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.

CHAPTER XXI

THE POISON BRACELET

We reached the Blake mansion and were promptly admitted.  Miss Betty, bearing up bravely under Reginald’s reassurances, greeted us before we were fairly inside the door, though she and her brother were not able to conceal the fact that their mother was no better.  Miss Sears was out, for an airing, and the new nurse, Miss Rogers, was in charge of the patient.

“How do you feel, this morning?” inquired Kennedy as we entered the sun-parlor, where Mrs. Blake had first received us.

A single glance was enough to satisfy me of the seriousness of her condition.  She seemed to be in almost a stupor from which she roused herself only with difficulty.  It was as if some overpowering toxin were gradually undermining her already weakened constitution.

She nodded recognition, but nothing further.

Kennedy had set the dog basket down near her wheel-chair and she caught sight of it.

“Buster?” she murmured, raising her eyes.  “Is—­he—­all right?”

For answer, Craig simply raised the lid of the basket.  Buster already seemed to have recognized the voice of his mistress, and, with an almost human instinct, to realize that though he himself was still weak and ill, she needed encouragement.