[356] The displacement of hydrogen by a metal, like sodium, is the result of the displacement of the hydrogen-ion (see Chapters XIV and XV). The hydrogen-ion in fused sodium hydroxide is probably formed chiefly by the secondary ionization of the hydroxide-ion (HO− ⇄ H+ + O2−) (see Chap. XIII). We cannot have positive ions, Na+, with negative ions, O2−, without having some ions NaO−. (O2− + Na+ ⥂ NaO−), NaOH, undoubtedly, is much too weak an acid to form salts with bases in the presence of water. Such salts would be decomposed by water (see below, p. [180]), as sodium oxide, indeed, is decomposed; we have Na─O─Na + HOH ⇄ 2 NaOH (see Chapter XIII for a detailed discussion of this action). These relations sufficiently account for the fact that salts of sodium hydroxide, in which it has the functions of an acid, are not commonly formed. (Cf. Abegg, Anorganische Chemie, II, (1) p. 247.)
[357] See J. J. Thomson, Corpuscular Theory of Matter, pp. 103–141.
[358] See Mendeléeff, Principles of Chemistry, I, 22 (1891), in regard to the rôle of "even" and "uneven" series in the system.
[359] In regard to the indications of the amphoteric character of stronger acids, see Chapter XV.
[360] An elaborate treatment of this problem is given by Walker, Z. phys. Chem., 49, 82 (1904), 51, 706 (1905).
[361] Kohlrausch and Heydweiller, Z. phys. Chem., 14, 317 (1894).
[362] See the table, p. [104].
[363] See p. [53] and van 't Hoff's remarks, ibid.
[364] This suggests a much broader, natural definition of a base than the conventional one. All salts of very weak acids, to a certain degree, which is determined by the weakness of their acids, do exactly what the ordinary bases do, e.g. neutralize acids. Metal derivatives of acids weaker than water, metal amides, like Zn(NH2)2, metal alkyls, like zinc methyl, Zn(CH3)2, react more vigorously than the hydroxides do, e.g. in neutralizing acids, and water attacks them and acts upon them, exactly as ordinary acids interact with metal hydroxides. We have, for instance, Zn(CH3)2 + 2 HOH → Zn(OH)2 + 2 CH4.
[365] See footnote, p. [177]. Similar considerations apply to the conventional definition of an acid.