Why had he done so?
He had been in the courtyard when Norbert threw the glass from the window.

Putting all these circumstances together, as soon as the inanimate body of the Duke had been laid upon a bed, Jean went into the dining-room, feeling sure that he should make some discovery which would confirm his suspicions.

The bottle from which the Duke had filled his glass stood half emptied upon the table.

With the greatest care, he poured a few drops of its contents into the hollow of his hand, and tasted it with the utmost caution.

The wine still retained its customary taste and scent.