sed 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.

It was undoubtedly cipher, but whether a simple or abstruse one Foyle
was in no position to judge. He had an elementary knowledge of the
subject, but he had no intention of attempting to solve it by himself.
There were always experts to whom appeal could be made. A successful
detective, like a successful journalist, is a man who knows the value of
specialists--who knows where to go for the information he wants. That
meaningless jumble of letters could only be juggled into sense by an
expert. Foyle nevertheless scrutinised them closely, more as a matter of
habit than of reading anything from them. They were--

UJQW. BJNT. FJ. UJM. FJTV. UIYIQL. SK. DQUQZOKKEYJPK. ANUJ. M.Q.
NG. N. AYUQNQIX. IGZ. ANUJ. SIO. IGZ. SMPPN. RT. 12845 HGZVFSF.

"We'll let Jones have a go at that," he said. "Anything else now?"

Some one handed him the knife that had been thrown at him on the landing
and a curious leather sheath that had been picked up near the bed. From
the bottom of the sheath depended a leather tassel. Foyle looked it over
and failed to discover any manufacturer's name. He slipped the weapon
into his pocket with the mental reflection that it looked Greek. The
search went on from attic to cellar, and profuse notes were taken of
eve