We paid our check and Kennedy and I sauntered in the direction Sherburne had taken, finding him ultimately in the cafe, alone. Without further introduction Kennedy approached him.

"So—you are a detective?" sneered Sherburne superciliously, elevating his eyebrows just the fraction of an inch.

"Not exactly," parried Kennedy, seating himself beside Sherburne. Then in a tone as if he were willing to get down, without further preliminary, to business, seemingly negotiating, he asked: "Mr. Sherburne, may I ask just what it is on which you base your claim on Mrs. Seabury? Is it merely meeting her here? If that is so you must know that it amounts to nothing—now."

The two men faced each other, each taking the other's measure.

"Nothing?" coolly retorted Sherburne. "Perhaps not—in itself. But—suppose—I—had—"

He said the words slowly, as he fumbled in his fob pocket, then cut them short as he found what he was looking for. Safely, in the palm of his hand, he displayed a latch-key, momentarily, then with a taunting smile dropped it back again into the fob pocket.

"Perhaps she gave it to me—perhaps I was a welcome visitor in her apartment," he insinuated. "How would she relish having that told to Mr. Seabury—backed up by the possession of the key?"

I could not help feeling that for the moment Kennedy was checkmated. Sherburne was playing a desperate game and apparently held the key, however he got it, as a trump card.

"Thank you," was all that Kennedy said, as he rose. "I wanted to know how far you could go. Perhaps we can meet you halfway."

Sherburne smiled cynically. "All the way," he said quietly, as we left the cafe.

In silence Kennedy left the hotel and jumped into a cab, directing the driver to the laboratory, where he had asked Mrs. Seabury to wait for him. We found her there, still much agitated.

Hastily Craig explained to her how he had saved the situation, but her mind was too occupied over something else to pay much attention.

"I—I can't blame you, Professor Kennedy," she cried, choking down a sob in her voice, "but I have just discovered—he has told me that it is even worse than I had anticipated."

We were both following her closely, the incident of the latch-key still fresh in mind.

"Some time ago," she hurried on, "I missed my latch-key. I thought nothing of it at the time—thought perhaps I had mislaid it. But today he told me—just after the dance, even while I was making him think I would pay him the money, because—because I liked him—he told me he had it. The brute! He must have picked my handbag!"

Her eyes were blazing now with indignation. Yet as she looked at us both, evidently the recollection of what had just happened came flooding over her mind, and she dropped her head in her hands in helpless dismay at the new development.

Craig pulled out his watch hastily. "It is about six, Mrs. Seabury," he reassured. "Can you be here at, say, eight?"

"I will be here," she murmured pliantly, realizing her own helplessness.

She had scarcely closed the door when Craig seized the telephone, and hurriedly tried to locate Seabury himself.

"Apparently no trace of him yet," he fumed, as he hung up the receiver. "The first problem is how to get that key."

Instantly I thought of Dunn's secret service girl. Kennedy shook his head doubtfully. "I'm afraid there is no time for that," he answered. "But will you attend to that end of the affair for me, Walter? I have just a little more work here at the laboratory before I am ready. I don't care how you do it, but I want you to convey to Sherburne the welcome news that Mrs. Seabury is prepared to give in, in any way he may see fit, if he will call her up here at eight o'clock."

Kennedy had already plunged back among his beakers and test tubes, and with these slender instructions I sallied forth in my quest of Sherburne. I had little difficulty in locating him and delivering my message, which he received with a satisfaction that invited assault and battery and mayhem. However, I managed to restrain myself and rejoin Craig in the laboratory, shortly after seven o'clock.

I had scarcely had time to assure Kennedy of the success of my mission, when we were surprised to see the door open and Seabury himself appear.

His face was actually haggard. Whether or not he had believed the hastily concocted story of Kennedy at the Vanderveer, his mind had not ceased to work on the other fears that had prompted his coming to us in the first place.

"I've been trying to locate you all over," greeted Craig.

Seabury heaved a sigh and passed his hand, with its familiar motion, over his forehead. "I thought perhaps you might be able to find out something from this stuff," he answered, unwrapping a package which he was carrying. "Some samples of the food I've been getting. If you don't find anything in this, I've others I want tested."

As I looked at the man's drawn face, I wondered whether in fact there might be something in his fears. On the surface, the thing did indeed seem to place Agatha Seabury in a bad light. At the sight of the key in Sherburne's possession I had grasped at the straw that he might have conceived some diabolical plan to get rid of Seabury for purposes of his own. But then, I reasoned, would he have been so free in showing the key if he had realized that it might cast suspicion on himself? I was forced to ask myself again whether she might, in her hysterical fear of exposure by the adroit blackmailer, have really attempted to poison her husband.

