300°. They all combine with ammonia, and also with various other haloid compounds, to form double compounds; e.g. ZnCl,. 2NH 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(CHO),, &c.

2

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 = HgCl2 + 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.

The salts of the metals we are considering are very 408 numerous; each metal forms salts of the form M2NO, 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. MgÖH ̧; 4ZnO.SO,; Cd2NO. CdOH; зHgO.SO2; 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,,

(X = Cl, &c. So, CO, NO, &c.);

2 2

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.

19

The molecular weights of these elements are unknown.

General formulae and chemical characters of compounds. 412 The compositions of the more important compounds of these four metals are expressed by the following formulae; but representatives of each formula are not known for all the elements, thus sesquioxides, M,O,, of tungsten or uranium have not been obtained, and the formula MX, is represented by WC, only.

2

6

Oxides. MO, M,O,, MO,, MO, hydrates of some of these are known.

Sulphides. MS, M,S,, MS,, MS,, MS,.

Haloid compounds; chiefly chlorides. MX, MX„ MX, MX,, MX,

Acids. HMO,, H,M,O,, &c., HMS.

Salts. MSO, M2NO, &c.; M.(SO), M,(NO), &c. when M= Cr. M(SO) M(NO), MO (SO), &c. when M = U. Salts of Mo and W are scarcely known.

The oxides MO are scarcely known; hydrates of CrO and 413 MOO are obtained by adding a solution of potash in air-free water to solutions of Cr.Cl, and Mo,Cl, respectively. These hydrates are rapidly oxidised in the air; neither yields corresponding salts by its reactions with acids.

3

2 3

The sesquioxides MO, are stable compounds when M = Cr or Mo (chromic and molybdic oxides); no sesquioxide of W or U is known. Cr2O, is prepared by precipitating a solution of a chromic salt by ammonia, washing, drying, and heating; in the case of Mo,O,, the hydrated oxide preciptated by potash is heated in hydrogen. Both oxides form dark coloured solids, insoluble or nearly insoluble in acids. Hydrated Cr2O (Cr 0.3H,O) dissolves readily in acids forming chromic salts, e.g. Cr,3S0. This oxide also seems to combine with a few basic oxides, e.g. with CaO. It is therefore basic but also slightly acidic.

2

The dioxides MO, are dark coloured solids, obtained by reducing the oxides MO, directly or indirectly. CrO, is more easily formed by passing nitric oxide into an aqueous solution of potassium dichromate (K,Cr,O,); MoO, and WO, by heating MOO, and WO, in hydrogen to low redness, or by digesting a solution of MoO,, or WO,, in hydrochloric acid with copper or zinc and then precipitating by ammonia. UO, may be prepared by digesting UCl, with water.

3

CrO, is decomposed by heat at 300° with evolution of oxygen and production of Cr,O,; the other oxides MO, are oxidised to MO, MOO, by heating with nitric acid, WỎ, by

2

2