dissolving Cr.02.2H,0 in the various acids. Many chromic salts exist in two forms, one violet to red, the other green. In some cases, e.g.Cr,(SO2), both varieties are known in the solid form and have the same composition ; in other cases, e.g. Cr,(NO3). only a violet salt is known in crystals but a green

solution is obtainable from these crystals. Aqueous solutions of most of the violet salts become green when boiled; many of these solutions become violet again on cooling or on standing for some time. Only the violet solutions yield

crystalline salts; the green solutions give amorphous gummy solids on evaporation. Various hypotheses have been suggested to account for these changes of colour; that which seems to rest on the best experimental evidence asserts that the violet salts are the normal salts and

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