«II. Precipitation of Zinc Sulphide.»—Now, if a solution of zinc sulphate is prepared so as to contain 28.7 grams of ZnSO_{4}, 7 H_{2}O, per liter, which would make it of the same molar concentration as the ferrous sulphate solution, we may, in view of the fact that analogous salts ionize approximately to the same extent in solutions of the same concentration, consider the concentration, [Zn^{2+}], of zinc-ion to be practically the same as the concentration, [Fe^{2+}], of the ferrous-ion in the ferrous sulphate solution. We may put [Zn^{2+}] = [Fe^{2+}]. If the solution of zinc sulphate is saturated with hydrogen sulphide, under the same conditions as were used with the ferrous salt solution (‹exp.›), or if we add the zinc sulphate solution to the mixture of ferrous sulphate and hydrogen sulphide (‹exp.›), we immediately obtain heavy white precipitates of zinc sulphide. We would decide, therefore, on the basis of the principle of the solubility-product, that in this case [Zn^{2+}] × [S^{2−}] > K_{ZnS}. Since we have used hydrogen sulphide under practically the same conditions, we may consider that [S^{2−}], in this experiment,[407] is the same as in the [p205] test with ferrous sulphate, and, by the conditions of the experiment, we have also made [Zn^{2+}] = [Fe^{2+}]. The two factors of the product are, therefore, the same, for the first moment, and we may put [Fe^{2+}] × [S^{2−}] = [Zn^{2+}] × [S^{2−}] = P. Since P is smaller than K_{FeS} and larger than K_{ZnS}, it is clear that the ‹solubility-product constant for zinc sulphide must be smaller than that for ferrous sulphide›. The solubility-product constants, for similar salts, are a measure of their solubilities in water. We may obtain their values by determining the solubilities of salts in pure water, whenever the solubility is not affected by other chemical changes. In the present instance, the quantitative measurements, that have been made in this way, are open to question, owing to the considerable hydrolysis which sulphides, as salts of a very weak acid, undergo in solutions of such extreme dilution.[408] Until such relations have been taken into account quantitatively, it is better to limit ourselves for the present to the more accessible question of ‹relative› solubility.