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The sulphides, MS, are solids. MgS resembles the sulphides of Ca, Sr, and Ba, it is an unstable compound which interacts with water to form MgS,H, and MgO,H,; MgSH is quickly changed to MgÒ ̧Î ̧ ́and H ̧S. The compounds MgS and MgS, have also been prepared. The other sulphides, MS, where M = Zn, Cd, or Hg, are more definite and stable compounds. They are produced by passing H.S into solutions of the salts of the metals; ZnS is soluble in and decomposed by most acids, the two others are insoluble in dilute acids. CdS and HgS are also obtained by directly combining the metals with sulphur, the reaction proceeds slowly and to a limited extent with Cd.

Pentasulphides of zinc and cadmium, MS,, seem to exist; they are unstable compounds. Mercuric sulphide exists in two forms; black and amorphous, by precipitating solutions of mercuric salts by H,S; red and crystalline, by directly combining Hg and S, or by subliming the amorphous form.

When HgS is precipitated from solutions of mercuric salts by excess of ammonium sulphide, and potash is added, the HgS dissolves and the solutions probably contain a compound of KS and HgS; the compound K,S. 2HgS is said to have been obtained. None of the other sulphides MS exhibit any tendency to form compounds with the sulphides of the strongly positive metals. HgS also forms many double compounds, chiefly with mercuric salts; e.g. 2HgS. HgCl,; HgS. HgI ̧; HgS.2HgSO,. The sulphides of Zn and Cd form a few oxysulphides, e.g. ZnO.ZnS.

Mercurous sulphide, Hg,S, if it exists is extremely easily decomposed to HgS and Hg.

407 The haloid compounds, MX,, are obtained by the direct combination of metal with halogen, or by dissolving the metals, oxides, or carbonates in aqueous solutions of the haloid acids HX. They are white solids, generally soluble in water and alcohol; the solubility decreases as the atomic weights of the metals increase. They all form oxyhaloid compounds of the general form xMCl,.yMO; a great many mercury oxychlorides have been obtained. MgCl, cannot be obtained by evaporating a solution in HClAq as when most of the water has been removed this solution is decomposed to MgO and HCl.

The haloid compounds all melt at temperatures below 1000°; e.g. MgCl, melts at about 700°, ZnCl, at approximately 500-600°, CdCl, at about 550°, and HgCl, at about

300°. They all combine with ammonia, and also with various other haloid compounds, to form double compounds; e.g. ZnCl2NH C1; 2CdCl,. SrCl,; 3HgCl.. MgCl. The haloid mercury compounds form a very great number of such double compounds; they also combine with various salts, e.g. with K.Cr.O,, Cu(CH ̧O2),, &c.

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The mercurous haloid compounds, HgCl, HgI, and HgBr, are obtained by heating the corresponding mercuric compounds with mercury; they are nearly insoluble in water, and are partially decomposed by heat into mercury and the corresponding mercuric compounds, e.g. 2HgCl = HgCl + Hg.

The following haloid compounds have been gasified: ZnCl2, CdBr,, HgCl,, HgI,; these formulae represent the compositions of molecules of the compounds, hence the atoms of Zn, Cd, and Hg are divalent in these molecules. It is probable, but not quite certain, that the formula HgCl represents the gaseous molecule of mercurous chloride; if this formula is molecular the atom of Hg is monovalent as well as divalent.

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The salts of the metals we are considering are very 408 numerous; each metal forms salts of the form M2ÑO,, MSO,, MCO,, &c. and besides these Hg forms a series of mercurous salts, M.SO,, MNO,, &c. Many of the salts are isomorphous ; some salts of mercury are isomorphous with corresponding salts of copper. The metals all form basic salts, e.g. 4MgCO. MgO,H.; 4ZnO. SO,; Cd2NO. CdOН; зHgO.SO,; 3HgO.Ñ,0,; a very great many basic mercury salts are known; Mg seems to form fewer of these salts than any other of the four metals. Double salts of all these metals are numerous, especially in the case of mercury. The salts of the four metals, as a class, are stable and well defined; those of Hg, on the whole, are less soluble in water than the others. The mercurous salts are considerably less stable than the mercuric salts; they are easily changed into the latter. Mercury salts form a very large number of compounds with ammonia; the composition of many of these is complex; in this respect mercury resembles copper, gold, platinum, and chromium.

