He took a needle and injected some of a liquid which he had isolated.  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, without struggle.  Its breath simply seemed to stop.

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

I was intent on what Craig was doing when Dr. Leslie broke in with a question.  “May I ask,” he queried, “whether, admitting that the first mouse died at least apparently in the same manner as the second, you have proved 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, that enough of it has been discovered in the blood of Mendoza to have caused his death?  In other words, I want the last doubt set aside.”

If ever Craig startled me, it was by his quiet reply: 

“I’ve isolated it in his blood, extracted it, sterilized it, and I’ve tried it on myself.”

In breathless amazement, with eyes riveted on him, we listened.  “Then that was what was the matter?” I blurted out.  “You had been trying the poison on yourself?”

He nodded unconcernedly.  “Altogether,” he explained, as Leslie and I listened, speechless, “I was able to recover from both blood samples six centigrams of the poison.  It is almost unknown.  I could only be sure of what I discovered by testing the physiological effects.  I was very careful.  What else was there to do?  I couldn’t ask you fellows to try it, if I was afraid.”

“Good heavens!” gasped Leslie, “and alone, too.”

“You wouldn’t have let me do it, if I hadn’t got rid of you,” he smiled quietly.

Leslie shook his head.  “Tried it on the dog and made himself the dog!” exclaimed Leslie.  “I need the credit of a successful case—­ but I’ll not take this one.”

Kennedy laughed.

“Starting with two centigrams of the stuff as a moderate dose,” he pursued, while I listened, stunned at his daring, “I injected it into my right arm subcutaneously.  Then I slowly worked my way up to three and then four centigrams.  You see what I had recovered was far from the real thing.  They did not seem at first to 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.”

“Good night!” I exclaimed.  “Didn’t that satisfy you?”

“Five centigrams considerably improved on it,” he continued, paying no attention to me.  “It caused a degree of lassitude and vertigo 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 a few minutes before you came in.”