In the laboratory, Kennedy quietly set to work. He began by tearing from the germ letter the piece of gelatine and first examining it with a pocket lens. Then, with a sterile platinum wire, he picked out several minute sections of the black spot on the gelatine and placed them in agar, blood serum, and other media on which they would be likely to grow.

"I shall have to wait until to-morrow to examine them properly," he remarked. "There are colonies of something there, all right, but I must have them more fully developed."

A hurried telephone call late in the day from Miss Sears told us that Mrs. Blake herself had begun to complain, and that Dr. Wilson had been summoned but had been unable to give an opinion on the nature of the malady.

Kennedy quickly decided on making a visit to the doctor, who lived not far downtown from the laboratory.

Dr. Rae Wilson proved to be a nervous little woman, inclined, I felt, to be dictatorial. I thought that secretly she felt a little piqued at our having been taken into the Blakes' confidence before herself, and Kennedy made every effort to smooth that aspect over tactfully.

"Have you any idea what it can be?" he asked finally.

She shook her head noncommittally. "I have taken blood smears," she answered, "but so far haven't been able to discover anything. I shall have to have her under observation for a day or two before I can answer that. Still, as Mrs. Blake is so ill, I have ordered another trained nurse to relieve Miss Sears of the added work, a very efficient nurse, a Miss Rogers."

Kennedy had risen to go. "You have had no word about your car?" he asked casually.

"None yet. I'm not worrying. It was insured."

"Who is this arch criminal, Dr. Hopf?" I mused as we retraced our steps to the laboratory. "Is Mrs. Blake stricken now by the same trouble that seems to have affected Buster?"

"Only my examination will show," he said. "I shall let nothing interfere with that now. It must be the starting point for any work that I may do in the case."

We arrived at Kennedy's workshop of scientific crime and he immediately plunged into work. Looking up he caught sight of me standing helplessly idle.

"Walter," he remarked thoughtfully adjusting a microscope, "suppose you run down and see Garwood. Perhaps he has something to report. And by the way, while you are out, make inquiries about the Blakes, young Baldwin, Miss Sears and this Dr. Wilson. I have heard of her before, at least by name. Perhaps you may find something interesting."

Glad to have a chance to seem to be doing something whether it amounted to anything or not, I dropped in to see Garwood. So far he had nothing to report except the usual number of false alarms. From his office I went up to the Star where fortunately I found one of the reporters who wrote society notes.

The Blakes, I found, as we already knew, to be well known and moving in the highest social circles. As far as known they had no particular enemies, other than those common to all people of great wealth. Dr. Wilson had a large practice, built up in recent years, and was one of the best known society physicians for women. Miss Sears was unknown, as far as I could determine. As for Duncan Baldwin, I found that he had become acquainted with Reginald Blake in college, that he came of no particular family and seemed to have no great means, although he was very popular in the best circles. In fact he had had, thanks to his friend, a rather meteoric rise in society, though it was reported that he was somewhat involved in debt as a result.

I returned to the laboratory to find that Craig had taken out of a cabinet a peculiar looking arrangement. It consisted of thirty-two tubes, each about sixteen inches long, with S-turns, like a minute radiator. It was altogether not over a cubic foot in size, and enclosed in a glass cylinder. There were in it, perhaps, fifty feet of tubes, a perfectly-closed tubular system which I noticed Kennedy was keeping absolutely sterile in a germicidal solution of some kind.

Inside the tubes and surrounding them was a saline solution which was kept at a uniform temperature by a special heating apparatus.

Kennedy had placed the apparatus on the laboratory table and then gently took the little dog from his basket and laid him beside it. A few minutes later the poor little suffering Buster was mercifully under the influence of an anesthetic.

Quickly Craig worked. First he attached the end of one of the tubes by means of a little cannula to the carotid artery of the dog. Then the other was attached to the jugular vein.

As he released the clamp which held the artery, the little dog's feverishly beating heart spurted the arterial blood from the carotid into the tubes holding the normal salt solution and that pressure, in turn, pumped the salt solution which filled the tubes into the jugular vein, thus replacing the arterial blood that had poured into the tubes from the other end and maintaining the normal hydrostatic conditions in the body circulation. The dog was being kept alive, although perhaps a third of his blood was out of his body.

