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The formulae for the compounds of the three metals, with
the exception of the haloid compounds, are the simplest by
which their compositions can be expressed.

Oxides. MO, M,04, M,0,: hydrates of all are known.
Sulphides. MS, MS.
Haloid compounds. M,X, MX.

MX4,
Salts. MX, M 3X; X = SO,, 2NO3, 3PO,, &c.

Oxides. · Ferrous oxide Feo is difficult to prepare free 486 from ferric oxide, as it combines very rapidly with oxygen. The hydrated oxide Feo.H,0 is obtained by pptg. ferrous sulphate dissolved in air-free water with potash in absence of oxygen.

Nickelous oxide Nio, and cobaltous oxide Coo, are obtained by ppg. solutions of the corresponding salts by alkalis and heating the ppts. out of contact with air. These oxides combine with oxygen when carefully heated in air, forming the oxides M.O, which at a higher temperature are decomposed to MO and oxygen,

The protoxides, MO, dissolve in acids forming salts MX.

The oxides M,0, are formed by heating the oxides MO in air; Fe, O, is also obtained by adding an alkali to a hot mixture of ferrous and ferric sulphates (or other salts) in the ratio FeSOn : Fe (SOA), Ferroso-ferric oxide, Fe,o,, interacts with

: acids to form both ferrous and ferric salts; e.g.

Fe, 0, +4H SO, Aq = FeSO, Aq + Fe (SO2)3Aq + 4H,0. The corresponding oxides of nickel and cobalt form nickelous salts only, and evolve oxygen, or chlorine if hydrochloric acid is used.

Ferric oxide Fe,o, is obtained by adding an alkali to a solution of a ferric salt, e.g. to Fe (SO.),Aq, and drying and heating the hydrated oxide, Fe,0,.31,0, so obtained. This oxide interacts with acids to form ferric salts.

Nickelic oxide Ni,Og, and cobaltic oxide Co,O.,, are obtained by oxidising solutions of nickel or cobalt salts in presence of an alkali; e.g. by passing chlorine into potash containing Ni0.wH, 6 or 000.XH, in suspension. These oxides dissolve in acids to form salts MX and evolve oxygen, or chlorine if hydrochloric acid is used ; they are decomposed to MO and oxygen when heated in air.

When ferric oxide is heated with potash and a little bromine, or when very finely divided iron is heated with potassium nitrate, and the product is poured into water, a reddish

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solution is obtained which is decomposed by addition of a little nitric acid giving a pp. of ferric hydrate and evolving oxygen. From the quantities of ferric hydrate and oxygen thus obtained, the existence, in the red solution, of a salt K, FeO, potassium ferrate, is inferred. The corresponding barium salt, BaFeO,, is said to have been obtained as a solid. Corresponding

nickelates or cobaltates are unknown. 487 Sulphides. The sulphides MS are obtained by adding

hydrogen sulphide or ammonium sulphide to aqueous solutions of salts of the three metals. None of these sulphides shews any acidic functions. When ferrous sulphide, Fes, is

heated with sulphur, iron disulphide, Fes, is obtained. 488 Haloid compounds. The metals dissolve in hydro

chloric acid to form solutions of the chlorides MCI,; crystals of the hydrated chlorides MCI,.&H 0 are formed on evaporation.

Ferrous chloride is easily oxidised to a basic ferric chloride by evaporating its solutions in air; nickelous and cobaltous chlorides are stable in air. Ferrous chloride has been gasified but the vapour density has not been finally determined ; the numbers obtained seem to point to the existence of gaseous molecules having the composition Fe, Cl, at moderate temperatures, and the composition FeCl, at higher temperatures.