It was a desperate situation. But Kennedy was apparently ready to meet it, though he seemed to take no great interest in the food samples Seabury had just brought.

Instead he seemed to rely wholly on the tests he had already begun with the peculiar tissue I had seen him boiling and the blood serum derived from Seabury himself.

Without a word he took three tubes from the incubator, in which I had seen him place them some time before, and, as they stood in a rack, indicated them lightly with his finger.

"I think I can clear part of this mystery up immediately," he began, speaking more to himself than to Seabury and myself. "Here I have a tested dialyzer in which has been placed a half cubic centimeter of pure clear serum. Here is another dialyzer with the same amount of serum, but no tissue, such as Mr. Jameson has seen me place in this first one. Here is still another with the tissue in distilled water, but no blood serum. I have placed all the dialyzers in tubes of distilled water and all are covered with a substance known as toluol and corked to keep them from contamination."

Kennedy held up before us the three tubes and Seabury gazed on them with a sort of fascination, scarcely believing that in them in some way might be contained the verdict on the momentous problem that troubled his mind and might perhaps mean life or death to him.

Carefully Kennedy took from each tube a few cubic centimeters of the dialyzate and into each he poured a little liquid from a tiny vial which I noticed was labelled "Ninhydrin."

"This," he explained as he set down the vial, "is a substance which gives a colorless solution with water, but when mixed with albumins, peptones, or amino-acids becomes violet on boiling. Tube number three must remain colorless. Number two may be violet. Number one may approximate number two or be more deeply colored. If one and two are about the same I call my test negative. But if one is more deeply colored than two, then it is positive. The other tube is the control."

Impatiently we waited as the three tubes simmered over the heat. What would they show? Seabury's eyes were glued on them, his hand trembling in the presence of some unknown danger.

Slowly the liquid in the second tube turned to violet. But more rapidly and more deeply appeared the violet in number one. The test was positive.

"What is it?" gasped Seabury hoarsely, leaning over close.

"This," exclaimed Kennedy, "is the famous Abderhalden test—serum-diagnosis—discovered by Professor Emile Abderhalden of Halle. It rests on the fact that when a foreign substance comes into the blood, the blood reacts, with the formation of a protective ferment produced as a result of physiologic and pathologic conditions.

"For instance," he went on, "a certain albumin always produces a certain ferment. Presence in the blood stream of blood-foreign substances calls forth a ferment that will digest them and split them into molecules. The forces of nature form and mobilize directly in the blood serum.

"Let me get this clearly. Albumin cannot pass through the pores of an animal membrane, since the individual molecules are too large. If, however, the albumin is broken up by a ferment-action, then the molecules become small enough to pass through."

Seabury was listening like a man on whom a stunning blow was about to descend.

"Thus we can tell," proceeded Kennedy, "whether there is such a ferment in blood serum as would be produced by a certain condition, for when the ferment is there blood from the individual possessing it will digest a similar proteid in a dialyzing thimble kept at body temperature.

"Why," cried Kennedy, swept along by the wonder of the thing, "this test opens up a vista of alluring and extensive possibilities. The human organism actually diagnoses its own illnesses automatically. It is infinitely more exact, more rapid, and more certain than all that human art can attain. Each organ contains special ferments in its cells in the most subtle way attuned to the molecular condition of the particular cell substance and with complete indifference to other cells.

"Don't you see? It diagnoses at the very first stage. You take a small quantity of blood, derive the serum, then introduce a piece of tissue such as you wish to find out whether it is diseased or not. The thing is of overwhelming importance. One can discover a condition even before the organ itself shows it outwardly. It means a new epoch in medicine. As for me, I call it the new 'police service' of the organism—working with perfect, scientific accuracy."

"Wh-what do you find?" reiterated Seabury.

"I have made tests for about everything I can suspect," returned Kennedy, taking the tubes and pouring the liquid from number two into number one until they were equalized in color, thus testing them, while we watched every action closely.

"You see," he digressed, "to get the two the same shade I have to dilute the first by the second. Now, the dialyzers are not permeable to albumin. Therefore the violet color indicates that the blood serum in this case contains ferments which the body is making to split up some foreign substance in the blood, such as I suspected and obtained from the hospital. The test is positive. Mr. Seabury, how long have you felt as you say that you do?"

"Several weeks," the man returned weakly.

"That is fortunate," cried Kennedy, "fortunate that it has not been several months."

He paused, then added the startling statement, "Mr. Seabury, I can find no evidence here of poison. As a matter of fact, the wonderful Abderhalden test shows me that you have one of the most common forms of internal disease that occur for the most part in persons at or after middle life, about the age of fifty, more common in men than in women—a disease which taken in time, as it has been revealed by this wonderful test, may be cured and you may be saved—an incipient cancer of the stomach."

