t the lower part of the body showed no wounds or
bruises whatever.

"Tracks of blood such as are left by dragging a bleeding body differ
very greatly from tracks of arterial blood which are left when the
victim has strength to move himself. Continuing my speculations,
supposing it to be a blood-spot, what did it indicate? Clearly that
Mr. Langley was struck by somebody on the head with a heavy instrument,
perhaps in another part of the room, that he was choked, that as the
drops of blood oozed from the wound on his head, he was dragged across
the floor, in the direction of the fireplace--"

"But, Professor Kennedy," interrupted Doctor Putnam, "have you proved
that the spot was a blood-spot? Might it not have been a paint-spot or
something of that sort?"

Kennedy had apparently been waiting for just such a question.

"Ordinarily, water has no effect on paint," he answered. "I found that
the spot could be washed off with water. That is not all. I have a test
for blood that is so delicately sensitive that the blood of an Egyptian
mummy thousands of years old will respond to it. It was discovered by
a German scientist, Doctor Uhlenhuth, and was no longer ago than last
winter applied in England in connection with the Clapham murder.
The suspected murderer declared that stains on his clothes were only
spatters of paint, but the test proved them to be spatters of blood.
Walter, bring in the cage with the rabbits."

I opened the door and took the cage from the groom, who had brought it
up from the stable and stood waiting with it some distance away.

"This test is very simple, Doctor Putnam," continued Craig, as I placed
the cage on the table and Kennedy unwrapped the sterilised test-tubes.
"A rabbit is inoculated with human blood, and after a time the serum
that is taken from the rabbit supplies the material for the test.

"I will insert this needle in one of these rabbits which has been so
inoculated and will draw off some of the serum, which I place in this
test-tube to the right. The other rabbit has not been inoculated. I draw
off some of its serum and place that tube here on the left--we will call
that our 'control tube.' It will check the results of our tests.

"Wrapped up in this paper I have the scrapings of the spot which I
found on the floor--just a few grains of dark, dried powder. To show
how sensitive the test is, I will take only one of the smallest of these
minute scrapings. I dissolve it in this third t