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It is a compound of iron and cyanogen, of varying composition, formed by adding yellow prussiate of potash to a persalt of iron, or by oxidizing the precipitate obtained from the prussiate and a protosalt.

From a numerous set of experiments the author showed that no salt of iron and no precipitate of iron equalled the common sulphate of iron—that is, the commercial copperas—for the purpose of ink- making; and that even the addition of any persalt, such as the nitrate or chloride of iron, though it improved the present color of the ink, deteriorated its durability.