Pigments. These are substances employed as coloring matter in mixing paints, etc.. The following receipts furnish the method of preparing the pigments and other coloring matters m general use, and their special appliances.

2674. Turnbull's Prussian Blue

2674. Turnbull's Prussian Blue. Ferricyanide (red prussiate) of potassium, 10 ounces; solution protosulphate of iron, 1 pint; water, 3 pints. Dissolve the ferricyanide of potassium in part of water, and add the solution, gradually, to the solution of protosulphate of iron previously diluted with the remainder of the water, stirring the mixture during the addition. Then filter the liquid, and wash the precipitate on the filter with boiling water until the washings pass nearly tasteless. Lastly, dry it, and rub it into fine powder. It may also be made by adding protosulphate of iron to a mixture of yellow prussiate of potash, chloride of soda, and hy-drochloric acid. This, mixed with water, makes an excellent bluing.

2675. Prussian Blue

2675.      Prussian Blue. Percyanide, ferrocyanide, or ferroprussiate of iron. Commercial Prussian blue is made by adding to a solution of prussiate of potash (or of prussiate cake), a solution of 2 parts alum and 1 part sulphate of iron, washing the precipitate repeatedly with water to which a little muriatic acid has been added, and exposing it to the air till it assumes a deep blue color. A purer kind is made by adding a solution of persulphate or perchloride of iron to a solution of pure ferroprussiate of potash. (See No. 2674 (Turnbull's Prussian Blue).)

2676. Action of Prussic Acid on Iron Solutions

2676.    Action of Prussic Acid on Iron Solutions. The Germans call prussic acid blausäure, because it produces a blue precipitate in certain iron solutions; but the following experiment undoubtedly proves that the prussic acid does not produce the color of that precipitate, since it can be made just as well without it. Prepare a saturated solution of green vitriol in water. Take 4/7 parts of the above solution and treat it with nitric and sulphuric acids, until it is changed into the sulphate of peroxide of iron. Mix this with the remaining 3/7 of the first solution, then add very gradually (to avoid its becoming heated) concentrated sulphuric acid, until a precipitate is formed. The result will be a beautiful blue precipitate, equal to Prussian blue. If water is added, the precipitate is dissolved and the color destroyed; but if the precipitate is separated from the acid and rubbed with phosphate of soda, we obtain a beautiful blue phosphate of iron, which will resist the action of water. ln all these cases the acids, which possess no color, are by no means the cause of the blue color, but favor only the production of it, by depriving the mixed hydrates of protoxide and peroxide of iron of certain equivalents of water, and likewise by preventing the same from entering into a higher state of oxidation in the atmosphere.