The concentration of the free bromine, ([Br_{2}]′ − ‹x›), under the new conditions of equilibrium, is smaller than the original concentration [Br_{2}]′—a result confirmed by experience. ‹It is in our power, therefore, arbitrarily to change the concentration of a reacting component›, in a case of equilibrium, ‹and thus to affect the reactivity of the system;› for instance, for brominating purposes, the new system would be less effective than the original one, and it might be of especial service where bromination is to be avoided. In the cases studied, are found the two fundamentally important relations expressed by the law of equilibrium: ‹the equilibrium constant is a measure of the stability of a certain system› and, in a way, of its ‹reactivity› at a given temperature; and the [p098] ‹concentration factors are variables›, which we may change to a very considerable extent, so as, to a certain degree, to subject the system to our own purposes. We shall repeatedly have occasion to refer to these two fundamental relations and we shall use them again and again in our analytical work. «Chemical Equilibrium of Electrolytes.»—Ionization of an electrolyte is a reversible chemical action and its relation to the law of chemical equilibrium will now be discussed. For acetic acid, ionization into hydrogen and acetate ions occurs thus: CH_{3}CO_{2}H ⇄ CH_{3}CO_{2}^{−} + H^{+}, and, in accordance with the law of equilibrium, at a given temperature, the following relation would hold: [H^{+}] × [CH_{3}CO_{2}^{−}] / [CH_{3}CO_{2}H] = K_{ionization}. If the total concentration of the acid is known, the concentrations of the ions and of the non-ionized acid may be calculated from the conductivity of the solution. For instance, if 60 grams of acetic acid (1 mole) is dissolved in sufficient water to make 10 liters, the equivalent conductivity of the solution (p. 50) is found to be 4.67 reciprocal ohms at 18°. The maximum conductivity of one mole of acetic acid, at infinite dilution, when all the acid would be ionized, would be 347. Therefore, in the acid under examination, 4.67 / 347, or 1.34 per cent, is ionized (p. 50). Since the total concentration of the acid is 0.1 mole ‹per liter› and 1.34 per cent is ionized, the concentration of the hydrogen-ion, [H^{+}], is 0.1 × 0.0134, and that of the acetate-ion, [CH_{3}CO_{2}^{−}], is the same. The concentration of the non-ionized acetic acid, [CH_{3}CO_{2}H], is 0.1 × 0.9866. If these values are inserted in the equation for the condition of equilibrium, we have (0.1 × 0.0134)^2 / (0.1 × 0.9866) = K_{ionization} = 18.2E−6.