The elements placed in Group II. are evidently closely 409 related in their chemical properties. The four elements Ca, Sr, Ba, and Mg, more nearly resemble one another than they resemble any of the other members of the group. This resemblance is well shewn in the alkalinity of their

oxides and hydroxides, in the formation of hydroxides by the direct union of water with the oxides, in the great stability of their oxides towards reducing agents, in the stability of their salts and the existence of but few basic salts, in the preparation and properties of their sulphides and hydrosulphides, &c. In its physical character however Mg differs from Ca, Sr, and Ba, and resembles Be, and to some extent Zn and Cd. Beryllium, although occurring in Series 2, is decidedly more like the odd-series, than the evenseries, members of the group; the properties of the oxide and hydroxide of Be are very similar to those of the corresponding compounds of Zn and Cd; in the comparatively large number of basic salts which it forms, and in the readiness with which oxyhaloid compounds of Be are produced, the metal also resembles the odd-series members, especially Hg. Some of the haloid compounds of Be, Zn, Cd, and Hg, are gasifiable without decomposition at workable temperatures; no compounds of the other elements of the group have been gasified.

Mercury is distinguished from the other members of the group by the fact that it forms two series of compounds, mercurous HgX, and mercuric HgX,,

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also by its physical properties, &c.

Looked at broadly, Ca, Sr, and Ba, are the most positive members of the group, next to them comes Mg, and Hg is the least positive. It is to be noted that the arrangement of Group II. in accordance with the periodic law shews a gap in Series 9; if an element is discovered to fill this gap it will probably resemble mercury on the one hand and zinc and cadmium on the other.

Beryllium to some extent summarises in itself the properties of the other members of the group.

It cannot be said that the even-series members of Group II. form a sub-group or family distinctly marked off from the odd-series members of the group; nor can it be asserted that there is a gradual change of properties from the first to the last member of the group. All the members shew distinct relations to each other; neither the family-character nor the group-character preponderates.

If we now turn to Group VI. we shall find that the 410 first member of the group, oxygen, to some extent summarises in itself, or is typical of, the properties of all the other members; and that the other elements placed in this group fall into two well marked families, one of which contains the even-series members and the other contains the odd-series members of the group.

M. E. C.

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Atom. weights spec. gravs. Occurrence and preparation.

Colour, appearance, &c.

General chemical properties.

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•10 (?) 7.7

The molecular weights of these elements are unknown.

Chrome-ironstone, (FeOCr2O3) and lead chromate, &c. occur in a few rocks; not widely distributed. Obtained by deoxidising Cr2O3 by C, or by action of K on CraCl, or by electrolysis of Cr2C14 containing Cr2Cl6. Very hard, brittle, powder composed of minute brilliant tinwhite crystals. Descriptions differ much; probably the metal has not yet been obtained approximately pure. Burns in stream of O; heated in air is superficially oxidised; oxidised by molten KNO3 or KClO3. Easily dissolved by dilute HCIAq or H2SO4Aq, but does not react with hot conc. HNO3. Combines readily with Cl and I when heated. Decomposes steam slowly at a red-heat. Replaces H of acids forming two series of salts. CrO3 is an anhydride; Cr2O3 seems to form a few salts by heating with basic oxides. Atom of Cr is perhaps hexavalent in gaseous molecules.

Infusible at full white-heat.

*072 (?)

11.3

Occurs in small quantities as oxide and sulphide, also as lead or cobalt molybdate.

Obtained by reducing oxide or chloride by H, or the oxide by C or KCN.

Ashen-grey powder; when compressed, is a silver-white, lustrous, hard, brittle, infusible, metal.

Not oxidised in air at ordinary temperature but burns at low red-heat. No reaction with HCIAq, HFAq, or H2SO4Aq; oxidised by conc. HNO3 to MoO3; oxidised by molten KOH, but no reaction with KOHAq. Combines with Cl and Br, but not directly with I. Salts formed by replacing H of acids by Mo are scarcely known.

MoOg is an anhydride, and also combines with many more negative anhydrides, e.g. P205. Atom of Mo is pentavalent in gaseous molecules.

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