Ferric chloride, Fe, Clo, is obtained by heating iron in a stream of chlorine; crystals of the hydrate Fe 01.12H, 0 are obtained by dissolving iron in aqua regia, or by passing chlorine into a solution of ferrous chloride, and evaporating. Nickelic and cobaltic chlorides, M Clo, are very unstable and

are easily decomposed to the chlorides MCI, and chlorine. 489 Salts. Iron forms two series of salts; the ferrous salts FeX,

and the ferric salts Fe,.3X. Nickel and cobalt form only one series of definite stable salts MX. Ferrous salts are very numerous; they are more or less easily oxidised to basic ferric salts. Several normal ferric salts exist, but the greater number are basic salts. Both series of salts form numerous double salts. Ferric sulphate forms alums, Fe 30,. M,80,.24H,O,

where M =K, Na, or NH4. 490 Iron is distinctly related to manganese, the last element of

Series 4. The relation is shewn in the composition of the oxides MO, M,0,, and M.O, of the salts MX and M.3X, and in the existence of ferrates analogous to the manganates, K,MO

Iron is distinctly metallic, but the formation of ferrates shews that it has negative tendencies.

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Nickel and cobalt are less like manganese than iron is; they are decidedly metallic in their chemical characters.

The three elements of Section 1 of Group VIII. form a link connecting the negative metals chromium and manganese, which are the highest members of Series 4, with the positive metal copper which forms the first member of Series 5. Section 2. RHODIUM RUTHENIUM.

PALLADIUM. Atomic weights 104 104:4

106.2 The molecular weights of these elements are unknown. Sp. grs. (approx.)

118
12.1

113 Atom, weights

8.8
8-6

9.4
spec. graus.
Sp. heats
*058
*061

-06 Melting points

abt. 2000° above m. p. of rhodium. abt. 1500°-1600° Occurrence, and These metals occur in small quantities in many platinum ores. They are preparation. usually separated in the form of ammonio-chlorides, 2NH4CI. ÁC14;

when these are strongly heated the metals are obtained. Appearance, and Greyish-white; very White; lustrous; less White; hard ; lustrous; general physical hard; much less ductile ductile and malleable ductile, and malleable; properties. than Pd; scarcely than Pd.

most fusible of the softened'in oxyhydro

metals of this section gen flame.

or of Section 3 of Group

VIII. General chemical Oxidises at a red-heat, Oxidised by heating Oxidised superficially properties. when in powder; also powdered metal in air. in ordinary air.

combines with Cl at Oxidised by heating Slowly dissolved by hot red-heat.

with KOH, or KNO3, HCIAT or H,804A9, Unacted on by any acid forming K,Ru04 which readily by HNO3Aq. when pure; when alloy- is soluble in water. ed with Pt, Pb, Cu, &c. Combines directly with dissolves in aqua regia. C1 when heated. Oxidised by fusion with Slowly dissolved by KNO3 or Ba02.

aqua regia.

Section 3.
IRIDIUM.
OSMIUM.

PLATINUM.
Atomic weights

192-5
193* (?)

194:3 The molecular weights of these elements are unknown. Sp. grs. (approx.)

21.1
21:4

21.2 Atom, weights

9.1
9.0

9.1
spec. graus.
Sp. heats
*0326
0311

*0324 Velting points

abt. 2500° infusible at full white-heat. abt. 2000° Occurrence, and These metals occur in small quantities associated (? alloyed) with each preparation. other and frequently with rhodium, ruthenium, and palladium. They

are usually separated as 2NH4C1.MC14, and are obtained by strongly

heating these compounds. Appearance, and White; lustrous; White with tinge of Silver-white; very lusgeneral physical brittle, but fairly mal- blue; hard; crystalline; trous; fairly hard; very properties. leable at red-heat. also a black, amor- malleable and ductile;

phous, powder.

expands by heat less

than any other metal. General chemical When finely divided Oxidises readily to Not oxidised by heating properties. oxidises slowly when 0.804, when heated in in air or oxygen.

heated in air, and dis- air in state of finely Oxidised by heating
solves in aqua regia; divided powder: in this with solid potash.
in compact form is in- state is also oxidised to Combines directly with
soluble in all acids. 0804 by nitric acid. Ci.
Oxidised by fusion with Combines directly with Dissolved by aqua
potash and potassium Cl.

regia.
nitrate.
('ombines directly with

Cl. * Exact value of atomic weight of osmium is doubtful; numbers vary from 193 to 199.

