Quest turned around.

“Doctor,” he said, “I happen to have my chemical chest with me, and some special testing tubes.  If you’ll allow me, I’d like to examine this cup of bouillon.  You might come round, too, if you will.”

The Captain nodded.

“I’d better stay here for a time,” he decided.  “I’ll follow you presently.”

The service of dinner was resumed.  Laura, however, sent plate after plate away.  The Captain watched her anxiously.

“I can’t help it,” she explained.  “I don’t know whether you’ve had any talk with Mr. Quest, but we’ve been through some queer times lately.  I guess this death business is getting on my nerves.”

The Captain was startled.

“You don’t for a moment connect Mrs. Foston Rowe’s death with the criminal you are in search of?” he exclaimed.

Laura sat quite still for a moment.

“The bouillon was offered first to Mr. Quest,” she murmured.

The Captain called his steward.

“Where did you get the bouillon you served—­that last cup especially?” he asked.

“From the pantry just as usual, sir,” the man answered.  “It was all served out from the same cauldron.”

“Any chance of any one getting at it?”

“Quite impossible, sir!”

Laura rose to her feet.

“Sorry,” she apologized, “I can’t eat anything.  I’m off on deck.”

The Captain rose promptly.

“I’ll escort you, if I may,” he suggested.

Harris, too, rose from his place, after a final and regretful glance at the menu, and joined the others.  The Captain, however, drew Laura’s arm through his as they reached the stairs, and Harris, with a little shrug of the shoulders, made his way to Quest’s stateroom.  The Doctor, the Professor, Quest and Lenora were all gathered around two little tubes, which the criminologist was examining with an electric torch.

“No reaction at all,” the latter muttered.  “This isn’t an ordinary poison, any way.”

The Professor, who had been standing on one side, suddenly gave vent to a soft exclamation.

“Wait!” he whispered.  “Wait!  I have an idea.”

He hurried off to his stateroom.  The Doctor was poring over a volume of tabulated poisons.  Quest was still watching his tubes.  Lenora sat upon the couch.  Suddenly the Professor reappeared.  He was carrying a small notebook in his hand; his manner betrayed some excitement.  He closed the door carefully behind him.

“I want you all,” he begged, “to listen very carefully to me.  You will discover the application of what I am going to read, when I am finished.  Now, if you please.”

They looked at him wonderingly.  It was evident that the Professor was very much in earnest.  He held the book a little way away from him and read slowly and distinctly.

“This,” he began, “is the diary of a tour made by Craig and myself in Northern Egypt some fourteen years ago.  Here is the first entry of import:—­