The precipitate shows certain differences in composition according to the exact method by which it is obtained. A different result is obtained when the solution of the ferric salt is poured into that of a ferrocyanide, to that produced by the reverse procedure, the precipitate produced in the one case has properties which it does not possess in the other. Similar differences often occur in precipitating pigments. If the solution of the ferrocyanide is poured into the iron solution, the latter being in excess, an insoluble precipitate is formed; but if the iron solution is poured into the ferrocyanide solution of which an excess is present, the blue precipitate is formed, but it is soluble in pure water, though not in water containing salts. Thus when this precipitate is separated from the liquid and washed, the washings are at first quite colourless and remain so whilst salts are present, but when these have been completely washed away, a fine blue solution of the ferric ferrocyanide is formed, from which the addition of a salt solution again separates the dissolved Prussian blue. Before proceeding to the manufacture of Prussian blue on the large scale, it is necessary to consider the behaviour of ferrous salts towards yellow prussiate, since these are commonly used instead of ferric salts. When a solution of potassium ferrocyanide is mixed with a solution of a ferrous salt, there is formed a pure white precipitate of potassium ferrous ferrocyanide, K₂FeFe(CN)₆. In order to obtain a completely white precipitate, it is necessary that the ferrous salt should be completely free from ferric salts and that the solutions should contain no dissolved oxygen; the solutions would therefore have to be boiled before mixing. If one of the solutions contains the smallest quantity of oxygen a bluish precipitate is formed. When the white precipitate of potassium ferrous ferrocyanide is exposed to the air it immediately acquires at the surface a dark blue colour, and is completely changed to Prussian blue by sufficiently long contact with air. In accordance with this behaviour of ferrous and ferric salts different methods may be used to obtain Prussian blue. Either the process is commenced with a ferric salt from which a precipitate of Prussian blue is at once obtained, or the solution of a ferrous salt is converted into ferric salt by a powerful oxidising agent, such as chlorine or nitric acid, or a solution of a ferrous salt is precipitated with yellow prussiate and the white or bluish precipitate, which consists chiefly of potassium ferrous ferrocyanide, converted into ferric ferrocyanide or Prussian blue by treatment with nitric acid.