"You see," he said at length, after we had watched the process a few minutes, "what I have here is in reality an artificial kidney. It is a system that has been devised by several doctors at Johns Hopkins.

"If there is any toxin in the blood of this dog, the kidneys are naturally endeavoring to eliminate it. Perhaps it is being eliminated too slowly. In that case this arrangement which I have here will aid them. We call it vividiffusion and it depends for its action on the physical principle of osmosis, the passage of substances of a certain kind through a porous membrane, such as these tubes of celloidin.

"Thus any substance, any poison that is dialyzable is diffused into the surrounding salt solution and the blood is passed back into the body, with no air in it, no infection, and without alteration. Clotting is prevented by the injection of a harmless substance derived from leeches, known as hirudin. I prevent the loss of anything in the blood which I want retained by placing in the salt solution around the tubes an amount of that substance equal to that held in solution by the blood. Of course that does not apply to the colloidal substances in the blood which would not pass by osmosis under any circumstances. But by such adjustments I can remove and study any desired substance in the blood, provided it is capable of diffusion. In fact this little apparatus has been found in practice to compare favorably with the kidneys themselves in removing even a lethal dose of poison."

I watched in amazement. He was actually cleaning the blood of the dog and putting it back again, purified, into the little body. Far from being cruel, as perhaps it might seem, it was in reality probably the only method by which the animal could be saved, and at the same time it was giving us a clue as to some elusive, subtle substance used in the case.

"Indeed," Kennedy went on reflectively, "this process can be kept up for several hours without injury to the dog, though I do not think that will be necessary to relieve the unwonted strain that has been put upon his natural organs. Finally, at the close of the operation, serious loss of blood is overcome by driving back the greater part of it into his body, closing up the artery and vein, and taking good care of the animal so that he will make a quick recovery."

For a long time I watched the fascinating process of seeing the life blood coursing through the porous tubes in the salt solution, while Kennedy gave his undivided attention to the success of the delicate experiment. It was late when I left him, still at work over Buster, and went up to our apartment to turn in, convinced that nothing more would happen that night.

The next morning, with characteristic energy, Craig was at work early, examining the cultures he had made from the black spots on the gelatine.

By the look of perplexity on his face, I knew that he had discovered something that instead of clearing the mystery up, further deepened it.

"What do you find?" I asked anxiously.

"Walter," he exclaimed, laying aside the last of the slides which he had been staining and looking at intently through the microscope, "that stuff on the gelatine is entirely harmless. There was nothing in it except common mold."

For the moment I did not comprehend. "Mold?" I repeated.

"Yes," he replied, "just common, ordinary mold such as grows on the top of a jar of fruit or preserves when it is exposed to the air."

I stifled an exclamation of incredulity. It seemed impossible that the deadly germ note should be harmless, in view of the events that had followed its receipt.

Just then the laboratory door was flung open and Reginald Blake, pale and excited, entered. He had every mark of having been up all night.

"What's the matter?" asked Craig.

"It's about my mother," he blurted out. "She seems to be getting worse all the time. Miss Sears is alarmed, and Betty is almost ill herself with worry. Dr. Wilson doesn't seem to know what it is that affects her, and neither does the new nurse. Can you DO something?"

There was a tone of appeal in his voice that was not like the self-sufficient Reginald of the day before.

"Does there seem to be any immediate danger?" asked Kennedy.

"Perhaps not—I can't say," he urged. "But she is gradually getting worse instead of better."

Kennedy thought a moment. "Has anything else happened?" he asked slowly.

"N-no. That's enough, isn't it?"

"Indeed it is," replied Craig, trying to be reassuring. Then, recollecting Betty, he added, "Reginald, go back and tell your sister for me that she must positively make the greatest effort of her life to control herself. Tell her that her mother needs her—needs her well and brave. I shall be up at the house immediately. Do the best you can. I depend on you."

Kennedy's words seemed to have a bracing effect on Reginald and a few moments later he left, much calmer.

"I hope I have given him something to do which will keep him from mussing things up again," remarked Kennedy, mindful of Reginald's former excursion into detective work.

Meanwhile Craig plunged furiously into his study of the substances he had isolated from the saline solution in which he had "washed" the blood of the little Pekinese.

