2 480 Salts. Salts which have been definitely isolated belong to the forms MX and M2X, X = SO1, 2NO,, PO1, &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. 2SnO,.P,O,.xH2O; 2SnO,.As,O,. PbO, dissolves in concentrated acetic and phosphoric acids without evolving oxygen, but definite salts have not been obtained from these solutions; with HCIAq, PbO, forms PbCl, and evolves chlorine, with HSO Aq it forms PbSO, and evolves oxygen. 481 482 5 5 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 SnSO1, and the arsenate SnHAsО.îН ̧0; a few basic salts are known, e.g. 2SnO.CO, and 5SnO. 2P,O,.xH2O. 2 The plumbous salts form a stable and very definite series, e.g. PbSO, PbCO,, Pb2NO, Pb(AsО ̧), PbСO̟, PbSiF ̧. Many basic carbonates, e.g. 4PbO. 3CO,. H2O; зPыO. 2CO,.H2O, 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; this is very marked in the cases of tin, placed in 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 dis covered. 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. Appearance, and characters. Occurrence, and General chemical IRON. Found native but not in Forms compounds re- Decomposes steam at 485 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. 6 Co2C1.10NH,.2H2O; Co2Cl ̧.10NH; Co,Cl. 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. |