Kennedy thought a moment.  “Of course,” he said, “your mother has had no such relative amount of the poison as Buster has had.  I think that undoubtedly she will recover by purely natural means.  I hope so.  But if not, here is the apparatus,” and he patted the vividiffusion tubes in their glass case, “that will save her, too.”

As well as I could I explained to Reginald the nature of the toxin that Kennedy had discovered.  Duncan listened, putting in a question now and then.  But it was evident that his thoughts were on something else, and now and then Reginald, breaking into his old humor, rallied him about thinking of Betty.

A low exclamation from both Kennedy and the surgeon attracted us.

Dora Sears had moved.

The operation of the apparatus was stopped, the artery and vein had been joined up, and she was slowly coming out from under the effects of the anesthetic.

As we gathered about her, at a little distance, we heard her cry in her delirium, “I—­I would have—­done—­anything—­for him.”

We strained our ears.  Was she talking of the blackmailer, Dr. Hopf?

“Who?” asked Craig, bending over close to her ear.

“I—­I would—­have done anything,” she repeated as if someone had contradicted her.  She went on, dreamily, ramblingly, “He—­is—­is—­ my brother.  I—­”

She stopped through weakness.

“Where is Dr. Hopf?” asked Kennedy, trying to recall her fleeting attention.

“Dr. Hopf?  Dr. Hopf?” she repeated, then smiling to herself as people will when they are leaving the borderline of anesthesia, she repeated the name, “Hopf?”

“Yes,” persisted Kennedy.

“There is no Dr. Hopf,” she added.  “Tell me—­did—­did they—­”

“No Dr. Hopf?” Kennedy insisted.

She had lapsed again into half insensibility.

He rose and faced us, speaking rapidly.

“New York seems to have a mysterious and uncanny attraction for odds and ends of humanity, among them the great army of adventuresses.  In fact there often seems to be something decidedly adventurous about the nursing profession.  This is a girl of unusual education in medicine.  Evidently she has traveled—­her letters show it.  Many of them show that she has been in Italy.  Perhaps it was there that she heard of the drug that has been used in this case.  It was she who injected the germ-free toxin, first into the dog, then into Mrs. Blake, she who wrote the blackmail letter which was to have explained the death.”

He paused.  Evidently she had heard dimly, was straining every effort to hear.  In her effort she caught sight of our faces.

Suddenly, as if she had seen an apparition, she raised herself with almost superhuman strength.

“Duncan!” she cried.  “Duncan!  Why—­didn’t you—­get away—­while there was time—­after you warned me?”

Kennedy had wheeled about and was facing us.  He was holding in his hand some of the letters he had taken from the trunk.  Among others was a folded piece of parchment that looked like a diploma.  He unfolded it and we bent over to read.