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 a.s.sault.

"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 acc.u.mulation 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!"

CHAPTER X

THE SIXTH SENSE

"I suppose you have read in the papers of the mysterious burning of our country house at Oceanhurst, on the south sh.o.r.e of Long Island?"

It had been about the middle of the afternoon that a huge automobile of the latest design drew up at Kennedy's laboratory and a stylishly dressed woman, accompanied by a very attentive young man, alighted.

They had entered and the man, with a deep bow, presented two cards bearing the names of the Count and Countess Alessandro Rovigno.

Julia Rovigno, I knew, was the daughter of Roger Gaskell, the retired banker. She had recently married Count Rovigno, a young foreigner whose family had large shipping interests in America and at Trieste in the Adriatic.

"Yes, indeed, I have read about it," nodded Craig.

"You see," she hurried on a little nervously, "it was a wedding present to us from my father."

"Giulia," put in the young man quickly, giving her name an accent that was not, however, quite Italian, "thinks the fire was started by an incendiary."

Rovigno was a tall, rather boyish-looking man of thirty-two or thirty-three, with light brown hair, light brown beard and mustache. His eyes and forehead spoke of intelligence, but I had never heard that he cared much about practical business affairs. In fact, to American society Rovigno was known chiefly as one of the most daring of motor-boat enthusiasts.

"It may have been the work of an incendiary," he continued thoughtfully, "or it may not. I don't know. But there has been an epidemic of fires among the large houses out on Long Island lately."

I nodded to Kennedy, for I had myself compiled a list for the _Star_, which showed that considerably over a million dollars' worth of show places had been destroyed.

"At any rate," added the Countess, "we are burned out, and are staying in town now--at my father's house. I wish you would come around there.

Perhaps father can help you. He knows all about the country out that way, for his own place isn't a quarter of a mile away."

"I shall be glad to drop around, if I can be of any a.s.sistance," agreed Kennedy as the young couple left us.

The Rovignos had scarcely gone when a woman appeared at the laboratory door. She was well dressed, pretty, but looked pale and haggard.

"My name is Mrs. Bettina Petzka," she began, singling out Kennedy. "You do not know me, but my husband, Nikola, was one of the first students you taught, Professor."

"Yes, yes, I recall him very well," replied Craig. "He was a brilliant student, too--very promising. What can I do for you?"

"Why, Professor Kennedy," she cried, no longer able to control her feelings, "he has suddenly disappeared."

"What line of work had he taken up?" asked Craig, interested.

"He was a wireless operator--had been employed on a liner that runs to the Adriatic from New York. But he was out of work. Someone has told me that he thought he saw Nikola in Hoboken around the docks where a number of the liners that go to blockaded ports are laid up waiting the end of the war."

She paused.

"I see," remarked Kennedy, pursing up his lips thoughtfully. "Your husband was not a reservist of any of the countries at war, was he?"

"No--he was first of all a scientist. I don't think he had any interest in the war--at least he never talked much about it."

"I know," persisted Craig, "but had he taken out his naturalization papers here?"

"He had applied for them."

"When did he disappear?"

"I haven't seen him for two nights," she sobbed.

It flashed over me that it was now two nights since the fire that had burned Rovigno's house, although there was no reason for connecting the events, at least yet.

The young woman was plainly wild with anxiety. "Oh, can't you help me find Nikola?" she pleaded.

"I'll try my best," rea.s.sured Kennedy, taking down on a card her address and bowing her out.

It was late in the afternoon before we had an opportunity to call at the Gaskell town house where the Rovignos were staying. The Count was not at home, but the Countess welcomed us and led us directly into a large library.