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),. M.SO.24H,O where M = Na, K, &c.; these salts are alums.

The uranic salts have not been fully investigated; the sulphate U(SO),, obtained by dissolving UO,.xH2O 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.

2

2

A fair number of uranyl salts have been prepared. The nitrate UO,(NO),.6H ̧O is obtained by dissolving UO.H2O in nitric acid and evaporating; various salts are obtained by treating the nitrate with different acids, e.g.

2

others are obtained by dissolving UO,H,O 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;

SO

M= X= Cl, Br, I, So, Cro,, NO, &c. X can be easily re

9 2

2

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

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,, but tungsten and uranium are characterised by the oxides MO, and MO,; similarly Cr2S, is the

420

highest sulphide of chromium known with certainty, but the important sulphides of the other elements are MS, and MS. Again the chlorides of chromium are CrCl, and CrCl, but the chlorides MC, MC, and even MC1, are characteristic of the other elements of the series.

The oxides MO, are all acidic; but the acidic character becomes less marked as the atomic weight of M increases. This decrease in acidic character is shewn by the production of such a salt as UO,(SO) from the oxide UO,, and by the fact that the most definite and stable molybdates, tungstates, and (probably) uranates, belong to the form XMO.MO(X = K,, Ba, &c.; M = Mo, W, U), whereas the most marked chromates are the normal salts XCrO. In other words, the combination of a relatively large quantity of the acidic oxides MO, with basic oxides seems to be necessary for the production of stable salts when MO, is MoO,, WO,, or UO,.

ვა

Thio-salts, usually of the composition MXS,(M=K„, Ba, &c. X= Cr, Mo, W), of all the elements except uranium are known.

The salts of molybdenum and tungsten have been so little examined that no generalisations regarding them can be made; salts of chromium and uranium are numerous, many of them are basic, and several form double salts chiefly by combining with salts of the alkali and alkaline earth metals.

The odd-series members of Group VI. are SULPHUR, SELENION, and TELLURIUM. The properties of these elements have been already considered (Chap. XI. pars. 170 to 179); it will suffice to summarise these properties here.

Sulphur and selenion are distinctly non-metallic in their chemical properties; tellurium inclines towards the metals but it is decidedly less metallic than chromium, molybdenum, tungsten, or uranium.

The existence of stable gaseous hydrides MH,; the distinctly acidic functions of the oxides MO, and MO1; the non-existence of salts produced by replacing the hydrogen of acids by M; the existence of strong acids HMO, and H.MO,, each giving a series of definite salts; these among other properties, mark the non-metallic character of the elements sulphur, selenion, and tellurium.

The negative character of sulphur and selenion is further marked by the fact that these elements exhibit allotropy; by their physical properties; by the possibility of forming oxy

3

chlorides directly from the acids H,MO, and H.MO; by the slightly acidic functions of the hydrides MH.

3

Tellurium does not exist in more than one form; the acids H.TeO, and H.TeO, are not produced by the direct interactions of the oxides TeO, and TeO, with water; the hydride TeH, shews no acidic properties; the anhydride Teo, combines with some acids (e.g. TeO,.2HCl is known); some of the physical properties of tellurium approximate to those of the metals.

The existence of the stable gasifiable tetrachloride TeCl1; the formation not only of ditellurates MTe,,, but also of ⚫tetratellurates MTe O, and of salts of the form MTe,O, (M = K,, Ba, &c.); the unreadiness to enter into chemical reaction with alkalis or alkaline carbonates of the oxide TeO,; these are some of the properties in which tellurium approaches the higher members (W and U) of the even series of Group VI.

2

OXYGEN is the first member of the even-series of the group now under consideration. Oxygen is a typical non-metallic or negative element both in its chemical and physical properties (8. Chap. VIII.). Nevertheless the properties of some of the compounds of this element suggest the properties of the other elements of the group in which oxygen occurs. Thus oxygen forms two compounds with hydrogen, water H2O and hydrogen peroxide H,O,, but neither is acidic; oxygen combines with the positive elements to form oxides the composition of which is frequently similar to that of the sulphides and selenides of the same elements, compare for instance the formulae MO and MS where M = Cu, Fe, Mg, Ni, Co, Mn, Ca, Ba, Sr, K., Na,, &c.; most of the oxides of positive elements are basic, some however as we have seen are acidic; most of the sulphides of positive elements interact with acids to form salts and hydrogen sulphide, some however interact with alkaline sulphides to form thio-salts.

2

2

The compounds of oxygen with chlorine CIO, and (?) Cl2O2 do not resemble the chlorides of the other members of Group VI. in composition, but the oxide OCl, is analogous in composition to MCI, when MS, Se, Te, Cr, Mo, or W. The compounds of oxygen with chlorine are very easily decomposed by heat and reagents generally; in this they resemble the compounds of sulphur and selenion with chlorine. The existence of the gaseous molecules O, and O2, Se, and Se, S, and S., emphasises the resemblance between oxygen, sulphur,

2

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421

422

and selenion. Oxygen forms a solid, stable, compound with iodine, O,I,; although the composition of this compound is not similar to that of the iodides of the other elements of Group VI., the fact of the existence of this stable compound suggests the existence of the stable iodides of tellurium, chromium, and tungsten. The existence of many definite and stable compounds of oxygen with non-metallic elements (oxides of As, B, C, I, N, P, Si) shews that oxygen is to some extent positive in its chemical properties: the compositions of these oxides are very frequently similar to those of the sulphides of the same elements when sulphides of these elements exist.

Group VI., then, is evidently divided into two well marked sub-groups or families; one of these families consists of the elements chromium, molybdenum, tungsten, and uranium; the other is formed of sulphur, selenion, and tellurium; oxygen, which is the first member of the group, to some extent summarises the properties of both families, but at the same time it differs from all the other members of the group. At the same time the elements of Group VI. taken as a whole more closely resemble one another than they resemble any other elements.

423 If we now turn to Group V. we shall find a group of ten elements shewing a gradation of properties from the first to the last member; we shall find that the group-character preponderates over the family-character, so that although the evenseries members are on the whole more like each other than they are like the odd-series members, yet it is not possible to divide Group V., as we have divided Group VI., into two distinct sub-groups or families.

liquid; s.G.885

The molecular weight of nitrogen is 28-02; the molecular weights of the other elements are unknown. Sp. grs. (approx.) '97 if air=1;

The melting points of most of these elements have not been determined.

In large quanti- In a few minerals, not widely distributed, chiefly as oxides and sulphides.

tasteless, odour- line, powder.

* There is some doubt whether the body known as didymium is or is not a mixture of two or more elements; many of the properties of compounds of didymium are probably the properties of mixtures.