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an acid towards strong alkalis; it dissolves in concentrated KOHAq, and the salt potassium aurate KAO,.31.0 may be obtained by evaporating in vacuo.

The sulphides M S (M = Cu or Ag) are obtained by the 440 direct union of the elements, or in the case of Ag S by precipitating a solution of a silver salt by hydrogen sulphide. When hydrogen sulphide is passed into a solution of a salt of copper, cupric sulphide, Cus, is precipitated. The two sulphides of gold, AuS and Au,$, seem to be obtained by the interaction of hydrogen sulphide with solution of auric chloride, AuCl, under varying conditions.

The sulphides Cu, S, CuS, and Ag,S interact with acids to form salts and hydrogen sulphide, cupric salts being obtained from both sulphides of copper. Both sulphides of gold, Aus and Au, S,, dissolve in solutions of alkali sulphides to form thio-salts, the salts Na AuS and KAUS, having been obtained ; these sulphides are therefore acidic.

The haloid compounds M,X,, M=Cu, Ag, Au, are 441 obtained; in the case of copper and gold by reducing the compounds CuX,; in the case of silver by the interaction of haloid acids with silver oxide Ag, 0. Cu,CI, is formed by heating a solution of Cuci, in HCIAq with copper, or with sulphur dioxide, and pouring into water; Au,cl, is produced by dissolving gold in aqua regia, evaporating to dryness, and heating cautiously. By the interaction between Aucl, and hydriodic acid, aurous iodide, Au, Ig, is obtained

(2AuCl Aq + 6HIAJ = Au, I, + 40 + 6HCIAq). Cuprous iodide is formed by reducing CuSO, Aq by sulphur dioxide and adding potassium iodide solution.

The haloid compounds of silver, AgCl, AgBr, AgI, are very stable compounds; Cu,Cl, has been gasified; Cu I, is also a stable compound; Au ci, is separated into its elements by heat or by the action of reducing agents.

The only representatives of the class of haloid compounds MX, are the cupric compounds. Cupric chloride, Cuci,, is obtained from the corresponding oxide, Cuo, by dissolving in HClAq, and evaporating. This chloride combines with ammonia to form various compounds CuCl .NH, it also combines with alkali chlorides to form double salts, e.g. CuCl2KC2H,0. Various oxychlorides are also derived from CuCl,; their composition may be expressed by the general formula «CuCl,. yCuO.

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Gold is the only element of the family which forms haloid compounds of the form MX. Auric chloride Aucl, is best obtained by adding water to aurous chloride. Solutions of this compound are very easily decomposed by reducing agents, or by light, with separation of gold. When gold is dissolved in aqua regia and the solution is evaporated, crystals of the monobasic chloro-auric acid, HAuCl, are obtained; bromoauric acid, HAuBr, has also been isolated ; and from each a

few definite salts have been formed, e.g. KAuCl, andKAuBr,. 442 The salts of copper and silver are numerous and are very

definite compounds; only a few salts of gold have been isolated, and they seem easily to undergo change.

A few cuprous salts are known; one of the most definite is cuprous sulphite Cu Son, prepared by adding ammonium sulphite to a solution of copper sulphate, and then passing sulphur dioxide into the solution. The cupric salts, Cux, e.g.

. CuSO,, Cu2NO3, CuCO2, Cu2C103, Cu,(PO.),, &c. are usually obtained by dissolving moist copper oxide in the various acids. Many form double compounds especially with the salts of the alkali metals, e.g. Cuso,(NH), 50, 61,0. Several basic copper salts are known, e.g." 3Cu0.0, 4Cuo. As 0,,

. , 8Ču0.280. Many copper salts combine with ammonia, e.g.

, CuSO,.4N8, Cu(NO3)2NH, The salts of silver all belong to the form Ag,X, e.g. Ag, ŠO,, AgNO3, Ag,S,O.,, Ag,PO Ag, Coz. A few basic salts of silver are known chiefly derived from the weaker acids, e.g. 3Ag,0.200,; many silver salts combine with ammonia, e.g. AgNO3.3NHj, Ag_SO4.4NH;. Silver sulphate forms an alum Ag,so, A1, (SO ),. 24H,0. Only a few salts of gold have been prepared ; Au,8,0, is obtained combined with Na S.O, by mixing concentrated solutions of sodium thiosulphate and auric chloride and adding alcohol. Hydrated auric oxide AuOH,O dissolves in nitric acid, and from this solution the nitrates Au(NO3)2.HNO3.3H, O and AuO. NO, are obtained. Sulphuric acid interacts with Au0.NO, to form the sulphate Au0.H.SO,, and this on heating gives the sulphate Auso,

Many of the compounds and salts of gold are soluble in water; e.g. AuCl2, Au,o (?), Auso. The oxides and haloid compounds, and the greater number of the salts, of silver are insoluble in water. The oxides of copper are insoluble in water; the cupric haloid compounds are soluble, the cuprous haloid compounds are insoluble, in water; many cupric salts are soluble in water.

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There are prominent differences between the three ele- 443 ments, copper, silver, and gold, placed in the odd series of Group I. Gold is marked off from the others by the great instability of its salts, by the acidic character of its hydrated oxide Au,0, and its sulphide Au S, as well as by the existence of the compounds Au X, (X=0, Cl,, &c.) and the acids HAuCl, and HAuBr, which find no analogues among the compounds of copper or silver. Although gold possesses the physical characters of a metal in a very marked way, yet chemically considered it rather inclines to the non-metals.

