=Soluble Prussian Blue.=—The modification of ferric ferrocyanide easily soluble in water is obtained by adding a solution of a ferric salt to a solution of yellow prussiate, the latter being in considerable excess. Brücke gives the following method: a solution is prepared of 217 grammes of yellow prussiate in 11 kilogrammes of water. Another solution contains 100 grammes of ferric chloride in 1 litre, 1 litre of which is mixed with 2 litres of a saturated solution of Glauber’s salt and the mixture added to the prussiate solution so long as a blue precipitate forms. The Glauber’s salt plays no part in the formation of the colour; it is added to prevent the solution of the precipitate, which is insoluble in salt solutions. The precipitate is brought on to a filter and washed with water until the washings begin to be blue, which is a sign that the salts have been almost completely removed. The precipitate cannot be further washed without the loss of a large quantity which would be dissolved by the water; without further washing it is slowly dried in the air. Soluble Prussian blue has but a limited application; anatomists use its solution to inject specimens, and it was formerly used for blue writing inks, for which purpose aniline dyes are now preferred. It may also be remarked that ordinary Prussian blue readily dissolves in a solution of oxalic acid; the solution was also used as a writing ink, but since it attacks steel pens, it has now been replaced by solutions of dyes which have no action on steel. =Special Processes for the Manufacture of Chinese Blue.=—In place of chlorine or nitric acid manganic chloride can be used to oxidise the white precipitate obtained from yellow prussiate and green vitriol. This process is based upon the property of manganic chloride to readily give up part of its chlorine and be changed into manganous chloride (the ordinary chloride of manganese). The conversion of the white precipitate into the blue is thus also in this case indirectly effected by chlorine. Many varieties of pyrolusite contain considerable quantities of manganic oxide which are of little use in the manufacture of chlorine, for which pyrolusite is chiefly employed. If such pyrolusite be digested with crude hydrochloric acid for several days, the manganic oxide is dissolved with the production of manganic chloride. The oxidation of the white precipitate is performed in the usual way; after running off the liquid an excess of manganic chloride solution is added, as soon as two tests taken at an interval of a few minutes show no further increase in the depth of the colour, the reaction is ended and the solution of manganous chloride is drawn off. Any powerful oxidising agent may be used to oxidise the white precipitate; chromic acid has been proposed for this purpose, but up to the present the chromates are too dear to be used in place of the cheap oxidising agents previously mentioned.