In many ink recipes soluble Prussian blue, which is a preparation of Prussian blue and ferrocyanide of potassium, is mentioned. This soluble blue is made thus. With a pestie and mortar thoroughly incorporate a quantity of ordinary Prussian blue with half its quantity of ferrocyanide of potassium. The mixture is then put into distilled water and thoroughly shaken from time to time; then it is allowed to stand and the sediment riltered off.