Kennedy paused a moment and listened. I fancied I heard someone in the hall. But he went on, "The person whom you suspect of poisoning you—"

There came a suppressed scream from the door, as it was flung open and Agatha Seabury stood there, staring with fixed, set eyes at Kennedy, then at her husband. Mechanically I looked at my watch. It was precisely eight. Kennedy had evidently prolonged the test for a purpose.

"The person whom you suspected," he repeated firmly, "is innocent!"

A moment Agatha stood there, then as the thing dawned on her, she uttered one cry, "Judson!"

She reeled as Kennedy with a quick step or two caught her.

Seabury himself seemed dazed.

"And I have—" he ejaculated, then stopped.

Kennedy raised his hand. "Just a moment, please," he interrupted, as he placed Mrs. Seabury in a chair, then glanced hastily at his watch.

She saw the motion and seemed suddenly to realize that it was nearing the time for Sherburne to call up. With a mighty effort she seemed to grip herself. She had just been shocked to know that she was charged unjustly. But had she been cleared from one peril only to fall a victim to another—the one she already feared? Was Sherburne to escape, after all, and ruin her?

The telephone tinkled insistently. Kennedy seized the receiver.

"Who is it?" we heard him ask. "Mr. Sherburne—oh yes."

Mrs. Seabury paled at the name. I saw her shoot a covert glance at her husband, and was relieved to see that his face betrayed as yet no recognition of the name. She turned and listened to Kennedy, straining her ears to catch every syllable and interpret every scrap of the one-sided conversation.

Quickly Craig had jammed the receiver down on a little metal base which we had not noticed near the instrument. Three prongs reaching upward from the base engaged the receiver tightly, fitting closely about it. Then he took up a watch-case receiver to listen through, in place of the regular receiver.

"Sherburne, you say?" he repeated. "H. Morgan Sherburne?"

Apparently the voice at the other end of the wire replied rather peevishly, for Kennedy endeavored to smooth over the delay. We waited impatiently as he reiterated the name. Why was he so careful about it? The moments were speeding fast and Mrs. Seabury found the suspense terrific.

"Must pay—we'll never get anything on you?" Craig repeated after a few moments further parley. "Very well. I am commissioned to meet you there in ten minutes and settle the thing up on those terms," he concluded as he clapped the regular receiver back on its hook with a hasty good-by and faced us triumphantly.

"The deuce I won't get anything. I've got it!" he exclaimed.

Judson Seabury was too stunned by the revelation that he had a cancer to follow clearly the maze of events.

"That," cried Kennedy, rising quickly, "is what is known as the telescribe—a new invention of Edison that records on a specially prepared phonograph cylinder all that is said—both ways—over a telephone wire. Come!"

Ten minutes later, in a cab that had been waiting at the door, we pulled up at the Vanderveer.

Without a word, leaving Judson Seabury and his wife in the waiting cab, Craig sprang out, followed by me, as he signaled.

There was Sherburne, brazen and insolent, in the cafe as we entered, from a rear door, and came upon him before he knew it, our friend, Dunn, whom we had met in the lobby, hovering concealed outside, ready to come to our assistance.

In a moment Kennedy was at Sherburne's elbow, pinching it in the manner familiar to international crooks.

"Will you tell me what your precise business is in this hotel?" shot out Craig before Sherburne could recover from his surprise.

Sherburne flushed and flared—then became pale with rage.

"None of your damned insolence!" he ground out, then paused, cutting the next remark short as he gritted, "What do you mean? Shall I send a wax impression of that key—"

Kennedy had quickly flashed the cylinder of the telescribe before his eyes and instinctively Sherburne seemed to realize that with all his care in using typewriters and telephones, some kind of record of his extortion had been obtained.

For a moment he crumpled up. Then Kennedy seized him by the elbow, dragging him toward a side door opposite that at which our cab was standing.

"I mean," he muttered, "that I have the goods on you at last and you'll get the limit for blackmail through this little wax cylinder if you so much as show your face in New York again. I don't care where you go, but it must be by the first train. Understand?"

A moment later we returned to the cab, where it had pulled up in the shadow, away from the carriage entrance.

"You—you'll forgive me—for my—unjust suspicions—Agatha?" we heard a voice from the depths of the cab say.

Kennedy pulled me back in time not to interrupt a muffled "Yes."

Craig coughed.

As he reached a hand in through the cab door to bid good-night to the reunited couple, I saw Mrs. Seabury start, then turn and drop into her handbag the key which Kennedy had extracted from Sherburne's pocket in the mÊlÉe and now conveyed back to her in the handshake.