His brows were drawn together tightly. The sheet of paper which Green had used was an ordinary piece of writing-paper. On its rough surface Foyle had noted a slight sheen, unusual enough to attract his attention. Even he would not have noticed it but for the angle in which he had happened first to look at it when he took it from Green. It might be an accidental fault in the manufacture of the paper. Yet, trivial as it seemed, it was unusual, and one of the chief assets in detective work is not to let the unusual go unexplained. "It's the same typewriter. There can be no question of that," said Green. "You can see that the 'b' is knocked about and the 'o' is out of line." "That's all right," said Foyle. "I wasn't thinking of that. It looks to me as if there's some sympathetic writing on this." He held the paper so that the heat from the gas-jet warmed it. Every moment he expected that the heat would bring something to light on the paper. He gave a petulant exclamation as nothing happened, and his eyes roved over the table whence Green had taken the paper. He believed that he was not mistaken, that there was something written which could be brought to light if he knew how. He knew that there were chemicals that could be used for secret communications which could only be revealed by the use of other chemicals--a process something akin to development in photography. It was unlikely, if the user of the room had used some chemical agent, that he would have thought of destroying and concealing it. But there was nothing on the table that suggested itself to Foyle as having been used in the connection. Keenly he scrutinised the room, his well-manicured hand caressing his chin. "Ah!" he exclaimed at last. He had noted a small bottle of gum arabic standing on the cast-iron mantelpiece. Now, gum arabic can be used for a variety of purposes, and it has the merit of invisible ink of being made decipherable by quite a simple process which minimises the risk of accidental disclosure. The superintendent held the paper to the gas again for a few minutes. Then from a corner of the room he collected a handful of dust--no difficult process, for it was long since the place had felt the purifying influence of a broom--and rubbed it hard on the rough surface of the paper. A jumble of letters stood out greyly on the surface. He looked at them hard, and Green, peeping over his shoulder, frowned. "Cipher!" he exclaimed.