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There were several jars of various liquids on the table and a number of chemicals. Among other things was a sort of gourd, encrusted with a black substance, and in a corner was a box from which sounds issued as if it contained something alive.

I did not trouble Kennedy with questions, for I was only too glad when he consented to take a brisk walk and join me in a thick porterhouse.

It was a large party that gathered in Kennedy’s laboratory that night, one of the largest he had ever had. Mr. and Mrs. Wainwright and Miss Marian came, the ladies heavily veiled. Doctor Nott and Mr. Whitney were among the first to arrive. Later came Mr. Vanderdyke and last of all Mrs. Ralston with Inspector O’Connor. Altogether it was an unwilling party.

“I shall begin,” said Kennedy, “by going over, briefly, the facts in this case.”

Tersely he summarised it, to my surprise laying great stress on the proof that the couple had been asphyxiated.

“But it was no ordinary asphyxiation,” he continued. “We have to deal in this case with a poison which is apparently among the most subtle known. A particle of matter so minute as to be hardly distinguishable by the naked eye, on the point of a needle or a lancet, a prick of the skin scarcely felt under any circumstances and which would pass quite unheeded if the attention were otherwise engaged, and not all the power in the world–unless one was fully prepared–could save the life of the person in whose skin the puncture had been made.”

Craig paused a moment, but no one showed any evidence of being more than ordinarily impressed.

“This poison, I find, acts on the so-called endplates of the muscles and nerves. It produces complete paralysis, but not loss of consciousness, sensation, circulation, or respiration until the end approaches. It seems to be one of the most powerful sedatives I have ever heard of. When introduced in even a minute quantity it produces death finally by asphyxiation–by paralysing the muscles of respiration. This asphyxia is what so puzzled the coroner.

“I will now inject a little of the blood serum of the victims into a white mouse.”

He took a mouse from the box I had seen, and with a needle injected the serum. The mouse did not even wince, so lightly did he touch it, but as we watched, its life seemed gently to ebb away, without pain and without struggle. Its breath simply seemed to stop.

Next he took the gourd I had seen on the table and with a knife scraped off just the minutest particle of the black licorice-like stuff that encrusted it. He dissolved the particle in some alcohol and with a sterilised needle repeated his experiment on a second mouse. The effect was precisely similar to that produced by the blood on the first.

It did not seem to me that anyone showed any emotion except possibly the slight exclamation that escaped Miss Marian Wainwright. I fell to wondering whether it was prompted by a soft heart or a guilty conscience.

We were all intent on what Craig was doing, especially Doctor Nott, who now broke in with a question.

“Professor Kennedy, may I ask a question? Admitting that the first mouse died in an apparently similar manner to the second, what proof have you that the poison is the same in both cases? And if it is the same can you show that it affects human beings in the same way, and that enough of it has been discovered in the blood of the victims to have caused their death? In other words, I want the last doubt set aside. How do you know absolutely that this poison which you discovered in my office last night in that black precipitate when you added the ether–how do you know that it asphyxiated the victims?”

If ever Craig startled me it was by his quiet reply. “I’ve isolated it in their blood, extracted it, sterilised it, and I’ve tried it on myself.”