The Manufacturer and Builder [date unknown]
Various chemical substances are used as symphathetic inks — a moderately dilute solution of chloride of cobalt is perhaps tho most popular. Letters written with it are at first scarcely perceptible, but when gently warmed they become quite visible, being at first of a rich red color, which rapidly changes to green. Advantage has been taken of this fact to produce drawings, in which the houses, branches of trees, etc, are drawn with ordinary India ink, while, for the foliage, the cobalt ink is used. So long as the pictures remain cold, they represent a cheerless winter scone; but when brought near the fire, they rapidly assume the brilliant and beautiful foliage of spring or summer. This phenomenon is mentionod in a book on alchemy, printed in the year 1703, and bearing a title which we tranalate, Clue for the Cabinet of the Secret Treasury of Nature. In 1736, a vagrant alchemist performed the trick in Paris for money; but it has now ceased to be a wonder, and is considered only as one of the many amusing playa of chemistry.
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