In a similar way, the conductivity of every solution of an electrolyte may be shown to represent the sum of the mobilities of the ions carrying the current (‹principle of Kohlrausch›). The limit of the conductivity of one equivalent of an electrolyte is the sum of the mobilities of the ions composing the electrolyte. The frictional forces being constant for infinitely dilute solutions, at a given temperature, an ion will always show the same mobility, irrespective of the nature of the ion of opposite charge, with which it forms the electrolyte. We may then put Λ_{∞} = (‹l›^{+}_{∞} + ‹l›^{−}_{∞}), if ‹l›^{+}_{∞} and ‹l›^{−}_{∞} are used to designate the limits of the mobilities of gram-equivalents of the positive and negative ions forming the electrolyte. The following table[90] gives the limits of the mobilities for gram equivalents of some of the most important ions at 18°. ‹Limits of Mobilities of Common Ions at› 18°. K: 65.3 ½ Ca: 53.0 I: 66.7 Na: 44.4 H: 318.0 NO_{3}: 60.8 (NH_{4}): 64.2 OH: 174.0 C_{2}H_{3}O_{2}: 33.7 Ag: 55.7 Cl: 65.9 ½ SO_{4}: 69.7 For quite dilute solutions, in which the friction may be assumed to be approximately constant, the conductivity will depend, not only on the mobilities of the ions, which may be taken to be the same as for solutions of extreme dilution, but also ‹on the proportion of electrolyte that is ionized›, ‹i.e.› on the degree of ionization, α. Then Λ_{‹v›} = α (‹l›^{+}_{∞} + ‹l›^{−}_{∞}), which is an elaboration of the original equation given on page 50. Now, Kohlrausch discovered the principle of the summation of the mobilities of ions a number of years before the theory of Arrhenius was advanced, and the proportion in which the ion is present in a given solution being unknown, the effect of what is here known as the degree of ionization was included empirically in the value of the mobility. It is not surprising, then, that an ion was found to have approximately the same mobility ‹only› in solutions of the same concentration ‹of strictly analogous and closely related salts›, which, according to present methods of investigation, are ‹now› found to have approximately the same degree of ionization. For instance, the mobility of the gram-equivalent of the chloride-ion was found to be approximately the same, 47.3 and 50.5 respectively, in molar solutions of sodium and potassium chloride at 18°, no account being taken of the degrees of ionization. However, the degrees of ionization of the two salts are approximately the same, 66.9% and 74.9% respectively, and might be ignored in a comparison of the conductivities, without affecting the result of the comparison in any marked way. [p057]