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Ges dissolves in potash solution, probably forming a thio-germanate.

Haloid compounds. The tetrachlorides MCI, M = Si, 479 Ge, Sn, are obtained by heating the elements in a rapid stream of chlorine; Sici, is better prepared by heating a mixture of silica and carbon in chlorine; Geci, by heating a mixture of germaniumi and mercuric chloride (HgCl); SnCl, by passing chlorine into stannous chloride, Snci,. When chlorine is passed into a solution of PbCl, in HClAq, the gas is absorbed ; on heating this solution, chlorine is evolved ; addition of water to the solution ppts. lead dioxide PbO,; the solution probably contains PbCl, but this compound has not yet been certainly isolated.

The tetrachlorides, Sici, GeCl, and SnCl,, are liquids boiling at moderate temps.; Sici, boils at 57°, GeCl, at 86°, Sncl. at 115o. SiCl, is decomposed by water to silica (SiO,) and hydrochloric acid ; SnCl, dissolves in water, and on evaporation various hydrates, SnCl,. xH 0, are obtained. Sncl. combines with many chlorides and other compounds to form double compounds; e.g.

SnCl2SeOCI,; SnCI.PCI,; SnCI,.2PH ; SnCI.N.Oz.

The tetrafluorides MF, M=Si, Ge, Sn, are obtained by the interaction of hydrofluoric acid with the oxides MO.. Sif is gaseous, the others are solids. They all combine with potassium fluoride to form characteristic salts K MF.. When SiF, is passed into water, silicic acid and fluosilicic acid H SiF. are formed (3SiF4 + 3H2O + Aq=2H SiF Aq+ H, SiO2). Potassium fluosilicate, K Sił, is obtained by neutralising this acid by potash. Acids of the form H MF, when M = Ge or Sn are not known.

Silicon trichloride, Si Cla, is obtained by heating the tetrachloride with silicon : no corresponding haloid compounds of the other members of the family are known.

The dichlorides Mci,, where M = Ge, Sn, or Pb are formed when the metals are heated in hydrochloric acid gas; PbCl, is usually obtained by adding HCIAq to the solution of a lead salt, and crystallising from hot water. GeCl, is a liquid ; the others are solids. SnCl, is decomposed by much water giving oxychlorides, xSnO.ySncl.zH0. PbCl, dissolves in hot water without change; oxychlorides of lead are obtained by heating this solution with lead oxide, PbO. No fluoride of lead has yet been isolated.

480

Salts. Salts which have been definitely isolated belong to the forms MX and M2X, X = SO., 2NO, PO,, &c. Silicon oxide does not shew any basic functions. The other dioxides, MO,, dissolve in acids. From the solutions of Geo, in acids no salts have yet been obtained. Sno, forms stannic salts by interacting with acids; only a few of these salts have been obtained as definite solids, and most of these are basic salts, e.g. 28n0,.P,03.XH,O; 28nO,.As, Og. PbO, dissolves in concentrated acetic and phosphoric acids without evolving oxygen, but definite salts have not been obtained from these solutions; with HClAq, PbO, forms PbCl, and evolves chlorine, with H SO, Aq it forms PbSo, and evolves oxygen.

The monoxides SnO and PbO interact with acids to form salts, MX; salts of germanium GeX have not yet been isolated

The chief stannous salts which have been obtained as solids are the sulphate SnSox, and the arsenate SnHAsO,. «H,0; a few basic salts are known, e.g.

28nO.CO, and 5Sn0.2P.05.XH,O.

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e.g. PbSO,, PbCO,, Pb2NO, Pb,(AsOn), PbCO, PbSiFe Many basic carbonates, e.g.

4Pb0.300,.H,0; 3Pb0.200,H,O, are known; basic lead nitrates are also very numerous. Many lead salts form double salts by combination with salts of alkali and alkaline earth metals.

Carbon and silicon, which are, respectively, the first evenseries, and the first odd-series, member of Group IV., shew most marked similarities; both are to some extent separated by their distinctly non-metallic characters from the other members of the group. The other seven elements are fairly closely allied. The properties of several elements in Group IV. seem to be distinctly conditioned by the properties of the elements coming before and after them in their respective

Series 7 between indium and antimony, cerium, placed in Series 8 between lanthanum and didymium, and lead, placed in Series 11 between thallium and bismuth.

The history of the element germanium, in Group IV., and of the elements scandium and gallium in Group III., is peculiarly interesting. When Mendelejeff published his memoir on the periodic law, these elements had not been discovered. Mendelejeff predicted the properties of the three elements; he stated the atomic weight, spec. grav., general physical properties, and the formulae and chemical characters of the chief compounds, of each element. The descriptions given by Mendelejeff of the elements in question, several years before these elements were discovered, might almost be adopted now as descriptions of germanium, scandium, and gallium, so exactly in nearly every particular have they been realised.

There were two gaps in Group III., in Series 4 and 5, respectively. The differences between the values of the atomic weights of the elements in Series 2 and 4, in the various groups beginning with Group I., and of course omitting Group III., are 32, 31, 36, 37, 36, 36.5; hence, it was argued, in Group III. the difference will probably be about 33. The differences between the values of the atomic weights of the elements in Series 3 and 4 are 16, 16, 20, 20, 20, 19.5; hence, in Group III. the difference will probably be about 17. Boron, 11, occupies the position III.-1; now 11 + 33 = 44. Aluminium, 27, occupies the position III.—3; now 27 + 17 = 44. Therefore, it was concluded that the atomic weight of the element which is to occupy the position III.—4 would be about 44. Similar reasoning led to the value 69 for the atomic weight of the element in III.-—5.

The elements in Group III., when Mendelejeff's prediction was made, shewed a gradation of properties from the non-metallic boron to the distinctly metallic thallium ; boron was succeeded by the metal aluminium ; the elements of the group did not fall very distinctly into two families. One of the unknown elements would find a place in Series 4 succeeding the positive metals potassium and calcium, and followed by the elements titanium, vanadium, chromium, and manganese, all of which are metals but several shew decidedly negative functions: the other unknown element would find a place in Series 5, following the decidedly metallic elements copper and zinc, and followed by the metal-like non-metal arsenic, which is again followed by the non-metals selenion and bromine. The relations of the unknown element in III.— 4 to aluminium should, it was argued, be fairly similar to those of titanium to silicon, or of vanadium to phosphorus; the unknown element would probably less closely resemble aluminium than calcium resembles magnesium, or potassium resembles sodium ; but it would more closely resemble aluminium than vanadium resembles phosphorus, or chromium resembles sulphur; because 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 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.

(Section 1. Fe= 55.9 Ni= 5866 Co= 59 Group VIII. {Section 2. Rh=104 Ru=1044 Pd=106.2

484 (Section 3. Ir=192.5 Os=193 (?) Pt=1943 Section 1.

COBALT. Atomic weights

586

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

8.6 Atom. weights

6.8 spec. gravs. Sp. heats

•108

•107 Melting points 1500°—1600

1400°–1500

1400-1500° (approx.)

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