Chapter 23

“My dear fellow!” said Mr. Mappin ruefully. “There are a hundred formulas! . . . In the Indian Mutiny when General Sales wished to advise his superior officer of his danger, he wrote a despatch in rice water with the single word ‘iodine’ for a clue. When iodine was applied to the paper it turned the writing bright blue. “That’s only one,” he went on; “the old books are full of recipes for making sympathetic ink. If you write with solution of Saturn as they used to call it—we know it under its more matter-of-fact na…