Here's the midrash.
- The conspirators got wind of their betrayal to the king, and in good time they removed the poison they had already placed in Ahasuerus's cup. But that the lie might not be given to Mordecai, God caused poison to appear where none had been, and the conspirators were convicted of their crime. The king had the water analyzed which he was given to drink and it was made manifest that it contained poison. (Legends of the Jews, volume 2, page 1145)
So even though they had removed all traces of their crime, God caused the poison to manifest. Better yet, the two tried to evade punishment through suicide, but were hanged instead.
Now, look to The Count of Monte Cristo.
- Just then, Madame de Villefort, in the act of slipping on her dressing-gown, threw aside the drapery and for a moment stood motionless, as though interrogating the occupants of the room, while she endeavored to call up some rebellious tears. On a sudden she stepped, or rather bounded, with outstretched arms, towards the table. She saw d'Avrigny curiously examining the glass, which she felt certain of having emptied during the night. It was now a third full, just as it was when she threw the contents into the ashes. The spectre of Valentine rising before the poisoner would have alarmed her less. It was, indeed, the same color as the draught she had poured into the glass, and which Valentine had drank; it was indeed the poison, which could not deceive M. d'Avrigny, which he now examined so closely; it was doubtless a miracle from heaven, that, notwithstanding her precautions, there should be some trace, some proof remaining to reveal the crime. (emph. mine) While Madame de Villefort remained rooted to the spot like a statue of terror, and Villefort, with his head hidden in the bedclothes, saw nothing around him, d'Avrigny approached the window, that he might the better examine the contents of the glass, and dipping the tip of his finger in, tasted it. "Ah," he exclaimed, "it is no longer brucine that is used; let me see what it is!"
Then he ran to one of the cupboards in Valentine's room, which had been transformed into a medicine closet, and taking from its silver case a small bottle of nitric acid, dropped a little of it into the liquor, which immediately changed to a blood-red color. "Ah," exclaimed d'Avrigny, in a voice in which the horror of a judge unveiling the truth was mingled with the delight of a student making a discovery. Madame de Villefort was overpowered, her eyes first flashed and then swam, she staggered towards the door and disappeared.
Oh, and interestingly, Madame de Villefort is given the choice between the scaffold and suicide, and chooses suicide.
Alexandre Dumas was on to something, wasn't he? Of course, it's the Count who plants the poison, but even so...