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General formulae and chemical characters of compounds of metals of Sections 2 and 3. The platinum metals—i.e. rhodium, ruthenium, palladium, iridium, osmium, and platinum-are characterised by their insolubility in most acids: the three metals placed in Section 3 are also characterised by their high specific gravities. Gold resembles the platinum metals both in being insoluble in most acids and in being very heavy. Gold and the platinum metals are often named the noble metals.

The double chlorides 2KC1. MCI, are characteristic of the platinum metals. The chlorides of these metals combine with ammonia and form several series of more or less complex ammonia-compounds. The higher oxides of these metals are acidic in their reactions with alkalis. Osmium is characterised by the easily gasified oxide OsO,; this is the only compound of the platinum metals the vapour density of which has been determined. The compounds of the platinum metals have not been fully investigated.

Oxides. All the metals form protoxides MO, and dioxides MO,. The protoxides are usually unstable. The dioxides usually dissolve in acids, but few definite salts have been isolated. The hydrated dioxides, which are not obtained by direct interactions between the oxides and water, generally dissolve in alkali solutions; in some cases salts of the form K. MO, or basic salts «K,O.YMO,, have been separated. Trioxides, MO,, are known where M = Rh, Ir, or Os : these oxides dissolve in alkali solutions. Tetroxides of ruthenium and osmium are known, MO,: they are solids with low melting points and boiling about 100°; they form salts by interacting with alkalis.

Chlorides. All the metals form dichlorides MCI,, and tetrachlorides MCI,. Trichlorides MClz, are known where M = Ru, Rh, or Ir. The di- and tetra-chlorides combine with alkali chlorides forming compounds of which 2KCI.MCI, and 2KCI.MCI, are representatives; these double compounds are usually known as chloro-platinites (rhodites, ruthenites, &c.), and chloro-platinates (rhodates, &c.), respectively.

Salts. Very few salts of the platinum metals have been isolated. They all seem to be unstable and ill-defined compounds. Most of the salts are either basic, e.g. Rh,Og. 31,05.4H,0; or double salts, e.g. Pt2NO,. 2AgNO,

The platinum metals are evidently possessed of most of the physical properties which we are accustomed to associate with

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the metallic elements; but chemically considered they are both metallic and non-metallic. They differ considerably from all the other elements.

The properties of these elements are in keeping with the position assigned them by the periodic law : each section follows a series of elements the lower members of which are metallic, and the higher members are physically metallic but chemically both negative and positive.

We have now learned something of the classification of 497 elements and compounds based on the periodic law.

The elements fall into series and groups : each series, with the exception of the first, is composed of seven elements; Series 1 contains a single element, hydrogen ; each group is divided into two families, those elements which occur in even series, and those which occur in odd series; a complete family consists of six elements. The nine elements, iron, nickel, cobalt, rhodium, ruthenium, palladium, iridium, osmium, and platinum, are placed to some extent apart from the others; the first section of this group (iron to cobalt) belongs both to Series 4 and Series 5, the second section (rhodium to palladium) belongs to Series 6 and 7, the third section (iridium to platinum) belongs to Series 10 and 11.

The properties of the elements in any series vary from that with the smallest atomic weight to that with the largest, so that the first and last members of the series are the most unlike one another. When the gradation of properties has been completed through a series, the properties, as it were, swing back to the starting point, and exhibit a similar gradation in the next series. The properties of the first members of Groups II. to VII., and of the first and second members of Group I., are to a certain extent typical of the properties of all the other members of these groups.

The properties of an element and its compounds are connected with the general properties of the group to which the element belongs and also with the gradation of properties in that group; they are likewise connected with the general properties of the series and with the gradation of properties in the series to which the element belongs. Inasmuch as the properties of each group and series are connected with the properties, and the gradation of properties, of the other groups and series, it follows that the relations of any element to other similar elements are to be elucidated only by studying the M. E. C.

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