By the conditions of the experiment we started with a concentration of the cadmium-ion, [Cd^{2+}], equal to the concentration, [Zn^{2+}], of the zinc-ion in the zinc sulphate solution, and with the same concentration,[415] [S^{2−}] / ‹x›, of sulphide-ion as was used when hydrogen sulphide failed to precipitate zinc sulphide. The corresponding factors of the products of the ion concentrations are equal, at the beginning of the two experiments, and we may put ([Cd^{2+}] × [S^{2−}] / ‹x›) = ([Zn^{2+}] × [S^{2−}] / ‹x›) = P′. We recall the fact, that we have already concluded, on the basis of the principle of the solubility-product, that ([Zn^{2+}] × [S^{2−}] / ‹x›), or P′, is ‹smaller› than K_{ZnS} (p. 207), and that ([Cd^{2+}] × [S^{2−}] / ‹x›), or P′, is ‹greater› than K_{CdS}. P′ being smaller than K_{ZnS} and larger than K_{CdS}, it is clear that [p210] K_{CdS} is smaller than K_{ZnS} and that cadmium sulphide must be the ‹less soluble› of the two sulphides. As a matter of fact, if ammonium sulphide is carefully added to a mixture of equal quantities of the two salt solutions, cadmium sulphide is precipitated first, and when practically all of the cadmium is precipitated, a final precipitate of white zinc sulphide is obtained (‹exp.›; see note, p. 205). Or, if zinc sulphide is first precipitated by the addition of a little ammonium sulphide to 25 c.c. of the 0.1 molar zinc sulphate solution, care being taken to have zinc sulphate in excess, and if 25 c.c. of the 0.1 molar cadmium sulphate solution is then added to the mixture, the white zinc sulphide immediately gives way to the less soluble yellow cadmium sulphide (‹exp.›; see p. 165). Cadmium sulphide is thus proved to be the ‹less soluble› of the two sulphides, a result which confirms the prediction made above with the aid of the principle of the solubility-product, and we may indeed conclude that the solubility-product constant K_{CdS} of cadmium sulphide must be smaller than the constant K_{ZnS} of zinc sulphide (see pp. 163–168, on fractional precipitation). We are, therefore, also justified in deciding that CdS may well be precipitated from acidulated solutions by hydrogen sulphide, when ZnS is not thus precipitated, simply because K_{CdS} is ‹sufficiently small› (CdS is sufficiently insoluble) ‹to make the product of the ion concentrations› [Cd^{2+}] × [S^{2−}] / ‹x›, ‹in spite of the extremely small value of› [S^{2−}] / ‹x›, ‹greater than the constant› K_{CdS}, whereas the same small value of [S^{2−}] / ‹x› makes it impossible for the product [Zn^{2+}] × [S^{2−}] / ‹x› to reach the value of the ‹larger constant› K_{ZnS}, required for the precipitation of ZnS. Since cadmium sulphide may be precipitated quantitatively under the conditions given, it is also evident that it may be precipitated even when the concentration of the cadmium-ion also has a rather small value. The relations, in regard to this point, will be discussed presently.