The existence and solubility in water of the oxide Au,0, and the existence of aurous chloride AuCl (or Au,cl,) suggest a slight resemblance between gold and the alkali metals.

The similarities between silver and the alkali metals, although feeble, are more distinct than those between these metals and either copper or gold. The compositions of the silver compounds and of the compounds of the alkali metals are expressible by the formula M.X, where X=0, CI, SO,, 2NO3, &c. and M = Ag, Li, Na, K, Rb, or Cs. In the general formula for the aluminium alums, M SO. A1 380,.24H,O, M may be any alkali metal except lithium, or it may represent Ag. Moist silver oxide Ag,0 may often be used, especially in organic chemistry, in place of a solution of caustic potash or soda ; it reacts as an alkali. The insolubility in water of the haloid compounds, and the comparative insolubility of many salts, of silver, as well as the physical characters of the silver compounds, mark off these compounds from those of the alkali metals.

The cuprous compounds belong to the form M.X, which includes the compounds of the alkali metals and the silver compounds, but the marked compounds of copper are the cupric compounds CuX; almost the only silver compound of this form is the unstable peroxide Ago (or Ag, 0); the gold compounds belonging to this form are also very few in number (AuO, Auso.).

When the compounds of gold have been more fully examined it is probable that further resemblances will be found between the chemical characters of gold and those of the other elements of Group I.

HYDROGEN is placed in Group I, series 1. The chemical 444 characters of hydrogen are peculiar. It is at once a typical metal and a typical non-metal. (For an account of some of the properties of hydrogen, s. Chap. viii.) In the electrical series

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hydrogen stands midway between the metallic and the nonmetallic elements. It forms stable compounds with all the distinctly non-metallic elements (? with boron); it also forms compounds with some of the metals and metal-like elements (e.g. Cu H,, AsHZ, SbH). In the acids hydrogen plays the part of a metal, as it can be replaced from acids by metals with formation of compounds analogous in composition to the acids. In the hydrocarbons (CH, CH, C,H, &c. &c.) hydrogen acts as a non-metal, as it can be replaced by nonmetallic elements such as chlorine, bromine, &c. The physical properties of hydrogen are very different from those which characterise the metals as a class, yet it seems to form alloys with such metals as palladium and platinum.

To mark the peculiar chemical relationships of hydrogen this element is placed apart from the others in Series 1, of which series it is the only member.

In Group I, then, the family-character plainly predominates over the group-character: there is the family of the alkali metalslithium, sodium, potassium, rubidium, and caesium ; silver is more or less allied to this family; copper, silver, and gold form another family the members of which differ very considerably from each other; for some reasons silver and copper are placed in one family, and gold is separated from them ; hydrogen must be placed apart from all the other elements.

It must be remembered that Group I. includes three elements which have not yet been isolated.

In Chap. XVIII. (par. 392) it was remarked that the properties of an element are determined by considering (1) the properties of the group to which it belongs, (2) the properties of the series in which it finds a place, (3) the position of the element in the group and in the series, (4) the relations between the properties of elements situated similarly to the given element and the properties of the other members of the groups and series to which these elements belong, and (5) the relations between the group and series in which the given element occurs and other groups and series.

Let us now apply these statements to the elements copper, silver, and gold, the positions of which in Group. I. seem peculiar.

We have already considered the properties of the group to which these elements belong, and the position occupied by each element.

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What are the properties of the series in which each of the three elements finds a place, and what is the position of each element in the series?

Copper is the first member of Series 5; it is succeeded by the metallic elements zinc, gallium, and germanium, which are followed by the metal-like non-metal ar nic, and the nonmetallic elements selenion and bromine. Between series 4 and 5 are placed the distinctly metallic elements iron, nickel, and cobalt, forming a section of Group VIII. (s. Tables I. and II., pars. 390 and 394).

Silver is the first member of Series 7; it is succeeded by the metals cadmium, indium, and tin, which are followed by the metal-like non-metals antimony and tellurium, and the nonmetallic element iodine. Silver comes immediately after the metals rhodium, ruthenium, and palladium which form a section of Group VIII.

Gold is the first member of Series 11; it is succeeded by the metals mercury, thallium, lead, and bismuth ; the sixth and seventh members of this series are not yet isolated. Gold, like copper and silver, immediately succeeds a section of Group VIII., the section, namely, comprising the metals iridium, osmium, and platinum.

The positions of copper, silver, and gold are peculiar; no other known elements are similarly situated in the classificatory scheme based on the periodic law. The change from the last member of an even series to the first member of the next odd series seems to be always less sudden and abrupt than the change from the last member of an odd series to the first member of the succeeding even series. Group VIII. differs considerably from the other groups; each section of it seems to impress its character on the elements which come before and on those which succeed it. We cannot expect the relations of copper, silver, and gold, to lithium, potassium, rubidium, and caesium, to resemble the relations of zinc, cadmium, and mercury, to beryllium, calcium, strontium, and barium, except in a broad and general way. (s. also Chap. XXVI.)

If we consider the relations of group to group and series to series we shall find, speaking broadly, that in the lower groups the first member of the odd series is very like the second and succeeding members of the even series, but that this similarity becomes less and less marked as we pass to the higher groups; magnesium, for instance, closely resembles calcium, strontium, and barium, but sulphur differs considerM. E. c.

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