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“Then came the search. Perhaps she was restless. Another injection settled that. At last the great diamond was found. But the twilight sleep meant not forgetfulness but death to Rawaruska!”

Craig paused. It was almost as if one could see the word picture of the scene as he painted it.

“What was to be done? The diamond must be recut–anything to hide its identity, at once, and at any cost. And Margot? The story of the Arkansas diamond and the sale is a blind. The case is perfect!”

Kennedy raised his eyes for the first time from the study of the little electrical machine before him, and caught the eye of Cecilie, holding it, unwilling.

“Did you ever hear of the great diamond, the Invincible?” Kennedy smashed out.

I felt that it might not have been exactly chivalrous, but it was necessary.

Cecilie’s breast, which had showed a wildly beating heart as Kennedy told of how her mistress had died, was calmer now. Her air of surprise at the mention of the diamond was perfect. Elsa Hoffman was gazing at her, too, in tense interest. De Guerre was outwardly cool, Margot openly cynical, Preston leaning forward in ill-suppressed excitement.

For a moment Kennedy paused again, as if allowing all to collect themselves before he took them by assault.

“I have lately been studying,” he remarked casually, “the experiments of Dr. Von Pfungen of Vienna showing the protective resistance of the human skin against an electric current. Normally, this resistance averages from seventy to eighty thousand ohms. In the morning, owing to the accumulation of waste products, the resistance may mount to almost double. In persons suffering from nervous anxiety, it decreases to five thousand and even down to a thousand ohms in cases of hysteria. Von Pfungen has also measured a human being’s emotional feelings by the electric current. I have a copy of his instrument here. There is one person who sits gripping the carbon electric handle connected with this galvanometer who, to begin with, had a resistance of over sixty thousand. But when I began to tell of how Rawaruska met her death, of the hypothetical case I have built up by my observations and experiments here in this very laboratory, the needle of the galvanometer started to oscillate downward. It went down until it reached thirty-eight thousand at the mention of murder. When I said the case was perfect, it had got as low as under twenty thousand, swinging lower and lower as the person saw hope depart!”

Kennedy was no longer paying any attention to the little instrument. As I followed him, I became more and more impatient. What was it he had discovered? Who was it?

“Preston,” cried Kennedy, suddenly wheeling on the young doctor, “through your regard–honorable, I am sure–for Rawaruska you have let yourself be drawn into doing a little amateur detective work. Let me warn you. Instead of clearing up the case, you merely laid yourself open to suspicion. Fortunately the galvanometer absolves you. You should have known that Cecilie was only a tool. De Guerre, your black wallet, that all diamond dealers carry–thank you, Wade–that’s it.”

Kennedy had turned from Preston to Cecilie, then to De Guerre so suddenly that no one was prepared for the signal he gave to the customs officer.

Wade had covered the surprised dealer and was now emptying out the contents of the wallet.

There, on the table, gleaming in the light of the laboratory, lay a wonderful brilliant, some three hundred carats–perfect in its blazing crystalline orange beauty. There it lay, a jewel which might charm and arouse the cupidity of two hemispheres. It shone like a thing of life. Yet back of its orange fire lay a black tragedy.

Margot was on his feet instantly.

“That is not the–“

“Just a moment, Mr. Margot,” interrupted Kennedy. “I think Mr. Wade will be able to show that it is the Invincible when he matches up the parts that have been hurriedly cut from–from the wonderful Arkansas diamond,” Craig added sarcastically. “Miss Hoffman, Dr. Preston tells us that before you were a diamond saleswoman you had been a trained nurse!”

The look Elsa Hoffman flashed, as her calm exterior refused to conceal her emotions longer, was venomous.

Kennedy was the calmest one of us all as he tapped the little galvanometer significantly with his index finger.

“De Guerre,” he exclaimed, leaning forward slightly, “you and your lover, Elsa Hoffman, planned cunningly to rob your own brothers. But, instead of robbers merely,” he ground out, “you are murderers!”