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

Tungsten trisulphide ws, is obtained similarly to Mos. The disulphide WS, is formed by heating together tungsten and sulphur. WS, is distinctly acidic, forming thiotungstates, e.g. KWS, 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 are 415 considering are important. Their compositions are shewn by the following formulae; CrCl, CrCl, CrBr,, CrBr, Crin, Crl, CrFz; Moči,, MoCl,, MoCi, MoCl., MoBr,, MoBrz, MoBr, ; wci, wci, wci, wci, WBr,, W Bry, WI,; Uől,, uci, UC1,' UBr,'UF, "The following have been gasified" Moch, wci, wỜIUCI, UBrı; 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, CrCl, is formed by heating CrCl, in hydrogen. The higher chloride is stable in the air, but when strongly heated it gives Cros; solutions of this chloride when heated give precipitates of various oxychlorides Cr,0,,cl,; the most important oxychloride of chromium is Croch Chromic chloride 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

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, Moci,, 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 M00,ci, and MoOCI,

Tungsten hexachloride, wci, 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 wci,, are obtained from Wci, by heating in hydrogen; the tetrachloride WCI, is produced by heating a mixture of wci, and WCl, in hydrogen or carbon dioxide. The oxychlorides WO, CI, and Wocl, are known.

When 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 ferric 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). This compound is basic, as it interacts with acids to form salts. By dissolving Cro, in a little cold water, warming, and again cooling, crystals of the hydrate Cro. .H.O(=H.Cro) are sai 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,.H,O( = 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,0 and Mo,0,.«H,O are known, but their interactions with acids have been little examined ; they appear however to possess only basic properties. Various hydrates of Mo0, are prepared indirectly (that is not by addition of water to the oxide); the chief are M00,H,O and M00g. 2H,O=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 WOg. 2H,0= H, WO, and H. W0s; these compounds are acidic, and possibly also basic. Another compound H,W2073.74,0 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,Oz, 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,O,, is formed; from this salt a series of dichromates MCr.), is obtained. Trichromates MCr, 0, 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,00 and tetramolybdates MM0,013, are obtained by boiling various 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 wo, with alkali or alkaline carbonates; several series of condensed tungstates or polytungstates exist belonging to the

13'

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forms MW,O, M,W,0, MWOX, MW,0,7, M,W,024, and MW,,,, (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,O, 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, Aso, &c. A few so called basic salts of chromium also exist, e.g. Cr,O,SO,, Cr,O(SO2), Cr,O(NO3)2, &c.

The salts of uranium also form two series, the uranic salts UX,, and the uranyl salts UO,X, where X = SO,, 2N03, PO, &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,O,),.H,0 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 CrSO4.7H,0, chromous oxalate CrCon, 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,(80), 18H,O, and the phosphates Cr,(PO ),.12H0,, Cr,(P,02)278,0, Cr,(PO3) These salts are generally prepared by dissolving Cr,

0.1,0 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)2, 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

that they are partially decomposed on boiling in aqueous solution into green basic salts and a little free acid.

Chromic sulphate forms double salts with the alkali sulphates of the composition Cr,(SO )3. M SO.. 24H,0 where M = Na, K, &c.; these salts are alums.

The uranic salts have not been fully investigated ; the sulphate U(SO2), obtained by dissolving UO,. «H,0 in sulphuric acid and evaporating in contact with excess of acid, is one of the most important. These salts are generally unstable and easily changed to basic salts.

A fair number of uranyl salts have been prepared. The nitrate UO, (NO2),.6H 0 is obtained by dissolving UO..HO in nitric acid and evaporating ; various salts are obtained by treating the nitrate with different acids, e.g.

UO SO, UO,Se0g. 2H,0; others are obtained by dissolving UO,H,0 in acids, or by double decomposition from the nitrate, e.g.

UO HPO, (UO), (ASO),.12H,0. Many of these compounds form double salts especially with salts of the alkali and alkaline earth metals.

A very large number of compounds of chromium with 418 chlorine, bromine, SO, NO, &c. and ammonia are known. These compounds are of complex compositions; most of them belong to one or other of the six following general forms ;

Cro M = X = Cl, Br, I,

&.

2 ? 2
placed by other negative radicles, M can be replaced only
with difficulty.

M,.Cr,. 8NH,.X, e.g. Cl. Cr, 8NH, CI, 2HO.
M.Cr.10NH,.X, e.g. Br,. Cr,.10NH,. (Cros),

Cr. 10NH,.Xe.g. Cr. 10NH (NO)
M. Cr. 10NH .X, e.g. (NO),. Cr,.10NH. Br

Cr.12NH .X e.g. Cr.12NH (CO)
M.Cr. 10NH,.X, e.g. OH.Cr. 10NH.I.

The four even-series elements of Group VI. are, then, 419 evidently very similar in their chemical properties. The compositions of the compounds of chromium differ to some extent from the compositions of the corresponding compounds of the other elements; thus the most marked oxide of chromium is Cr,o,g, but tungsten and uranium are characterised by the oxides MO, and MO, ; similarly Cr,S, is the

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