“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.”