“The rest may be conjectured,” interrupted M. Lecoq.  “She had gone to secrete the manuscript in some safe place; and when her new husband asked her to give it up to him, she replied, ‘Look for it.’”

“Sauvresy had enjoined on me to give it only into her hands.”

“Oh, he knew how to work his revenge.  He had it given to his wife so that she might hold a terrible arm against Tremorel, all ready to crush him.  If he revolted, she always had this instrument of torture at hand.  Ah, the man was a miserable wretch, and she must have made him suffer terribly.”

“Yes,” said Dr. Gendron, “up to the very day he killed her.”

The detective had resumed his promenade up and down the library.

“The question as to the poison,” said he, “remains.  It is a simple one to resolve, because we’ve got the man who sold it to her in that closet.”

“Besides,” returned the doctor, “I can tell something about the poison.  This rascal of a Robelot stole it from my laboratory, and I know only too well what it is, even if the symptoms, so well described by our friend Plantat, had not indicated its name to me.  I was at work upon aconite when Sauvresy died; and he was poisoned with aconitine.”

“Ah, with aconitine,” said M. Lecoq, surprised.  “It’s the first time that I ever met with that poison.  Is it a new thing?”

“Not exactly.  Medea is said to have extracted her deadliest poisons from aconite, and it was employed in Rome and Greece in criminal executions.”

“And I did not know of it!  But I have very little time to study.  Besides, this poison of Medea’s was perhaps lost, as was that of the Borgias; so many of these things are!”

“No, it was not lost, be assured.  But we only know of it nowadays by Mathiole’s experiments on felons sentenced to death, in the sixteenth century; by Hers, who isolated the active principle, the alkaloid, in 1833 and lastly by certain experiments made by Bouchardat, who pretends—­”

Unfortunately, when Dr. Gendron was set agoing on poisons, it was difficult to stop him; but M. Lecoq, on the other hand, never lost sight of the end he had in view.

“Pardon me for interrupting you, Doctor,” said he.  “But would traces of aconitine be found in a body which had been two years buried?  For Monsieur Domini is going to order the exhumation of Sauvresy.”

“The tests of aconitine are not sufficiently well known to permit of the isolation of it in a body.  Bouchardat tried ioduret of potassium, but his experiment was not successful.”

“The deuce!” said M. Lecoq.  “That’s annoying.”

The doctor smiled benignly.

“Reassure yourself,” said he.  “No such process was in existence—­ so I invented one.”

“Ah,” cried Plantat.  “Your sensitive paper!”

“Precisely.”

“And could you find aconitine in Sauvresy’s body?”

“Undoubtedly.”