It is a comparatively easy matter to determine the ‹relative solubility› of zinc and ferrous sulphides. If equal quantities of the equivalent solutions are mixed and a precipitant, ammonium sulphide, ‹which would precipitate either sulphide, if its salt were present alone›, is carefully and gradually added to the mixture, it will precipitate first the less soluble one (see p. 163); and that one alone can be present permanently (‹i.e.› in equilibrium) in contact with the solution containing the two salts. As a matter of fact,[409] [p206] ‹we find that zinc sulphide is precipitated first, under conditions permitting the precipitation of ferrous sulphide if no zinc sulphate were present›, and the precipitate of zinc sulphide remains unchanged in the presence of a mixture of the composition indicated (‹exp.›). It is clear, therefore, that the prediction, based on the conclusion drawn from the application of the principle of the solubility-product, is verified by experiment. Now, closer examination of the solution of zinc sulphate, from which zinc sulphide has been precipitated by the action of hydrogen sulphide, shows, after the hydrogen sulphide has precipitated as much sulphide as it can and the solution has been passed through a filter, that a very considerable proportion of zinc salt is still present in the filtrate, and we must ask why hydrogen sulphide fails to precipitate the zinc completely. The concentration of the zinc-ion has grown somewhat smaller, but that is not the cause of the nonprecipitation of zinc sulphide under the new conditions, since hydrogen sulphide will precipitate the sulphide, if it is passed into a solution of zinc sulphate which contains even a smaller concentration of zinc-ion than the filtrate, in which it fails to give any further precipitate. If the filtrate is examined, it is found to be ‹strongly acid›, since sulphuric acid has been liberated by the action of hydrogen sulphide on zinc sulphate: ZnSO_{4} + H_{2}S → ZnS ↓ + H_{2}SO_{4}. Sulphuric acid is a strong acid, which is very highly ionized, much more so than the exceedingly weak acid hydrogen sulphide, and consequently, as the precipitation of zinc sulphide proceeds, the ‹concentration of hydrogen-ion in the solution rapidly grows larger and larger›. But, the greater the concentration of the hydrogen-ion, the smaller is that of the sulphide-ion, since the product [H^{+}]^2 × [S^{2−}] is a constant [equation (IV), p. 201] for a solution kept saturated with hydrogen sulphide. The sulphide-ion is reduced in concentration very much more rapidly than is the zinc-ion.[410] As [S^{2−}] is a factor in the [p207] solubility-product of zinc sulphide, it is clear that the value of this product must grow rapidly smaller during the precipitation of zinc sulphide from the solution, and that it may well, eventually, grow too small to surpass the value of the solubility-product constant K_{ZnS}. Precipitation of zinc sulphide will then cease.