«Summary.»—We find, thus, that the general principle of the quantitative relations governing oxidation and reduction gives us the means of interpreting ‹the differences in results› in (qualitatively) similar combinations, which, qualitatively, might lead to an oxidation-reduction reaction, and which, in certain cases, do produce such reactions (ferric-ion with iodide-ion), and in other cases do not (ferric-ion with chloride-ion). FOOTNOTES: [514] The oxidation by chlorine may also be represented on the basis of the conception that the chlorine molecule contains a positive and a negative chlorine atom, Cl^{+}Cl^{−}. (‹Vide› W. A. Noyes, ‹J. Am. Chem. Soc.›, «23», 460 (1901); Stieglitz, ‹ibid.›, «23», 796 (1901); Walden, ‹Z. phys. Chem.›, «43», 385 (1903); J. J. Thomson, ‹Corpuscular Theory of Matter›, p. 130 (1907)). We may consider the action to take place as follows: 2 Fe^{2+} + Cl^{+} → 2 Fe^{3+} + Cl^{−}. [515] ε^{−} is used to indicate an electron. [516] The whole device is an adaptation of Ostwald's "Chemometer" [see ‹Z. phys. Chem.›, «15», 399 (1894)]. It has been found best to convert a Weston voltmeter into a lecture table apparatus by lengthening its index to 10 inches, with the aid of a very light, hollow aluminium wire carrying an index and playing over a scale 10 inches wide, drawn on glass and divided into 150 divisions. The scale is illuminated by means of five small one-candle-power lamps. The whole is encased in a simple wooden frame. The voltmeter shows a range of 0.7 volt, but, on account of its low resistance (78 ohms), it is used only for qualitative purposes and does not register the true potentials, quantitatively. (Such adaptations of Weston voltmeters may be purchased from the Weston Electrical Instrument Co., or a similar instrument obtained from Hartmann and Braun, Frankfurt a/M, Germany.) [517] Chemical Action at a Distance, Ostwald, ‹Z. phys. Chem.›, «9», 540 (1892). [518] Peters, ‹Z. phys. Chem.›, «26», 229 (1898). [519] See below, in regard to the ‹quantitative› relations for reactions of this nature. [520] EXP. Ferric sulphate solution is tested with a ferricyanide. [521] Rigorous quantitative examination of the relations shows (p. 275) that these reductions and oxidations ‹do take place›, but equilibrium is reached when they have proceeded to so slight an extent, that, qualitatively, they are not always obvious or discernible. [522] A change in the nature of the solvent changes the value of the equilibrium constant, just as it changes the ionization constant of electrolytes. See p. 61 and see remarks by Sackur, ‹Z. Elektrochem.›, «11», 387 (1905). [523] For exceedingly thin films of copper we cannot make this assumption, and for such films the conclusions, that follow, are, in fact, found not to hold. (Overbeck. ‹Vide› Le Blanc, ‹Electrochemistry›, p. 252 (1896)). [524] ‹Z. phys. Chem.›, «4», 129 (1889).