[183] Owing to the instability of carbonic acid, which breaks down into carbon dioxide and water (H_{2}CO_{3} ⇄ H_{2}O + CO_{2}), the carbonic acid is, in turn, in equilibrium with the carbon dioxide. For the sake of simplicity, this relation is not included in the detailed discussion, and wherever the symbols H_{2}CO_{3} and [H_{2}CO_{3}] are used, they are intended to represent ‹the total carbonic acid, present› as such and as carbon dioxide. The detailed discussion of the complication mentioned will be given in connection with the analogous case of ammonium hydroxide and ammonia, and, as will be shown there, ‹with the significance just attached to the symbols›, the relations developed in the text are a rigorous expression of the facts. [184] [H^{+}], in all the equations used, represents as usual, the total concentration of hydrogen-ion. [185] Walker and Cornack, ‹J. Chem. Soc.› (London), «77», 20 (1900). [186] McCoy, ‹Am. Chem. J.›, «29», 437 (1903); Stieglitz, ‹Carnegie Institution Publications›, No. «107», p. 245 (1909). [187] In a solution of phosphoric acid, all the possible forms of ionization, described in the text, occur simultaneously, but the secondary and tertiary forms of ionization, as far as the concentration of the hydrogen-ion is concerned, are entirely subordinate to the primary ionization. [188] ‹Vide› footnote, p. 79. [189] Luther, ‹Z. Elektrochem.›, «3», 296 (1907). Noyes and Eastman, ‹Carnegie Institution Publications›, «63», 274 (1907). Noyes and Stewart, ‹J. Am. Chem. Soc.›, «32», 1133 (1910). [190] The apparatus described on p. 77 is used. ‹Vide› Noyes and Blanchard, ‹loc. cit.› [191] Hydrogen ions move five to ten times as fast as the anions and carry 80–90 per cent of the current; see p. 56. [192] The solution of these salts is due to the action of hydrogen ions on their anions; see Chap. VIII. [193] Goodwin, ‹Z. phys. Chem.›, «21», 1 (1896). [194] See Kohlrausch and Holborn, ‹loc. cit.›, p. 194. Their values for K (last column) must be divided by 100 to express the constants in terms of the same units as those used in the above table. [195] J. H. Long, ‹J. Am. Chem. Soc.›, «18», 693 (1896). Ley, ‹Z. phys. Chem.›, «30», 322 (1899); Bruner, ‹ibid.›, «32», 133 (1900); Walker, ‹J. Chem. Soc.›, (London), «67», 585 (1895). [196] Instances are found in the laboratory experiments, Parts III and IV. [197] The ionization of salts of other types, ‹e.g.› MeX_{2}, Me_{2}Y, etc., likewise fails to conform to the law of chemical equilibrium. [198] See below (pp. 112–4), also, further, confirmatory evidence, derived from the application of the same law to the influence upon the degree of ionization of weak acids and weak bases, exerted by the presence of their own salts.