Thorndyke had just handed the card to the coroner when a very singular disturbance arose.  While my friend had been giving the latter part of his evidence, I had observed the man Petrofsky rise from his seat and walk stealthily across to the door.  He turned the handle softly and pulled, at first gently, and then with more force.  But the door was locked.  As he realized this, Petrofsky seized the handle with both hands and tore at it furiously, shaking it to and fro with the violence of a madman, and his shaking limbs, his starting eyes, glaring insanely at the astonished spectators, his ugly face, dead white, running with sweat and hideous with terror, made a picture that was truly shocking.

Suddenly he let go the handle, and with a horrible cry thrust his hand under the skirt of his coat and rushed at Thorndyke.  But the superintendent was ready for this.  There was a shout and a scuffle, and then Petrofsky was born down, kicking and biting like a maniac, while Miller hung on to his right hand and the formidable knife that it grasped.

[Illustration:  SUPERINTENDENT MILLER RISES TO THE OCCASION.]

“I will ask you to hand that knife to the coroner,” said Thorndyke, when Petrofsky had been secured and handcuffed, and the superintendent had readjusted his collar.  “Will you kindly examine it, sir,” he continued, “and tell me if there is a notch in the edge, near to the point—­a triangular notch about an eighth of an inch long?”

The coroner looked at the knife, and then said in a tone of surprise:  “Yes, there is.  You have seen this knife before, then?”

“No, I have not,” replied Thorndyke.  “But perhaps I had better continue my statement.  There is no need for me to tell you that the fingerprints on the card and on the candle are those of Paul Petrofsky; I will proceed to the evidence furnished by the body.

“In accordance with your order, I went to the mortuary and examined the corpse of the deceased.  The wound has been fully and accurately described by Dr. Davidson, but I observed one fact which I presume he had overlooked.  Embedded in the bone of the spine—­in the left transverse process of the fourth vertebra—­I discovered a small particle of steel, which I carefully extracted.”

He drew his collecting-box from his pocket, and taking from it a seed-envelope, handed the latter to the coroner.  “That fragment of steel is in this envelope,” he said, “and it is possible that it may correspond to the notch in the knife-blade.”

Amidst an intense silence the coroner opened the little envelope, and let the fragment of steel drop on to a sheet of paper.  Laying the knife on the paper, he gently pushed the fragment towards the notch.  Then he looked up at Thorndyke.

“It fits exactly,” said he.

There was a heavy thud at the other end of the room and we all looked round.

Petrofsky had fallen on to the floor insensible.