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.

"Within three minutes of the time of injection the dizziness and vertigo
had become so great as to make walking seem impossible. In another
minute the lassitude rapidly crept over me, and the serious disturbance
of my breathing made it apparent t