It has been long known that the Prussian blue of commerce contains an admixture, in greater or less quantity, of alumina, sulphate of potash, and common alum, one or all of them being easily detected in every specimen. The sulphate of potash and the alum may be separated by frequent boiling in water, but they are seldom in such quantity as to render this process necessary. The alumina may be removed by digestion in muriatic acid, and the washing consequent upon this mode of treatment will free it from all the soluble impurities.

When the alkalies or earths are digested with Prussian blue, in order to form the common Prussiates, and the yellow solution is evaporated, it almost uniformly happens that after the first or second crop of crystals is separated, there remains a dark brownish-red liquid, which either does not crystallize at all, or gives crystals of the required prussiate of a dirty brown colour, and mixed with a greater or less portion of a red matter, either massive, or in small, red, four-sided needles and prisms. This may be observed in preparing the prussiates of lime or soda by the common process, but has been more frequently taken notice of in forming the cyanide of mercury; because the least colouring matter in this salt is at once perceptible, and because in the preparation of it, a partial loss is of greater consequence. To the presence of a portion of this red salt, particularly in extemporaneous prussiates, I attribute those differences in the colour of the precipitates which they give with the metallic oxides, and which have led some to doubt the accuracy of their indications. The grounds of this opinion will appear in the sequel of the present paper.