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when members of Series 3 and 4 are compared it is found that the resemblance is most marked in the lower members of the series.

The unknown element to be placed in III.-4 would shew analogies with boron; therefore although it must be similar to aluminium, it probably would not form an alum. But the unknown element to be placed in III.- 5 must resemble aluminium more distinctly than the other unknown element does; therefore it probably would form an alum.

As calcium, which occupies in Group II. a position similar to that to be occupied by one of the unknown elements in Group III., is distinctly more positive than the first member of its own family (beryllium), but is very similar to the other members of its own family (strontium and barium), so probably would the unknown element to be placed in III.—4 closely resemble the succeeding even-series members of its group (yttrium, lanthanum, ytterbium).

The elements coming in Series 3, 5, and 7, are unlike each other in Group I., are similar but not very closely and intimately related in Group II., are very similar in Group V., and yet more similar in Groups VI. and VII.; therefore it was concluded that the unknown element in III.-5 would be distinctly similar to, but yet would shew differences from, both aluminium and indium.

Reasoning such as this guided Mendelejeff when he tabulated the properties of the elements scandium and gallium in Group III., and the element germanium in Group IV., while yet these elements were unknown.

CHAPTER XXVI.

THE ELEMENTS OF GROUP VIII.: AND RECAPITULATORY.

THE elements of Group VIII. are divided into three 483 sections. These elements are not placed in not placed in any of the ordinary series; but they find their places, one section, iron, nickel, and cobalt, between Series 4 and 5; another section, rhodium, ruthenium, and palladium, between Series 6 and 7; and the third section, iridium, osmium, and platinum, between Series 10 and 11.

It is probable that Group VIII. will some day be completed by the discovery of three elements to come between Series 8

and 9.

Copper, which is the first element of Series 5; silver, which is the first element of Series 7, and gold, which is the first element of Series 11, that is, the three elements which in order of atomic weights immediately succeed the respective sections of Group VIII., are sometimes placed in Group VIII.

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Section 1. Appearance, and general physical characters.

Occurrence, and
preparation.

General chemical
properties.

IRON.
Greyish-white; lus-
trous; crystalline; mal-
leable; ductile; fair
conductor of electricity;
hard: magnetic.
Iron obtained by elec-
trolysis of FeCl2Aq is
said to be silver-white
and very soft.

Found native but not in
large quantities; oxides,
sulphides, carbonates,
&c. occur in enormous
quantities, and very
widely distributed.
Prepared by reducing
Fe2O3 by Cat very high
temperatures; or by re-
ducing Fe2O3 or FeCl2
by H or by electrolysis
of FeCl2Aq.
Oxidised, chiefly to
Fe3O4, by strongly heat-
ing in oxygen.
Slowly oxidised
exposure to ordinary
moist air.

by

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General formulae and characters of compounds. Nickel and cobalt very closely resemble each other in their chemical properties. The salts of cobalt are generally pink when hydrated and blue when anhydrous; the hydrated salts of nickel are usually green, and the anhydrous salts, yellow. Cobaltic chloride Co,Cl, form a large series of compounds with ammonia, e.g.

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Co2C1.10NH,.2H2O; Co2Cl ̧.10NH ̧; CoCl ̧.12NH ̧. These compounds resemble the chromium-ammonia compounds; corresponding nickel compounds are not known.

The cyanides of iron and cobalt form compounds with potassium cyanide of the forms K,M(CN), and K,M(CN),, (M = Fe or Co); the acids of which these compounds are salts, viz. H,M(CN), and H.M(CN), have been obtained. Nickel cyanide does not form a corresponding salt; the compound K,Ni(CN), is known.

The only compound of the three metals the vapour density of which has been determined is Fe,Cl; the valency of the atom of iron cannot be decisively determined from the composition of this molecule; the atom is probably tetravalent.

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.

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Oxides. MO, MO,, M,O,: hydrates of all are known.
Sulphides. MS, MS,.

Haloid compounds. M,X, M.X..

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Salts. MX, M,3X; X = SO1, 2NO, PO, &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. H2O 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.

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

The oxides MO, are formed by heating the oxides MO in air; FeO is also obtained by adding an alkali to a hot mixture of ferrous and ferric sulphates (or other salts) in the ratio FeSO, Fe,(SO4)3 Ferroso-ferric oxide, Fe,O,, interacts with acids to form both ferrous and ferric salts; e.g.

Fe2O + 4H2SO,Aq = FeSO2Aq + Fe ̧(SO) ̧Aq + 4H2O.

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The corresponding oxides of nickel and cobalt form nickelous salts only, and evolve oxygen, or chlorine if hydrochloric acid is used.

3

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,.3H2O, so obtained. This oxide interacts with acids to form ferric salts.

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Nickelic oxide Ni,O,, and cobaltic oxide Co ̧ ̧, are obtained by oxidising solutions of nickel or cobalt salts in presence of an alkali; e.g. by passing chlorine into potash containing NiO.xH2O or CoO.xH2O 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.

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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.

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.

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Haloid compounds. The metals dissolve in hydrochloric acid to form solutions of the chlorides MCI,; crystals of the hydrated chlorides MCl,. xH O 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,Cl, is obtained by heating iron in a stream of chlorine; crystals of the hydrate Fe Cl. 12H,O 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.Cl, are very unstable and are easily decomposed to the chlorides MCl, and chlorine.

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,зSO. M2SO.24H2O, where M = K, Na, or NH.

Iron is distinctly related to manganese, the last element of Series 4. The relation is shewn in the composition of the oxides MO, M,O,, and M,O,, of the salts MX and M.3X, and in the existence of ferrates analogous to the manganates, K.MO1.

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

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