[528] In other words, the greater the concentration of cupric-ion, the greater its osmotic pressure must be, and the repelling electric force, required to overcome the pressure of the cupric-ion, would be correspondingly greater. [529] ‹Cf.› Nernst, ‹Theoretical Chemistry› (1904), pp. 720–723, in regard to the derivation and the general form of his formula. [530] For elements that form ‹negative ions›, ‹e.g.› for chlorine, bromine, oxygen, etc., the ‹equation reads› (see pp. 273, 275 and the table at the end of Chapter XV): ε_{Elem., Electrolyte} = −(0.0575 / ‹v›) log(C / K). Note the ‹changed› sign of the expression on the right. The difference in sign expresses the fact that, when negative ions discharge on an electrode, they render it negative, and when they are formed by an electrode, they leave the latter positive; for positive ions, it will be recalled, the conditions are just the ‹reverse› (see above). Where a ‹soluble› element (‹e.g.› chlorine) or a solution of a metal (‹e.g.› sodium amalgam) is used as an electrode, its concentration, in general, is not constant, as in the case of a pure, solid metal like copper (p. 258). In such cases, the quantity in the denominator of the ratio in the logarithm cannot be expressed by a constant K, but is expressed by K × C_{Element}, C_{Element} being used to indicate the concentration of the element in the experiment in question. [531] The convention, adopted in the text, for the use of the positive and negative signs in expressing potentials, is that proposed by Luther (‹cf.› Le Blanc's ‹Lehrbuch der Elektrochemie› (third edition), p. 212). The ‹sign› always ‹denotes› the character of the ‹charge› on the ‹first component› written in the subscript to ε. Thus, for a copper plate in contact with a solution of cupric sulphate, when C > K, the logarithm, log(C / K), has a ‹positive› value and ε_{Cu, CuSO_{4}} is ‹positive›, which means that the ‹metal› will be ‹positive›, the electrolyte negative. For instance, for [Cu^{2+}] = 1, ε_{Cu, CuSO_{4}} is found to be +0.606 (see the table at the end of Chapter XV). ε_{Cu, CuSO_{4}} = −ε_{CuSO_{4}, Cu′}. By this use of the signs one is never in doubt as to their meaning. Unfortunately, widely different definitions of the signs have been used (‹cf.› Le Blanc, ‹Electrochemistry› (1896), pp. 209, 219, and Lehfeldt, ‹Electro-Chemistry› (1904), p. 159). Care must be taken, in using the data of original papers, to be informed as to the definition used.