It is well known to all chemists that sulphuretted hydrogen does not give a precipitate of zinc sulphide in a solution of zinc chloride or zinc sulphate if a sufficient (quite small) quantity of a strong acid, such as hydrochloric or sulphuric acid, has been added to the solution. It is also quite well known that if we add a sufficient quantity of a solution of sodium acetate to the clear solution containing the zinc salt sulphuretted hydrogen and strong acid, we throw down the whole of the zinc as sulphide. This used to be explained as follows:—Hydrochloric acid acts on zinc sulphide thus, ZnS + 2HCl = H2S + ZnCl2, and therefore the opposite action ZnCl2 + H2S = ZnS + 2HCl cannot take place in the presence of hydrochloric acid. But as acetic acid is too weak an acid to act on zinc sulphide, the presence of acetic acid does not prevent the precipitation of zinc sulphide, and the hydrochloric acid originally there, as well as that produced by the reaction, acts on the sodium acetate to form sodium chloride and acetic acid. On the modern theory the explanation is this:—The zinc sulphide is attacked not by the hydrochloric acid, but by the hydrogen ions; these are present in the solution of hydrochloric acid; and that acid being to a great extent “ionised,” we have a great concentration of hydrogen ions. There is a limiting value for the concentration of hydrogen ions, above which the action ZnS + 2H. = Zn.. + H2S takes place, so that zinc sulphide cannot be formed by the action of sulphuretted hydrogen on a zinc salt in a solution containing hydrogen ions with a concentration above this limiting value.