“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.”

In breathless amazement, with eyes riveted on Craig, we listened.

“Altogether I was able to recover from the blood samples of both of the victims of this crime six centigrams of the poison,” he pursued.  “Starting with two centigrams of it as a moderate dose, I injected it into my right arm subcutaneously.  Then I slowly worked my way up to three and then four centigrams.  They did not produce any very appreciable results other than to cause some dizziness, slight vertigo, a considerable degree of lassitude, and an extremely painful headache of rather unusual duration.  But five centigrams considerably improved on this.  It caused a degree of vertigo and lassitude that was most distressing, and six centigrams, the whole amount which I had recovered from the samples of blood, gave me the fright of my life right here in this laboratory this afternoon.

“Perhaps I was not wise in giving myself so large an injection on a day when I was overheated and below par otherwise because of the strain I have been under in handling this case.  However that may be, the added centigram produced so much more on top of the five centigrams previously taken that for a time I had reason to fear that that additional centigram was just the amount needed to bring my experiments to a permanent close.