Manufacturer and builder 3, 1888
Can you give me a receipt for an invisible ink suitable for writing a secret postal-card message? - I. M. S., Syraence, N. Y.
Answer. Several such compositions have been proposed and used to some extent. A slution of nitrate (or chloride) of cobalt mixed with a little gum or sugar, produces a "magic" ink which may be made visible by warming, either by holding near a hot fire or over a burning match. A solution of ferro-cyanide of potassium n water may be used for te same purpose; but this has the objection that it requires a "developer," for which a solution of either sulphate of copper (blue stone) or sulphate of iron (green vitriol) may be used. When the copper salt is used, the ink will appear in brown, and when the iron salt is used, blue.
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