There was nothing more to be done at Grave Street. Heldon Foyle remained in the house while Green walked to the chief divisional station, and in an hour or two the divisional inspector with a couple of men arrived. Then Foyle saw to a strict search of the house from top to bottom. Nothing there was that seemed to possess any great importance as bearing on the case. The man who had fled over the roof had used a single room, apparently as bed- and sitting-room, so it was to this place that the detectives devoted chief attention.

"He must have been sleeping in his clothes," grumbled Green. "He hadn't time to dress. There's the typewriter the note was written on."

He sat down before a rickety table and, inserting a piece of paper in the machine, slowly tapped out the alphabet, and after a brief inspection passed the paper on to the superintendent, who scanned it casually, and was about to throw it away when something gripped his attention.

"This looks queer," he muttered, and held the paper up slantingly away from the gas-jet in order to examine it by what photographers call transmitted light.

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.

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 everything found, with its exact position. The elaborate trouble taken by these men to describe minutely in writing every little thing would have seemed absurd to any one not versed in the ways of the Criminal Investigation Department. Yet nothing was done that was not necessary. An error of an inch in a measurement might make all the difference when the case came on for trial.

Foyle and Green left the house in charge of the divisional man. Already a description had been circulated of the man they had failed to surprise; but as neither had caught more than a glimpse of a shadowy figure in the darkness, they had had to rely on the descriptions given by Israels and his wife. And even if that estimable pair had really tried honestly to give a fair description of the man—which the detectives thought was extremely doubtful—there could be little hope that it was accurate. If the average man tries to describe the appearance of his most intimate friend and then asks a stranger to identify him, he will realise how misleading such descriptions may be even at the best of times. Yet the Criminal Investigation Department had to work with such material as they had.

Heldon Foyle was very silent as they trudged side by side out of Whitechapel into the silent City streets—for there are no taxicabs to be found in the East End at such hours. The case was developing; but though he was beginning to have a hazy glimpse into some of its workings, there was much that remained a mystery to him. His questionings of Israels had satisfied him that the man who had escaped was neither Grell nor Ivan. He could not blame himself for not effecting an arrest. Looking back over the night's events, he could not see that he could have taken further precaution. If he had taken more men the escape would have occurred just the same over the roofs, for he would still have felt it his duty to question Israels. He could not have foreseen that the ready-witted Lola was there, nor that she should have so ingeniously given the alarm. The luck had been against him.

Nevertheless he had gained an important fact. Lola was in London and was obviously acting in concert with Grell. It was easier to look for two persons than one. Sooner or later he would lay hands on them and solve the mystery of the murder. He clenched his fists resolutely as his thoughts carried him away. Meanwhile there was the cipher. If that could be de-coded it might be valuable.

Green's voice broke in upon his thoughts.

"We didn't find anything bearing on Waverley."

"Waverley?" repeated Foyle. "Oh yes, I had almost forgotten him."

For an hour after they had reached Scotland Yard the superintendent laboured at his desk, collecting reports and writing fresh chapters in the book which held all the facts in relation to the crime, so far as he knew them. He slipped the result of his labours at last in an envelope and left them over to be dealt with by the inspector in charge of the Registry, which is a department that serves as official memory to Scotland Yard.

"That is all right," he said, and stretched himself.

Some one knocked at the door. The handle turned and an erect man with his right arm carried in a black silk handkerchief improvised into a sling entered the room. It was Detective-Inspector Waverley.