"There's no use doing anything in the dark," he explained. "Until we know what it is we are fighting we can't very well fight."

For the moment I was overwhelmed by the impending tragedy that seemed to be hanging over Mrs. Blake. The more I thought of it, the more inexplicable became the discovery of the mold.

"That is all very well about the mold on the gelatine strip in the letter," I insisted at length. "But, Craig, there must be something wrong somewhere. Mere molds could not have made Buster so ill, and now the infection, or whatever it is, has spread to Mrs. Blake herself. What have you found out by studying Buster?"

He looked up from his close scrutiny of the material in one of the test tubes which contained something he had recovered from the saline solution of the diffusion apparatus.

I could read on his face that whatever it was, it was serious. "What is it?" I repeated almost breathlessly.

"I suppose I might coin a word to describe it," he answered slowly, measuring his phrases. "Perhaps it might be called hyper-amino-acidemia."

I puckered my eyes at the mouth-filling term Kennedy smiled. "It would mean," he explained, "a great quantity of the amino-acids, non-coagulable, nitrogenous compounds in the blood. You know the indols, the phenols, and the amins are produced both by putrefactive bacteria and by the process of metabolism, the burning up of the tissues in the process of utilizing the energy that means life. But under normal circumstances, the amins are not present in the blood in any such quantities as I have discovered by this new method of diffusion."

He paused a moment, as if in deference to my inability to follow him on such an abstruse topic, then resumed, "As far as I am able to determine, this poison or toxin is an amin similar to that secreted by certain cephalopods found in the neighborhood of Naples. It is an aromatic amin. Smell it."

I bent over and inhaled the peculiar odor.

"Those creatures," he continued, "catch their prey by this highly active poison secreted by the so-called salivary glands. Even a little bit will kill a crab easily."

I was following him now with intense interest, thinking of the astuteness of a mind capable of thinking of such a poison.

"Indeed, it is surprising," he resumed thoughtfully, "how many an innocent substance can be changed by bacteria into a virulent poison. In fact our poisons and our drugs are in many instances the close relations of harmless compounds that represent the intermediate steps in the daily process of metabolism."

"Then," I put in, "the toxin was produced by germs, after all?"

"I did not say that," he corrected. "It might have been. But I find no germs in the blood of Buster. Nor did Dr. Wilson find any in the blood smears which she took from Mrs. Blake."

He seemed to have thrown the whole thing back again into the limbo of the unexplainable, and I felt nonplussed.

"The writer of that letter," he went on, waving the piece of sterile platinum wire with which he had been transferring drops of liquid in his search for germs, "was a much more skillful bacteriologist than I thought, evidently. No, the trouble does not seem to be from germs breathed in, or from germs at all—it is from some kind of germ-free toxin that has been injected or otherwise introduced."

Vaguely now I began to appreciate the terrible significance of what he had discovered.

"But the letter?" I persisted mechanically.

"The writer of that was quite as shrewd a psychologist as bacteriologist," pursued Craig impressively. "He calculated the moral effect of the letter, then of Buster's illness, and finally of reaching Mrs. Blake herself."

"You think Dr. Rae Wilson knows nothing of it yet?" I queried.

Kennedy appeared to consider his answer carefully. Then he said slowly: "Almost any doctor with a microscope and the faintest trace of a scientific education could recognize disease germs either naturally or feloniously implanted. But when it comes to the detection of concentrated, filtered, germ-free toxins, almost any scientist might be baffled. Walter," he concluded, "this is not mere blackmail, although perhaps the visit of that woman to the Prince Henry—a desperate thing in itself, although she did get away by her quick thinking—perhaps that shows that these people are ready to stop at nothing. No, it goes deeper than blackmail."

I stood aghast at the discovery of this new method of scientific murder. The astute criminal, whoever he might be, had planned to leave not even the slender clue that might be afforded by disease germs. He was operating, not with disease itself, but with something showing the ultimate effects, perhaps, of disease with none of the preliminary symptoms, baffling even to the best of physicians.

I scarcely knew what to say. Before I realized it, however, Craig was at last ready for the promised visit to Mrs. Blake. We went together, carrying Buster, in his basket, not recovered, to be sure, but a very different little animal from the dying creature that had been sent to us at the laboratory.