“Let us assume it was a blood-spot, however.  What did it show?  It was just a little regular round spot, quite thick.  Now, drops of blood falling only a few inches usually make a round spot with a smooth border.  Still the surface on which the drop falls is quite as much a factor as the height from which it falls.  If the surface is rough the border may be irregular.  But this was a smooth surface and not absorbent.  The thickness of a dried blood-spot on a non-absorbent surface is less the greater the height from which it has fallen.  This was a thick spot.  Now if it had fallen, say, six feet, the height of Mr. Langley, the spot would have been thin—­some secondary spatters might have been seen, or at least an irregular edge around the spot.  Therefore, if it was a blood-spot, it had fallen only one or two feet.  I ascertained next that 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.